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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Feeder antenna effect. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Antennas. Theory The normal operation of the antenna-feeder path largely determines the effectiveness of an amateur radio station as a whole. The effect discussed in this article can significantly reduce it, since it manifests itself in most practical antenna designs (including factory-made ones). The first part of the article reveals the causes of the feeder antenna effect and its influence on the operation of the antenna-feeder path. In the second part, practical recommendations will be given to eliminate this influence. Almost every shortwave is well aware of the situation when the transmission work interfered with the electronic equipment in the house - the neon light glows when brought to the body of the switched on transmitter, and the reception is accompanied by strong interference of local origin. These are the most striking manifestations of the long-familiar, but relatively little-studied feeder antenna effect, the essence and features of which are described in the article. The essence and causes of the antenna effect of the feeder It is customary to call the antenna effect the phenomenon of radiation or reception of radio waves by objects not intended for this. The feed line should only be used to transmit high frequency energy from a transmitter to an antenna or from an antenna to a receiver. Consideration of the causes of the feeder antenna effect (AEF) will begin with the transmission mode. As you know, the electromagnetic field emitted by the antenna is created by alternating currents flowing through its constituent conductors. Almost always the antenna is not in free space. In the immediate vicinity of it (for example, within the wavelength l) can be many objects. These are wires of power supply, broadcasting and communication lines, conductive masts, supports and guys, pipes, rigging, fittings, bodies and fuselages of vehicles, roofs and walls of buildings, the operator's body and the ground surface. If currents somehow arise in the objects of the environment (induced, for example, by the near field of the antenna), then the radiation field created by these currents will add up to the field from the antenna currents. The antenna together with the environment will be called the antenna system (AS). Under these conditions, the characteristics of the speakers may differ greatly from the calculated characteristics of the antenna itself. In order for the characteristics of the speakers to be less dependent on the environment, they try to raise the antenna higher, install it further from conductive structures, make non-metallic masts, braces. One of the nearest and fundamentally unremovable objects of the antenna environment is the feeder that feeds it. The simplest feeder is an open two-wire line. In the ideal case, the instantaneous values of the currents in the line wires in any section of the feeder and at any time are the same in magnitude and opposite in direction, i.e. the sum of the currents of both wires of the feeder in any section is equal to zero. We will call such currents antiphase. An open two-wire line will radiate even under this condition, the reason for this is the finite distance d between the wires of the line. A vertical line radiates vertically polarized waves in the horizontal plane with maxima in the plane of the line and horizontally polarized waves with maxima perpendicular to this plane. The radiation field is proportional to the ratio d/l. The radiation of a two-wire line is minimal at a matched line load and noticeably increases with a mismatch, when standing current waves appear. The described phenomenon (under the condition of strictly antiphase currents in the feeder wire system) is called the antenna effect of the feeder of the 2nd kind (AEF-2) [1]. In practice, it manifests itself very weakly. For example, at a frequency of 145 MHz, a line from a KATV (or KATP) television cable with a length of l / 2 at d \u10d 50 mm radiates a field about XNUMX times weaker due to this effect than a half-wave loop vibrator connected to this line. There are many reasons why the sum of the currents of all wires in the cross section of the feeder line can be different from zero. The vector diagram (Fig. 1) shows that with an arbitrary phase and amplitude difference of the currents I1 and I2 in separate wires, these currents can be represented as the sum of anti-phase I1p = -I2p and in-phase I1c = I2c components (the latter are sometimes called single-cycle). The fields created by the common-mode currents of different wires are not compensated (as anti-phase), but summed up. If the length of the feeder is comparable to l, then their sum can create a large additional radiation. This phenomenon is called the antenna effect of the feeder of the 1st kind (AEF-1) [1]. It is noticeably more serious than AEF-2, which will be discussed below.
Since AEF of the 1st kind (hereinafter simply AEF) is associated with common-mode currents, the problem of determining its causes can be reduced to finding the causes of the appearance of common-mode currents of the feeder line in the transmission mode (in the receive mode, such currents always arise under the influence of external electromagnetic fields). Consider a horizontal dipole antenna with a two-wire feeder without taking into account the "ground". We will assume that the AU consists only of an antenna and a feeder. The radiation field of the AS at each point in space is the vector sum of the fields created by the currents of all AS conductors. The total field at each point depends on the distribution of currents along the conductors of the system. This distribution at a given frequency is uniquely determined by the shape, size and placement of the AC wires, as well as the method of excitation. Sufficiently obvious considerations lead to the conclusion (confirmed by calculation and practice) that with the geometric symmetry of the AU and symmetrical (strictly antiphase) excitation, the distribution of currents will also be symmetrical both along the antenna wires and along the feeder wires. In this case, the sum of the common-mode currents of all feeder wires will be equal to zero. An example of such a case is shown in the model in Fig. 2a. The currents of the wires of a symmetrical feeder are the same in amplitude and antiphase, this is determined by the symmetry of the arms of the vibrator antenna and the symmetrical location of the symmetrical feeder relative to these arms, as well as the symmetrical connection of the generator to the beginning of the feeder line.
