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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Transceiver KB antennas. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / HF antennas About antenna installation height When choosing the design of a receiving-transmitting antenna for his amateur radio station, a shortwave operator has to take into account many factors, look for compromise solutions for many technical issues. One of them is the height of the antenna installation. The possibilities of a radio amateur in this area (regardless of where he lives - in a city or in a village) are very, very limited. Are there any optimal solutions here? To some extent, the experiment conducted by DJ2NN[1] provides an answer to this question. It should be emphasized that it is not easy to measure the dependence of the antenna efficiency on the height of its installation at short waves. Of greatest interest, of course, are these data for long paths (i.e., for DX links), which means that the measurement results are significantly affected by the propagation of radio waves in the ionosphere (especially fast propagation fluctuations). Moreover, in the general case, these dependences can have a different character for paths with different lengths and azimuth directions. The reliability of the results can be increased only by multiple repeated measurements, a set of statistical data.
DJ2NN measured the dependence of the antenna efficiency on the height of its installation on the amateur bands 14. 21 and 28 MHz in the mode of receiving signals from DX stations (the length of the route is at least 5000 km). In addition, similar dependences were measured from the signals of stations located in the "near" zone, where the connection is due to the surface wave. In these experiments, DJ2NN used "wave channel" antennas, the installation height of which could be very quickly changed within 2,5 ... 25 m. He took special measures that would eliminate measurement errors due to antenna detuning at low installation heights (due to the influence of the "earth"). The results of these experiments for the 14 and 28 MHz bands are shown in Figs. 1a and 1b. The general course of similar dependences for the 21 MHz band is very close to the data shown in Fig. 1, a. The curves marked with number 1 refer to measurements based on the signals of DX stations, and those marked with number 2 refer to measurements from stations located in the "near" zone. An analysis of these curves allows us to draw several conclusions. First, measuring the parameters of a short-wave antenna and working out its radiation pattern by the field strength in the "near" zone can not always provide objective information about its effectiveness when making DX communications. In other words, measurements in the "near" zone are a necessary, but sometimes insufficient step in establishing a directional KB antenna. Secondly, in the height range of 2,5 ... 15 m, the efficiency of such an antenna on the 14 and 21 MHz bands changes very much. A situation may arise when a simpler and lighter two-element antenna, raised to a height of 10 ... 12 m, will be more effective than, say, a three-element antenna, which a radio amateur cannot raise above 5 ... 7 m (due to more mass, more bulky and heavy rotating device, etc.).
And thirdly, increasing the height of the antenna installation above about 17 m is not justified. The efficiency gains are marginal, and the manufacturing costs and technical complexities associated with the installation and operation of the antenna increase many times over.
Omnidirectional Antennas Most of the shortwaves are forced to confine themselves to installing only one antenna, which, of course, they are trying to make multi-band and omnidirectional. There are many designs of such antennas in which these requirements are met to a greater or lesser extent. One of these antennas - "G5RV" (according to the call sign of the radio amateur who proposed it [2] - is designed to operate on amateur bands 3,5 ... 28 MHz. The dimensions of the antenna and two-wire matching line are shown in fig. 3.a, the antenna is powered by a coaxial cable with a characteristic impedance of 75 ohms. The recommended installation height of the antenna above the ground or above the roof is about 10 m. If the span in which the antenna is installed is less than 32 m, then the end sections of the antenna web up to 3 m long can be left hanging down (i.e., to install the antenna in this case a span of approximately 26 m is suitable). The "G5RV" antenna in principle allows installation using only one mast in the form of "INVERTED V", but in order to not noticeably degrade its performance, the apex angle must be at least 120°.
A self-made two-wire matching line is formed by two wires, the distance between which is maintained by constant insulators (Fig. 3, b) made of a good, non-hygroscopic dielectric (plexiglass, textolite, etc.) After appropriate impregnation, you can also use wood or plywood. The wires of the line are placed in V-shaped cutouts at the ends of the insulators and fixed with small pieces of wires (Fig. 3. 0) passed through the holes in the insulators. The matching line must run perpendicular to the antenna web for at least 6 m. For effective operation of the G5RV antenna on all bands, its feeder must be connected to the transmitter through a matching device. Since this antenna in the feeder almost always has a standing wave to one degree or another, it makes no sense to use a balancing device (BALUN) to go from a matching line to a coaxial cable. However, to reduce radiation from the outer braid of the cable (this, in particular, may cause interference to television), it is advisable [3] to make a high-frequency choke from the upper part of the feeder (Fig. 3. d). The number of turns is 8 .. 10, the winding diameter is about 180 mm, the turns are fastened in three places with adhesive tape.
