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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Experimental antenna at 144 MHz. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / VHF antennas The experimental antenna (EA) described below exacerbated another problem of electromagnetic compatibility of communication means - the problem of suppressing the image channel of radio receivers. When experimenting with an EA and a transmitter with an output power of 2 W in the city, there were complaints from a professional-commercial organization about interference from side-frequency radiation. A subsequent check with a scanning receiver in the specified frequency range (145 MHz) near the transmitter did not give a result. The previous work for two years at the same place and with the same equipment did not cause any complaints, and there was only one difference - another antenna: before the incident - a "double square", then - described by EA. The radiated energy of the two-watt transmitter turned out to be so concentrated in the direction of the main lobe of the antenna pattern that it became equal in level to the signal in the main (not mirror) channel of the "commercial" receiver, where the reception of the two-meter transmitter signal became possible exactly as if the transmission was carried out on frequency 2Ffr higher. I ask radio amateurs to pay the most serious attention to this problem: although it really is “not yours”, you will have to eliminate it. since merchants (and others like them) don’t care about this: they “paid money” and you can’t force them to fork out for an additional high-pass filter or a band-pass filter. Having carried out some measurements, the author (out of harm's way) decided to transfer experiments with EA to field conditions - to the dacha. As the antenna is light in weight and very easy to fold and unfold, there are no transport problems. A few words about why the "square" was chosen as a portable antenna. Firstly, it is half as long as, for example, a dipole antenna (in terms of the length of the elements). Secondly (and this is the main thing), the "square" can be operated at very low suspension heights and is insensitive to surrounding objects (the influence of a hand brought to the antenna from the side affects only at a distance less than 150 ... 200 mm). Thirdly, such an antenna to a certain extent suppresses local noise and impulse interference. Fourth (in the author's version), it has an active element closed in direct current. The basis for the construction of the EA was a "double square" [75] powered by a 1-ohm cable with a distance between the vibrators of 0,2 (see Fig. 1), the elements of which (1 - active vibrator, 3 - reflector) were simply hung on the window window 2 inside the room.
A similar "window" -traverse can be fixed with hinges on the wall of the house or the corner of the balcony. The rotation of such an antenna, depending on the location of the reflector, is possible within 120 ... 150 °. Hooks and loops can be used for fastening in the chosen direction. Such a design, of course, taking into account specific local conditions, can be convenient both for conducting communications and for receiving television. How did the described EA begin? Indoors (a room on the second floor of a wooden house) a "stand" was made for experimental work with VHF antennas: two pieces of thick fishing line were stretched under the ceiling at a distance of 250 ... 300 mm from one another. Elements were hung to them with the help of rings from the same forest or winding wire (Fig. 2): first two, then three, and so on up to 13 (the room contained so much). The lengths of the active vibrator (AB) and reflector (P) elements were calculated using the formulas from [1], after which they were checked using an XI-48 frequency response meter (FCC). Directors (D1 -D11) are made with a decrease in each subsequent (per side) by 5 mm. The material for the manufacture of elements is aluminum wire in PVC insulation from a three-phase APV cable (even better - copper-plated aluminum wire
