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