Another version of the multi-band KB antenna based on "G5RV" [4] is shown in fig. 4. a. On the central mast 1, about 12 m high, at an angle of about 30° to each other, two "G5RV" antenna sheets are suspended. The ends of these webs are attached through insulators 4 to four auxiliary masts 3 about 6 m high. In the center, the web antennas are connected in pairs to a common two-wire line 5 (see Fig. 4.b), which is the same as in the usual "G5RV" , made air on insulators 6. For fastening the ends of the cloths on the mast 1, the central insulator 2 serves. It should be noted that the given dimensions are not critical. They can be varied within a fairly wide range, focusing on the capabilities of the radio amateur and the place available to him for installing the antenna. In amateur literature, there are often descriptions of multi-band horizontal antennas, which are radiators connected in parallel (for example, half-wave dipoles) for separate KB bands. This principle can also be applied to create antennas with vertical polarization. The design of such a three-band KB antenna [5] is shown in Fig. 5. A metal mast 3, which serves as a radiator in the 14 MHz range, is mounted on a support insulator 2. In its upper part, at a distance of about 350 cm from the support insulator, a dielectric spacer 9 is fixed. Wire emitters 4 are attached to the base of the mast (and connected to it electrically) on the 21 and 28 MHz bands. The tension of the emitters is provided by nylon extensions 5, which are connected to them through insulators 6. The antenna is powered by a coaxial cable 8 with a characteristic impedance of 50 Ohm, the central core of which is connected to the mast 3, and the braid to the system of counterweights 7. The lengths of all emitters differ from the value */4 for the corresponding range, which is due to the mutual influence of the emitters. Shown in fig. 5, the dimensions of the radiators were selected experimentally according to the minimum SWR values in the operating ranges.
A variant of a broadband antenna [b], operating on all KB bands, including 160 m, is shown in fig. 6. The antenna is a wire emitter 22,6 m long, at a distance of one third from the end of which an LR circuit is connected, expanding the operating frequency band.
This circuit (Fig. 6, b) is formed by a resistor R with a resistance of 370 Ohm (6 resistors with a resistance of 2,2 kOhm and a maximum dissipation power of 1 W) and a coil L (55 turns of wire 1 mm in diameter, ordinary continuous winding on a frame with a diameter of approximately 50 mm ). The antenna is connected to the feeder (impedance 50 Ohm) through a matching transformer (Fig. 6, c). It is made on a ferrite ring with a diameter of about 50 mm with an initial magnetic permeability of about 20. Each of the windings has 24 turns of wire with a diameter of 1 mm. The antenna is connected to the tap from the 18th turn of the secondary winding. The connection point is selected experimentally when setting up the antenna. The antenna is tuned by selecting, first of all, the inductance of the coil L and the point of connection of the antenna to the matching transformer. The criterion is the minimum SWR within the amateur bands. Although the article notes the possibility of antenna operation even on the 160 m band, in reality, apparently, it is possible to obtain satisfactory characteristics only at frequencies of 7 MHz and higher. The influence of the "earth" The antenna described above, as well as many other "wire" and whip antennas, requires a good "electronic ground" for its normal (effective) operation. In urban (and not only in urban) conditions, it is usually provided by connecting an equivalent - counterweights. How many counterweights and how long can create a good "radio-technical earth"? Measurements show [7] that their number should exceed 20 ... 30. With several balances (a very typical case in amateur radio practice), the loss resistance is approximately 30 ohms. This means that about 50% of the transmitter power is lost. In other words, it is worth considering: what is easier - to conflict with the State Telecommunications Inspectorate, increasing the transmitter power beyond the permitted limits, or to add several dozen balances to the antenna and get the same efficiency of the radio station as a whole.
Typical dependences of the input impedance of a quarter-wave pin (theoretical value 37 Ohm) on the number of quarter-wave balances for various conditions (1 - dry soil, 2-wet, 3 - theoretical value) are shown in fig. 7. Given these dependencies, it should not be surprising that a GP with three counterbalances provides an SWR of ~ 1 when powered by a 75 ohm coaxial cable (theoretical SWR value of ~ 2). It becomes clear and effective operation of some vertical antennas in a wide frequency band - losses in the "ground" significantly expand it. Notch circuits for KB antennas Antennas with notch circuits ("W3DZZ" and the like) are widely used in amateur radio practice. They have quite acceptable characteristics, but from a constructive point of view, they are not very convenient. Particular difficulties (in manufacturing or purchasing) are caused by a capacitor included in the LC rejector circuit. It must have a well-defined rating and very high electrical parameters, working under conditions of exposure to a moisture atmosphere. The notch circuit for "W3DZZ" type antennas can be made from a piece of coaxial cable, the braid of which will form the necessary inductance, and the "central core-braid" will create the necessary capacitance | 8].
