Topband: Detuning Xmit Antenna while RXing.

Tom Rauch w8ji at
Sun Jul 27 08:02:00 EDT 2003

> Simple.  Just use the TX GND jack on the MP's rear panel with the rear
> panel LIN switch in the ON position.  It provides ground when
> transmitting and is open when receiving and can be used to
> energize/deenergize the vacuum relay.

Better watch the current required and the relay sequencing or timing. Vacuum
relays often use fairly low resistance coils, and generally surplus relays
are 24VDC or higher voltage.

Adding a backpulse canceling diode across the relay coil slows release time
considerably. Running low voltage slows pull-in time. You have to know what
the relay timing is in the actual system in actual operating conditions, or
carefully copy a system that is known to have good timing.

The last thing you want to do is transfer a load while RF power is on. Not
only will it cause a click, it will ruin things like band switches, PIN
diodes, or other expensive components (it can even eat up a tube over time).
This is especially problematic when running QSK.

Grounding of TX antennas for detuning may not always be best either. They
might require an open, a short, or anything in between. I'd be really
careful doing fast RF switching, especially when running QSK or using
tetrodes or metal oxide cathode tubes!!!

73 Tom

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