|
Arabic
Bengali
Chinese
English
French
German
Hebrew
Hindi
Italian
Japanese
Korean
Malay
Polish
Portuguese
Spanish
Turkish
Ukrainian
Vietnamese|
ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Vertical top feed. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / HF antennas The article discusses the principles of creation and practical designs of multi-band vertical antennas with top feed. They are especially convenient for work in the field or expeditionary conditions, but nevertheless they can be used in the home "shack", taking up little space and providing good parameters. The problem of creating a simple and effective multi-band antenna is still of concern to almost every shortwaver. Most often, attention is drawn to the designs of vertical antennas, since they take up little space, are easier to install and have an optimal radiation pattern (DN) for DX communications: with zero in the zenith direction and a maximum in the direction of the horizon and the absence of azimuthal directivity, allowing radio communications with correspondents in any direction. Numerous well-known designs of verticals fed from below suffer from disadvantages associated with inefficient use of the entire height of the mast on high-frequency ranges and the difficulty of setting up barrier circuits (ladders) or other devices located at a considerable height and, in fact, turning the antenna into a multi-band one. In the first part of the article, we will consider what advantages and conveniences appear when the power point is shifted up along the vertical radiating conductor. For brevity, let's call the described antenna GDP - the vertical of the upper feed. GDP design Along the radiating conductor of the vertical, as in any other antenna, a standing current wave with zero at the top is installed, so the feed point cannot be placed near the top itself - the input impedance will be too large. By moving the feed point down from the top, we get to a place where the current is already significant, and the voltage is less than at the top, so the input resistance (equal to the ratio of voltage to current) decreases. At the power point, we will attach the central conductor of the coaxial feeder to the upper part of the vertical, and the braid ... let's not attach it anywhere at all. Then the current will flow from the power point along the outer surface of the braid, and in the same direction as in the upper part of the vertical. This concept is presented in the article [1], in its third part, referring to Fig. 19. There, the current on the braid is proposed to be used to improve the DN. Following these recommendations, we will make the current on the braid part of the main, radiating current. Note that the currents on the outer and inner sides of the feeder braid are not related to each other in any way due to the very small thickness of the skin layer in the volume of the conductor, they are only equal to each other on the upper cut of the braid. On fig. 1, a schematically shows the projected vertical, and in fig. 1b - current distribution in it. Feeding point A is indicated by a circle (graphics of the MMANA program). Here the center conductor is connected to the top 3 meters long, and the braid is left free. The sinusoidal current distribution will be preserved both on the upper part of the vertical and on the braid. At point B, at a distance of half a wave from the top of the antenna vibrator in the range of 10 meters, a current node is formed (see the leftmost current distribution graph in Fig. 1, b). In this place, a blocking circuit must be placed in order to stop the further flow of current down the braid.
The contour is easiest to make in the form of a cable bay, without violating the integrity of the latter [2, 3]. We have already got a vertical antenna of a range of 10 meters. Its design is shown in Fig. 2, a. The antenna can be made entirely of coaxial cable, using only the braid of the upper cable section for the upper part. Whether or not to connect the inner conductor with it is indifferent, the current will still flow only through the braid. They hang the antenna on a dielectric guy (thick fishing line) from a tree branch, etc., it is only necessary to ensure a strong mechanical bundle of cable segments at feed point A, since the central conductor is unlikely to withstand the weight of the entire feeder and "balun". Another option is to attach the antenna to a thin dry spruce or pine mast (damp wood introduces noticeable losses) or to a fiberglass rod. In this case, it is advisable to make the upper part of a metal tube. Let's go back to the contour. The cable bay has a significant inductance L and at the same time capacitance between individual turns, the main role is played by the capacitance between the first and last turn. The total equivalent capacitance C closes the bay. Thus, a cable bay for HF currents is a parallel circuit, the equivalent circuit of which is shown in fig. 2b. The frequency of its tuning can be changed by selecting the number of turns, their diameter and the stacking order - placing the first turn closer to the last, we increase the capacitance and lower the frequency To tune to a frequency of 28,5 MHz, three turns with a diameter of 13 cm are sufficient [3]. It is curious that even if the current is not completely blocked on the