To use a special receiving (RX) antenna for DX reception, especially on the lower KV bands, it is necessary to separate the input for the receiving and the output for the transmitting antenna. Transceiver IC746 does not have this option, if we do not want to interfere inside the device. The adapter described below is used to separate the antennas.
You will read in the article
Adapter for separating RX and TX antennas
Because I also did not want to limit (or complicate) the possibility of connecting the power output stage behind the transceiver, I did not use the SEND output (connects "clean" ground during transmission). I used the 7-pin ACC(2) connector on the back of the transceiver. I built a relatively simple DC amplifier, which provides antenna switching, into a 145x70x30 mm tinned iron box that I had at home, as well as other components ("drawer" supplies - except for a suitable relay). I bought the relay in the GM Electronic store in Prague 8 (relay M12E12, Uss=12V, winding resistance: 640 Ohm, one switching contact sufficiently sized for current and voltage) for about 80 CZK. The connection diagram of the adapter is attached. It works without problems in the 3.5 MHz band (and also on higher bands up to 28 MHz) with a vertical Delta Loop 83 meter transmission antenna in both CW (Full BK-IN) and SSB (VOX) operation. The adapter will certainly work in the 1.8 MHz band, but I haven't tested it yet.
To the wiring diagram and connection with the transceiver
The capacitors used have the task of reliably unblocking the RF signal during transmission. The PL connectors are connected to the switching contact of the relay using pieces of RG58 coaxial cable. Be careful with the 7-pin plugs of the connection cable (shielded wires are used). To be sure, it is good to separate pin 7 and pin 3 in the plugs from each other (eg by wrapping a strip of insulating tape). These pins are located next to each other. A random connection of these two pins would not allow the transceiver to survive "in good health".
Potentiometric trimmer settings
We start in the extreme position, when the runner of the trimmer is connected to the emitter of the GC509 transistor (certainly a silicon PNP transistor would be more suitable here). With the TRANSMIT button pressed, turn the trimmer runner until the Re relay reliably closes. We press the TRANSMIT button back and try F-BK-IN with CW ("it works" without problems even with QRQ 200 bpm).
I will be happy to answer any questions on the band or via ok1dn@hotmail.com.
Vašek, OK1DN
Literature: INSTRUCTION MANUAL – VHF/HF ALL MODE TRANSCEIVER IC746 ICOM, Inc. 1997