in the same insulation that can be soldered). The insulation was not removed from the wire (it is convenient to alternate elements with white, black and red insulation - it is easier not to confuse them during tuning operations: after two elements, the difference in size becomes more noticeable). The lengths of the sides of the frames and the distances between them are shown in Fig. 2 (values of their perimeters are given in parentheses). The input impedance of the antenna is about 45 ohms. For power supply, the author used a piece of RK-50 coaxial cable with a diameter of 4 mm and a length of approximately 1 m (Fig. 3). At the point of connection to vibrator 1, a ring 2 with a diameter of 20 mm made of 20VCh ferrite is installed, on which one turn of cable 3 is made. -, and with a 4-ohm cable. It is also possible to move the first director relative to the active vibrator for coordination, and then pull up the remaining directors. It should be noted that antennas with a large number of elements must have a rigid structure - the distances between the elements must not change during operation. As experiments in the field have shown, two segments of the forest are not enough: the slightest breath of the breeze - and the antenna began to "play" - the elements swayed like linen on a rope. The best option is a rigid traverse, but this is undesirable for field conditions, so I propose a design schematically shown in Fig. 5: add two more pieces of 1 fishing line or strings for tennis rackets, i.e. bring their number to four. The segments should be stretched at the corners inside the frames 2 and the latter should be fixed (after the final adjustment), for example, using the same line (3), at the required distance from one another in accordance with Fig. 2. The length of the scaffold segments must be chosen in such a way that 3 ... 4 m are left from each edge of the antenna for tying to supports, for example, to trees. To increase reliability, you can place frames 2 (Fig. 6) made of wooden slats along the edges of the structure, attach the ends of the segments of the scaffold 5 to them at the corners, and stretch the antenna beyond the frame using, for example, nylon strings 3 (here 1 are supports , 4 - antenna elements). If on one or both frames a gutter is made of wooden bars 4 (Fig. 7), then the antenna vibrators 3 and extensions 2 can be placed in them as in a case in a folded state and in this form the antenna can be stored and transported to any distance. To fasten the cover frame 1 to the frame 4, you can use hooks or rings of electrical tape. In this case, the power cable can be laid along with the antenna along the perimeter of the frames or disconnected (if there is a detachable connector). Antenna elements should be made of well-aligned wire. The easiest way to do this is by pulling it out, securing one end in a vise and holding the other in pliers. When cutting blanks, it is necessary to provide an allowance for the connection (twisting or welding) of the ends of the wire, for which they should be freed from insulation. A small "tail" of twisted wires does not affect the operation of the antenna, it is only important that the calculated perimeters of the frames are observed. It is better to place the joints of elements on one side, for example, from below. There should be no distortions in the plane of the frames. They should be installed relative to one another strictly parallel and "concentric" (when viewed from the side of the reflector).
You can orient the antenna to clarify the direction to the correspondent as shown in Fig. 8, i.e. holding it by the wooden frame 5 (or brace 6) behind the reflector - in this case, the influence on it from the operator is minimal. Stretching 2, fixed on a wooden frame 3 from the side of directors 4, it is desirable to tie it to supports 1. Having found the correct direction to the correspondent, it is better to tie the frame around the corners - the antenna will spin less in the wind. The experiment with EA (with vertical polarization) was carried out in the immediate vicinity of the earth, in a drained swampy area, in a lowland. The upper parts of the antenna elements were at a height of 1,8 m. The EA was stretched between the wall of the shed and a small board dug into the ground as a support and a scarf reinforced from the side of the antenna. The distance to correspondents reached 22...24 km. In the "alignment" of the EA there was a road running along the embankment and dividing the "target" in half, about 200 m to the road, and behind it - a forest of 350...500 m (the situation schematically depicted in Fig. 9). With careful manufacture and rigid design of the EA, the "spot" outlined by the main lobe of the radiation pattern (at the level of 0,7) is 25 ... 30 °. With a fuzzy installation of the elements, the "spot" is blurred, and the gain drops. If it is not possible to ensure the mechanical stability of a multi-element antenna (on four segments, the EA scaffolding is quite rigid) and sufficient accuracy of its manufacture, it is better to limit yourself to four or five elements, and take a larger diameter wire for their manufacture. In this case, the antenna will have to be raised higher to avoid ground reflections near the antenna due to broadening of the main beam. However, you still have to fasten the elements quite rigidly.
