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Alteration of the Len-M radio station at 29 MHz - FM. 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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I have been working with phase modulation in the 29 MHz range since June 1991. The impetus was the article by A. Koval (UA3AFO) in RL N 2/1991 "And no accidents."

For work I use the factory transceiver "LEN". Of the existing modifications of "LEN" (B, C, M), "LEN-M" was rebuilt, which had an operating frequency of about 34 MHz.

The radio station, rebuilt by me, showed the simplicity of alteration, high reliability and convenience for more than a year of operation. Correspondents (in frequency, foreign) invariably rate the modulation quality as very good.

If there is a choice, it is necessary to take a device with an operating frequency as close as possible to 29 MHz - this will facilitate the restructuring.

The receiver of the radio station "LEN-M" is a superheterodyne with two conversions (IF frequencies - 10,7 MHz and 100 kHz). The block diagram of the receiver is shown in Fig.1.

Alteration of the Len-M radio station at 29 MHz - FM

The transmitter of the radio station is made according to the scheme of quadruple frequency multiplication (Fig. 2).

Alteration of the Len-M radio station at 29 MHz - FM

Location of boards on transceiver frames:

Fixed frame-transmitter (view from the side of the radio elements):

Quartz. Modulator LPF Amplifier
generator + и capacity
+ doublers ant. (KT920A,
staby + commut. KT920)
lysator mic. mustache

Movable receiver frame (view from PCB side):

auto- ULF+ Driver + Quartz 0
teak UPC1 + 2-circuit generator
UPC2+ filter
quartz. sq. gene.
filter +
UHF+ 0

In the receiver, the local oscillator, UHF must be tuned; in the transmitter - master oscillator, modulator, frequency doublers, power amplifier, output low-pass filter.


1. Local oscillator

The simplest option that requires minimal costs: you need to unsolder the quartz, the antiparasitic resistor, and replace the capacitor connected between the quartz and the base of the transistor with a jumper. Instead of soldered parts, it is necessary to install the elements shown in Fig. 3.

Alteration of the Len-M radio station at 29 MHz - FM

Coil L1 is wound on a correction coil frame (connected in series with quartz) and has 15 turns of PEL 0,3 wire wound turn to turn. Despite the fact that the coil is wound on a plastic frame, the frequency stability turned out to be sufficient for FM operation. A better coil can be made from a resistor VS-0,5, VS-1, VS-2 by removing the resistive layer from it and winding 12 - 15 turns of PEL 0,3 wire. It is desirable to fill the coil with BF glue and place it in the screen. Varicap - type KV102G or having a similar capacity (KV102, D901, KB 109). Capacitor C2 has a capacitance of 5-15 pF and is selected during tuning. It is advisable to use a trimmer with an air dielectric as C4, because. ceramic trimmers KPK-MP strongly "float".

In the factory version, the receiver's crystal oscillator operates according to the scheme: Fkg = Fsignal + Fpch, i.e. the local oscillator frequency is greater than the signal frequency, so it is necessary to rewind the coils of the loop at the output of the GPA and the coils of the bypass filter at the output of the GPA. The coils are wound turn to turn on the existing frames with PEL wire 0,31 15 turns (circuit capacitance 51 pF) and are tuned to resonance at a frequency of 18,85 MHz at the maximum RF voltage. At the output of the GPA should have a tuning range of 18 - 19,7 MHz.

Unsuccessfully, from the point of view of frequency stability, the GPA board is placed in the radio station case: it is located above the transmitter PA board, as a result, the heat from the terminal transistor, when operating at full power, quickly reaches the frequency setting circuit through the case, and the frequency "floats".

You can deal with this in the following way:

a) careful selection of the TKE-contour (total TKE should be equal to 0);

b) rearrange the GPA board on the transceiver frame away from the PA;

c) radical method: use an external GPA and apply RF voltage via a coaxial cable.

2. UHF

To tune UHF, you must have a generator at a frequency of about 29,6 MHz with output control and an attenuator. It is necessary to set the signal level so that it can be heard in the noise, and adjust the circuit to resonance, each time reducing the signal level at the output of the RF generator. You can use the GIR in the "soft" generation mode for tuning by turning on a small piece of wire at the output of the GIR. After that, try to catch the signal with the receiver, if this fails, increase the signal amplitude. Further - the same as in the case of the generator. To adjust the circuits, it is necessary to solder the tuning capacitor KPK-MP 8 - 30 pF from the side of the printed wiring in parallel with the circuit capacitor, to "catch" the resonance. Having determined the capacitance of the tuning capacitor by eye, we solder a constant instead of it and finally adjust the circuit with the core of the coil (be careful, the cores are fragile and filled with mastic, one careless movement can destroy them!).

