Tuesday, June 10, 2008

Receiver Saver Circuit

I recently assembled and delivered to W4BXI a customized version of a circuit developed by AD5X to permit the safe use of a secondary (spotting) receiver on its own antenna while operating a transceiver on a main antenna. The circuit is intended to protect the spotting receiver from excessive RF energy when the main rig transmits on its nearby antenna. The original circuit was published in the January 2008 issue of CQ magazine.

Here is a copy of the customized schematic, which adds a power switch, status indication LEDs, and supplementary protection elements.

Some views of the assembled unit are shown below.

W4BXI is testing this unit at his station, and I expect to incorporate some refinements as I get his feedback. One change may be to use shielded leads from the two SO-239 jacks to the circuit board, to improve the isolation between the receiver and the antenna when the receiver is grounded. Another change I am considering is the introduction of a time delay for restoring the antenna-to-receiver connection when the control signal changes to the receive state. A few seconds of delay here would minimize relay cycling if the operator is using full QSK, while not significantly affecting the utility of a spotting receiver.