When working in the forest (especially with vertical polarization), you should choose places that are sparse or open towards the correspondent (even better - elevated), hanging the antenna between trees or supports in such a way as to avoid the presence of trees in the antenna "range" near it. So, the described EA can be folded and unfolded like harmonica bellows. This is useful for rolling, carrying, and then quickly deploying, but is only good for relatively slow turning. However, if everything is prepared in advance (hooks for fastening, for example), then the antenna can be turned together in ten seconds, which makes it possible to use it in Field Day competitions on the road. The 13-element EA was calculated to operate at a frequency of 145,5 MHz. With little or no adjustment, the antenna can be used over the entire 15m amateur band. Gain EA - not less than 16...30 dBd. The width of the main lobe of the radiation pattern both in the vertical and horizontal planes is no more than 0,7° (at the level of 45). Input impedance - about 145,5 ohms, SWR at a frequency of 50 MHz when using a RK-3 coaxial cable and a matching device shown in fig. 1,8, - XNUMX. The methods used by the author for assessing the quality of the antenna are amateur, approximate. During the experiments, foreign equipment was used: IC-706, FT-11, FT-270. At a distance of 24 ... 25 km with a low-hanging EA and a power of 0,3 W, the correspondents gave maximum marks of 3-4 points on the scales of the available S-meters. For comparison: in their equipment, the noise suppressor "opens" and "holds" and the intelligibility of the signal is 100% at signal levels when the S-meter shows nothing at all. But it is known that with auditory control 1 point - reception is impossible, so the signal level in the city turned out to be significant even at such low power. When it was increased to 4 W, the maximum ratings were 59, 59 + 10 and even 59 + 20 dB! True, the last "decibels" sometimes "winked". The experiment was carried out in the FM mode. For reception in the city, a vertical dipole, a four-element collinear antenna and a vertical five-element "wave channel" located on the roofs of houses were used, and at the far (from the EA) end of the city, the "wave channel" stood "slightly sideways". The influence of the wet ground surface and vegetation on the signal transmission near the ground was noticed. As soon as the rain passed and the sun looked out, the strength of the signals fell by 2 points. The ratio of signal levels between the standard for the F-11 "rubber band" and the EA was estimated: only "change in the noise spectrum - reception is impossible" - to 59 with decibels, which clearly speaks in favor of the EA. An antenna "wave channel" at such a low altitude would be hopelessly upset. The large linear dimensions of the elements of such an antenna require greater care in operation and a greater suspension height, which is not always possible. Although when walking, it is probably more convenient to carry "Yagi" in a folded state, for example [2]. Experimenting with VHF antennas, as mentioned above, is possible on a "stand" of two stretched segments of the forest. The elements of dipole antennas, for example, are simply placed on top and moved relative to each other during tuning. From accidental displacement, they can be secured with any clips, for example, plastic clothespins with cuts. As a model, you can use the signal of the "beacon" [З], installed in the "site" of the antenna in the center of the main lobe of the radiation pattern at a distance of at least 10 traverse lengths (the distance from the reflector to the last director).
The cable from the active vibrator is connected to the input of the receiver, the setting is carried out to the maximum of the "beacon" signal. In the same way, you can "work out" a directional antenna for receiving television signals beyond the zone of reliable reception. In this case, the cable from the antenna is connected to the TV, and the setting is carried out, achieving maximum contrast and minimum noise (moiré) on the screen, or better, by controlling the AGC voltage. The setup sequence is as follows. First, they pull the traverse scaffolding in the direction of the television center, hang the active vibrator and connect it to the TV. Then a reflector is installed behind the vibrator and moved along the traverses until the maximum possible signal level is obtained (perhaps this will be just an increase in noise in the sound channel). Having fixed the reflector in the position found, the first director is installed and in the same way a further increase in the signal is achieved, then the second, and so on until the last director. Next, the direction to the television center is specified by turning the traverses from the side of the reflector, after which the position of all antenna elements is again corrected to the maximum of the received signal. In a similar way, the author made a ten-element antenna for the sixth television channel for receiving broadcasts in the mountains of Karachay-Cherkessia (the signal reflected from the mountain was received). For lack of suitable material, the active vibrator had to be cut out of a sheet of duralumin (Pistohlkors vibrator). A few words about connecting the power cable to an active vibrator. At the break point in its middle part, a dielectric insert is installed, on which a coaxial cable or a coaxial socket of a detachable connector is fixed. For quick manufacture of the antenna, the option used by the author is also possible: the ends of the wire of the active vibrator are bent at a right angle, flattened for better fixation of the cable, after which they are laid one on top of the other (with insulation on the insulation) and fastened with a bandage of strong - nylon, lavsan or ordinary "severe ", impregnated with bitumen - threads (it is undesirable to use a wire, since a bandage from it gives parasitic resonances in the electromagnetic field of the antenna). In this case, the frame is somewhat deformed, but this is not terrible, it is only important that the working perimeter of the vibrator is equal to the calculated one. To the ends of the vibrator with a bandage of a single-core tinned wire, a braid and a central core of a coaxial cable are attached, previously passed through a ferrite (HF20) ring with an outer diameter of 16...20 mm (Fig. 3). At the other end of the cable, a coaxial plug is mounted to connect to the antenna jack of the radio station.
