[Elecraft] KPA500 RFI Problem

[Jim Brown]

On 1/22/2014 7:36 PM, Randy Farmer wrote:
> I've put a bunch of ferrites on the cable and this has reduced the 
> current on the cable but not eliminated the RFI problem. 
  "A bunch of ferrites on a cable" won't do much at HF. You must wind 
multiple turns through the core to make a dent in RFI at 14 MHz.

I think it's unlikely that the problem is generated in the KPA. I 
suspect either the K3 or the Microham box, so I would work on choking 
the cables connected to those boxes.  Also, try disconnecting everything 
from this system except the bare minimum -- the mic, the K3, and the 
KPA.  remove the AUX cable, use the RCA cable from the K3 to the KPA. 
{The KPA works fine without the AUX cable -- it samples the first bit of 
RF drive it sees, counts the frequency, and switches bands very 
quickly.) If that's clean, go back to the AUX cable. If that's clean, 
add the Microham.

BTW -- 5-6 turns is about right for 20M for a 2.4-in #31 core.

That 300W number is important information -- it tells us that we only 
need about 3dB of suppression to solve our problem. That's not a lot 
once you figure out which cable(s) is (are) letting the RF in. .

Another thing to look at is the coax between the K3 and KPA. Is this 
first quality coax with a real manufacturer's brand name (Belden, Times, 
Commscope) with Amphenol connectors, properly installed, or is it some 
off-brand advertised for cheap in QST or CQ, or even from a flea 
market?  Flaky coax or a lousy connector or a connector poorly installed 
can cause things like this.

Also look at the interconnecting audio cables. Many (most?) stereo 
cables sold by the big box stores have flimsy shields (or no shield at 
all), even when they're charging a lot of money for them.

73, Jim K9YC