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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Rapid deployment antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / HF antennas In many cases, a short handheld radio antenna is not enough for reliable communication on the 27 MHz band, and installing an effective full-size antenna requires an unacceptable amount of time and money. The most striking examples are various kinds of tourism, sports, mountaineering, parking, camps, expeditions, hunting, various operational groups, rescuers, radio communication services for various events on land, on water and in the air. Most often, in such cases, elongated surrogate antennas are used, as a rule, poorly matched and low-lying, the effectiveness of which is low.
The proposed antenna is a slightly shortened half-wave dipole fed in the middle, made of available materials, easy to manufacture and tune. The antenna folds easily and can be quickly deployed in almost any environment. Such an antenna can be made for different bands, but it is especially advantageous to use it on the 27 MHz band. The upper arm of the dipole (see figure) is made of stranded copper wire, and the outer surface of the outer conductor (braid) section of the coaxial feeder is used as the lower arm. This section is electrically separated from the rest of the feeder by a coil. The dipole arms are undulated to match a 50 ohm feed. The preservation of this shape is ensured by means of a nylon fishing line (or cord), to which the dipole conductors are attached at several points. The line also serves to hang the antenna. The length of the free end of the fishing line and the length of the feeder part below the reel depend on the required height of the antenna suspension. The last dimension should not be a multiple of half the wavelength, the best values are 2,6 m or 8 m. A safe load is tied to the end of the line, which is thrown, for example, over a dry tree, and then the antenna can be pulled up by the end of the line. There should be as much free space around the antenna as possible. On a helicopter or a balloon, the antenna is simply hung down. No counterweight or ground is required for the radio. The coil is wound with the same continuous cable from which the lower arm of the dipole and the feeder are made. It should have an inductance of about 15 µH and should be protected from water, snow, dirt, etc. between the turns. It is better to use a cable with a stranded inner conductor, such as PK50-2-12 or PK50-2-16. For the upper arm of the dipole, you can use any insulated wire that more or less retains its shape when suspended. The number of "waves" of the wire in the arms of the dipole is not critical. It is only important to maintain the total length of each arm (B and H) and the lengths of the wires of the arms indicated in brackets in the figure. To tune the antenna, it is convenient to use a multi-channel radio station, preferably with several grids (with a wider frequency range), and an SWR meter. By changing the size of the upper arm B (by uniform stretching or compression along the length along the fishing line without changing the length of the wire), the dipole is tuned to resonance at the selected frequency, and then by changing the size of the lower arm H (without changing the length of the cable section in this arm), the minimum SWR is achieved at resonance. In this case, the resonance frequency may change, and you may have to do the whole process several times. After setting, firmly fasten the fishing line with the wires of the shoulders to prevent displacement. The authors fabricated and tested two copies of the antenna with a coil on a polyethylene cylinder with a diameter of 60 mm and a length of 80 mm, containing 19 turns of cable, and with a coil on a ring 65x40x6 mm made of M55NN-1 ferrite - 22 turns of cable. The mass of the antenna without load is about 300 g. Both antennas have almost the same parameters. On a panoramic SWR meter of the R2-73 type, SWR values of no more than 1,5 in the 0,72 MHz band and no more than 2,0 in the 1,32 MHz band at a center frequency of 27,3 MHz were obtained. The calculated efficiency improvement of 16 dB due to the transition from the efficiency=2% of the helical antenna to the efficiency=80% of the dipole is confirmed experimentally. On flat terrain, an additional 1,5 dB can give an additional effect by increasing the height of the antenna from 5 to 10 m. The total gain of 26 dB is equivalent to a power increase of 400 times, which allows you to increase the communication range by about 4,5 times. Stationary variants of such an antenna are possible, including those with a rigid upper arm. This antenna has been successfully used by the authors since 1994. Authors: A. Grechikhin (UA3TZ), V. Morozkin, Nizhny Novgorod; Publication: N. Bolshakov, rf.atnn.ru
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