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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Whip antennas. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / VHF antennas 1. Definition and concepts Asymmetric (whip) antennas are called antennas located directly at the ground (or a metal screen) perpendicular (less often obliquely) to its surface. If we consider the earth to be ideally conducting and take into account the mirror image, then the unbalanced vibrator can be considered half of the equivalent balanced vibrator (Fig. 1).
The radiation resistance of an asymmetric vibrator is two times less than that of an equivalent symmetrical vibrator, since at the same currents the former emits half the power (there is no radiation into the lower half-space) [1]. The input resistance of an asymmetric vibrator is two times less than that of an equivalent symmetrical vibrator, since with the same supply currents, the first supply voltage is two times less (Fig. 1). The directional action coefficient of an asymmetric vibrator is twice that of an equivalent symmetrical vibrator, since, at the same radiation power, the former provides twice the angular power density, since all its power is radiated into one half-space (Fig. 2). All of the above is true for an ideal asymmetrical vibrator, that is, when the earth is an ideal conductor. If the earth has poor conductive properties, the radiation field of the vibrator changes. In addition, this leads to a decrease in the amplitude of the current in the vibrator and, consequently, to an increase in its resistance and a decrease in emitted power. Soil is a dielectric with a high dielectric constant (almost 80), which leads to a change in the electrical length of the imaginary dipole, as well as the path length of the displacement currents. The result is a complete distortion of the radiation pattern (raising the lobes upward and the disappearance of radiation at small angles to the horizon) and an increase in the resistance of the pin. For this reason, soil is practically not used as "land", but artificial land is used. 2. Whip ground Theoretical calculations show that the greatest losses occur in a zone with a radius of 0,35 wavelengths, therefore, in this zone it is desirable to "metallize" the earth: connect the radial wires to each other with jumpers (Fig. 3). It is very good if this metallization is carried out over the entire distance of the counterweights.
Counterweights should be isolated from the ground. If they lie on the ground, then from moisture their electrical length will not be resonant for the antenna. Also, their ends must be isolated from the ground. Only in one case it is possible not to isolate the ends of the counterweights from the ground: if they are securely connected by a jumper ring (Fig. 3). It should never be forgotten that an ideal whip antenna has an efficiency of 47%, while an antenna with 3 counterweights has an efficiency of less than 5%. So, when working with a rod antenna with three counterbalances, out of your 200 watts supplied to the rod, 180 watts (!!!) are wasted in vain, creating TVI along the way. Many processes in the ionosphere are nonlinear, i.e. The reflection of radio waves starts at, say, 7 watts of power to your antenna, and is no longer completely at 5 watts. This means you're missing out on the unique DX QSO experience by saving on weight wire. It should also take into account the distortion of the radiation pattern with a small number of counterweights. From spherical, it becomes petal, having a direction along the counterweights. The problem of finding the optimal number of counterweights was solved by me using a computer. The solution is shown in fig. 4. It can be seen from it that the minimum required number of counterweights is 12. With a larger number of them, the efficiency increases slowly. Counterweights should be located at the same distance relative to each other.
The angle of their location relative to the pin should be from 90 ° to 1350. At larger and smaller angles, the efficiency and d.n. is distorted. Counterweights must be at least as long as the main pin. This can be explained by the fact that the bias currents flowing between the pin and counterweights occupy a certain amount of space, which is involved in the formation of the radiation pattern. By reducing the length of the counterweights, and, consequently, by reducing the amount of space that serves to form the DP, we significantly worsen the characteristics of the antenna. With great approximation, we can say that each point on the pin corresponds to its own point on the counterweight. However, it is not necessary to use counterweights longer than the main pin. Counterweights and the pin itself must be covered with protective paint. This is necessary so that the material from which the antenna is made does not oxidize. Oxidation of the vibrators renders the antenna unusable due to the fact that the thin oxide film has significant resistance, and since the surface effect is strongly pronounced on the RF, the transmitter energy is absorbed and dissipated into heat by this film. It is highly desirable to use radio paint for this (the one with which locators are painted). Conventional paint contains dye particles that absorb RF energy. But, in extreme cases, you can use ordinary paint. 3. Whip Antenna Dimension As is known, the radiation resistance of the Rizl antenna is proportional to the ratio L/d, where L is the length and d is the diameter of the antenna. The lower the L/d ratio, the wider the antenna and the higher the efficiency. It should be noted that when using thick vibrators, the “end effect” affects. It is determined by the capacitance between the ends of the vibrator and the ground. Physically, this is expressed in the fact that the antenna turns out to be “longer” than the calculated one. To reduce it, broadband pins are usually tapered in shape. Calculations show that the minimum required thickness of counterweights should be d=D/2,4n, where d is the diameter of the counterweights, D is the diameter of the pin, n is the number of counterweights. Often radio amateurs cannot install a quarter-wave pin and use a smaller pin. In principle, it is possible to match a pin of any length with the help of matching devices. However, short pins have low active and high reactance [3] and will be matched very non-optimally (up to 90% of the energy can be dissipated on the matching devices themselves). And if surrogate short counterweights are also used, then the efficiency of such an antenna system will be very low. However, in mobile communications, such surrogate antennas are often used. But this is only because other types of shortened antennas will perform even worse! 4. Directional patterns of whip antennas Many are interested in how the height of the pin rises on its radiation pattern in the horizontal plane and whether its resistance depends on the height of the suspension. The most important result [4] is that the distribution of currents in the pin does not depend on the height of its suspension in the presence of an ideal "ground". In practice, this means that no matter how high the pin is, its resistance will be constant. The overall result of the solution shows that if the pin is tuned to resonance, then its lower end can be grounded. Moreover, it can be powered at any point. Based on the results of this important conclusion, whip antennas (flag antennas, mast antennas) were created, the lower end of which is connected to the "ground" and which are fed through gamma matching. The radiation patterns of the vertical plane of the half-wave pin are shown in fig. 5. This figure shows that the higher the antenna rises, the flatter the radiation angle to the horizon. This is due to the fact that the addition of the wave emitted by the pin and the wave reflected from the ground takes place. If the soil has poor conductive properties, then the radiation pattern will be close to that of a pin above the ground. Raising the antenna to a height of more than a wavelength does not make sense, because. in this case, there is no longer a decrease in the radiation angle, but only the upper side lobes begin to fragment.