braid, the remaining current below the circuit will flow in the same direction as in the antenna - after all, the circuit inverts the phase, having equal and antiphase oscillations at the terminals. Therefore, the remaining current on the bottom of the cable will not spoil the DP, even improve it somewhat. Now the important advantages of the GDP have been outlined: first, you can tune the antenna (select the diameter of the cable coil and its position along the vertical height) from below, five meters below the top point, and second, feed point A can be located anywhere in the vertical, achieving the desired input impedance antennas, no additional balancing devices are required. Focusing on an available 75-ohm television cable, it is advisable to slightly shift the feed point A down relative to the middle of the current half-wave, while the input resistance rises slightly compared to the resistance of a half-wave vibrator fed in the middle (73,1 ohms for an infinitely thin and somewhat less for a vibrator of finite thickness). Taking into account the frequently encountered length of duralumin pipes, equal to 3 meters, the length of the upper part was chosen. Why not 2 meters? In order for the antenna to work better on other bands. In the range of 15 meters, circuit B is no longer tuned into resonance and represents only some inductive resistance for these frequencies (see Fig. 1 in [3]), being, as it were, an extension coil. As a result, the half-wavelength decreases from 7,1 to 5,82 m (see Fig. 1). At this distance from the top of the vertical there will be a current node, and here we turn on the second blocking circuit C, tuned to a frequency of 21,2 MHz (middle frequency of the range of 15 meters). Continuing the process further, we turn on the third circuit D, already tuned to a frequency of 14,15 MHz (the middle of the 20-meter band), and we will see that for the 40-meter band, the length of our half-wave vertical was only 9 meters. Such a significant shortening in the range of 40 meters was due to the combined influence of circuits B, C and D, which at a frequency of 7 MHz have inductive resistance and serve as "extending" coils. When the half-wave vibrator is shortened, its radiation resistance, referred to the antinode (place of maximum) of the current, decreases. On the other hand, the feed point A, as the frequency decreases, turns out to be higher in relation to the maximum current and the input resistance, equal to the radiation resistance, recalculated to the feed point, increases. The two processes cancel each other out to a great extent, and the input impedance remains roughly constant across ranges. All this design was easily and quickly done using the MMANA program, and after some optimization (I'm not sure that it can not be improved further), the antenna shown in Fig. 1. The input impedance of the antenna in the ranges of 10, 15, 20 and 40 meters turned out to be 78, 67, 69 and 61 Ohms, respectively, with zero reactance, which provides good matching (SWR less than 1,2 at medium frequencies of the ranges). When calculating, the following values of the parameters of equivalent circuits (frequency, inductance, capacitance) were obtained: V - 28 MHz, 5 mH, 1,6 pF; C - 19,5 MHz, 21,2 mH, 2 pF; D - 28 MHz, 14,15 mH, 3,2 pF. Perhaps the most important advantage of the designed vertical is that it does not require either "earth" or radials. It remains to decide how to bring the feeder further down from the lower point of the vertical (see Fig. 1, a). We already know how to wind another coil of the same cable so that it forms a loop tuned to 7,05 MHz. Another solution is also possible - just below the D contour, attach three to four short (about 1,5 m long) horizontal or inclined radials to the cable sheath. They will bring the electrical length of the antenna to half a wave in the range of 40 meters. The short radials do not eliminate the need for a barrier loop, but it will now be positioned directly below the radial connection point. The inductive connection of this circuit with circuit D (after all, now they are close) is undesirable. Instead of a circuit in this embodiment, chokes wound with the same feeder on ferrite rings are suitable. The process of setting up the GDP seems simple and fairly obvious. Start with the highest frequency range of 10 meters. By selecting the winding density (diameter) and, within a small range, the position along the height of bay B, an acceptable SWR is achieved in this range. Having fixed the bay with electrical tape, they switch to the 15-meter range and repeat the same operation with bay C, without touching the tuned circuit B. And so on, until the entire antenna is tuned on all bands. Cable antenna, for example, RK-75-4-11 is especially good for field conditions. It is configured, it can be in the field if the transceiver is equipped with an SWR meter. Under stationary conditions, the GDP can probably be made from duralumin tubes separated by dielectric inserts in places B, C, D and at the lower end. Over the inserts are placed coils bent from a soft copper or aluminum tube (tape can be used). The capacitors of the circuits must be high-voltage, since the circuits are located in the antinodes of the voltage. In this case, the cable should run straight inside all pipes, but in order to avoid current on the braid, a