[Elecraft] RX Ant Preamps and the K3 12v Accessory Jack

[Tom W8JI]

> If you are planning to use closely spaced Tx and Rx antennas, I would
> suggest that you consider using a Reed relay or similar "fast acting" 
> relay
> to short circuit the external preamp's input during transmit periods.

So long as the relay is accuated before RF appears, and held until after RF 
goes away.

> Of course if diodes are used for protection instead of a relay, the diodes
> could generate Tx harmonics which are then radiated by the Rx antenna.

Worse yet diodes destroy dynamic range. A typical switching silicon diode or 
fast rectifier starts to conduct at about 2.5 dBm, and that can be from an 
accumulation of many signals...even out of Ham bands. It takes special 
circuitry to not reduce dynamic range with diodes.

<<To reduce my bench time creating my own
amplifier and such like I was looking into using the RPA-1 from
DXEngineering which has an automatic bypass mode.  I don't know if it will
cope with QSK, but as a first stop that's what is in my mind to buy.  >>

It wasn't planned to do that. The relay would be held up by a capacitor as 
it discharges, but it could be modified.

<<I think the spec says the RPA
handles 30dBm signals as normal, so I'm thinking it will prevent damaging
signals getting to the rx input.>>

The RPA can output well over 1/2 watt, so don't expect it to act like a 
limiter. It makes a good QRP transmitter.

73 Tom