VHF antenna. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / VHF antennas

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The zone of reliable reception of VHF FM radio stations is very limited. So for transmitters with a power of about 10 kW, its radius is 50-80 km. In this zone, the field strength level is 200 µV/m, and outside it it rapidly decreases and at a distance of 140...160 km from the transmitter drops to 5...10 µV/m. The sensitivity of VHF FM tuners with a signal-to-noise ratio at the LF output of 50 dB is usually 10 μV in mono mode and 100 μV in stereo. From what has been said, it is clear that high-quality stereo reception using simple antennas (built-in, telescopic whip) is not always possible even in the area of ​​\u5b\u6bgood reception, not to mention more distant distances. In this case, more complex antennas with an effective height of XNUMX...XNUMX m are required.

In the article published below, a description of such a VHF antenna is offered to the attention of readers. Its gain is 11,5 dB; beam width in the horizontal plane - 42°, in the vertical - 60°; protection ratio (reverse radiation coefficient of the antenna, equal to the ratio of the voltages at its output when irradiated from directions 0 and 180 °) - 18 dB. The design of the antenna is shown in fig. one.

Fig.1 (click to enlarge)

In its manufacture, one should be guided by the general recommendations that were repeatedly given in the Radio magazine when describing television antennas of the "wave channel" type. It is not critical to small errors in the design and has a stable phase characteristic, which allows it to be used as part of gratings. The active element of multi-element antennas is usually made in the form of a Pistohlkors loop vibrator. The vibrator itself is well matched to a 75 ohm cable with a half-wave loop that reduces its resistance by a factor of 4.

However, as part of a multi-element antenna, its resistance decreases significantly (approximately 10 times) due to the influence of closely spaced directors and a reflector and is about 30 ohms. When a half-wave loop is connected, its resistance decreases by another 4 times, and as a result, a cable with a characteristic impedance of 75 ohms is connected to an eight-ohm load. In this mode, the antenna pattern is sometimes distorted and the efficiency decreases sharply. It is known that the coefficient of the traveling wave is approximately: Kn=R/W, where R is the wave impedance of the load, W is the wave impedance of the feeder. The efficiency of signal transmission from the load to the line is determined by the formula: Efficiency = 4Kn / (1 + Kn)². In our particular case, we get: Кн=8/75=0,107, and efficiency=4*0,107/(1+0,107)²=0,35. In other words, 65% of the received power will be lost in the feeder and the real gain will be not 11,5, but 7 dB, which corresponds to a matched four-element antenna. To increase the efficiency, a vibrator of three tubes with an input resistance of 657 Ohm and a matching device with a transformation ratio of 1:1 are used as an active vibrator. As part of a multi-element antenna, the resistance of a vibrator of three tubes drops to 66 ohms, and in this case Kn = 66/75 = 0,88, and efficiency = 4 * 0,88 / (1 + 0,88)²\u0,996d 2, i.e., almost complete matching of the antenna with the cable and the maximum use of its directional properties are achieved. The electrical circuit of the matching device is shown in fig. XNUMX.

Ris.2

The wiring diagram of the connections in the antenna box is shown in fig. 3. A similar matching device is used in industrial ATVC antennas.

Ris.3

The coils are wound on 50VCh (7X4X2 mm) ferrite rings with PEL 0,23 wire and contain eight double turns each. The described antenna can be used as part of an array of two antennas. In this case, they should be connected in accordance with Fig. four.

Ris.4

The location of the antennas (top view) in the case of receiving waves with horizontal polarization is shown in fig. 5.

Ris.5

To receive waves with vertical polarization, the upper part of the mast must be made composite by installing an insulating insert at a distance of 700 mm from the upper end of the mast (Fig. 6).

Ris.6

The total gain of the array will be 14 dB, and the effective height will be 7 m. An antenna amplifier can be used to improve the sensitivity of the receiving system. The author has an industrial amplifier UTKTI-66 rebuilt for the range of 73 ... 2 MHz. Tuning is easy to carry out with tuning capacitors of the amplifier, using a frequency response meter. The input transistor of the amplifier (GT329A) was replaced by KT3115A. The sensitivity of the system with an antenna amplifier with a signal-to-noise ratio at the tuner output for AF of 50 dB was 60 μV in mono mode and 60 μV in stereo. The boundary field strength for high-quality stereo reception is: Еgr=60/hd=7/9=XNUMX μV/m.

The described antenna array with an amplifier is used to receive stereo transmissions from Krosno (Poland), a distance of 160 km. Transmitter power 10 kW, transmitting antenna height 300 m, vertical polarization. The height of the receiving antenna is 30 m (the roof of a nine-story building). Polish stereo broadcasts are conducted using a pilot tone system, so the author had to make a stereo decoder on the A290D chip.

Author: A. Gerasimov, Stryi, Lviv region; Publication: N. Bolshakov, rf.atnn.ru

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