Spiral antenna.

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The radiation pattern of the helical antenna is stable over a wide frequency band; for example, a spiral with a gradually changing diameter of individual turns had an operating frequency range of 120 - 450 MHz; the initial diameter was 60 cm, and after 10 turns, the axial length of which was 112 cm, the diameter decreased to 20 cm; the point of excitation was at the apex. It has been shown that the dimensions of the conductor have little effect on the radiation characteristics.

Since the radiation pattern of an equiangular helix antenna rotates as the frequency changes, when detailed study To change the pattern with frequency, it is necessary to rotate the antenna for each frequency shift. At the usual way During operation, the antenna is usually stationary and it is desirable to know changes in the position of the diagram relative to the initial state of the antenna.

The width of the radiation pattern of a helical antenna decreases with an increase in the elevation angle, the number of turns of the helix, and a decrease in the diameter of the screen.

The radiation pattern width of a helical antenna decreases in inverse proportion to the square root of the helix length in wavelengths. The relationships (21 - 21) - (21 - 23) will be illustrated in the following example.

The multiplier / s (6) of the radiation pattern of a helical antenna has a maximum directed along the axis of the helix in the positive direction for the current wave Jv (n), in the negative direction for the current wave Jv (n 1), if the values of ka are selected in intervals corresponding to strong dispersion of the phase velocity of these current waves.

The results of solving the problem of excitation of natural waves T by given field sources, discussed in the chapter, are used in the next chapter to analyze the dependence of the radiation patterns of helical antennas on the parameters of the exciting sources.

In this case, RP - k - Ap, the phase velocity of the nth spatial harmonic is close to the speed of light in free space, and is directed in the opposite direction compared to the current wave in the spiral conductor. The multiplier Ус (9) of the directivity pattern of a helical antenna has a maximum along the axis of the system, but is directed towards the current wave.

IN technical literature There is a large number of theoretical and experimental works devoted to the study of the radiation patterns of equiangular helical antennas. However, in these works the radiation patterns of equiangular helical antennas with angular parameters fro and a limited to small limits are studied. Formulas have been obtained for the radiation patterns of conical helical antennas with values of cone and winding angles that satisfy the condition sindotgaCl, but their use for engineering calculations is difficult, since the formulas are sums of complex fields of turns. For the same reason, the expressions for the radiation field given in are not convenient for obtaining formulas for the phase and polarization characteristics of equiangular helical antennas. In the known literature there are no formulas for calculating the radiation pattern of multi-pass equiangular helical antennas, and there are also no sufficiently extensive families of calculated graphs of the radiation patterns of equiangular spiral antennas for various angular parameters before and a and at various numbers antenna approaches.

Thus, the wave field Tn, when conditions (3.12) are met, is mainly determined by the lth spatial harmonic and has a phase velocity close to the speed of light in free space. In this case, the multiplier /c (9) of the helical antenna radiation pattern has a maximum along the helix axis in the direction of propagation of the current wave. Therefore, the interval ka, in which conditions (3.12) are satisfied, is called the region of strong phase velocity dispersion.

In the technical literature there is a large number of theoretical and experimental works devoted to the study of the radiation patterns of equiangular helical antennas. However, in these works the radiation patterns of equiangular helical antennas with angular parameters fro and a limited to small limits are studied. Formulas have been obtained for the radiation patterns of conical helical antennas with values of cone and winding angles that satisfy the condition sindotgaCl, but their use for engineering calculations is difficult, since the formulas are sums of complex fields of turns. For the same reason, the expressions for the radiation field given in are not convenient for obtaining formulas for the phase and polarization characteristics of equiangular helical antennas. In the known literature there are no formulas for calculating the radiation pattern of multi-pass equiangular helical antennas, and there are also no sufficiently extensive families of calculated graphs of the radiation patterns of equiangular spiral antennas for different angular parameters up and a and for different numbers of antenna passes.

Pages: 1

1. Radiation modes of helical antenna 2

2.Calculation ratios for a cylindrical helical antenna 5

3.Planar arithmetic helical antenna 8

4. Equiangular (logarithmic) spiral antenna 11

5.Example of calculation of a cylindrical helical antenna 14

References 16

1. Radiation modes of a helical antenna.

1.1. A helical antenna is a wire rolled into a spiral (1), which is fed through a coaxial feeder (2) (Fig. 1, a). The inner wire of the feeder is connected to the spiral, and outer shell feeder - with a metal disk (3). The latter serves as a reflector and also prevents the penetration of currents from the inner to the outer surface of the feeder shell. The spiral can be not only cylindrical, as in Fig. 1, a, but also conical (Fig. 1, c) and flat (Fig. 7) or convex.

Fig.1. Helix antennas:

a - cylindrical; b – unfolded coil; c – conical.

A cylindrical spiral antenna is characterized by the following geometric dimensions: radius a, pitch s, length of one turn, number of turns p, axial length, elevation angle.

As can be seen from the diagram of the antenna and the image of the unfolded turn of the spiral (Fig. 1, b), there are the following dependencies between the dimensions of the antenna:

, ,

1.2. Helical antennas are used on VHF in the traveling wave mode with axial radiation and rotating polarization. This mode requires certain relationships between antenna dimensions and wavelength. Let us identify these relationships.

A high-frequency current passing through a spiral causes the emission of electromagnetic waves. Ten to eleven turns are enough so that all the energy supplied to the antenna is radiated into space and no waves are reflected from the end of the spiral. Such a traveling current wave propagates along the spiral wire with phase speed, i.e., with deceleration .

Fig. 2. Helical antenna turn

The wave passes one turn (from section 1 to section 5 in Fig. 2) in time. Electromagnetic waves excited by the spiral current propagate in the air with speed c and wavelength.

If all the turns merged, then it would be enough to set the time equal to the oscillation period , i.e. so that the fields of any pair of opposite elements (1-3,2-4) of the spiral are in phase and completely add up at the points of the 0"0" axis, which is equidistant from the contour of the coil. This is explained by the fact that within one turn the current amplitudes are almost the same, and the difference in phase by angle in the diametrically opposite sections of the turn (1-3, 2-4) is compensated by the opposite direction of the currents in them.

In the case of a cylindrical spiral with a step s, the condition for maximum axial radiation is formulated somewhat differently: during the passage of current through the turn, the electromagnetic wave must travel in the air a distance greater than the wavelength, by step s:

; respectively

(1)

With such a retardation factor, currents in any two sections located at an angle of 90° (for example, in 1 and 2, 2 and 3, 3 and 4, 4 and 5) cause fields on the O "O" axis that are shifted in phase by 90°, and waves that are polarized at an angle of 90°. As a result of the addition of these linearly polarized waves, circularly polarized waves are obtained.

1.3. It has been experimentally established that as the wavelength increases, the phase velocity decreases, and the retardation coefficient increases by the same factor. Thanks to this, the condition of axial radiation (1) is maintained in a wide range of waves:

(Fig. 3, a).

Fig. 3. Pattern of a cylindrical helical antenna

at different lengths of the spiral turn

With a coil length of 360°, a phase shift occurs when a current wave passes several turns of the spiral. In this case, the antenna is likened to an electrically small frame of N turns of wire, which has a pattern in the form of a figure eight with radiation maxima in a plane perpendicular to the axis of the spiral (Fig. 3, b). If, then two, three or more waves fit on one turn of the spiral, and this leads to inclined radiation and a conical shape of the spatial pattern (Fig. 3, c).

