[Elecraft] RFI in audio chain

[Vic Rosenthal]

A vertical should have an RF choke at the feed point. This is sometimes called a 'balun', but in this application it is common mode choke.

To reduce RF pickup with ferrite beads or toroids at HF, especially on the lower bands, you need to wind multiple turns around the ferrites. Also they should be the appropriate type of ferrite material. 31 mix is good for 160-40 meters.

Vic 4X6GP/K2VCO

> On 23 Nov 2015, at 1:02 AM, Jan Ditzian <ditzian at windstream.net> wrote:
> 
> I have a problem that appears to be changing, but I could use some help.  The problem is RFI in the audio input (microphone input) when I use my vertical antenna on 40 meters.  Here is a rundown of what has happened:
> 
> I have a 67-foot elevated vertical that I can use on 80 and 40, and it works fine on CW.  However, on SSB, both bands, there clearly is feedback; there is no feedback on the other bands where I use a C3 yagi.  Initially, the feedback was so bad that the rig would go into oscillation, and I had to turn the amp to standby.
> 
> I recently purchased the new KSYN3A and decided to install it.  I replaced the K3 with my IC-730 backup, using the hand microphone that came with the 730.  The 730 did not have any RFI.  I finished the modification and returned the K3 as the operating rig.  Now, the RFI on 40 seems to have diminished substantially or disappeared, but it still happens on 75/80.  However, it seems to be much less there as well.  I do not suspect that the KSYN3A had anything to do with this, but perhaps I tightened connections better when I returned the rig to service.  I also redid some ground connections.
> 
> The microphone for the K3 is an Audio-Technica that works well everywhere else.  It has a long cord, though.  I put a few toroids on the cord near the microphone connector and that has possibly reduced RFI a little, but it is still there.
> 
> Is there a possibility that a bypass capacitor is bad, or has someone else had the problem and solved it externally to the K3? For instance, has anyone found that a long string of ferrite beads has cured this problem?
> 
> Despite decades of operating, I am hardly a troubleshooting hotshot, and I would appreciate guidance.
> 
> Thank you,
> 
> Jan, KX2A
> 
> 
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