The polarization of the antenna can be easily changed by turning only the active vibrator 90 ° (the rest of the elements do not need to be touched). Some inconvenience in this design is the lack of compensation for the weight of the cable with vertical polarization. With a small length, there are no problems - the operator himself holds the cable, with a large one. length has to be supported by an additional slingshot stuck into the ground near the active vibrator. It is desirable to place the cable perpendicular to its side (with vertical polarization, it must be located strictly horizontally). The author hopes that the simplicity of the design and manufacture of the described EA will encourage radio amateurs to experiment with antenna technology, because it is known that the best RF amplifier is a good antenna. Such an antenna will allow you to feel much more confident on a hike, in the country, in a word, wherever you need to provide reliable communication with low power over long (by the standards of VHF and QRP) distances. After all, low power is the small dimensions of the equipment itself and, most importantly, its power sources. Remember the test results above: only the change in the noise spectrum on the standard radio station antenna at an output power of 4 W and 3-4 points on the "rough" S-meter at 0,3 W - the difference is significant! The antenna is called experimental - the radio amateur himself will decide how best to make it from the available materials. In the traveling version (without wooden frames or a case and cable), it weighs less than a kilogram, it is easy to carry - you can carry both the antenna and the bag with one hand (inside the vibrator frames), and it is easy to assemble the ends of the forest traverse into a bundle and temporarily fix it with PVC rings electrical tape or KLT. The antenna allows you to be near it (on the side) at a distance of up to 150 ... 200 mm, which, in turn, allows you to use a short cable. No less important is the fact that it works fine at low suspension heights (although a higher height, if circumstances permit, does not hurt at all). In practice, the upper edge of the vibrators should be at a height of at least 1 m (preferably 1,5 ... 2 m) from the ground. The distances between the vibrators are chosen taking into account their easy memorization, which simplifies the manufacture of the antenna as needed (impromptu), as well as, if necessary, adjusting the position of the vibrators in case of their accidental displacement. It should be noted that when using uneven (non-aligned) wire for the manufacture of frames, an error occurs, which is expressed in the elongation of the perimeter of the elements. The use of a thicker wire leads to an increase in the self-capacitance of the frames, which requires a corresponding reduction in their perimeter. Roughly, the bandwidth F (in megahertz), which increases with increasing diameter of the frame conductor (including in the form of a tape), can be calculated using the formula given in [1]. For example, for an active vibrator F - Рmax - Fmin - 304635/Рmin - 304635/Рmax, where Fmax and Fmin are the upper and lower cutoff frequencies of the passband corresponding to the minimum and maximum frame perimeters (Fig. 10). A tape vibrator can be modeled from several wires by electrically connecting them to each other (Fig. 10, b), which has long been successfully used in the manufacture of zigzag television antennas. Sometimes, when making an antenna according to the description, it is better to slightly increase the diameter of the wires of the elements, and thus "remain" in the passband, losing a little in the gain of the antenna. Taking this opportunity, I want to express my gratitude to those who (voluntarily or involuntarily) assisted the author in the experiment: RA9LO, RA9LZ, RA9LE, UA9LFJ, RA9LT, UA9LAJ.UA9LP, UA9LDG, RA9LY. UA9LAC, UA9LR, RA9LAP, UA9LBG, as well as radio amateurs of the Sverdlovsk region, who installed a repeater (IARU R1 channel 145025 kHz - TX / 145625 kHz - RX) and encouraged me to this venture. After long vigils while experimenting with EA, I still managed to detect fuzzy signals from the S2 QSB of the repeater. But, of course, two