1.4. The most advantageous mode is axial radiation, which, as is known, requires a turn length and provides a passband. This band can be significantly expanded by moving to a conical antenna (Fig. 1, b), in which section (2) with an average turn length satisfies the condition, and the outer sections (1, 3) with larger () and smaller () turn lengths satisfy similar conditions, but for the maximum and minimum wavelengths of the operating range:

,. Depending on the operating wavelength, only one of the zones of the spiral radiates intensely, and only this active zone determines the sharpness of the pattern.

The main problems that arise when using 802.11b/g wireless networks are that the connection is not stable enough due to weak level received signal, and the strong dependence of the transmission speed on the distance between the wireless network adapter and access point. So, if within a room (office) one access point is able to ensure stable operation of wireless clients, then it is hardly possible to guarantee stable communication with a client located behind the wall, and not every access point can “break through” two walls.

If we talk about operating a wireless access point in an apartment or office, the situation when wireless clients are in different rooms and separated from the access point by a wall or even two is quite real. It would seem that the problem can be solved quite simply: you just need to purchase an access point with high transmitter power. However, the transmission power of 802.11b/g wireless devices is regulated by regulation. In particular, in the frequency band 2400-2483.5 MHz (i.e. devices of the 802.11b/g standard) to create radio data networks without frequency planning and on a license-free basis, it is allowed to use transmitters with radiated power that is equivalent to isotropically radiated power (EIRP) , no more than 100 mW. If this indicator is exceeded, it is necessary to obtain a license from the Ministry of Communications to create and operate a departmental radio data transmission network.

There is another obstacle, and a more serious one. The fact is that access points and wireless adapters with a transmission power of more than 100 mW, which is equivalent to 20 dBm (we will talk about how these units are connected to each other below), are not on sale at all (we are, of course, talking about devices aimed at end users).

What can be done in this situation? You can, of course, focus on distributed wireless networks. However, this solution cannot be called cheap, since to increase the coverage area wireless network requires the use of not one, but several wireless access points. Another way to increase the coverage area of a wireless network is to use directional antennas, which do not change the EIRP parameter (and therefore do not violate the law), but amplify the signal in a certain direction. In this article we will look at the most typical examples of directional antennas and talk about how you can make them yourself from scrap materials.

Characteristics of directional antennas

Although directional antennas are often called amplification antennas and are even characterized by a gain, they do not actually amplify the transmitted signal. That is, if the transmitter power, for example, is 50 mW, then no matter what antenna we install, the power of the transmitted signal will not change. This is understandable: all antennas of this kind are passive, so they simply have nowhere to get energy to amplify the transmitted signal.

What then is the effect of amplifying the signal by a transmitting antenna? Imagine an electric lamp illuminating a room. The light from this bulb spreads approximately evenly in all directions, making the entire room bright. However, the same light bulb can be installed in a lantern by creating a parabolic mirror reflector behind it. In this case, we will get directional propagation of light, that is, a beam of light that will not illuminate the entire room, but will be able to transmit light to a significantly larger area. longer distance. External antennas work precisely on this principle: they do not change the power of the transmitted signal, but change its directional pattern.

Isotropic emitter

Antennas radiate energy in all directions, but the efficiency of signal transmission for different directions may not be the same and is characterized by a radiation pattern. In order to evaluate the efficiency of signal transmission for different directions, the concept of an isotropic emitter (omni), or isotropic antenna, was introduced.

An isotropic emitter is an ideal point source of electromagnetic waves, emitting a spherical field that is uniform in energy density. Isotropic emitters do not occur in nature. Each transmitting antenna, even the simplest one, emits energy unevenly, but there is always a direction in which the maximum energy is emitted. The concept of an isotropic emitter is considered exclusively as some idealized standard emitter, with which it is convenient to compare all other antennas.

Antenna radiation pattern

The directional properties of antennas are determined by the dependence of the strength of the field emitted by the antenna on the direction. Graphic image This dependence is called the antenna radiation pattern. The three-dimensional radiation pattern is depicted as a surface described by a radius vector emanating from the origin, the length of which in each direction is proportional to the energy emitted by the antenna in a given direction.

In addition to three-dimensional diagrams, two-dimensional diagrams are often considered, which are built for the horizontal and vertical planes. In this case, the radiation pattern is a closed line in the polar coordinate system, constructed in such a way that the distance from the antenna (the center of the diagram) to any point on the radiation pattern is directly proportional to the energy emitted by the antenna in a given direction.

In the case of an ideal isotropic antenna, radiating energy equally in all directions, the radiation pattern is a sphere, the center of which coincides with the position of the isotropic emitter. In this case, the horizontal and vertical radiation patterns of the isotropic emitter are circles.

In the case of directional antennas, the so-called lobes can be distinguished in the radiation pattern, that is, the directions of preferential radiation. The direction of maximum radiation from antennas is called the main direction, and the corresponding lobe is called the main direction. The remaining lobes are side lobes, and the radiation lobe oriented in the direction opposite to the main direction is called the back lobe of the antenna radiation pattern. The directions in which the antenna does not receive or radiate are called zeros of the radiation pattern.

The radiation pattern is also characterized by its width. The width of the radiation pattern is understood as the angle within which the gain decreases relative to the maximum by no more than 3 dB. Almost always, the gain and the width of the diagram are related: the higher the gain, the narrower the diagram, and vice versa.

The most simple type antenna, which is often used in wireless devices, is a Hertz dipole in radio engineering it is equivalent to a small antenna, the size of which is much smaller than the radiation wavelength (Fig. 1).

The Hertz dipole radiation pattern shown in Fig. 2, resembles a torus, the cross section of which consists of two touching circles. The antenna patterns of various transmitters have approximately the same shape. It should be noted that the maximum flow of electromagnetic energy is emitted in a plane perpendicular to the dipole axis. The dipole does not radiate energy along its axis. The vertical and horizontal radiation patterns of the Hertz dipole are shown in Fig. 3 and 4.

Antenna gain

One more important characteristic directional antenna is the gain, which shows how many times the efficiency of this antenna is higher compared to an isotropic emitter.

Antenna gain is defined as the ratio of the energy flux density emitted in a particular direction to the energy flux density that would be detected using an isotropic antenna. Thus, the antenna gain determines how much more field strength a given antenna will create compared to an isotropic antenna at the same distance, all other things being equal. Antenna gain is measured in so-called isotropic decibels (dBi or dBi):

,

Where k antenna gain in a given direction; E field strength created by the antenna at a certain point; E omni field strength created by an isotropic antenna at the same point.

Let's say the antenna gain in a given direction is 5 dBi this means that in this direction the radiation power is 5 dB (3.16 times) greater than the radiation power of an ideal isotropic antenna. Naturally, an increase in signal power in one direction entails a decrease in power in other directions.

When they say that the antenna gain is 10 dBi, they mean the direction in which the maximum radiation power is achieved (the main lobe of the radiation pattern).

For example, when using a wireless access point with a transmitter power of 20 dBm (100 mW) and a directional antenna with a gain of 10 dBi, the signal power in the direction of maximum gain will be 20 dBm + 10 dBi = 30 dBm (1000 mW), that is, 10 times more than when using an isotropic antenna. Therefore, with such an antenna, two reinforced concrete walls will not become a problem.

In physics, power is usually measured in watts (W), but in communications theory, decibels (dB) are more often used to measure signal strength. This unit measurement is logarithmic and can only be used to compare physical quantities of the same name. So, if two values A and B of the same physical quantity are compared, then the A/B ratio shows how many times one quantity is greater than the other. If we consider the decimal logarithm of the same ratio (), then we get a comparison of these values, but expressed in bels (B), and the expression determines the comparison of these values in decibels (dB). If, for example, they say that one value is 20 dB greater than another, then this means that one value is 100 times greater than the other.