One more interesting feature of pins should be remembered, the height of which is equal to the wavelength or more. Such antennas are used in professional communications as anti-fading antennas [5]. This means that such an antenna will receive without problems a signal arriving with fading on a quarter-wave pin or dipole. 5. Whip antenna matching For successful operation, the whip antenna must be matched. Despite all the apparent variety of matching devices and pins, they can be divided into 3 groups. 1. The pin is matched, the electrical length is equal to a quarter of the wavelength; 2. A pin with an electrical length greater than required, this length is "removed" using a container; 3. The pin is less than a quarter wavelength long. The missing length is "added" by an inductor. It must be remembered that the capacitor and coil must have the highest possible quality factor, and it is also desirable that TKE and TKI be as good as possible. Typically, the capacitance of the shortening capacitor can be within 100 pF at 28 - 18 MHz, the parameters of the extension coil are a few microns up to 21 MHz, tens up to 3,5 MHz. In conclusion, it should be noted that this matching practice is applicable to pins with a length that is a multiple of a quarter wavelength. 6. Types of whip antennas Asymmetric vibrator with a screen of finite dimensions (Fig. 3). This antenna is mainly used by radio amateurs. As a screen, counterweights with a length of at least a quarter of a wavelength are usually used. Asymmetrical loop vibrator (Fig. 6). His Ph.D. coincides with d.s. classic pin. However, it has the advantage that one end is grounded. By choosing the thickness dl and d2, you can change its input resistance over a wide range. With d1=d2, the vibrator resistance will be 146 ohms.
The resistance of an asymmetric vibrator having different thicknesses is calculated by the formula /1 /: Ra=(1+n2).36n, where n=ln(d/d1)/ln(d/d2). Wide range vibrators are made of thick pipes, pins, plates. They can be both conical and rhombic, cylindrical, solid and lattice (Fig. 7). Frequency coverage depends on the I/O ratio. The smaller it is, the wider the vibrator. The well-known antenna UW4HW is a broadband monopole, and the vertical radiator UA1DZ is a broadband dipole . Conical antennas are a special case of broadband vibrators (Fig. 8).
The radiation field is created by currents flowing around the cone, and the disk plays the role of a screen and almost does not radiate. At an opening angle of 600, the highest range overlap coefficient is achieved, equal to five, with BV > 0,5 in a feeder with a characteristic impedance of 50 Ohms. In this case, the maximum wavelength is 3,6. The radiation pattern of a KB and VHF discone antenna is approximately the same as that of an ordinary rod. The KB uses a wire version of the cone antenna (Fig. 8b), in which a flat wire fan is used instead of a cone, and a grounding system of radial wires is used instead of a disk. Separately, I want to pay attention to the antenna-masts. A feature of such antennas is that their lower end is grounded.
The top feed antenna (Fig. 9) is excited using a feeder laid inside the mast. It is fundamentally. D.Sc. its the same as that of a conventional pin, but the losses during transmission and reception are greater, since the radio wave is reflected from the ground when emitted. The medium feed antenna (Fig. 10) is a two-part mast, excited in series at points 1 and 2 by a voltage supplied by a feeder installed inside the lower part. The antenna resistance at the feed points is Ra=Rb/cos2kll, where k is the shortening coefficient, Rb is the resistance of the “pure” vibrator at point 3. By choosing a ratio between 11 and 12, you can match the antenna with the power feeder. It is of fundamental importance that the feeder must pass inside the bottom of the antenna. The disadvantage is difficulties with the insulator for its upper part.
The shunt power antenna (Fig. 11) is excited in parallel using a shunt connected to the mast at a certain height 11. Typically, the input reactances of the lower and upper parts of the antenna are inductive and, accordingly, capacitive in nature, and in terms of the input impedance at point 1, the antenna is equivalent to a parallel circuit. Selecting value 11 ensures the best coordination with the power feeder. The distribution of currents is such that it partially attenuates the radiation of the antenna, so the shunt should be made of minimal dimensions. The classic implementation of shunt power is gamma matching. Often, especially when building antennas for low-frequency ranges, it is not possible to position the vibrator vertically relative to the ground. When the pin is inclined relative to the ground, the radiation pattern will, of course, be distorted. Place as many counterweights as possible under the part of the antenna that is tilted. It is also necessary, if possible, to raise the counterweights so that they form an angle of no more than 135 ° with the antenna. It should be remembered that such an antenna is more difficult to match due to the presence of a significant reactive component. Literature
Author: I. Grigorov (UZ3ZK); Publication: cxem.net
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