Portable radio station at 28 MHz. Scheme, description

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Portable radio station on 28 MHz. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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From time to time in the editorial mail there are requests to publish descriptions of a simple portable radio station that can be used for making contacts over short distances (up to 2...3 km). The 2-meter range is best suited for these purposes, but it is not so easy for a radio amateur who does not have much experience to make a simple and reliable radio station for this range. A 10-meter radio station can be a reasonable compromise: the radio station itself is relatively simple and does not contain scarce parts, and a short telescopic antenna is still quite effective for making communications over not very long distances.

About five or six years ago, Vasily Volkov (UA3DGU) from the Moscow region developed such a radio station. Its feature is the simplified requirements for quartz resonators, the selection of which usually causes difficulties when repeating such designs. The transmitter of a radio station needs a resonator with a frequency whose third harmonic falls into the telephone section of the amateur band of 10 meters. For the receiver, any resonator is suitable, the frequency of which is separated from the operating frequency of the transmitter by an arbitrary value in the range from 300 kHz to 1 MHz. Such freedom of choice is explained by the fact that the selective properties of the receiver's IF path are determined by LC circuits, and, therefore, the value of the intermediate frequency can be set arbitrarily within certain limits.

(click to enlarge)

The transmitter of the radio station (Fig. 1) is two-stage. The resonator ZQ1 of the master oscillator (transistor VT1) is excited at the third harmonic. Amplitude modulation is carried out in the output stage. The modulating audio frequency voltage is fed through the transformer T1 to the emitter circuit of the transistor VT3. The modulator consists of a pre-amplification stage on a VT2 transistor and a main amplifier on a DA1 chip.

(click to enlarge)

The radio station receiver (Fig. 2) is assembled on two microcircuits in a standard inclusion. A small feature is that a certain closing voltage can be applied to the VD1 detector diode from the variable resistor R7 engine, which allows the simplest means to implement the noise suppression function (more precisely, the elimination of annoying receiver noise with a confident connection). At the frequencies of the quartz resonators of the receiver and transmitter shown in Fig. 1 and 2, the receiver IF frequency will be 860 kHz. Switching "reception-transmission" is carried out by the SA1 switch (Fig. 1), which, with the SA1.1 contacts, switches the antenna either to the transmitter output or to the receiver input, and the SA1.2 contacts - power either to the transmitter or to the receiver. A telescopic antenna with a length of approximately 1 m is tuned to the operating frequency with an extension coil L5 (see Fig. 1).

The radio station in the author's version was made by the method of surface mounting (the printed circuit board was not developed). The receiver coil L1 has 9 turns, L2 - 4 turns. They are wound with PEV-0,31 wire on a frame with a diameter of 5 mm with a trimmer made of carbonyl iron from the SB-12a magnetic circuit. L3 is a standard high-frequency choke from the DM series (inductance 160 μH). Coils L3 and L4 use the same frames. They have, respectively, 5 (wire PEV-0,51) and 15 turns (wire PEV-0,31). Modulating transformer T1 - output from the Selga-404 receiver. It is made on a W-shaped magnetic circuit with a cross section of 5x6 mm. The primary winding has 60 turns of PEV-0,51 wire, and the secondary winding has 450 turns of PEV-0,31 wire.

Coils L1-L3 of the receiver are wound with PEV-0,31 wire on a frame with a diameter of 5 mm with a trimmer made of carbonyl iron from the SB-12a magnetic circuit. They have 10, 3 and 10 turns, respectively. Coils L4-L6 are used from the IF circuits of a small-sized receiver. When repeating the design, the values of capacitors C12, C19 and C4 must be taken such that they provide resonance with these coils at the selected IF frequency. To increase the sensitivity of the receiver and, consequently, increase the communication range, an input stage on a field-effect transistor can be introduced into it (Fig. 3). Coil L1 - the same as in the main version of the receiver.

28 MHz portable radio

When adjusting the transmitter by the trimmer, the L1 coils achieve stable excitation of the quartz resonator of the master oscillator at the third harmonic. Then, by sequentially adjusting the coils L4 and L5, the maximum radiated power is achieved (it is controlled by an external field indicator).

After that check the modulation. If at the same time overmodulation is observed, the gain of the cascade on the transistor VT2 is reduced (the resistor R9 is installed with a higher rating). And, conversely, if the modulation is "small", then install this resistor with a lower rating. Setting up the receiver is reduced to similar procedures. First, by tuning the L3 coil, stable operation of the local oscillator is achieved, and then by tuning the coils L1 and L4-L6 - the maximum sensitivity of the receiver.

Author: Vasily Volkov (UA3DGU); Publication: N. Bolshakov, rf.atnn.ru

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