Decibels are used not only to compare values, but also to express absolute values. However, for this purpose, a certain reference value is taken as the second value with which comparison is made. To express absolute value signal power in decibels, a power of 1 mW is taken as a reference value, and the power level is compared in decibels with a power of 1 mW. This unit of measurement, called decibels per milliwatt (dBm), shows how many decibels the power of the measured signal is greater than the power of 1 mW.

Making directional antennas with your own hands

In most cases, 802.11b/g access points are equipped with miniature whip antennas (Fig. 5), which can be either removable or non-removable. In the horizontal plane, such antennas are omnidirectional with a gain of no more than 4 dBi.

The height of such an antenna is 88 mm, but if you disassemble such an antenna (Fig. 6), you will notice that the length of the antenna itself is only 30 mm.

Rice. 6. Standard whip antenna disassembled

It is clear that you cannot expect anything serious from such an antenna, which is why many manufacturers of wireless Wi-Fi equipment also produce directional antennas with a higher gain as accessories for their access points. The main problem with all such external antennas is their unreasonably high price: on average, you will have to pay at least $50 for such a directional antenna, although, by and large, there is nothing special in its design. So why not try making such an antenna yourself?

In this article we will look at several schemes homemade antennas for the 2400 MHz band, which can be found on various Internet sites. However, before you take practical steps and run to the Chip and Dip store, it’s a good idea to familiarize yourself with the types of connectors used to connect antennas to cable.

RF connectors for connecting antennas to cable

To connect the antennas to the cable, specialized high-frequency (HF) connectors are used, which can be purchased at specialized stores (for example, Chip and Dip). There are several options for such connectors, which differ from each other in the type of thread (inch or metric), and the type of cable (RG-58, RG-8, etc.), and other characteristics. In addition, RF connectors also differ in the way the cable is fastened: crimped, soldered, or with a nut.

All connectors are classified by series. Thus, there are connectors of the N, BNC, F, FME, SMA, SMB, TNC, UHF series. Unfortunately, there is no uniform standard for marking connectors, and therefore each manufacturing company uses its own connector designation.

In most cases, it is recommended to use N-series connectors to create directional antennas. However, it is worth considering that N-series connectors are the largest and that their installation may be inconvenient in some cases. From our own experience we can say that it is not at all necessary to use N-series connectors. The main thing is that the Male type connector (male) matches the Female type connector: one of these connectors is installed on the antenna reflector, and the second is mounted on the cable. It is clear that the connector mounted on the antenna must have either a flange or nuts that allow it to be mounted on the reflector (such connectors are called instrument connectors).

In Fig. 7-12 show connectors of various series. It is easy to notice that SMA connectors are the smallest, and N connectors, on the contrary, are the largest.

with fastening nut. Marking TNC-7401A

Marking TNC-7422

for mounting on the antenna cable.
Marking TNC-7422

Let us also note that in most cases, both on the points themselves and on the antennas for them, miniature SMA-type connectors are used, and on the antennas the Female type connectors with a union nut are used, and on the access points - the Male type. The problem is that it is quite difficult to find an SMA connector to mount on the antenna cable so that it matches the connector on the access point. There are three ways to solve this problem. First, replace the rather rare SMA connector on the access point itself to match the connector mounted on the cable. Secondly, you can completely get rid of the connector on the access point and simply bring out the antenna cable directly; this method is also used in the case when the access point has a non-removable antenna and there are no connectors. Thirdly, you can make the required SMA connector from the smallest antenna.

Software modeling of antennas

After some clarity has been brought into the matter of connector types, we will proceed directly to the modeling and production of antennas.

To model antennas, you can use the free utility EZNEC Demo v.4.0.15 (www.eznec.com), which has a number of limitations, but in the simplest cases it can be used for modeling antennas. In particular, in the demo version of the program the number of segments that make up the antenna is limited. In addition, it is impossible to use reflectors with given dimensions, much less with a given geometry. Therefore, it is better to buy or find it online full version programs.

The EZNEC Demo v.4.0.15 utility is compatible with 32-bit versions of Microsoft Windows XP/2000/2003. Let's take a closer look at how antennas can be modeled using this utility.

After starting the program, we find ourselves in the main window (Fig. 13), which contains the main characteristics of the antenna. To design a new antenna, it is best to select the most suitable one from the list of models provided with the program and modify it. To access the database of antenna models, you just need to click on the Open button.

Rice. 13. Main window of the EZNEC Demo v.4.0.15 program

To create a whip antenna, select the Dipole1.ez model in the database. And to view the type of the selected antenna, that is, its diagram in the Cartesian coordinate system, click on the View Ant button. In our case, it will be a regular rod (Fig. 14).

To model the desired antenna, you must first set the radiation frequency, so in the main program window, instead of 299.793 MHz, you must set the frequency to 2473 MHz (the radiation frequency on the sixth channel in 802.11b/g networks).

Next, you can start drawing the antenna itself. The antenna consists of individual pieces of wire, and in order to draw the antenna, it is necessary to specify the coordinates of the start and end points of each individual piece of wire. In addition, you can set the wire diameter. All necessary antenna geometry parameters are set in the Wires window.

For example, if we want to depict a vertical whip antenna 30 cm long, then we need to set the values 0, 0, 0 as the coordinate of the starting point, and 0, 0, 30 as the coordinate of the end point. If our antenna should have G -shape, you will have to use two pieces of wire.

In addition, the main window allows you to specify the position of the signal input point to the antenna (the point of connection with the feeder) (Sources window), set the type of grounding, and also make many other specific settings.

After the antenna model is generated, you can view its radiation pattern in the FF Plot window. The EZNEC Demo v.4.0.15 utility allows you to build both three-dimensional and two-dimensional radiation patterns. For the considered example Dipole1.ez, the three-dimensional and vertical two-dimensional radiation patterns are shown in Fig. 15 and 16.

In addition to constructing a radiation pattern, the EZNEC Demo v.4.0.15 utility allows you to calculate the antenna gain and the width of the main lobe. In our case (see Fig. 16), the gain is 2.16 dBi, and the width of the main lobe is 77.2°.

Whip antenna with perpendicular reflector

The simplest antenna option is a whip antenna, and it is these antennas that are most often used in wireless access points.

A whip antenna is often called a monopole. The radiation pattern of such an antenna differs little from the radiation pattern of a Hertz dipole. In the horizontal plane, the antenna radiates energy in all directions evenly, therefore, in the horizontal plane such an antenna is omnidirectional, and therefore there is no need to talk about preferential radiation in a certain direction. Using the EZNEC Demo v.4.0.15 utility, you can simulate the radiation pattern for different antenna lengths. Typical options are when the antenna length is a quarter or half the radiation wavelength, and in most standard access point antennas, the antenna length is a quarter of the radiation wavelength (30 mm). So, for an antenna length of 1/4, the gain is 1.71 dBi, and for a length of 2.11 dBi. If we continue modeling the antenna length, then for a length of 3/4 the gain is 3.33 dBi, and for a length of 3.43 dBi. The radiation pattern for an antenna length of 3/4 is shown in Fig. 17.

design whip antenna can be improved by using a reflector perpendicular to the antenna - a metal surface (screen) that functions as an ideal grounding surface. For an antenna length of 1/2, in the case of an ideal reflector, the directivity coefficient will be already 7 dBi. The radiation pattern of such an antenna is shown in Fig. 18.

