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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Vertical antenna for low bands. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / HF antennas The problems of creating a sufficiently effective antenna for the 160, 80 and 40 meters bands to work in confined spaces are of concern to many radio amateurs, including me. Previously used LW, Dipole and Inverted Vee no longer satisfied me. On my roof, it was somehow possible to place a dual-band version of IV at 3,5 and 7 MHz, but there was nowhere to install the third antenna, at 1,8 MHz. In April 1995 I made a 7 MHz capacitive loaded GP antenna. After two years of successful operation, I was convinced of its sufficient effectiveness. Its height is 11,9 m, plus four wires of 5 m each, fixed on the top of the antenna and stretched with nylon ropes at an angle of 90 ° to each other. The effective operation of this antenna suggested the idea of converting it into a tri-band version. After a long study of various publications on vertical antennas [1...3], I settled on a design [3], which had a not very high height and made it possible to successfully work not only with DXs, but also effectively carry out short-range communications. Design In my version, the mast has a height of 16,8 m and is assembled from four pipes with a diameter of 55 mm (from the antenna mast of the R-401 radio station), three pipes with a diameter of 40 mm and one with a diameter of 32 mm. All pipes fit well together. Used four tiers of nylon stretch marks. The mast is lifted by a lift from R-401. The hoist together with the mast is mounted on an insulator (plate made of thick fiberglass). The insulator with the help of steel corners is fixed on a concrete slab measuring 50x50x8 cm (these slabs line the footpaths). This slab lies on the flat roof of the house and is filled with resin around the entire perimeter. Two capacitive load wires are made of a steel cable with a diameter of 2,5 mm and a length of 8,5 mm and are part of the braces of the upper tier. The counterweights are made of aluminum wire with a diameter of 2...2,5 mm (a seven-wire wire was taken from overhead power lines and untwisted). For the 7 MHz band, 6 resonant counterbalances are used, for the 3,5 and 1,8 MHz bands - 4 counterweights each. It is best if all counterweights are located at equal angles to each other, but due to the fact that the width of the house is only 10 ... 11 meters and it is arched, it was necessary to place the counterweights simply by placing them on the roof. In my version, the antenna is powered by a 75 ohm cable.
Setting. I had difficulties with tuning the 7 MHz band, apparently, the close location of the 2el QUAD antenna affected the HF bands. When measuring with an RF bridge, the input impedance at resonance was 38 ... 40 Ohm, and, accordingly, the SWR was about 2. Therefore, we had to change the matching device circuit (see figure), adding a third short-circuit relay, which, through normally closed contacts, shorts the tap to the case coils L1 in the range of 7 MHz. When you turn on any of the relays K1, K2, i.e. ranges of 1,8 and 3,5 MHz, the short circuit relay is activated, its contacts open, and it does not affect operation in the 1,8 and 3,5 MHz bands. Now the antenna can be tuned perfectly. The fact is that in [3] only the resonant frequency is regulated in the 7 MHz range, and the input resistance Rin is not, therefore, it is impossible to achieve an SWR equal to exactly 1, since Rin is determined not only by the height of the mast and the angle of the capacitive loads, but and surrounding objects. In the proposed version of the matching device (CD), by moving the middle tap of the coil L1, the resonant frequency is set, and then, by moving the upper tap, Rin is adjusted, i.e. achieve SWR \u1d 1. If SWR at resonance> 7,050, this only indicates that the tuning was not done carefully enough. When setting the resonant frequency to 2 kHz by moving the middle tap, you must simultaneously shift the top tap - so that there are always 3 ... XNUMX turns between them. The 1,8 and 3,5 MHz bands were tuned without problems in accordance with the methodology described in [3], i.e. resonance at 1,8 MHz was achieved by changing the total number of turns of the coil L1, and SWR = 1 - by moving its lower tap. In the 3,5 MHz band, by selecting the capacitance C1, the minimum SWR was achieved in the middle of the range. It is better to solder a KPI with an air dielectric from broadcast receivers with a capacity of 12/495 pF when tuning, tune the antenna, and then, having measured the capacitance of the capacitor, solder a capacitor of constant capacitance. After the preliminary adjustment is completed, the matching device is closed with a lid and, if necessary, everything is adjusted again. Due to a change in the control system circuit, the range switching relays are powered by a separate control line. In my version, the relays are controlled by a voltage of +15 V. Coil L1 is wound with copper wire 2,5 mm in diameter on a ceramic frame 55 mm in diameter with a pitch of 2 ... 2,5 mm. It contains 33 turns, taps from - 8, 22 and 25 turns, counting from the grounded end. The exact number of turns and the position of the taps are determined during setup. Capacitors: C1 - type KSO for a voltage of 500 V, C2 ... C4 any blocking. Diodes VD1 and VD2 can be used with any rectifier that can withstand the current of the relay. Relays are used high-frequency on ceramics, the distance between the contacts is about 1,5 mm. To switch relay t1, a two-wire line is used, which is shunted on the roof by capacitors C2 and C3, and its other end, when entering the radio station, is passed through a ferrite ring with a permeability of 2000NN (5 ... 10 turns). The results.
I have been operating this antenna for only two summer months so far and I use a RA with a power of 250 ... 300 watts. On 40 meters I can easily "reach out" any DX-a that I hear. On the first or second attempt, it is possible to break through almost any pile up. Conducted many communications with all continents. However, unlike the usual GP, this antenna makes it possible to work confidently with close correspondents. Working during the day on 40m, I almost never get a report below 59+20dB. On 80m I could hear immediately and was heard by stations I could never reach on Dipole and IV. LU, ZP, CE, PY, UAO, VK, JA, Africa, Antarctica come to the common call. An example of excellent antenna performance is communication with Carlos (TI4CF), whom I always come to greet, the report from him is never lower than 58. With even a weak long-range transmission, even with my not very high output power, I manage to keep the general call frequency in the DX window (3790...3800kHz) for quite a long time. Those who have worked for CQ in this area will understand what this means. For those who are just getting ready to try their hand at the 80m DX window, keep in mind that if no DX station answers your calls for several minutes, and your signals are not heard in Europe very loudly, at best you will be politely asked not to occupy the narrow DX-OKHO in vain, and at worst they will simply occupy the frequency, ignoring your presence on it. When working in the evening with close correspondents, I get a report of 59 + 10dB or more from stations that have antennas with vertical polarization. If the correspondent has a horizontal antenna, the report is usually lower - 58, 59. On the 160-meter band, during the two summer months, a lot of contacts were made at distances of 2000...5000 km. On short-range communications (up to 1000 km), the antenna does not work as well as at 40 and 80 meters - from the stations of the Baltic states, Poland, the report is rarely more than 57. However, on longer routes - DL, G, F, UA9, UN - they hear me much better, and the report is rarely lower than 59. I hope that in winter the results will be more impressive, since in the summer the passage to the low frequencies is noticeably worse. Literature 1. Benkovsky 3., Lipinsky E. Amateur antennas KB and VHF. - M.: Radio and communication, 1983.
Author: G.Tsymbal (EU1AI), Minsk; Publication: N. Bolshakov, rf.atnn.ru
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