Any of the following reasons can lead to the appearance of common-mode feeder currents: antenna asymmetry (geometrical asymmetry of the arms, power supply is not in the middle, Fig. 2,b); feeder asymmetry (different diameters or lengths of wires, Fig. 2, c); asymmetry of the SS as a whole (asymmetric relative position of the antenna and feeder, Fig. 2, d). When the "earth" is taken into account, the geometric asymmetry of the AS relative to the "earth" (Fig. 2e) and the electrical asymmetry of the source relative to the "earth" (Z1 is not equal to Z2, Fig. 2f) will also be added here. If in the previous situation complete symmetry is possible in principle, then when a symmetrical antenna is powered by a coaxial (fundamentally asymmetric) feeder without taking special measures, AEF-1 is simply inevitable, although such a feeder is free from AEF-2. A feature of the coaxial line is that at high radio frequencies it can be considered not as a two-wire line, but as a three-wire line. The currents on the inner and outer surfaces of the cable sheath may differ due to the skin effect. To analyze common-mode currents on the model, you can represent the outer surface of the cable sheath with one wire, and connect the generator directly to the antenna. In the case when the central conductor of the cable is connected to one arm of a symmetrical antenna, and the braid to the other (model - Fig. 3, a), then even with a geometrically symmetrical location of the cable relative to the antenna, AEF will occur in the speaker. The reason is the electrical asymmetry of connecting an equivalent source to a geometrically symmetrical speaker (the source is supposed to be a point source and turned on exactly in the center of the antenna, but on the left is one antenna arm, and on the right is the other plus the outer surface of the cable sheath!). In this case, the current distribution strongly depends on the electrical length of the outer surface of the cable sheath (due to external insulation, it is approximately 1% larger than the geometric length). At resonant length (an integer number of half-waves including the ground length for the grounded lower end, or an integer number of half-waves plus l/4 for the ungrounded end of the cable, as in our case), the maximum amplitude of the common-mode current Ic of the cable is maximum and can reach 43% of the maximum amplitude of the current l1 left arm of the antenna (Fig. 3b).
In this example, it is convenient to show a simplified "mechanism" of inducing currents along the outer surface of the braid, which will help to more clearly present the physical processes leading to the AEF. One of the reasons for the common mode current is obvious: it is an equivalent excitation source, to one of the terminals of which an outer conductor is connected. However, this conductor is also in the near field of the antenna arms, the currents in which are not the same. As a result, there is another reason for common-mode currents: asymmetric, and therefore uncompensated at the location of the feeder, the near field of the antenna itself. Such an idea is, of course, very primitive, but sometimes in the practice of combating AEF, for some reason, this second reason is not taken into account at all. Significantly asymmetric with respect to the "ground" (or roof) are vertically polarized antennas located at a low height. Even if we ensure the formal relative symmetry of the antenna and the feeder (vertical dipole when fed from the side), AEF is inevitable. Thus, in transmission operation, feeder common-mode currents can occur for any of the following main reasons: - electrical asymmetry of the AC excitation source or an equivalent antenna excitation source; - geometric asymmetry of the antenna system as a whole: by itself and relative to the ground. In the reception mode, under the action of external electromagnetic fields on the feeder line, both antiphase and common mode currents can occur in its wires. The first ones arise in open two-wire lines and directly affect the input of the receiver (AEF of the 2nd kind). Common mode currents occur in any feeder line. By virtue of the principle of reciprocity, the effect of these currents on the input of the receiver (AEF of the 1st kind) is the stronger, the greater the relative intensity of the common-mode currents of the feeder of this AS in the transmission mode. Only anti-phase currents of the feeder can act directly on a correctly made input of the receiver. The "mechanism" for converting common mode currents in the receive mode into antiphase currents is similar to that described above for a coaxial feeder in the transmit mode. One of the ways is to connect the outer surface of the braid with the inner one at the point of connection of the antenna, and the second one - through the antenna, by means of the common-mode near-field currents, which are asymmetric for different arms of the antenna, with an asymmetric speaker. The characteristics of the AU, taking into account the feeder as part of it, differ from the calculated characteristics of the antenna without taking into account the influence of the feeder. Thus, the AEF is not only the reception or transmission directly by the feeder, so the concept can be expanded. AEF in a broad sense is the influence of the feeder on the characteristics of the antenna system (both during reception and transmission). Let's consider this influence in more detail. Manifestations of the antenna effect of the feeder The most striking manifestations of AEF were noted above. Let us consider these and possible other significant manifestations of AEF in more detail. As examples, let's take a horizontal half-wave vibrator and a well-known vertical antenna GP with a height of l/4 with three counterweights of the same length, installed at an angle of 135 "to the radiator. The input impedance of such an antenna in free space and without taking into account the influence of the feeder is purely active and is about 50 Ohm The vertical pattern (DN) and the distribution of currents on the wires of the pin (I4) and counterweights (I1 - I2) for this case are shown in Fig. 4. All characteristics given here are obtained using computer simulations without taking into account losses.