Of course, in reality the radiation pattern will have a slightly different shape and the gain will be lower. The fact is that when calculating the radiation pattern using the EZNEC Demo v.4.0.15 utility, it is assumed that the reflector is an infinite, ideally conducting plane. In addition, signal loss during propagation in the antenna itself is not taken into account. The EZNEC Demo v.4.0.15 utility partially allows you to make corrections for the “non-ideal” grounding screen and take into account signal losses in the antenna itself.

In order to construct such an antenna, we will need a copper pin (copper wire core), with a diameter of 1.5-2 mm and a length of 65 mm, as well as a metal reflector in the shape of a square with a side of about 100 mm or a disk with a diameter of 80-85 mm. This disc is made from the lid of a metal tin can. In addition, you will need an N-series Female connector with a flange for mounting on the antenna reflector (for example, N-7317), an N-Series Male connector for mounting on the antenna cable (for example, GN-7301A), required for connection to the antenna cable , and the cable itself with a resistance of 50 Ohm/m (RG-58).

An N-series Female connector with a flange must be secured to the reflector by drilling a hole in the center of the reflector. Fastening can be done using either four bolts or epoxy glue. On one side of the N-series connector with a flange, you need to insert a copper rod and additionally solder it. If the thickness of the copper rod is slightly larger than the hole in the connector, then you can use a file to reduce the diameter of the rod at the attachment point.

The copper rod should protrude 60 mm above the surface of the reflector, which is half the radiation wavelength. The production process of this antenna is shown in Fig. 19.

Whip antenna with parallel reflector

Another way to modify a whip antenna is to use a parallel reflector rather than a perpendicular one. Before we begin manufacturing such an antenna, let’s simulate it using the EZNEC Demo v.4.0.15 utility. Let's place the antenna parallel to the XY plane along the X axis. As the signal input point, select point E1 (antenna starting point) with coordinates (0, 0, z). The coordinates of point E2 (antenna end point) will accordingly be (x, 0, z), where the x coordinate is determined by the length of the antenna, and the z coordinate by the distance from the antenna to the flat reflector). As the “ground” we will choose an ideal conducting surface (Perfect). By varying the length of the antenna and the distance to the reflector, you can select the desired radiation pattern and gain.

In Fig. Figure 20 shows the radiation patterns, the width of the main lobe and the gain for an antenna with a length of 1/2 (60 mm) at different distances to the reflector.

for whip antenna length 1/2l (60 mm)
at different distances to the parallel reflector

To make this antenna, as in the previous case, we will need a copper pin (copper wire core), with a diameter of 2 mm and a length of 65 mm, two metal reflectors (one is also in the shape of a square with a side of about 100 mm, and the second is in the shape of a rectangle with dimensions approximately 100x170 mm). In addition, you will again need an N-series Female connector with a flange for mounting on the antenna reflector and an N-Series Male connector for mounting on the antenna cable.

The easiest way to assemble such an antenna is by slightly modifying the previous circuit.

The reflective screen will consist of two mutually perpendicular parts - horizontal and vertical. The N-series connector with copper rod is attached to the horizontal part of the reflector, and the vertical reflector is mounted at a distance of 12 mm (0.1l) perpendicular to the horizontal reflector and therefore parallel to the antenna itself. The diagram of this antenna is shown in Fig. 21.

with parallel reflector

Symmetrical half-wave vibrator with reflector

The next antenna option, which is easy to make at home, is a symmetrical half-wave vibrator with a reflector, or a half-wave dipole antenna.

This antenna consists of two symmetrical multidirectional arms, one of which is grounded (connected to the reflector), and the other is connected to the central core antenna cable. Each of the two arms of such an antenna is made in an L-shape. The part of each arm parallel to the reflector plane is 1/4, so the total length of such an antenna is 1/2. That is why such a dipole antenna is called a symmetrical half-wave vibrator.

Before we begin constructing such an antenna, we will model it using the EZNEC Demo v.4.0.15 utility in order to determine the optimal distance of the antenna from the reflector plane.

In Fig. Figure 22 shows the radiation pattern of a symmetrical half-wave vibrator with a reflector with a distance between the reflector plane and the antenna arms equal to 0.1 (12 mm). The antenna gain is 8.88 dBi, and the shape of the radiation pattern indicates the sectoral nature of the antenna.

with reflector

When making this antenna, we, as always, will need an N-series connector with a flange for mounting on the antenna reflector and an N-series connector for mounting on the antenna cable. In addition, you will need copper wire with a diameter of 2 mm, and to make a reflector you can use a copper or aluminum sheet in the shape of a circle or square (it can even be a frying pan). The dimensions of the reflector can be any, but it is advisable to make the minimum size twice the radiation wavelength (262 mm).

The diagram of this antenna is shown in Fig. 23.

Spiral antenna with reflector

Another example of common antennas for the frequency range from 2 to 5 GHz is helical antennas with a reflector. Such antennas were invented back in 1947 by John Kraus. A helical antenna is characterized by the number of turns N, the diameter of the turns D and the helix pitch d.

Without delving into complex theoretical calculations, we will only present final result. Basically, the more turns an antenna contains, the higher the gain. In this case, the coil radius is usually selected based on the condition that the coil length corresponds to the radiation wavelength, that is: 2 P R = , and the spiral pitch should be equal to a quarter of the radiation wavelength: d = /4 .

The size of the reflector, which is installed perpendicular to the axis of the spiral and can be in the shape of a disk or square, must be no less than the wavelength of the radiation. With a radiation wavelength of 123 mm (frequency 2437 MHz), we find that the coil diameter should be approximately 40 mm, and the spiral pitch should be 30 mm.

Unfortunately, modeling this antenna using the EZNEC Demo v.4.0.15 utility is impossible due to restrictions on the number of segments that make up the program. Therefore, to calculate helical antennas, it is necessary to use the full-featured version of the program, which is what we did. An example of the radiation pattern of such an antenna for 12 turns is shown in Fig. 24. Note that the calculated gain is 10.72 dBi. A further increase in the number of turns does not allow a significant increase in the antenna gain.

40 mm, spiral pitch 30 mm.

To make this antenna, we need a plastic pipe with a diameter of 40 mm (such pipes can be purchased on the construction market) and a length of about 40 cm, insulated stranded copper wire with a cross-sectional diameter of 1.5-2 mm. The wire is wound around the pipe and glued to it. This antenna design has an impedance of about 150 ohms and requires proper matching with a standard 50 ohm cable. The most elegant matching method is to use a piece of copper in the shape of a right triangle, which is an extension of the wire wound around the pipe. The legs of the triangle measure 71x17 mm. On one side, the triangle is soldered to the wire, and on the other, it is connected to the central pin of the N-series connector. The reflector is made from a square shaped copper plate and the pipe is attached to the reflector using a pipe plug. The antenna diagram is shown in Fig. 25.

In conclusion, we note that this antenna causes circular polarization, which can be either right- or left-handed depending on how the helix is wound. Such antennas should only be used in pairs, that is, if one access point uses a helical antenna, then the other access point should also have a helical antenna, and with the same helix winding.

Antenna with biquad quarter-wave emitter and reflector

From previous this option differs in the shape of the antenna itself. The main structural elements of the antenna are the reflector, which is made of any metal, and the emitter itself. The reflector can be in the form of a disk with a diameter of 140 mm or a square with a side of 123 mm (); in the latter case, it is also recommended to use edges with a height of 31 mm (1/4). A hole is drilled in the center of the reflector to mount the N-series connector.

The emitter is made of copper wire with a diameter of 2 mm, which is bent to form two square frames with sides of 31 mm (1/4). The free ends of the wire are soldered to each other. Next, wire legs 31 mm high are soldered to the emitter frame, as shown in Fig. 26. One of these legs is soldered to the central core of the antenna cable, the second to the reflector.