If there is no RF generator, you can adjust the UHF according to the level of operation of the PN system as follows: set the sensitivity of the IF with the "per PN" knob to the response limit, but so that the noise is not heard, then adjust the circuit with a trimmer capacitor until noise appears at the output, reduce the PN level again etc., until resonance is found. You can also tune the receiver to an over-the-air signal, if available.


The restructuring of the transmitter begins in the same order as the receiver: it is necessary to solder the quartz that shunts its resistor, and the capacitor connected in series with the quartz. Instead of a capacitor, a jumper is installed, and instead of quartz, a circuit shown in Fig. 4 is installed.

Alteration of the Len-M radio station at 29 MHz - FM

Then, the frequency of 7,4 MHz is set by the capacitors of the frequency-setting circuit and the circuit at the GPA output is tuned to resonance. With the specified data, the operating range of the transmitter is 29,3-29,7 MHz. The FM modulator usually requires a small adjustment of the coil core for the best signal quality (requires a monitoring receiver).

The doublers are tuned as follows: a frequency meter and an RF voltmeter are connected to the output of the doubler through a small capacitance and, selecting the capacitances of the circuits, tune the circuit at the output of the first doubler to 14,75 MHz, and at the output of the second to 29,55 MHz. The power amplifier in my case did not require adjustment (due to broadband), however, with a large difference in the operating frequency from 29 MHz, it will be necessary to select capacitors in the PA circuits. At a dangerous voltage level at the output transistor (high SWR), the red LED of the built-in SWR meter starts flashing. You can make a slight refinement to the PA circuit: instead of a trimmer resistor that regulates the output power, install either a 2,2 k variable resistor and bring it to the front panel instead of the "Dur. Receive / Receive" button, or use this button for discrete power switching. To do this, instead of a tuning resistor, you need to install two constants (3 k and 5 k), selecting them according to the required power. The QRP mode is necessary for conducting local communications and for experimental work.

To facilitate the combination of RX and TX frequencies, you can turn on the GPA TX in the receive mode, for which you need:

1) install a diode (KD522) in the power supply circuit of the GPA TX, breaking the printed conductor between the key and the GPA;

2) apply +10 V to the GPA TX through the free contact group of the "call" button (thus it will perform two functions:

"call" in TX mode and "tuning" in RX mode);

3) install a zener diode, KS 168, in the power supply circuit of the GPA TX (after the 100 Ohm ballast resistor).

Setting console

Initially, the knobs for the RX and TX tuning potentiometers were installed inside the case instead of the "Day receive / receive" and "Call" buttons, but this turned out to be very inconvenient, because. lack of space does not allow a sufficiently accurate scale to be established. Therefore, it was decided to mount the potentiometers and the stabilizer as a separate console (Fig. 5).

Alteration of the Len-M radio station at 29 MHz - FM

The console is connected to the transceiver using a multicore cable (4 cores), optionally shielded. There are "beaks" on the axis of the potentiometers, and graduations at 50 kHz are applied to the panel of the remote control. As experience has shown, such discreteness is sufficient for work. To control the frequency and work through repeaters, I use a frequency meter (TsSh-01) with a divider by 10 on K500IE 137 (TsSh-02).

A few words about the tactics of work on 29 FM.

The bulk of the stations operate in the frequency range 29,45 - 29,6 MHz. About the frequency 29.600 - especially. This is an international general call frequency, you cannot make QSOs here, but you can only work on a general call, and as soon as you are answered, immediately switch to another frequency. For example: "PSE QSY 29.540", or "All 10 FM, here ........ listening on 29,500".

The frequencies of the section 29,600 - 29,700 MHz are used for repeater transmitters: in these sections you can hear their call signs. For example, the Hungarian repeater HA29BME (Budapest) operating at a frequency of 685 MHz, when opened, issues its call sign by tone telegraph. The American repeater in New Boston W5NTE (1) gives out a kind of three-tone message, and occasionally the machine "pronounces" the call sign "voice".

To work through the repeater, you need to set the TX frequency to 100 kHz below the RX frequency and, after giving full power, try to open the repeater by briefly pressing the PTT. The repeater transmitter should respond with "pops" and give out its "business card".

If this fails, try scanning the transmitter at ±10 kHz by quickly depressing and releasing the PTT. Some repeaters, for example, HA4BME, are connected to a single network with 145 MHz repeaters.

Some foreign NAMs use portable (/p) and car (/m) radio stations. Agree, it is interesting to contact a radio amateur who is currently driving in a car somewhere in the vicinity of London.

For everyday communications over short distances, it is desirable to use frequencies below 29,500 MHz, but try not to interfere with satellite lovers.

In order not to miss the passage, I constantly keep the receiver turned on at 29,600.

See you on 29FM!

Author: A.Snopov (UA4CGL); Publication: N. Bolshakov,

See other articles Section Civil radio communications.

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