Topband: K9ay loop not performing

Guy Olinger K2AV k2av.guy at
Tue Dec 10 13:54:09 EST 2019

The need to disconnect the feedline shield is to block common mode signals.
A K9AY approved **EXCELLENT** common mode block (not a regular "balun" with
likely awful blocking on 160m), or (GASP) an isolation transformer will
take care.

In the case quoted, with the coax center contductor opened, the
stinky-on-160-blocking "balun" is how the noise gets in. Further since Pete
saw a 4 S unit drop by simply disconnecting the TX wire, it's likely that
his common-mode provision, even if not intended as such, would appear to be
satisfactory, and therefore a double pole relay (lot more expensive) is not
called for.

73, Guy K2AV

On Sun, Dec 8, 2019 at 7:30 AM Rob Atkinson <ranchorobbo at> wrote:

> If you open the tx antenna feedline for rx, from my experience, you
> need to completely open the entire feedline, namely the coaxial cable
> shield as well as the center conductor.   coaxial relays that maintain
> conductivity of the shield when N.O. won't detune/isolate the tx
> antenna.  I use a pair of open frame relays for this.  Fortunately on
> 1800 kc the Z bump is almost nonexistent.   What will govern your
> choice of relay is tx power and tx time, and whether or not you
> require QSK.  I do not, but if you do, you may have to consider
> something like multiple paralleled reed relays tor speed and power, or
> vacuum relays.
> 73
> Rob
> K5UJ
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