4m transmit-receive switching


I have had some enquiries about antenna changeover arrangements for my OZ2M transverter.  I have opted to remove the Omron relay that comes with the kit, and to have separate 28MHz input and output.  This allows the inclusion of an attenuator in the transmit line, switched out on receive.  The attenuator can be designed to permit use with a transceiver running a few watts on 28MHz.

For the OZ2M transverter running 30W or less, I used "BT47" type DPCO relays. There are lots of different makes to this specification, eg www.rapidonline.com  part number 60-4465, or Maplin code N17AW.
I chose them for their fast switching speed - specified as 6ms/4ms and usually better than this - I measured mine using a scope against the rf from my IC575.  The relay switches before the RF arrives, so there is no need for a sequencer.  (For the 160W PA, I use a SDS Relais type S2 from Barend.)

I measured the isolation at 70MHz.  For a simple open circuit, it is 31dB, but by using both poles and switching the receiver to a 47 or 56ohm load, it improves to 42dB. Two relays are then used - one to switch the antenna, and one to switch in the attenuator on 28MHz.

The circuit I use to drive them assumes that PTT is grounded on transmit and otherwise floating. It is arranged to energise the relays on receive - see circuit (a).  This connects the antenna to the transmit output if the 12V supply fails or is turned off.  Alternatively use circuit (b) to energise them on transmit.  The OA91 prevents interaction when more than one such circuit is in use, but L1 and C1 are then advisable to prevent the circuit becoming liable to RFI.