Adapter for separating RX and TX antennas of IC746 from OK1DN

For special reception use (RX) antennas for DX reception, especially in the lower HF bands, it is necessary to separate the input for the receiving and the output for the transmitting antenna. The IC746 transceiver does not have this option, if we don't want to interfere inside the device. The adapter described below is used to separate the antennas.

Because I didn't want to limit either (or complicate) the possibility of connecting a power output stage behind the transceiver, I didn't use the SEND output (connects to a "clean" ground when transmitting). I took advantage 7 – ACC pin connector(2) on the back of the trasnsceiver. In a tinned iron sheet box with dimensions of 145x70x30 mm, which I had at home, as well as other components ("drawer" supplies – except for a suitable relay), I built a relatively simple DC amplifier, which provides antenna switching. I bought the relay in a GM Electronic store in Prague 8 (relay M12E12, Uss=12V, winding resistance: 640 Ohm, one switching contact with sufficient current and voltage dimensions) perhaps for 80 CZK. The adapter wiring diagram is attached. It works without problems in the band 3,5 MHz (and also on higher bands up to 28 MHz) with vertical Delta Loop transmitting antenna 83 meters in CW operation (Full BK-IN) i SSB (VOX). The adapter will certainly work in the band 1,8 MHz, where I haven't tried it yet though.

To the circuit diagram and connection with the transceiver:
The capacitors used have the task of reliably unblocking the RF signal during transmission. The connection of the PL connectors to the switching contact of the relay is made with pieces of RG58 coaxial cable. Caution with the 7-pin plugs of the connection cable (shielded conductors used). To be sure, it is good to separate them from each other (e.g. by wrapping it with a strip of insulating tape) pin 7 a pin 3 in plugs. These pins are placed next to each other. Accidentally connecting these two pins, the transceiver might not survive "healthy"..

Potentiometric trimmer settings:
We start in the extreme position, when the trimmer runner is connected to the emitter of the GC509 transistor (a PNP silicon transistor would certainly be more suitable here). With the TRANSMIT button pressed, rotate the trimmer runner until the Re relay reliably closes. Press the TRANSMIT button back and try F-BK-IN in CW ("walks" without problems even with QRQ 200 zn./min.).

I will be happy to answer any questions on the band, or via ok1dn@hotmail.com.

Vasek, OK1DN

Literature: INSTRUCTION MANUAL – VHF/HF ALL MODE TRANSCEIVER IC746 ICOM, Inc. 1997