[Elecraft] My KAT500 seems very sensitive to common mode RFI

[Jeff Hall]

Thanks Jim, I'll give it a read and order up some mix-31 toroids.  Does it surprise you the KAT500 is getting power cycled as I described?  Is it really that sensitive? 10 of the ferrite beads I got from DXStore (rated for 150MHz or lower) work fine for 10m-20m. But no dice on 40m, so I guess the frequency is just too low for them to be effective (or perhaps I'd need like 50 of the suckers!) 

-Jeff



On Saturday, February 14, 2015 10:06 AM, Jim Brown <jim at audiosystemsgroup.com> wrote:
On Sat,2/14/2015 9:48 AM, Jeff Hall, W6UX wrote:
>   can anyone suggest some further experiments to try?

Yes. The problem is that ferrite beads slipped onto a transmission line 
does NOT make an effective common mode choke. Those beads are inductive, 
and, depending on the length of the line and the operating frequency, 
many lines look capacitive. The inductance resonates with the 
capacitance, and the common mode current increases.

To be effective as a common mode choke, we need a high value of common 
mode impedance, and the best way to get there is with a high value of 
RESISTANCE. We obtain that by forming a low-Q parallel resonant circuit 
-- we wind multiple turns of the feedline through a lossy ferrite toroid 
or clamp-on. The number of turns needed depends on the ferrite material 
and the operating frequency. The best ferrite material for the HF bands 
is Fair-Rite #31, because it has the lowest effective Q over the widest 
frequency range.

Study k9yc.com/RFI-Ham.pdf  and use the Cookbook as a guide to wind the 
chokes you need. The chokes are most effective at the feedpoint of your 
antennas.

73, Jim K9YC

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