This antenna design allows for a gain of 9.5 dBi. The radiation pattern of such an antenna, calculated using the EZNEC Demo v.4.0.15 utility, is shown in Fig. 27.

with biquad emitter

Antennas made from tin cans

Directional antennas made from cans have become widespread due to not only their simple design, but also their high efficiency. There are many options for manufacturing such antennas, differing in size. The main idea behind the design of such antennas is that a tin can acts as a waveguide in which standing waves are formed, and therefore in this case An important condition is the exact observance of dimensions.

Before moving on to the description of specific models of such antennas, let us briefly consider the main theoretical aspects. Let us introduce the following notation:

0 wavelength in vacuum (open space); if the frequency is measured in GHz and the wavelength in mm, then

.

C minimum critical wavelength that can propagate along the waveguide. This wavelength depends on the inner diameter of the waveguide: c = 1.1706· D ;

G is the length of the standing wave in the waveguide, which depends on c and 0.

In the following, we will consider waveguides in the form of a pipe, which is open on one side and closed on the other. Such a waveguide is similar to a short coaxial cable: the incoming high-frequency signal is reflected from the end of the waveguide, and the reflected wave is superimposed on the incident wave. As a result of the superposition of these waves, a standing wave effect occurs. For example, if the waves add up in antiphase, then they weaken each other, and if they are in phase, then, on the contrary, they strengthen each other.

The following relationship exists between the wavelengths c, 0 and g:

.

From this equation we can obtain the formula for the standing wave length:

Thus, for a frequency of 2.437 GHz and for the internal diameter of the waveguide D = 83 mm, we obtain that the standing wave length is 248.4 mm, and the minimum critical wavelength is 141.6 mm.

Now, based on the introduced notations, let’s consider an example of creating waveguide antennas from cans. When constructing waveguides from cans, the length of the waveguide should be 3/4 g, and the signal source should be installed at a distance g/4 from the closed end of the can. Typically, the diameter of a tin can is 83 mm. It is easy to calculate that the length of the waveguide in this case will be 186 mm. And since such long cans do not exist, the waveguide will have to be joined from two cans.

At a distance g/4 from the closed end of the can, a hole is drilled for an N-series connector with a flange. A copper rod with a diameter of 2 mm is inserted into the N-connector, which acts as a wave source inside the waveguide. The length of this rod should be a quarter of the radiation wavelength in our case 0 /4 = 31 mm. The model of this antenna is shown in Fig. 28.

If this antenna is used outdoors, it is also necessary to ensure that the can can be closed to prevent dirt, snow, rain, etc. from getting inside. You can use a plastic lid for this, but regular jar lids will not work as they may weaken the signal somewhat. The best option use plastic from a dish intended for a microwave oven as a lid, which can simply be glued to the jar with epoxy glue.

Belarusian State University of Informatics and Radioelectronics

Department of Antennas and Microwave Devices

COURSE WORK

in the discipline "Antennas and Microwave Devices"

Topic: Calculation of a helical antenna circular polarization

Minsk, 2010

Introduction

1. Basic relationships, selection of operating wave type and feeder

2. Description of the antenna design and ADF at its aperture

3. Calculation of geometric and electrical characteristics antennas

3.3 Calculation program electrical parameters

3.4 Results of numerical simulation of the antenna

Conclusion

Bibliography

Introduction

helical antenna circular polarization

An antenna is a necessary part of any radio system. Antennas are classified according to many characteristics and parameters.

Based on the direction of radiation and reception, a distinction is made between weakly directional antennas, the linear dimensions of which are either much smaller or comparable to the wavelength; moderately directional - dimensions of the order of several wavelengths; highly directional - dimensions of the order of tens of units of wavelengths.

According to the principle of operation and design, antennas are divided into: wire and whip, used at kilo-, hecto- and decameter waves; slotted, consisting of slits in screens or walls of waveguides, used at decimeter and centimeter waves; antennas surface waves, where radiation into external space occurs as a result of slowed or accelerated wave propagation along the surface of the antenna; aperture antennas, in which radiation occurs with a larger area compared to the square of the wavelength; multi-element antennas- antenna arrays, where weakly directional antennas serve as radiating elements.

By frequency band: narrowband (frequency band is 5-10% of the average frequency), wideband (bandwidth 40-50%) and ultra-wideband (bandwidth more than 50%).

Based on their area of application, antennas are divided into communications, broadcasting, television, radar, etc.

The development of various branches of radio electronics has created a practical need for antennas that provide radiation and reception of an elliptically polarized field in a wide frequency range. Among the various types of broadband antennas, an important place is occupied by helical antennas, which are weakly and mid-directional broadband antennas of elliptical and controlled polarization. They are used as independent antennas, feeders of mirror and lens antennas, exciters of waveguide-horn antennas, elements antenna arrays. In most cases, the main requirements are the ability to operate in a wide frequency range, providing elliptical and near-circular polarization.

A significant number of types and designs of helical antennas have been developed and are used, differing in range properties, field polarization and other properties. A cylindrical regular single-filament helical antenna in the axial radiation mode has a frequency overlap coefficient of 1.8 and emits a field with circular polarization (right or left - depending on the direction of twist of the helix) in the direction of the axis.

The helix antenna guide can be made in the form of a conical helix, which increases Kf, or a flat helix, which reduces the longitudinal size of the antenna (although it also reduces Kf). The number of entries (branches) of the spiral can be several. This also increases Kf. If the leads are wound in different directions (right and left spirals), it becomes possible to control the polarization of the radiation by changing the amplitudes and phases of the currents that excite individual leads. Depending on the ratio of the helix diameter to the wavelength, the radiation pattern can be axial or conical.

WITH In order to reduce the longitudinal dimensions of the antenna, flat spirals are used as a guide. A flat spiral antenna has less range than a cylindrical antenna, since the spiral itself radiates equally towards the screen and in the opposite direction. For in-phase addition of these fields in the direction from the screen, the distance between the spiral and the screen must be close to a quarter of the wavelength.

A cylindrical helical antenna with a variable pitch has a higher range compared to a cylindrical regular helix antenna.

Figure 1.1 – Flat helical antenna, variable pitch antenna, cone antenna

1. Basic relationships, selection of operating wave type and feeder

All waves in a spiral line have longitudinal and transverse components of the vectors E and H with respect to the axis and are analogues of the waves HEmn and EHmn in a circular waveguide. The difference is that they propagate at a phase velocity less than the speed of light in free space, and are therefore superficial.

The amplitudes of the E and H vectors as they move away from the spiral axis in the radial direction in the region t>R decrease approximately exponentially. The lower the phase velocity, the faster the field amplitude decreases with increasing r.

In a regular (infinite along the Z axis) decelerating system, there is a power flow only along the Z axis. This is a general pattern for slow waves in any decelerating systems. In a regular spiral line, the current distribution in a spiral turn along the coordinate φ is a periodic function of φ with a period equal to 2π. This follows from the fact that the observation points P(r,φ,z) and P(r,φ+2π,z) in space coincide. Therefore, the current in the spiral conductor I(φ,z) can be expanded into a complex Fourier series:

Each member of this series is called a spatial φ - harmonic, Im(z) - the amplitude of the harmonics. The field of a spiral line can be represented similarly:

Depending on the size in rows (1) and (2), one of the harmonics will be predominant (resonating). The wave field Tm in the general case can be written in form (2), with the harmonic number m resonating in the field.

