[Elecraft] K3 Sub Receiver and protecting the Sub RX input

[Edward R Cole]

An alternative to a relay (which should ground the sub-receiver ant) 
is using a pin diode to short the input.

Typically one uses a coupling capacitor to isolate the antenna from 
the circuit which will have a low value of dc bias to turn on the pin 
diode which will conduct to ground and short the antenna ckt.  I 
assume the K3 sub-receiver antenna ckt is capacitively coupled to 
isolate it dc wise.  Use a RF choke between pin diode and antenna to 
keep from loading the antenna in normal use.  hint: google pin diode 
antenna switching.

Use KEY OUT ckt from the K3 to control the pin diode via a transistor 
switch that applies the bias thru a dropping resistor.  You will have 
to do some design work - not plug-n-play freebie

Many ham rigs that do QSK use pin-diode TR switching.

73, Ed - KL7UW
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