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
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