number of ferrite rings should be put on it, and a blocking choke or several chokes on large-diameter ferrite rings should be wound near the lower edge of the GDP. This version of the GDP was not calculated and was not produced. In conclusion of this part - one more prospective variant of GDP. To make the antenna work also in the range of 80 meters, at the lower point of the vertical (see Fig. 1, a) it is necessary to install a barrier circuit tuned to a frequency of 7,05 MHz, and below its cable sheath (lower pipe in the stationary version) ground or connect to a system of radials 20 m long. Then the antenna will operate at a frequency of 3,6 MHz as a quarter-wave GroundPlane shortened by inductances with a raised feed point. Portable Dual Band GDP The first practical version of the GDP was made urgently, "on the knee" when it became necessary to deploy the radio station of the editorial office of the "Radio" magazine at the NTTM-2002 exhibition. A huge pavilion with openwork metal ceilings and metal fittings of glazed walls excluded the placement of the antenna inside the building due to the complete shielding of signals and a high level of interference. Fortunately, we managed to install a vertical on the roof of the ventilation booth and pass the cable into the ventilation shaft. A year later, a few days before the opening of the exhibition "Expo-Science 2003" (see "Radio", 2003, No. 8, first cover), fate presented an unpleasant surprise. The roof of a similar pavilion, where the exhibition took place, was a flat field, larger than a football one, covered with roofing material. Picking it, driving in nails, hooks, etc., as well as using ventilation shafts, was strictly prohibited. We could only talk about a free-standing antenna with a feeder descending along the outer wall and entering the building through a gap at the door. The situation seemed hopeless, but a few hours of modeling using the MMANA program and two evenings of "finishing" the GDP solved the problem. We needed at least two ranges: 20 and 40 meters. It was on them that the antenna was designed. When disassembled and folded, it fit into a bag with a diameter of 30 and a height of 160 cm, it was easily carried with one hand (they were not weighed, but the cable coil is many times heavier) and brought to the exhibition in the subway. After an hour and a half spent on its installation and solving organizational problems (feeder wiring, network, table, etc.), it provided communications with Siberia, Western Europe, and then with more distant correspondents. The sketch of the antenna is shown in fig. 3. The upper part of the GDP above the feeding point A is made of three duralumin tubes inserted one into the other (the middle one is a ski pole, the upper one is very light and thin-walled). Power points A to circuit B as a radiating element 1 is a cable braid, its central conductor is connected to the upper part of the antenna 2. Below circuit B, four radials 3 are connected to the cable braid, made of a thin-walled steel profile of rectangular section (from window curtains). The outer ends of the radials are interconnected by segments of a coaxial cable that has served its age, 2,5 m long (only a braid was used). This increases the effective surface of the resulting "virtual earth".
Since the antenna was designed as a dual-band antenna, it was decided to use one parallel circuit B, tuned slightly above the frequency of 7 MHz. In the range of 40 meters, it has an inductive reactance and serves as an extension coil, tuning the antenna into resonance. In the range of 20 meters, the circuit has a capacitance and shortens the electrical length of the antenna, again tuning it into resonance. The contour parameters for given antenna dimensions were optimized using the MMANA program by placing the radials at a height of 0,2 m above perfectly conducting ground (this is how we tried to take into account the effect of the reinforced concrete roof of the pavilion). The simulation yielded a loop tuning frequency of 7,6 MHz with an inductance of 1,24 μGy and a capacitance of 355 pF. It is impossible to make a circuit with such a large capacity from a cable coil, so conventional capacitors and a cylindrical coil of cable were used, which provide a high quality factor. The design features of the manufactured GDP are illustrated in Fig. 4. The contour is placed in a cylindrical body 4, which has a solid bottom, cast from an aluminum alloy, and relatively thin duralumin walls. The author used a spin tank from an old washing machine (for example, "Siberia"). The body dimensions are not critical (25...30 cm in diameter and height). The holes in the bottom are not closed - they serve their intended purpose for draining rainwater and condensate that accidentally got in. Radials 4 are attached to the bottom of the body with 3 screws. Special strength in these connections is not required, since the radials lie freely on the roof surface. The lower bearing element of the vertical 1 is made of a piece of plumbing plastic pipe with a diameter of 2.5...3 inches. To fix the pipe 1 to the bottom of the housing 4 and to fasten the upper radiating element 2, cylindrical bosses 5 are used. They can be made both from metal and from a dielectric material. A radial hole is drilled in the upper boss, through which the central