[Elecraft] KPA500 RFI Problem

[Don Wilhelm]

Randy,

You might have better success with applying the 2.4 inch mix 31 toroids 
to your feedline rather than trying to eliminate RF intrusion on every 
cable in the shack.

In other words, kill the common mode current at its source and you may 
not have to add additional ferrites on all your shack cables.  3 to 5 
turns through a stack of 5 2.4 inch mix 31 cores at the point where the 
feedlines enter the dwelling may do the job nicely. Another similar 
choke at the antenna feedpoint(s) may be helpful as well.

The place to deal with RF-in-the-Shack is at the antenna field, not in 
the shack.

Just for the record, OCF antennas are infamous for these problems, and 
with any antenna where the feedline are run under the radiator rather 
than dropping at 90 degrees from the feedpoint for at least 1/4 
wavelength can be expected to pick up radiation from the antenna and 
feed it into the shack as common mode currents.

73,
Don W3FPR

On 1/26/2014 6:42 PM, Randy Farmer wrote:
> Thanks to all the folks who replied with advice, both on and off the 
> list. I spent the better part of the weekend working the problem and 
> thought I'd update y'all on what I found.
>
> It turns out there were two major problem areas. The first was RF 
> getting into the K3 via the ACC jack from the connections between the 
> radio and the KPA500, the XV144 and the microHAM MK2R+ box. The KPA500 
> is NOT affected by the common mode RF on the ACC cable. The power 
> gyrations were clearly due to RFI effects within the K3. I *mostly* 
> fixed this by building a short extension cable for the ACC jack and 
> wrapping six turns of it around a 2.4" mix 31 toroid. This lets me use 
> up to 500W output at the problem antenna heading without making the 
> radio go nuts. If I increase power slightly it still begins to act up, 
> so I haven't completely cured the problem. It's easier to just back 
> the power down a few watts than it is to figure out how to get another 
> few dB suppression on this path. I plan to live with it.I still have a 
> little RF getting into the receive audio line at the worst case 
> antenna heading and causing a bit of hum on the audio, but this is not 
> really a big deal.
>
> The second problem turned out to be the 13.8V power supply I use for 
> the antenna switching relays and powering the Station Masters. This 
> thing is a cheap knockoff of an Astron supply that uses a 723 voltage 
> regulator. RF was apparently getting into the supply and modulating 
> the DC output. This caused the antenna relays in the Station Master to 
> buzz. This was undoubtedly the source of the "buzz" on my signal 
> reported by listeners. I took several turns of the output DC cable 
> around another mix 31 toroid near the supply output terminals and this 
> seems to have cured the problem.
>
> Thanks again to those who weighed in with suggestions. You gave me a 
> lot of troubleshooting approaches that let me home in on the problem 
> pretty quickly once I got started.
>