RDM-61M

is a directional radio from the former czechoslovakia.
It was built by "Research Institute 060 in Prague" around 1965, using miniature Soviet tubes and also partially semiconductors.

Primarily it was meant to build up quick connections on the battle field, between regiment and division, before fixed telephone lines could be installed.

It was used in command vehicles or as a portable station. The housing is watertight.

The tuning is common both for transmitter and receiver - it has only one knob (it was only control element). There are two sub-bands (ranges), red and blue, each described with channel numbers (not frequencies).

During operation, one radio was set on given channel in one sub-band (suppose red) and its radio counterpart on the same channel, but on opposite sub-band (i.e. blue).

RDM-61M also transparently transmitted ring signal. If you turn the crank on the manual telephone (or switchboard), the other-end telephone rang - no difference between straight wire line and radio lap.

Two RDM-61M could build up a "retranslation point" by their simple interconnecting with 2-wire cable attached to the LINKA socket.

Usually, as a power source a NiCd 6 V / 100 Ah accumulator was for each radio. It was much heavier than radio itself.

We even succesfully tried to tranfer data using regular 4800 bps modems after 1990.
Despite only 100 mW output power and quite short Yagi, we worked out at distance about 100 km (of course, you have to find suitable hills/mountains with radio visibility).

The yagi antenna can be mounted as well horizontally as vertically.

The measuring instrument shows the received signal strength or the battery voltage.

The originally intended silver- zinc- batteries were very expensive and delicate, so a 6V- cable for external 6V is enclosed.

False operation often led to the destruction of the radio. So the circuit was modified using a diode. Modified devices can be recognized by the letter F on the nameplate.

Most of the informations were provided by Josef, OK2PWD.
Thank you very much, Josef!

Frequency range:

230 ... 260 MHz in two ranges of 12 channels each

HF output:

100 mW

Channels:

2x 12

Mode:

F3 (FM) Duplex

Sensitivity:

5 µV

Range:

50 km with the Yagi- antenna

Antennas:

Rod antenna, Foldable Yagi antenna

Battery:

Silver- zinc, 6V; 40 AH

Size and weight of the wooden box:

120 x 205 x 135mm, 25 kg