[TowerTalk] Topband: Receiver protectors.

[Richard (Rick) Karlquist]

On 3/18/2020 6:41 AM, Rob Atkinson wrote:
> Last year I purchased an Airspy HF+ Discovery SD receiver.  Eventually
>
> grommetted hole for the USB cable's exit.  I mounted a pair of relays
> in the feedline to the receiver inside the box and put RF chokes in
> series with the DC line to the relay coils (24 v. DC) at the entrance.
> I figured two relays in series in the line would add some protection.
> Amazingly none of this  helped much as a 20 w. carrier on 160 m.
> produced a disturbingly strong signal trace on the receivers
> panadaptor.  I wanted to see little or no signal at a few hundred
> watts to feel comfortable about operating at higher power.  I was
> using typical ice cube style relays.  The contact spacing isn't much
> so I think the relays were just acting like low value air dielectric
> capacitors in the line.  RF went right through them.
> 

This behavior is completely predictable.  When you should have done is
have one set of relay contacts in series with the signal and another
set of relay contacts after those that shunt the signal to ground.
Then you would get a lot more isolation.  This is what all well designed
RF switches use.  You already had two relays, so you had all the parts
you needed on hand.

> Rob
> K5UJ

73
Rick N6RK