[Elecraft] RFI in audio chain

[Joe Subich, W4TV]

> 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.

Elevated verticals will treat the [outside of the] feedline as a
random length radial.  The current on the feedline will be proportional
to how close the feedline length is to a quarter (or half) wavelength
depending on whether the chassis of the transceiver/tuner/amplifier is
floating/grounded.  It is *imperative* that there be a high quality
common mode choke on the feedline at the feed point of the antenna.
"Ugly balun" or other random solenoid wound chokes are not reliable -
see the work by K9YC, GM3SEK, etc. and select a choke that shows at
least 5 K (preferably 10K) choking impedance on 80 and 40 meters to
disconnect the outside of the coax shield from the antenna.

 > The microphone for the K3 is an Audio-Technica that works well
 > everywhere else.

Assuming the Audio-Technica has an XLR connector, make *sure* the
shield (pin 1) is *not* connected to the mic return (pin 7, pin 8)
and depending on the age of your K3 make sure there is no RF choke
between pin 7 or pin 8 and the circuit board ground.

73,

   ... Joe, W4TV


On 11/22/2015 6:02 PM, Jan Ditzian 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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