[Elecraft] KPA500 RFI Problem

[Jim Brown]

On 1/26/2014 7:18 PM, Randy Farmer wrote:
> Unfortunately, in my case the shack IS part of the antenna field and 
> there's nothing I can do about it short of removing the antenna.

Yes, when the antennas are in close proximity, they radiate RF into the 
shack, independent of common mode on the feedline. That said, OCF 
antennas are a really bad idea in today's world, both because of noise 
coupling and the high common mode stress they put on a choke. I don't 
know anything about the Balun Designs productcs, but even the best 
chokes can be fried by running high power through them to a badly 
unbalanced antenna.

Some other thoughts. It is critically important that the shield of all 
cables be connected to the shielding enclosure at both ends, and ONLY to 
the shielding enclosure. You should make sure that the cables you are 
using do that, and it may not be easy.  I terminate the signal returns 
of all lines in my DB-connectors to the DB connector shell. This 
requires that the connector shell be bonded to the shielding enclosure, 
and with most gear, that works. It did NOT work with a cable I made to 
go between the K3 and a KAT500, so I suspect that there is a Pin One 
Problem at the AUX jack one or both of those products. What you are 
describing is symptomatic of a Pin One Problem.

Pin One Problems are tough to diagnose, because the places at which they 
inject RF are at the whim of the circuit layout designer. The choke 
you've put on the AUX cable sounds about right for 20M, and should be 
effective from 30-10M. At this point, I'd look at what other cables are 
connected to the radio and choke them.

There's another thing that might be causing your problem that I ran into 
only a few days ago helping W6OAT solve a similar problem -- his big 
SteppIR is about 60 ft above the shack on a tower, and he gets RF 
feedback into the radio that locks up CW keying . He's using a Microham 
box (don't know model number) and the interface cable between that box 
and the K3 is a train wreck waiting to happen. There's a big multi-pin 
plugged into the Microham with a lot of wires coming out of it that have 
labels on them like Line In, Line Out, Key, etc. but they are all 
unshielded, and there's no signal return -- the setup is depending on 
the coax shield for that. That works for DC, and maybe even for audio 
(if there's no big power transformer nearby), but NOT for RF.

The problem with using coax (or even a chassis bond between the 
equipment) as the signal return is that total circuit forms a big loop 
consisting of the signal wires, the chassis, and the coax shield. You 
are certainly in the near field of your antennas, and in the near field 
of a current maximum (in this case, the antenna feedpoint), the magnetic 
field dominates and couples into that open loop, and the strength of the 
coupling is proportional to the loop area!  In addition, that loop forms 
a receiving antenna, which picks up the electromagnetic field.

I have no idea where Rusty's cable came from, but it's clearly 
commercially made, and seems to be specific to the Microham and the K3. 
Cable adapters like this may be simple to hook up, but they are a 
sitting duck for RFI.

73, Jim K9YC