During transmission, there may be the following manifestations of AEF. 1. Appearance of AS radiation with non-fundamental polarization. If the main polarization of the antenna is vertical and the feeder is not vertical, the feeder radiation will appear with a horizontal component. If the main polarization of the antenna is horizontal and the feeder is not horizontal, the feeder radiation will appear with a vertical component. Example - DN in the vertical plane fig. 5 for a horizontal dipole. Vertical component of the field EQdue to the AEF is about 30% of the useful horizontal Ej. And this is a very undesirable effect, for example, for television reception. 2. Change in RP with the main polarization. The radiation of the feeder with the main polarization can lead to a significant change in the main RP (for example, for vertical antennas in the vertical plane): the directivity factor changes in the main direction (it can be either a decrease or an increase), unwanted lobes appear in other directions. An example is fig. 6 for GP antenna with 9l/4 ungrounded cable length. If the cable with the main polarization does not radiate, then the pattern may change as a result of violation of the symmetry of the excitation (Fig. 7 for Eph of a horizontal dipole).
3. Change in the complex input resistance. For the GP antenna, depending on the length of the coaxial feeder, the active component R of the complex resistance at the excitation points Zin = R + jX can vary from 42 to 100 ohms, and the reactive component X - from -40 to +17 ohms. 4. A change in the input resistance is associated with a change in the standing wave ratio (SWR) in the feeder line. On fig. Figure 8 shows the dependences of the SWR for the GP antenna at l=10,9 m: 1 - with a "normal" cable connection to the antenna; 2 - with perfect "isolation" of the outer surface of the braid at the point of connection to the antenna. As can be seen from the graphs, the SWR in both cases depends on the length of the feeder, which should not occur in the absence of common-mode currents (AEF) and losses in the feeder [2]. We note here that it is the common-mode currents that lead to a change in SWR (through Zin), but not vice versa! The dependence of AEF-2 on SWR has a different "mechanism".