In the case when a harmonic with m=1 resonates in the field of the Tm wave. Neglecting all other harmonics, the current I(φ,z) in accordance with (1) can be written as:

Since there is a traveling current wave in the spiral (the reflection from the end of the spiral is weak and can be neglected in an approximate consideration of the processes and calculations), the current I1(z) is determined by the expression:

describing a wave propagating along the Z axis. In (4) I1 is the current amplitude, β is the phase coefficient.

From expression (3) it follows that the current I(φ,z) is the sum of two currents I"(φ,z)= I1(z)-cosφ and I""(φ,z) = iI(φ,z) - sinφ. Each of them has the same dependence on the z coordinate, the same amplitudes I1(z), but different dependences on the φ coordinate. Moreover, the currents are phase shifted by 90°. Figure 2.1 shows the distribution of currents I"(φ, z) on a turn of the spiral depending on φ. Figure 2.1, b shows the distribution of current I (φ,z) depending on φ. Figure 2.1a also shows:

Elementary emitters of turns 1 and 2;

Vectors e1 and E2 of the field created by these elements on the spiral axis (Z axis);

Vector E", equal to the sum of vectors E 1 and E2.

As you can see, the vector E" is oriented along the Y axis, i.e., it is polarized linearly vertically. Similarly, for any two elementary emitters located symmetrically relative to the Y axis, the vector E of their total field is oriented along the Y axis. Therefore, the vector E of all coil elements will be oriented along the Y axis and we can assume that vector E" is the vector of the electric field of one turn of the spiral on its axis for current I"(φ,z). Moreover, the turn radiates equally along the +Z axis and in the opposite direction - along the -Z axis , and the maximum of the radiation pattern of one turn is oriented along the Z axis. The E plane is the YZ plane, the H plane is the XZ plane.

Figure 2.1 also shows the current distribution I""(φ) = I""(φ,z)│z = const and the vector E" of the field on the spiral axis created by a turn of the spiral with this current. Vector E" is oriented along the X axis. Planes E and H of the turn field with current I""(φ) change places compared to the current field I"(φ). Since the currents I""(φ) and I"(φ) have the same amplitudes and are phase shifted by 90°, vectors E" and E" are also identical in amplitude, shifted in phase by 90° and mutually perpendicular in space. As a result, the resulting vector E=E"+E" of the field of one turn of the spiral has circular polarization along the axis of the spiral.

The main lobe of the DN of the spiral turn in the E plane is narrower than in the H plane. This is due to the fact that the elementary emitter of the turn, the Hertz dipole, emits non-directionally in the H plane, but does not radiate along the axis in the E plane.

In the total field of a spiral turn, which has circular polarization, planes E and H rotate around the Z axis with the field frequency. Therefore, radiation patterns based on the components Eθ and Eφ are considered.

These radiation patterns are defined by the following expressions:

where J0 is the zeroth order Bessel function; k - wave number free space; R – radius of the spiral.

A traveling current wave propagates along the system of turns, so a linear phase distribution is established. The fields of all turns in the direction of the Z axis (in the direction of the phase velocity vector of the current wave) add up with the same phases, and in the opposite direction they compensate each other. As a result, the helical antenna on the wave T 1 forms a field with an axial radiation pattern.

Similarly, considering the current distribution in the spiral turn at wave T2, it can be shown that the spiral turn has a conical pattern. In coil elements located diametrically opposite, the currents are antiphase, therefore their total field on the spiral axis is zero. At a certain angle to the axis, the fields of these elements are already shifted in phase due to the path difference, and their total field is not equal to zero. The same is observed on all Tm waves. Moreover, as the number m increases, the number of side lobes of the pattern increases, and the direction of the main maximum approaches the spiral axis - the angle Θm decreases.

In the T0 mode, when the zero spatial harmonic resonates (m=0), the current throughout the entire turn of the spiral has the same phase (same direction). Therefore, such a turn is equivalent to a magnetic dipole that does not radiate along the axis of the turn. The antenna radiation pattern in T0 mode has the shape of a toroid.

A harmonic with number m resonates in the field of the spiral if m wavelengths fit on the perimeter of the spiral cylinder, i.e. 2πR=mλ or

A detailed analysis of the types of waves in a spiral line shows that condition (6) determines the average wavelength of the operating range in which the Tm wave exists. Thus, to create a T1 wave in a spiral line that meets the requirements for the antenna in this work, it is necessary to

The radiation pattern and directivity of a helical antenna can be approximately calculated using formulas obtained analytically for a linear antenna array with a uniform amplitude and linear phase distribution of excitation; more precisely - numerically, having previously solved the internal problem. You can also calculate the antenna and its parameters using empirical formulas obtained by processing a large number of experimental results.

The analytical method is as follows. A regular helical antenna with the number of turns n can be considered a linear antenna array. The radiation pattern of such an array in terms of components eθ and Еφ is determined by the expression:

The directivity patterns of one emitter - a spiral turn - are described by formulas (5). The system multiplier Fc(θ) for a grating with a uniform amplitude and linear phase distribution is determined by the expressions:

where S is the distance between adjacent emitters (spiral pitch).

Deceleration factor , where β is the phase coefficient of the slow wave propagating along the axis of the spiral.

An analytical solution to the problem of determining the types of waves in a regular (infinite) spiral shows that the retardation coefficient exceeds unity by 0.01-0.001 and can be considered equal to 1. In this case, we can apply the expression for the efficiency obtained for a linear antenna in the axial radiation mode:

where l=n×S is the axial length of the spiral (length of the guide).

Expression (10) gives an underestimated value of the efficiency factor. This is due to the fact that in a spiral of finite length the deceleration coefficient is greater. It is approximately determined from the condition of in-phase addition of the fields of all turns in the direction of the spiral axis (although this is not sufficiently substantiated), which leads to the following expression:

This value of the deceleration coefficient for L/λ > 1.5 is close to the optimal value in linear antenna in axial radiation mode and equal to:

At an optimal deceleration coefficient, the efficiency coefficient is determined by the expressions

which give more accurate values.

Expressions (8), (9) are valid for an integer number of spiral turns N. If N is not an integer, the spiral antenna for calculating the pattern is considered a linear antenna with a uniform amplitude and linear phase distribution of length L. In this case, the system multiplier is determined by the expression:

Where

Formulas (8), (9) and (14), (15) give similar results if N>5.

The analytical method for calculating the radiation pattern and efficiency of a helical antenna is approximate due to the assumptions used above (neglecting the waves emitted by the exciter and the end of the director) and the inaccurate value of the retardation coefficient. In addition, this calculation does not take into account a metal screen with a diameter De » (0.6 - 0.7)λ., which is always used to reduce rear radiation and increase the efficiency of excitation of a slow wave in the spiral. Therefore, the following empirical expression is often used to calculate the efficiency, in which k is the wave number of free space:

The input impedance over a wide frequency band has a small reactive part. Active resistance is approximately determined by the expression:

The main mode of a regular helical antenna is the axial radiation mode observed at wave T1. Therefore, let's consider the range properties in this mode.

The T1 wave in a single-start helical line exists in the wavelength range λmax-λmin, which is related to the free space wave number k and the helix radius R ratio:. The following expressions were obtained for the values of (kR)min and (kR)max:

where (kR)0max limits the value of kR on the side of smaller values and is the upper limit of the region of existence of the T0 wave;

kR" limits the region of existence of the T1 wave in which the spatial harmonic with m=1 resonates (axial radiation mode is ensured);

(kR)2min limits the area of existence of the T2 wave on the side of smaller values.

