When and where to ground is not as simple as it sounds at first thought.? Depending on circuit design, several grounding schemes for microphones might have to be tried to ensure proper shielding while at the same time not creating a ground loop.? Too, ferrite beads on each and every lead going to/from the rig might help.
Just some random thoughts from a fellow Drake enthusiast... -----Original Message----- From: Jim Shorney <[EMAIL PROTECTED]> To: drakelist@zerobeat.net <drakelist@zerobeat.net> Sent: Wed, 2 Jul 2008 10:35 pm Subject: [drakelist] Pondering RF Feedback (TR7/SP75) "Jim Shorney" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- Some readers may recall that last year I was plagued by RF feedback at Field Day with the TR7/SP75 combination. Careful balancing of the mic gain and SP75 input and output levels could eliminate it most of the time, but not always. This year, I connected a ground strap from the SP75 to the TR7 chassis, as was suggested here. Well, that helped quite a bit. However, I was still seeing some RF feedback this year. It was happening with the Yagi (4-el 20 meter monoband) pointed generally southwesterly, which would have been pretty much over my head. As usual, were stationed in an EMCOMM trailer on a concrete parking lot, with no station ground available. The feedback could be mostly eliminated, depending on the actual position of the Yagi, by reducing the TR7 mic gain. I couldn't nail down exactly where it was getting in, but it seemed to have something to do with the microphone wiring - a Telex Echelon 150 aircraft active noise-cancelling headset with a homebrew interface box. The box was solid aluminum, mic cable to the SP75 was RG-174/U coax, and power for the ANC amplifier and mic bias was well bypassed. Sort of a puzzler. Then I found this surfing the web: http://www.audiosystemsgroup.com/RFI-Ham.pdf This may be old news to some of you, but what caught my eye was the author's description of what is called "the Pin 1 problem". I recalled from having been inside these boxen that the mic ground connection inside both the TR7 and the SP75 is not tied to the chassis at the point of entry, but rather, is wired to the circuit board. According to the referenced article, this is bad. I've made wiring changes to both the TR7 and SP75 to establish proper grounding, as described in the article. There is also a tech note on Heil's web site regarding this problem: http://www.heilsound.com/amateur/tips/rfi_removal_mics.htm Heil provides a method of grounding at the mic connector that is pretty easy, but I have a lot of microphones and I like to try out different things. It seems possible that this may be where my feedback was entering, but I have no way of duplicating the Field Day conditions at home (and I doubt the radio club will want to drag out the trailer, Yagi, and tower just to test my rig). I don't recall ever seeing this sort of RF feedback at home, so it will have to wait for next year to test out the "Pin 1 problem" theory. I'm thinking I will take along some snap-on ferrite chokes next time as well - or maybe I can get the club to add some to the Field Day kit. Any thoughts from the gallery? 73 -Jim NU0C (BTW, the Echelon headset knocked the generator noise right out of the headphones...) ---------------------------------------------------------------------- Submissions: drakelist@zerobeat.net Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net ----------------------------------------------------------------------
