Active antenna of the VHF FM range. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / VHF antennas

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As the surveys of radio listeners show, today many of them listen to the transmission of radio stations in the VHF FM band. And this is not surprising, since it is in this range that you can find a radio station for almost every taste, and with sound quality that is unattainable in the ranges where AM is used. But the trouble is: due to the peculiarities of the propagation of VHF radio waves, the reliable reception area is small, and already at a distance of several tens of kilometers from the city, the signal weakens so much that it becomes difficult, if not impossible, to receive it on a conventional broadcasting receiver. And so you want outside the city - at the dacha, picnic or on a camping trip - to listen to the broadcasts of your favorite radio station ... The situation is fixable to some extent - you just need to take with you an active antenna made according to the description below. The simplicity of the design makes it accessible for repetition even by novice radio amateurs.

The portable active antenna brought to the attention of readers is designed to improve the quality of reception of VHF FM radio stations at the border of the so-called reliable reception zone. It is fixed on a wooden pole several meters long and, after installation, is connected to the radio receiver with a coaxial cable of the appropriate length.

The scheme of the active antenna is shown in fig. 1. It consists of WA1 telescopic whip antenna, L1-L3C1-C3 bandpass filter, VT1 transistor RF amplifier, drop cable and power unit. The bandpass filter, which suppresses out-of-band signals and improves the noise immunity of the device, is formed by the L1C1L2 high-pass filter with a cutoff frequency of about 60 MHz and the C2L3C3 low-pass filter with a cutoff frequency of about 110 MHz.

Fig. 1

The RF amplifier increases the voltage of the signal received by the antenna and compensates for the losses in the drop cable. It is made on a low-noise microwave transistor KT3120A. Due to the negative DC voltage feedback (through the R2R1 divider), the transistor collector voltage is maintained at a level of about 2 V when the supply voltage changes from 3 to 15 V. The current I consumed by the amplifier depends on the supply voltage Upit and the resistance of the resistor R3: l = (UnMT- 2)/R3.

With an increase in current from 2 to 6...8 mA, the gain increases by 2...3 dB. The minimum noise figure is provided at a current of 5...6 mA. Diodes VD1-VD3 protect the transistor from powerful electromagnetic pickups and lightning discharges.

The RF amplifier is powered by a GB1 battery or an external source, which is connected to the XS1 socket. The supply voltage is supplied through the L4 choke and a coaxial cable of several meters length reduction when joining the parts of the XW1 connector. Capacitor C6 - blocking, C7 - separating. If desired, a separate power switch can also be provided. It is installed in a break in the wire going from socket XS1 to the junction point of capacitor C6 and inductor L4.

The telescopic antenna, the elements of the band pass filter and the RF amplifier are mounted on a printed circuit board made of double-sided foil fiberglass, the drawing of which is shown in fig. 2. Through the holes marked with four dots, the printed conductor of the common wire is connected to the foil on the opposite side of the board with pieces of tinned wire. The device uses resistors MLT, S2-33 and capacitors K10-17. Coils L1-L3 are wound with wire PEV-2 0 21 on a mandrel with a diameter of 3 mm. The first two contain 7, the third - 5,5 turns.

Fig. 2

Inductor L4 - unified DM-0,1 or imported, for example, the EC24 series with an inductance of 10 ... 100 μH. Telescopic antenna WA1 - any length (in the extended state) of at least 1,1 m (the author used an antenna from a portable TV). When receiving radio stations in the VHF-1 range (65,8 ... 74 MHz), it is extended to its full length, and in the VHF-2 range (87,5 ... 108 MHz) - by about 75 cm. External view of the board fragment (of the part on which the parts are installed) is shown in Fig. 3.

The device is connected to the radio receiver with a piece of coaxial cable 0,5 .... 2 m long. If the receiver has an antenna jack, then the corresponding mating part (pin) is mounted at the end of the cable, and if not, crocodile clips, while the one that is soldered to the center conductor is connected to the telescopic antenna, and the one connected to the braid is connected to the common wire of the receiver.

After checking the performance, the board is covered with a moisture-resistant varnish or paint. From above, it is desirable to close the part with a metal or plastic cover, and fill the gaps between it and the board with silicone sealant. In the part-free part (on the right in Fig. 2) of the board, two holes can be drilled for fastening to a wooden pole with screws. Dimensions of the active antenna in the folded state (Fig. 4) - 28x30x190 mm. The appearance of the device in the working position (fixed on a pole) is shown in fig. 5. When using the antenna as a stationary telescopic antenna, it is advisable to replace it with a metal pin or tube of the appropriate length.

The power supply unit is mounted in a plastic case of suitable dimensions. A container for the battery cells is fixed inside, and on one of the walls there is a socket of the XW1 connector and the leads of the capacitor C7, the inductor L4 and the braid of the cable going to the receiver are soldered directly to it. If necessary, the socket XS1 is installed to connect an external source.

For power supply, a battery with a voltage of 3 ... 6 V is used, composed of several galvanic cells or batteries connected in series. An old cell phone battery is also suitable, which can no longer be used for its intended purpose due to a significant loss of capacity. If you plan to constantly use an external power source with a voltage of 12 ... 15 V (for example, the car's on-board network or a network power supply), the resistance of the resistor R2 should be increased to 47 kOhm, and R3 - up to 1,5 ... 2 kOhm.

Author: I. Nechaev, Moscow; Publication: radioradar.net

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