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

[goldtr8 at charter.net]

Hi Jeff does not suprise me.

Read the link in Jims email single beads do not cut it.


~73
Don
KD8NNU
FH#4107
-.- -.. ---.. –. –. ..-


On Sat, Feb 14, 2015 at 1:56 PM, Jeff Hall wrote:

> 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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