K3 RF in SSB audio on 6M

[Howard Sherer]

Only on 6M when I operate above 250W, I have RF getting into my SSB
audio, using either the the R or F mic connector but slightly worse with
the R mic connector. I have installed Fer chokes on the audio input
lines with no improvement. Any suggestions.

--
Howard Sherer

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[Don Wilhelm-4]

It is difficult to impossible to cure RF-in-the-shack at the transceiver
end.
The most certain solution is to work on the antenna system.  Baluns at
the antenna feedpoint and RF Chokes at the point where the feedline
enters the shack are the most effective.
In other words, keep the RF inside the feedline where it belongs and
prevent it from getting onto the outside of the coax.

For those with attic antennas (or otherwise quite close to the shack),
the situation is difficult to cure because you can have direct radiation
from the antenna in the shack, and the only cure I know for that is to
reduce the power.

73,
Don W3FPR

On 10/8/2011 8:26 AM, Howard Sherer wrote:
> Only on 6M when I operate above 250W, I have RF getting into my SSB
> audio, using either the the R or F mic connector but slightly worse with
> the R mic connector. I have installed Fer chokes on the audio input
> lines with no improvement. Any suggestions.
>
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[Jim Brown-10]

On 10/8/2011 5:26 AM, Howard Sherer wrote:
> I have installed Fer chokes on the audio input
> lines with no improvement. Any suggestions.

What sort of ferrite choke(s) have you installed?  A single turn through
a #43 or #31 ferrite core puts the resonant peak in the 150 MHz range,
while a single turn through #61 puts it in the 500-800 MHz range.  If
you want a choke to be effective at 50 MHz, you must wind at least two
turns through the core, and you will likely need multiple two-turn
chokes in series on whatever cable is picking up the RF.

While I completely agree with Don that it's critical to keep RF off the
coax, a 6M antenna that is close to the shack could easily radiate
enough signal to excite a Pin One problem, and you won't solve it by
choking the coax, you must choke whatever outboard cables are receiving
the RF.  Also, VERY important -- the nature of Pin One problems is that
it is not ONLY audio cables that can have the problem. ANY cable whose
shield (or common) goes to the circuit board rather than to the chassis
can be a Pin One problem, including key lines, PTT lines, headphone
lines, speaker lines, serial cables, and the AUX cable.

A completely different mechanism that SOUNDS like RFI is NOT RFI at
all.  That mechanism is modulation of the V- line from the power supply
by the IR drop that results from modulation of the DC that powers the
radio.  That modulation can be as much as a volt, depending on the
current, the line length, and the wire size.  If some piece of
unbalanced audio equipment that feeds the transmitter is powered from
the power supply end of the cable, and if V- is bonded to the chassis in
the power supply, that modulation will appear in the unbalanced signal
return, and will be added to the audio. It SOUNDS very much like SSB
audio detected by an AM detector.  Why might it appear on 6M and not on
other bands? It might if that the power supply is also feeding a 6M
brick amp, or if whatever outboard gear is powered from the power supply
end of the cable.

If you have NO outboard boxes connected, then it's almost certainly an
RFI problem, and almost certainly a Pin One problem (which means it can
be killed with the right choke(s) in the cable(s) that are acting as the
RX antennas.

73, Jim K9YC
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[Jim Brown-10]

On 10/8/2011 5:26 AM, Howard Sherer wrote:
> Only on 6M when I operate above 250W, I have RF getting into my SSB
> audio, using either the the R or F mic connector but slightly worse with
> the R mic connector. I have installed Fer chokes on the audio input
> lines with no improvement. Any suggestions.
>

More followup on this issue.  Today, I made up cables to interface two
KRC2s to two K3s. In doing so, I studied how cable shields are
terminated in the K3 and KRC2.  What I learned isn't pretty.  Both the
KRC2 and the K3 have "screaming" Pin One Problems at their DB9 and DB15
connectors. In the K3, there are RF chokes between the designated shield
contacts and the shielding enclosure!  To provide shielding, a shield
must be continuous. The RF chokes make this impossible. In the KRC2, the
DB9 connectors are insulated from the shielding enclosure by paint. To
get to the shielding enclosure, the DB9 shields must go through wiring
on the PC board to a single point on the far side of the enclosure where
the PC board makes intentional contact with the chassis -- IF the inside
tooth washers manage to get through the heavy layer of paint on the
inside of the chassis.  Not only that, but the top and bottom sections
of the chassis are insulated from each other by paint, even with the
hardware that mounts them together tightly screwed down.  I didn't build
the KRC2s, they belong to N6RNO, a member of our CQP team.

The ONLY proper connection for a cable shield is the SHIELDING ENCLOSURE
of equipment. To solve these problems, I took the two KRC2s apart,
scraped the paint from the inside of the chassis where the DB9s are
mounted, where the PC board is designed to be tied to the shielding
enclosure, and where the two pieces of the shielding enclosure are
mechanically tied together with those nice rectangular mounting parts
that Elecraft uses. Not wanting to modify internal wiring of the PC
board, I've terminated cable shields directly to the DB9 shells.

So far, I haven't had time to chase down details of how (or whether) DB
connectors contact the shielding enclosure of the K3,but I'm not
optimistic about what I'll find. For about seven years, I've been using
serial cables that connect signal returns and cable shields (if the
cable is shielded) to the DB9 shells, and I have yet to find a radio or
computer where that doesn't work, so there's a DC path there somewhere. :)

So -- two of the first cables I would choke thoroughly to solve that RFI
problem would be any cables connected to those DB9 connectors.  I have
also seen evidence of RF noise coming in on the main DC power connector!

73, Jim Brown K9YC
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