5. Poor SWR means the presence of a significant proportion of standing waves in the feeder currents that are not involved in the transfer of RF energy. In a real cable, losses increase, as a result, the efficiency of the antenna-feeder system decreases. Common-mode currents themselves also lead to additional losses of energy supplied to the AC. 6. Deterioration of DN and SWR, decrease in efficiency reduce the energy potential of the radio link. The range of reliable reception decreases, and in order to achieve the calculated communication quality, it is required to increase the power. And this is an additional cost of energy. At the same time, the problems on points 7-9 are exacerbated. 7. Changing the pattern leads to the appearance of radiation in unforeseen directions, which can create intense interference or field levels that are unacceptable according to sanitary standards. 8. If the feeder is located near other lines, for example, power or telephone lines, the presence of an inductive connection with them in the presence of AEF can lead to serious difficulties in ensuring the joint operation of the radio station with other electronic means (strong mutual interference during transmission and reception). 9. Near the feeder of the transmitting device, a noticeable electromagnetic field may arise, comparable to the fields near the active parts of the AU. Everything related to changes in the general characteristics of transmitting speakers applies equally to receiving speakers (DN, input impedance, SWR, efficiency). External sources of interference with non-primary polarization or in the area of additional lobes of the radiation pattern, or near the feeder, in the presence of an AEF, will create an additional interference background during reception. We note some general features of the manifestation of AEF: 1. AEF manifests itself more strongly with resonant dimensions of the feeder and weaker - with non-resonant dimensions. 2. The nature of the change in RP in the presence of AEF depends on the length of the feeder. The longer the vertical feeder, the more indented the DN becomes in the vertical plane. 3. The amplification of the AS in the main direction in the presence of the AEF can be both greater and less than without taking into account the AEF. 4. The AEF manifests itself the stronger, the stronger the near field of the antenna is the feeder. In this sense, the considered GP antenna is one of the most vulnerable. 5. For vibrator (dipole) antennas, AEF is more pronounced than for loop ones. 6. For vertically polarized antennas, AEF appears more often and stronger than for horizontally polarized antennas. 7. The influence of the feeder on the characteristics of the AU is the stronger, the smaller the size of the antenna and the lower its efficiency. Therefore, AEF is very dangerous for electrically small antennas. 8. AEF is especially dangerous for highly directional and, in particular, DF antennas. 9. The manifestation of AEF in receiving AS is no less, but even more serious than in transmitting. It was for receiving speakers that this problem first arose. AEF prevention and mitigation measures Ways to weaken the AEF are largely determined by the reasons that cause it. They are discussed in the first part of the article. Note that AEF can be completely eliminated only theoretically. Therefore, the terms "prevention" and "suppression" should be understood as different ways to reduce the harmful effects of the AEF, respectively, at the stages before and after the installation of the antenna. In the same order, the means of mitigation are listed in general and for each specific situation: design - installation - operation. For symmetrical two-wire feeders in a symmetrical AS with symmetrical connection (in the absence of common-mode currents), AEF of the 2nd kind can be significantly weakened in various ways and their combinations:
For any feeders, the fight against AEF of the 1st kind is more significant, especially dangerous and associated with the presence of common-mode currents in the feeder. First, we give a brief overview of technical means suitable for eliminating AEF of the 1st kind. In essence, this is a struggle either with the appearance of common-mode currents in the transmit mode, or with their transformation into anti-phase currents in the receive mode. Balancing devices or devices for interfacing symmetrical systems with asymmetric ones (for brevity, we will use the English abbreviation BALUN - from balanced-to-unbalanced). In the transmission mode, the electrical symmetry conditions [3] are determined by the equalities (Fig. 10): Z1=Z2; (one) U1=U2; (2) l1=l2; (3) la=lb; (four) lc=0. (five)
There are more than 100 [3] varieties of BALUNs and many different classifications, among which the simplest one is the most interesting for our purposes. Most of these devices can be divided into two groups [4]: the first - providing U1=U2 (voltage BALUN, V-BALUN); the second - providing I1=I2 (current BALUN, C-BALUN). The first group includes, for example, the well-known U-bend, small-sized transformers [5] on ferrite magnetic cores (Fig. 11, a), the second group includes devices that block common-mode tics. They are both resonant (quarter-wave glass) and aperiodic (choke type). The latter are also sometimes made on ferrite magnetic cores (Fig. 11b, see [6]). Strictly speaking, the former ensure the equality of the EMF in circuits with Z1 and Z2, so condition (2) is valid only when condition (1) is met. For symmetric systems, condition (1) is satisfied. But the latter simply represent a large resistance for the current Ic (and only for it). Therefore, we can assume that the current Ic at the point of connection of the cable to the antenna is close to zero, hence I1~I2. However, we have eliminated only one cause of common mode currents. In an asymmetric speaker (with geometric asymmetry or with asymmetric excitation), the still uncompensated near field of the antenna acts on the outer surface of the braid.
Isolating devices (Line Isolator, LI) are used to electrically separate the outer surface of the feeder sheath into non-resonant sections in order to attenuate the common-mode currents induced by the near field in an unbalanced speaker. To do this, on the path of common-mode currents, it is necessary to provide a large resistance in several places with an interval of l / 4. As LI, both resonant and aperiodic choke devices of the C-BALUN 1:1 type can be used (Fig. 11, b and c). In fact, C-BALUN 1:1 is a line insulator used for balancing. It has been established that for good efficiency of aperiodic LI, the impedance of the inductor winding must be at least 2 ... 3 kilo-ohms. If it is impossible to make a compact inductor on a ferrite ring from a thick cable, you can make a coil from a cable without a magnetic circuit or insert a small inductor into the cable break (both the central conductor and the braid!) 11b, wound with a two-wire line corresponding to the wave impedance of the cable and the power of the transmitter. Such a device does not lead to large losses, since with a large resistance, the common-mode current is negligible. The magnetic circuit in this case is not strongly magnetized, which, however, is typical of all LI and balancing devices of this type. Absorbers of common-mode surface current waves on a coaxial feeder are made using coatings of ferromagnetic or lossy dielectric materials. An example is the installation of ferrite rings or tubes on a coaxial feeder. For good attenuation on the KB bands, 50-70 ferrite rings (Fig. 12) with an initial magnetic permeability m=400...1000 are required. The gap between the cable sheath and the ring should be as small as possible. An absorber of this kind can be considered as a distributed linear isolator with losses.