The design of such a rejector circuit is shown in Fig. 8. A coaxial cable 1 is wound on the dielectric frame 2. The ends of the cable 3 are passed into the holes of the frame and soldered (5) in accordance with the figure. Brackets 4 are used to connect the antenna sheets 6. For simple antennas with rejector circuits, the choice of coil parameters is quite arbitrary (it is only necessary to provide the required rejection frequency). In the antenna "W3DZZ". in addition, it is necessary to have a well-defined ratio of the inductance of the coil L and the capacitance of the capacitor C - without this it is impossible to realize the multi-range properties of the antenna. Directional Antennas A rotating directional KB antenna is the dream of all shortwavers. However, many radio amateurs cannot afford to make a full-size antenna ("wave channel", "double square", etc.) One of the reasons for this is the very limited area on the roof of a residential building that a shortwave can use to install an antenna (especially towers). That is why in amateur radio magazines so often there are descriptions of various options for small-sized single- or multi-band KB antennas.
The antenna, the sketch of which is shown in fig. 9, was called "DOUBLE-D" ("double delta") [9]. Small in size, light, it may well be the first design of a shortwaver who wants to increase the efficiency of his amateur radio by installing a rotating directional antenna. On the mast 1, at a distance D from its top, there are four spacers 2 made of bamboo or wood impregnated with moisture-proof compounds. The webs of the active element 5 and the reflector 3 are attached to the ends of these spacers and through the extensions 4. Both webs are made of copper wire or an antenna cord, and the extensions are made of a nylon cord. The configuration of the active element and the reflector resembles the Latin letter D, hence the name of the antenna. The antenna is fed through a coaxial cable 6 with a wave impedance of 50 ohms. The length of the antenna wire elements in meters is calculated using the following formulas (f is the operating frequency in MHz): A = B = 85,1/f C = 60,2/f D=17,8/f E = 34/f The frequency value f is chosen either in the middle of the corresponding amateur range, or in the middle of its section, which is of most interest to the shortwave (for example, in the middle of the telegraph section). Based on the data [9], the "DOUBLE-D" antenna is practically not inferior to the two-element "wave channel" antenna in terms of directivity and back-to-forward radiation ratio. However, it has a lower bandwidth, as shown in Fig. 10, which shows the SWR versus frequency (28 MHz band) for the "DOUBLE-D" antenna (curve 1) and the full-sized "wave channel" (curve 2).
This antenna is tuned by selecting the length of the active element and the reflector. At the resonant frequency, its input impedance is purely active and is approximately 40 ohms. Using this principle of constructing an antenna, it is possible to manufacture a multi-band design. In this case, it is desirable to power each of the active elements with a separate coaxial cable. Experiments with a dual-band antenna (14 and 21 MHz) showed that setting the elements to the second range on the same design does not change the antenna patterns. When both active elements were powered, even through one coaxial cable, the SWR did not exceed 2 within both amateur bands. A compact tri-band (14, 21 and 28 MHz) "double square" (Fig. 11) has been proposed. 9H1GL [10]. In terms of dimensions, it does not exceed the two-band "double square" at 21 and 28 MHz. This antenna essentially consists of two full-sized "double squares" for the 21 and 28 MHz bands, and the third band, 14 MHz, is obtained by connecting load capacitances to the 21 MHz band elements.
On the mast 1, a short bearing beam 2 is fixed, to which, in turn, brackets 3 "hedgehogs" are attached. The use of a combination of "bearing traverse" - "hedgehogs" (each of them separately is widely used in "double squares") made it possible to obtain a very high attachment point for guy wires 6. The antenna rotates together with mast 1 (a motor with a gearbox is installed at its base), therefore guys are attached to the intermediate bearing 5. The height of the mast is approximately 5,5 m, the bearing is installed 0,8 ... 1 m below the attachment point of the carrier beam. In this case, with the maximum allowable angle between the mast and guys of 30°, the points of attachment of the guys to the roof will be approximately 2.7 m from the base of the mast. The configuration of the "hedgehogs" elements 3 (they are made of a steel angle) is shown in Fig. 11. c. Bamboo spacers 4 are fastened to the bent parts of these elements with U-bolts or clips. The length of the spacers is about 2,4 m. The length of each side of the frame for the 21 MHz band is 3,6 m, and for the 28 MHz band it is 2,75 m. The load capacitance elements that ensure the operation of the antenna on the 14 MHz band are located inside the 21 MHz band frames (slightly closer to the mast than these frames). They are "turned off" by four notch circuits - two for each frame. The resonant frequency of the notch circuits (before connecting to the antenna) is -20,2 MHz. Structurally, they are made of coaxial cable in the same way as described in the previous section of the review. The circuits are connected between the frame and capacitive loads at the points indicated in fig. eleven. The method of tuning the antenna elements on the bands 28 and 21, MHz does not differ from the standard. On the 14 MHz band, the antenna is tuned by selecting the length of the elements - capacitive loads. If changing the length of these elements significantly affects the parameters of the antenna on the 21 MHz band, then this indicates that the notch circuits are not tuned accurately (i.e., they do not completely “turn off” the capacitive load when operating on the 21 MHz band). When feeding the antenna with a 50-ohm coaxial cable, the SWR did not exceed 2 on all three bands. Literature
Author: B. Stepanov (RU3AX); Publication: cxem.net
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