conductor of the cable is connected to the upper radiating element 2 by terminal 6. It also gives mechanical strength to this assembly. Before screwing the terminal on pipe 1, put on a light plastic cover (not shown in Fig. 4), in which holes for the pipe and cable are made. The cover is lowered to body 4, protecting the circuit from precipitation. The upper end of the cable must be equipped with a contact lug with a hole suitable for terminal 6. The lug must be firmly fixed to the outer insulation of the cable, insulating it from the braid. The central conductor is connected to the petal without its tension, which will protect the conductor from breaking during assembly and disassembly of the GDP. Four more terminals are fixed at the outer ends of the radials 3, and contact lobes are pre-soldered to the ends of the "artificial earth" cable segments 7, which greatly speeds up the assembly of the antenna. The final strength of the whole structure is given by four extensions of thin fishing line, shown by dashed lines in Fig. 3. They are tied to element 2 at the top joint of the tubes and to the terminals at the ends of the radials. The design of the circuit is clear from Fig. 4. On the side wall of the housing 4, a coaxial connector 8 is fixed, preferably the same as in the radio station (this will allow you not to think when assembling the antenna which end of the main feeder should go to the antenna and which to the transceiver), and a mounting plate with two petals 9. Another lobe, which has contact with the body 4, is fixed under the connector screw 8. The braid of the cable from which the coil is wound is soldered to it, and one terminal of the capacitor 10. The petals of the mounting plate 9 should not have contact with the body 4. Two central conductors are soldered to one of them, and the braids of the cable segments and the other terminal of the capacitor 10 are soldered to the other. The capacitor is made up, for reliability, of two KSO capacitors connected in series for an operating voltage of 500 V with a capacity of 680 pF. It is acceptable to use other high voltage capacitors with a sufficient degree of encapsulation to withstand atmospheric influences. The circuit coil contains 7 turns of the PK-75-4-11 cable, wound tightly on a plastic pipe 1. The coil inductance is adjusted in two ways: either by moving the entire coil along the height of the pipe (bringing it closer to the bottom of the housing 4 reduces the inductance, increasing the circuit tuning frequency), or by raising the upper turns, increasing the length of the winding due to the resulting gaps between the turns (in this case, the inductance also decreases). After setting, the turns are fixed with insulating tape or wired twine. Antenna tuning is easy. Having assembled it and installed it at the working position (in case of strong wind, it is useful to "weight" the ends of the radials 3 with sandbags or other heavy objects at hand), connect the antenna to the transceiver with the main cable. Having removed the frequency dependence of the SWR in the range of 40 meters, it is determined where the loop tuning frequency should be shifted so that the SWR minimum falls into the middle of the range. For example, if the minimum SWR is below 7 MHz, the coil inductance must be reduced, and if it is above 7,1 MHz, it must be increased. As a rule, one, maximum two corrections are enough. Then check the SWR in the range of 20 meters. There, the antenna is very broadband, and correction, as a rule, is not required. If, nevertheless, such a need arose, then it is necessary to change the ratio of the L and C contours and again adjust the antenna in the range of 40 meters. An increase in the inductance of the circuit while reducing the capacitance lowers the tuning frequency of the antenna in the range of 40 meters and increases it in the range of 20 meters, i.e., it “spreads” the resonant frequencies of the antenna. In our country, after a single adjustment, the antenna mounted on a reinforced concrete roof provided an SWR close to unity in both ranges. During the operation of the antenna, it turned out that it works well in the range of 15 meters, although the SWR is higher there. The automatic tuner capabilities of the IC-746 transceiver were quite enough to tune it. The proposed concept of the VHF opens up wide possibilities for designing simple multi-band vertical antennas. Even if a radio amateur fails to tune the GDP well, he can still be sure that the upper, approximately five-meter, part of his vertical will radiate, and to the right place - in the direction of the horizon, and this is the key to successful results in DX- inge. Literature
Author: Vladimir Polyakov (RA3AAE), Moscow
A New Way to Control and Manipulate Optical Signals
05.05.2024 Primium Seneca keyboard
05.05.2024 The world's tallest astronomical observatory opened
04.05.2024
▪ Bitcoin tower to be built in Dubai ▪ Viable artificial lungs created for the first time
▪ section of the site Stories from the life of radio amateurs. Selection of articles ▪ article What is the solar corona? Detailed answer ▪ article Gachny knot with a hose. Travel Tips
Home page | Library | Articles | Website map | Site Reviews
www.diagram.com.ua |
See other articles Section 
Latest news of science and technology, new electronics:
All languages of this page