A significant weakening of the common-mode current also occurs when there is a lossy dielectric around the cable (water, soil, concrete). You can verify this even by wrapping your fingers around the place of the cable with the voltage antinode. In this sense, it is advisable to run the cable not in free space, but in the ventilation duct (along the wall, in the ground, etc.), not to mention special coatings of the cable with compounds containing graphite. Consider possible measures and means to combat AEF-1 in different situations. 1. Symmetrical antenna, symmetrical feeder: - ensuring the geometric symmetry of the AU with respect to the ground; - ensuring the electrical symmetry of connecting the AU (feeder) to the radio station (in particular, BALUN between the feeder and the radio station, if the connection of a symmetrical feeder to the station is not provided). 2. Symmetrical antenna, unbalanced (coaxial) feeder: - balancing devices: V-BALUN with a geometrically symmetrical speaker (Fig. 13, a), however, with a significantly asymmetric speaker, this will not help (Fig. 13, b) and C-BALUN will be required;
- HF insulation of the outer surface of the feeder braid at the point of connection to the antenna is actually C-BALUN (Fig. 13, c for a non-resonant cable length; Fig. 13, d for a resonant one); - fragmentation of the outer surface of the feeder braid along the HF (a series of HF line insulators LI, at least two, with a pitch of l / 4, starting from the antenna); - Common-mode wave absorbers (ferrite rings); - geometric balancing of the AU (in the presence of a balancing device); - selection of non-resonant length of the feeder (Fig. 13, c). 3. Unbalanced antenna, balanced feeder (not often, but used): - ensuring the geometric symmetry of the AU; - ensuring symmetrical connection of the feeder on both sides. 4. Unbalanced antenna, unbalanced feeder (one of the most common combinations and the most vulnerable, balancing devices like V-BALUN do not save here): - C-BALUN as a line isolator at the point of connection of the feeder to the antenna (provides lc=0 at this point - the measure here is necessary, but most often insufficient); - quarter-wave counterweights, sleeves on the cable sheath, locking chokes, cable loops and coils; - fragmentation of the outer surface of the feeder braid along the HF (a series of HF linear insulators LI in possible current antinodes through l / 4); - absorbers of common-mode current wave (ferrite rings); - selection of non-resonant length of the feeder. A few examples of GP antenna characteristics are given in Figure 14:
a - without AEF suppression, resonant length; b - effect of selection of non-resonant cable length; c - C-BALUN at resonant length; d - C-BALUN plus LI; e - C-BALUN plus two LI (compare with Fig. 4 without AEF). Grounding can significantly weaken the AEF, but not always, but only if the transition to a non-resonant length of the feeder + ground wire line occurs. If, in the absence of grounding, your cable already has a non-resonant length (which in itself does not ensure the absence of AEF), then in the presence of grounding, the effective length of the feeder and ground lines can become closer to resonant. Moreover, in cases where it is far from the ground or the ground wire is used for other equipment, it is advisable to abandon RF grounding altogether, retaining only protective grounding (to protect against the effects of short circuits and static electricity). The simplest tool for good RF decoupling from the mains and ground line is a filter choke on a ferrite ring from parallel wires of the network and ground (Fig. 15).
On fig. 16 shows the general scheme of AEF suppression by the technical means discussed above.