watts were not enough for transmission (QRB 300 km) to open the repeater. I had to make a sound generator of sinusoidal oscillations on an electromechanical AF filter with a frequency of 1343 Hz and a bandwidth of 9 Hz (ShY2.067.064 according to the specifications of the Kama-S radio station) so that when the repeater was "opened" by the Sverdlovsk stations, a weak telegraph one could "crawl through" against the background of the voice signal. But it was not there. It takes time and an excellent passage, which happens "only once a year", for example, as in November 1996, when the World Championships worked directly with the Sverdlovsk team, without repeaters. In the meantime, using FM telegraphy and pressing on the vocal cords, I could only shout to our "firms". They duly appreciated the quality of my EA and called the Gossvyaenadzor control point, since the level of my signal (in the direction of Yekaterinburg, and in other directions during experiments) compensated for the suppression of the frequencies of the two-meter amateur band by the input circuit of their receiver (suppression of the mirror channel). The experiment had to be stopped. A few words about other experiments with loop antennas. Tests of a two-element "double square" showed that for communication within the city it is suitable as a "non-directional" antenna with vertical polarization at a transmitter power of 1 ... 5 W. Being installed higher above the roof, it "reaches the very ground" in any direction, both during reception and transmission (author's experiments with UA9LFJ). The acquisition of non-directional properties by the antenna is explained by re-emissions, reflections, for example, from buildings, wires, metal poles and other structures.
When using such an antenna at the edge of the city, its directivity pattern comes into force, which has a rather wide (about 60 ° at a level of 0,7) main lobe and a gain of about 8 dBd (with a reflector located at a distance of 0,2 from the vibrator and an input impedance of 75 Ohm ). Thanks to this, it is not necessary to turn the antenna, it is enough to direct it to the city. As you move away from the city, the latter occupies a smaller and smaller angle on the horizon, and the signal level drops in proportion to the square of the distance, which corresponds to a narrower main lobe of the radiation pattern (higher gain) for antennas with an increased number of elements. A seven-element EA was also tested, located inside a wooden shed. Its main lobe width turned out to be about 40°, and the gain was about 12 dBd. As it turned out, the influence on the tuning of the active element (in terms of resonant frequency and input resistance) from the fourth and subsequent directors can be ignored and their number can be chosen as needed. At the same time, one should not forget that with a large number of directors, although it is possible to concentrate the energy to a small "spot", it is not long and "miss" in the direction of the correspondent both in azimuth and in elevation. At the same time, multi-element antennas are capable of operating at a lower height. An increase in the signal by one point was noted when the EA was raised from the initial height by only 300 mm. When the polarization changes to horizontal (for the correspondent - vertical), the signal strength drops by four points. More precise matching of the feeder with the antenna can be achieved by moving the ferrite ring along the cable. Some lowering of the middle elements of the EA and the rise of the last directors (due to sagging of the traverses from the scaffold), as well as the suspension of the upper sides of the elements at the same level (aconcentrically) creates additional conditions for a slight rise in the main lobe of the radiation pattern. This also allows for a low suspension above the ground without the risk of reflecting and dissipating concentrated RF energy in the vicinity of the antenna. At the same time, the conditions for the propagation of this energy over the very surface of the earth remain within the opening of the main lobe, Literature
Author: Victor Besedin (UA9LAQ), Tyumen; Publication: cxem.net
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