Specified values kR are defined by the expressions:

Figure 2.2 shows the dependences of the given values of kR from the spiral winding angle a. The range of kR and α values in which conditions (19), (20) are satisfied is shaded. In this region there is a T1 wave, and a spatial harmonic with number m=1 resonates in it, i.e. In a spiral antenna there is an axial radiation mode. As can be seen, this region has a maximum width on the scale kR = 2πR/λ (hence, on the wavelength scale λ) at a certain optimal spiral winding angle αopt. The maximum width of this region is limited by the values of kRmiu and kRmax, and on the wavelength scale by the values of λmax and λmin. From the condition of equality of the values of kR" and (kR)min at α=αonT it is easy to obtain αopt=19.5°. The values limiting the region of the axial radiation mode are equal to:

In this case, λmin≈4.5R; λmax≈9R frequency overlap coefficient is equal to

Figure 2.2 - Region of the axial radiation mode of a helical antenna

2. The average value of the wavelength in the range is equal to the perimeter of the cylinder of the 2πR spiral.

To determine the SWR and antenna gain, a number of formulas are given in the literature; in the context of solving the problem, we will use the following:

where K0 is the reflection coefficient:

The antenna will be excited by a coaxial cable RK-2-11 (50 Ohm). The parameters of this coaxial line are: inner core diameter – 0.67 mm, dielectric diameter – 2 mm, outer diameter – 3.9 mm. An SMA connector will be used for connection.

The type of SMA connector is illustrated in Figure 2.3.

Figure 2.3 – ViewSMAconnector

Since usually the characteristic impedance of the feeder is fixed, and the input impedance of the spiral can be different, in this case it is necessary to use a microwave matching device. The input impedance of a spiral antenna in the axial radiation mode remains purely active, since in this mode a traveling wave mode is established in the spiral wire. Therefore, for matching, you can use a cone-shaped transition (Figure 2.4) from coaxial transmission lines.

Figure 2.4 – Coaxial transformer

If the length of the conical part () is taken equal to l/4, then this transition works as a quarter-wave transformer to match a line with different characteristic impedance.

The wave impedance of the conical part of the line should be:

, Where

Wave resistance of the conical part of the transition

Characteristic impedance of the supply feeder

Characteristic impedance of a helix antenna

Based on the known wave impedance, the ratio of the diameters of the elements of the coaxial path can be determined:

lg (Ohm)

For an air-filled coaxial device, the ohm ratio is .

2. Description of the antenna design and ADF at its aperture

The main operating mode of the antenna is the axial radiation mode, in which a radiation pattern (hereinafter referred to as DP) is formed along the helix axis.

The helix antenna consists of the following components:

Figure 3.1 - General view of the antenna

Figure 3.1 shows: 1 - a spiral made of a copper tube, 2 - a dielectric frame, 3 - a metal mesh screen, 4 - a matching device, 5 - a supply feeder.

In this case, to make an antenna it is better to take copper tube, to facilitate the design, because High frequency currents flow only along the surface of the metal.

Solid foam can be used as a frame. In this case, the calculated ratios will remain unchanged because the foam is homogeneous in the azimuthal and longitudinal directions, and its dielectric constant is almost equal to the dielectric constant of air.

To reduce rear radiation, we use a screen, which in the UHF range is made of metal mesh.

Calculation of the characteristics and parameters of a helical antenna, naturally, cannot be done taking into account all its structural elements and the characteristics of the current distribution in it. Therefore, a simplified model of the spiral is used, provided that the axial length is greater than 0.5l, and the reflection of the current wave from the free end of the spiral is small (the conditions are met when the antenna operates in the traveling wave mode). Research shows that in this case, a real helical antenna can be replaced by a segment of a regular helix with a current distribution uniform in amplitude and linear in phase along the approach axis (Figure 3.2). :

Figure 3.2 – APR antenna

Figure 3.3 – Spiral development

where l= c/f, where c is the speed of light, f is the operating frequency, n is the number of turns of the spiral, a is the angle of rise of the turn, R is the radius of the spiral, S is the pitch of the spiral turn, L is the length of the spiral turn, l is the length throughout the spiral.

Thus, the design ratios for a cylindrical spiral:

obtained in accordance with Figure 3.3.

Knowing the required directivity coefficient, you can calculate the antenna length

The spiral pitch is calculated by the formula

The number of turns of the spiral is calculated using the length and pitch of the spiral

The length of the spiral turn at which the radiation is maximum along the axis must be selected from the interval

The radius of the spiral is found from relation (27)

We find the angle of elevation of the spiral from (28)

The diameter of the spiral wire is taken on the order of (0.03...0.05)l, and the thicker the wire, the higher the level of side lobes and the lower the input resistance. As the diameter increases, the radiation patterns along the q and j components also become closer. To simplify the design, we will take a smaller diameter of 0.03l.

To reduce the level of back radiation, a spiral is usually used in conjunction with a screen round shape, and in the UHF range it is made of metal mesh. The grid cell size is made less than one tenth of the wavelength, and the distance from the screen to the first turn of the spiral is made 0.25S. The screen should be made of wire with a thickness of at least 3 mm, because it is a carrier for the spiral feeder and any slewing support device.

The experiment shows that the antenna characteristics are most stable in the frequency range at , and the input resistance practically does not change. Therefore, let's accept .

The radiation characteristics of the antenna are influenced by the shape and size of the transition section from the inner conductor of the feeder to the spiral conductor. This element is flown around by a current of large amplitude and has no axial directional pattern. Its influence can be reduced by reducing its length.

3. Calculation of geometric and electrical characteristics of the antenna

3.1 Calculation of geometric parameters of the antenna

In the course of this work, the MathCAD program was used to carry out analytical calculations and plot dependency graphs.

Knowing the average operating frequency (f = 910 MHz), we determine the average wavelength of 0.33 m. The operating frequency range is 100 MHz, which is 11% of the carrier frequency.

To determine the beam width at which the required pressure gain will be achieved, it is necessary to carry out calculations.

Since 2θ E 0.5° = 2θ H 0.5° = 45°, then the efficiency will be equal to:

A cylindrical regular single-filament helical antenna has a frequency overlap coefficient of 1.8 in the axial radiation mode and emits a circularly polarized field, which is quite satisfactory technical requirements. Therefore, we will choose the antenna configuration described above.

Knowing the directivity coefficient, you can calculate the length of the antenna.

Based on the known wavelength, the helix pitch is calculated.

We calculate the number of turns using the length and pitch of the spiral.

Let's take 6 turns, but it is necessary to recalculate the length of the spiral so that the winding angle does not change.

The length of the spiral turn must be selected from the interval (0.75...1.3), for example, take L = the radius of the spiral turn is found from the following relationship:

We find the angle of elevation of the spiral by unraveling the spiral turn.

Let's take the diameter of the spiral tube as 0.03*=0.01m=1 cm. Let's choose the distance from the screen to the first turn of the spiral as 0.25*S=0.018m=1.8 cm. Let's choose the diameter of the screen as 0.7*=0.231m.

3.2 Electrical calculation of the antenna

To calculate radiation patterns on the average operating frequency by components, we substitute the found geometric dimensions of the antenna into formulas (5). To determine the antenna efficiency in the operating frequency range, we use relation (16).

To determine the input resistance, we will use the Kraus formula, which in the UHF range gives acceptable results.

As can be seen from the calculation, the input impedance of the antenna differs from the characteristic impedance of the selected feeder. This implies the need to use a matching device, namely a coaxial resistance transformer. Since RK-2-11 with a central core diameter of 0.67 mm was chosen as a feeder, the dimensions of the coaxial transformer can be determined (Figure 2.4).

