K3: A cure for RF feedback with electret mic

[K1LI]

I'm posting this only to add to the reflector's knowledge base of solutions
to mic problems. I do not claim that this problem occurs with all electret
mics. This information will come as no surprise to Jim, Tom and other
frequent contributors. While the solution falls into the realm of "good
engineering practice," I operate from a variety of locations, the
engineering of which is generally outside of my direct control. I've
identified a solution that I can apply to the stations I visit - and leave
behind, at no great expense.

It's worth noting that my problem was *not* solved by shorting the mic
ground to the shield as recommended in the Elecraft app note on this
subject. The problem behavior does not depend on whether the front- or
rear-panel mic jack is used.

I use a ClarityAloft headset, which employs an electret element. (To the
peanut gallery: yes, there are far cheaper headsets that may work "just as
well," but I've never found a match for the long-wearing comfort of these
units.) The headphones and mic are split at an unshielded box into separate
headphone and mic cables. I've been plagued with RF feedback which varies in
severity from installation to installation. The feedback is band-dependent,
can occur at a power output as low as 10-watts or less and is evidenced by
artifacts ranging from distorted audio in the headphone monitor to
uncontrollable, full-power output. Interestingly, I don't have this problem
with a dynamic mic. Until now, I've never expended the energy necessary to
identify the root cause and solve the problem, I've only applied enough of
the right kinds of band-aids to bring the transmitted audio from my home
shack into the "acceptable" range.

For these tests, the only connections to the K3 were coax to the ANT 1 jack,
headset connection to the rear-panel mic and headphone jacks and power from
a well-known 25-A switching supply. Since the feedback did not occur when
the feedline was terminated in a proper dummy load, my solution started by
applying a ferrite choke at the load end of the feedline to ensure that RF
is not flowing back to the rig on the outside of the antenna coax shield
from a "real world" load. Choking the load end eliminates the uncontrollable
output and brings the audio into the "very acceptable" range, but I could
still detect distortion on some bands and at higher output power levels.
Applying the same choke to only the K3 end of the feedline was not nearly as
effective, though it did reduce the severity of the feedback. Choking both
ends of the feedline eliminated the problem as evidenced by no change in
headphone-monitored audio quality from zero watts to full 100-watts output.

Choking the load end of the feedline and the mic cable going into the K3
also eliminated the problem. Choking only the mic cable, or the mic cable
and the K3 end of the feedline, was not sufficient to eliminate the
feedback.

Using an AIM4170 to measure choking impedances, I learned quite a bit about
ferrite chokes in the process of solving this problem. Suffice it to say
that all ferrites are not alike (plug: see
www.audiosystemsgroup.com/RFI-Ham.pdf ). The chokes used in these tests
consisted of five turns of the subject cable - whether mic or LMR200
feedline - through a Fair-Rite 2631102002 type 31 core.

Please contact me off-list if you have questions.

--
73 -- Brian -- K1LI
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

[Jim Brown-10]

On 3/29/2011 9:53 AM, Brian Machesney wrote:
> The feedback is band-dependent,
> can occur at a power output as low as 10-watts or less and is evidenced by
> artifacts ranging from distorted audio in the headphone monitor to
> uncontrollable, full-power output.

Thanks for the post, Brian.  Band-dependency comes from three important
variables -- how much RF current is on your feedline at the frequency of
interest, how much RF is radiated into your shack at the frequency of
interest (both are antenna-dependent), AND the frequency-dependent
nature of the Pin One Problem(s) in the radio. The latter depends on
internal construction and wiring layout within the radio.

When I started using an FT1000MP for contesting, I immediately noticed
RF feedback on 15M and 75M.  I set up my RF signal generator and did the
tests for RF Pin One problems on the MP's mic jack.  I found a broad
peak just below 5 MHz that extended down to 75M, and another peak at 21
MHz.  I already had chokes on the feedlines, so I added one on the mic
cable that fixed the problem.  BTW -- I was using an EV RE16 at that
time, which is a dynamic mic, the antenna was a fan dipole about 150 ft
from the shack, and I was running 1.5KW!

If I'm not mistaken, the procedure for using the generator to test for
RF Pin One problems is in the RFI tutorial.  It's also documented in one
of the AES papers on RFI, which are on my website.  
http://audiosystemsgroup.com/publish.htm

73, Jim Brown K9YC
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html