Topband: protecting rx input

[Pete Smith]

At 10:11 AM 4/4/2007, Roy Koeppe wrote:
>Concerning,
>
>"... I thought about adding a relay to disconnect
>and/or ground whatever antenna is connected to the RX input during
>transmit, but since I will also be working on station mods to allow
>QSK I am interested in keeping the number of relays to a minimum.
>Are there other ways of protecting the receiver? Or is the relay my
>best bet?...(etc.)"
>
>
>Devices for that are on the market. But Icom recommended a small lamp in
>series with the antenna, followed by back-to-back shunt diodes to ground
>(chassis) at the rx input. A #47 lamp or a grain of wheat, available
>down to 25 ma, and 1N914 are typical suggestions.


I agree, except I was advised to use a string of 3 1n914s in each 
direction, to raise the clipping point above 2 volts and avoid IMD from 
strong signals.  I believe that's still safe for a 50-ohm input and the 
resistors commonly used in receiver attenuators these days.  For one 
homebrew approach, see http://www.pvrc.org/n4zr/rxprotector.htm .

73, Pete N4ZR