We take the length of the conical part equal to /4 = 0.0825 m so that the transition works as a quarter-wave transformer to match a line with different characteristic impedances.

The wave impedance of the conical part of the line should be:

Based on the known wave impedance, the ratio of the diameters of the elements of the coaxial path can be determined using formula 26. For a coaxial waveguide with air filling and Z=100 Ohm, the ratio d/D=0.17, for Z=140 Ohm - 0.096, for Z=50 Ohm - 0.435.

In order to conveniently connect the spiral tube and the central core of the output part of the matching device, we take the latter with a diameter less than the diameter of the tube, for example 3 mm.

Since the antenna is matched at the average operating frequency, the SWR under these conditions will be minimal. As the frequency changes, the SWR will increase. To calculate SWR in the frequency range, we use formula 24. The dependence of SWR on frequency is shown in Figure 4.1.

Knowing the geometric dimensions of the antenna, it is possible to calculate its electrical parameters. Formulas for calculating radiation patterns are obtained for the Eq and Ef components of the T1 wave field. These expressions are given in the literature. It can be approximately assumed that the radiation patterns do not depend on the angle f, i.e. are bodies of rotation, although there is still a slight dependence. As a result, the DP for an integer number of turns of the spiral is determined by the expressions below.

Figure 4.1 – SWR of the antenna in the frequency band

The radiation patterns look like:

Figure 4.2 – Antenna pattern at mid-range frequency in polar coordinates

Figure 4.3 - Antenna pattern at the middle frequency range in rectangular coordinates (plane E)

Figure 4.4 – antenna pattern at the middle frequency range in rectangular coordinates (H plane)

Figure 4.5 – Antenna pattern at the lower frequency of the range in polar coordinates

Figure 4.6 – Antenna pattern at the upper frequency of the range in polar coordinates

The dependence of the efficiency on the wavelength within the operating range has the form:

Figure 4.7 – Dependence of efficiency on wavelength

3.3 Program for calculating electrical parameters

To solve the external problem and to calculate the electrical parameters of the cylindrical helical antenna, the MMANA program was used. The program was developed by Japanese specialist Makoto Mori and translated into Russian by specialist I. Goncharenko.

The program implements the method of integral equations for thin-wire antennas. According to user-specified geometry wire antenna MMANA allows you to:

· calculate any types of antennas that can be represented as an arbitrary set of wires;

· perform calculations at any frequency;

· create and edit antenna descriptions, both by indicating digital coordinates and in a graphic editor (simply draw the antenna with the mouse);

· consider many different types antennas;

· calculate radiation patterns (RP) in the vertical and horizontal planes;

· build three-dimensional radiation patterns;

· Simultaneously compare the simulation results of several different antennas;

· edit each antenna element, including the ability to change its shape;

· edit the description of each antenna wire by simply dragging with the mouse;

· calculate combined (consisting of several, different diameters) wires;

· create stacks, you can use any element as a stack element;

· optimize the antenna by flexibly setting optimization goals: Zin, SWR, gain, F/B, minimum vertical radiation angle;

· specify a change when optimizing more than 90 antenna parameters; it is possible to describe a joint (dependent) change in several parameters;

· save all optimization steps in the form of a separate table for subsequent analysis;

· build a variety of different frequency graphs: Zin, SWR, gain, F/B, DN;

· automatically calculate different matching devices, with the ability to turn them on and off when plotting graphs;

· create tables for all variable design data: currents at each point of the antenna, dependence of gain on angles, changes in the main parameters of the antenna on frequency, antenna field strength in a given area of space;

· calculate coils, circuits, control systems on LC elements, control systems on sections of long lines (several types), inductances and capacitances made from sections of coaxial cable;

· Has no restrictions on the relative position of wires. Maximum number: wires - 512, sources - 64, loads - 100.

Figure 4.8 - Appearance programsMMANA

After specifying the geometry, sources (required), loads (optional) and frequency, the characteristics and parameters of the antenna can be calculated. To do this, select the “Calculations” menu item and click on the “Start” button in the screen that opens. After calculation, the antenna parameters are displayed in the table of this screen. The radiation pattern can be viewed by selecting the “Radiation Patterns” menu item.

Figure 4.9 - View of the model in the programMMANA

Antenna geometry can also be created using the built-in graphic editor. In order to enter the editor and draw wire segments in it, you must:

Or on the “Geometry” screen in the main menu, select the “Edit” item, and in the expanded submenu, click on the “Wire Editing” item;

Or on the "Calculations" screen at the bottom, click on the "Edit Wire" item. The drawing technique will be clear after entering the editor. The created file of geometry and calculation results can be saved by selecting “File” in the main menu.

To specify the geometry of the helical antenna, a program was used that, based on the basic parameters of the antenna and screen, calculates the initial and final coordinates of the linear segments (segments) into which they are divided, and creates a file with the extension *.maa. Further calculations are made in the MMANA program.

Figure 4.10 - Appearance of the program for calculating geometry

In the program, the initial data for the spiral are: radius, number of turns, winding angle, conductor radius, approximation step. For the screen: radius, number of radial conductors, number of screen rings, delta - cell size relative to the wavelength. Also specified: frequency and name of the output file. To get started, click the “Calculate and save” button. A file with the specified name is created in the program folder.

3.4 Results of numerical simulation of the antenna

Simulations in the MMANA program will be performed in order to verify the correctness of the results that were obtained as a result of the theoretical calculation. If necessary, the necessary adjustments will be made to the antenna geometry.

Figure 4.11 – Antenna pattern in polar coordinates

Figure 4.12 – Dependence of SWR on frequency

Figure 4.13 – Dependence of efficiency andF/B of frequency

Figure 4.14 – Dependence of the active and reactive part of the input resistance on frequency

Figure 4.15 – Dependence of pattern on frequency

When the antenna operates in real conditions, the efficiency will increase due to the ruggedness of the pattern in the vertical plane. Figure 4.14 shows the antenna pattern located at a height of 5 meters above the ground.

Figure 4.16 – antenna pattern located at a height of 5 meters above the ground

Figure 4.17 – Volumetric radiation pattern of an antenna located at a height of 5 meters above the ground

The active resistance is about 140 Ohms, which allows you to connect the antenna to a standard coaxial cable with a resistance of 50 Ohms using the matching device calculated in section 4.2.

As for the radiation pattern, it remains virtually unchanged throughout the entire range of operating frequencies. Also, in a given frequency band, the SWR does not exceed the established limits.

In addition, it must be said that theoretical calculations and modeling give similar results in studying the characteristics of a helical antenna.

A feature of the MMANA program is that it does not provide for displaying a graph of the pattern in rectangular coordinates, which makes it difficult to determine the width of the pattern. However, judging by the graphs presented in Figures 4.3 and 4.4, it corresponds to the specified parameters.

Conclusion

In this course project, a regular cylindrical helical antenna was analyzed. Comparing the results obtained, we can say that the helical antenna is a broadband antenna with axial radiation of circularly polarized waves.

During the work, a program for WINDOWS was used, which allows you to create the geometry of various helical antennas and study them in the MMANA package.

The designed antenna is simple in design and can be used as independent antenna, and as an element of an antenna array.

Bibliography

1. Yurtsev O.A., Runov A.V., Kazarin A.N. Spiral antennas M.: Radio and communications 1974.

2. Yurtsev O.A. Elements of the general theory of antennas. Toolkit. BSUIR: 1997

3. Fradin A.Z. Microwave antennas. M: Soviet radio 1957

4. Markov G.T., Sazonov D.N. Antennas M.: Communication 1977.