We list the general directions of the fight against AEF: - anticipate and eliminate the possibility of AEF at the planning stage; - take reasonable maximum measures to prevent its occurrence; - good AEF suppression is ensured by the combined use of several of the above measures; - after installing the AU, check the presence of AEF and, if necessary, weaken it using available means; - to carry out continuous or periodic monitoring of the AEF in the process of operation; - it is absolutely necessary to suppress the AEF for safety reasons when the transmitter power is more than 100W. AEF controls of the 1st kind For testing, monitoring and carrying out work to suppress the AEF, control tools are needed. In the transmission mode, control is carried out using the simplest indicators. The simplest of them is a neon light bulb. The pointer indicator of common-mode currents can be made on the basis of a current transformer on an annular magnetic circuit made of ferrite grade M55NN-1, size K65x40x6 (Fig. 17, a). The primary winding is a cable threaded into a ring, the secondary winding - L1 has 10 turns of PEV-2 wire with a diameter of 0,15 mm. It is desirable to make the sensitivity of the measuring head adjustable. The ring is moved along the cable so that it is always in the center of the ring (Fig. 18, a)
The pointer indicator of the electric field (see Fig. 17, b) is quite simple to make. The length of the arms WA1, WA2 of the antenna is no more than 20 cm. When moving the end of one of the arms along the cable (Fig. 18, a), you must ensure that the distance between this end and the cable does not change. Of course, other types of indicators are also possible: with a detachable magnetic circuit, with an electrostatic screen, resonant or broadband, with an amplifier, light or sound, etc. By moving the indicator along the feeder in transmission mode, follow its reaction. In the presence of an AEF, it is possible to determine the position and evaluate the levels of antinodes (maxima) of current or voltage. AEF control in the transmission mode is also carried out with the help of instruments, using a laboratory generator (GSS) and a receiver (Fig. 18, a). However, the result may not match the real AEF if the signal generator is located and grounded differently than your transmitter. It is much more convenient to control the AEF in the receive mode (Fig. 18,6). Here the antenna is connected to its receiver, you just need to make sure that if the generator is not connected to the cable, the signal from the generator does not enter the receiver through the antenna.
Using the AEF In general, it is believed that AEF is always bad for everyone. But sometimes, with the help of an artificially created distribution of the common-mode current of the feeder, it is possible to improve some characteristics of the AU (as a rule, at the cost of degrading others). Using AEF to improve SWR by selecting the length of the feeder. A high SWR can damage the transmitter if it does not have automatic protection (low power or simply shutdown). Radio amateurs have long noticed that sometimes it is possible to achieve an improvement in SWR by changing the length of the feeder. However, not everyone correctly represents the nature of such a phenomenon. This is explained by the dependence of the complex input impedance of the speaker, and hence the SWR, on the length of the feeder in the presence of an AEF (see Fig. 8 in the first part of the article). In particular, a decrease in SWR can occur when moving from a resonant cable length to a non-resonant one (which is easy to check using an indicator). It is possible that the best way out in this case would be to eliminate the causes of AEF in more effective ways, which are described above. Using the DEF to improve the radiation pattern. Analyzing the dependence of the gain of vertical antennas on the length of the feeder, it can be seen that the AEF does not always lead to deterioration. If the field in the right direction and with the right polarization from the feeder currents is added in phase with the field from the antenna currents, additional gain can be obtained. The most striking and useful examples of this improvement are the creation of a symmetrical counterweight from the feeder section to form vertical antennas with a total length of 2xl / 4, 2xl / 2 and 2x5l / 8. In the simplest case, this is done using a shut-off choke with an inductive resistance of at least 2000 ohms. In order to well weaken the currents through the feeder in its "unused" part, it is advisable to install one or two more such chokes below the main one at l/4 intervals. As a result, you can get close to ideal diagrams in the vertical plane (Fig. 19). For vertical antennas, this is perhaps the easiest way to improve speaker performance when fed from below. It is only necessary to make sure that there are no parasitic resonances of the mast and guys.
The absence of a noticeable AEF is the first and main requirement for any antenna-feeder systems [8]. The antenna of the radio installation should be the only source and receiver of radio emission. The problems associated with the AEF are quite serious and they need to be solved already at the design stage of antenna-feeder devices. When developing antennas, devices for reducing the AEF should be provided. Antenna manufacturers should develop appropriate guidelines for antenna installation and feeder location. It is important for users to know the causes and manifestations of AEF, to be able to prevent and control them, and to deal with them. The possibility of the appearance of strong fields near the feeder must be taken into account when determining electromagnetic safety, drawing up a sanitary passport. Literature 1. Pistohlkors A. A. Receiving antennas. - M.: Svyaztekhizdat, 1937.
Authors: Anatoly Grechikhin (UA3TZ), Dmitry Proskuryakov, Nizhny Novgorod; Publication: N. Bolshakov, rf.atnn.ru
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