4m transmit-receive switching
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.
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.
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
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.
The circuit was amended on 13 April 2019.