[Elecraft] RFI With KPA500 Full Output on 40m

[Jim Bennett]

Problem solved!

I tried a little aluminum shielding on the control box (the plastic one that supports their wireless sensors), and that was a waste of time. I gave it a shot while waiting for my delivery of ferrite cores from Mouser. Didn't cost anything and was worth a try.

The security system RFI was cured by inserting, at the vertical's feed point, a choke made from a stack of five ferrite cores (Fair-Rite part number 263180033802) with seven turns of RG-8X passing through the stack. I cranked the KPA500 up to 550 watts and not a sound was heard from that alarm system. Cool.

A side benefit to this was that I have a lower SWR at my desired "sweet spots" on 40 - 10 meters. So, placing the string of five ferrite beads on the coax (per a manufacturer's "RFI Kit"), simply wasn't getting the job done. The five mix 31 cores from Fair-Rite (actually from Mouser) did the trick.

Thanks for everyone's suggestions! And especially thanks for the good information on K9YC's web pages.

73, Jim / W6JHB


On   Sunday, Mar 18, 2012, at  Sunday, 8:51 AM, Jim Brown wrote:

> On 3/18/2012 7:05 AM, Jim Sheldon wrote:
>> I trip every GFI outlet in the house.  I haven't found enough big snap-on ferrites to hit all the romex coming out of the panel, but when I do, I'll certainly put 'em on.  A really big one that would snap around the incoming line at the panel would be nice, but so far I haven't found a source of snap on's that I can afford.  Not sure either on what material would be best for that.
> 
> As was noted in another post, the ones that trip are defective (poor 
> design). The solution is to replace them with GOOD ones.
> 
> As K2VCO noted, ferrite beads clamped onto a wire are next to USELESS 
> for HF RFI.  Ferrite chokes work by adding a parallel resonance in 
> series with the wire that they surround, and a single bead or clamp-on 
> is resonant around 150 MHz. To make it useful at HF we must move the 
> resonance down to the HF bands by winding multiple turns through it.
> 
> Study http://audiosystemsgroup.com/RFI-Ham.pdf  for a detailed 
> explanation and lots of detailed advice.
> 
> 73, Jim Brown K9YC
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