Hi Jim, Well, the ferrite bead may not be self-resonant on 20 meters, but it still provides SOME impedance to the common-mode current. I found experimentally that it did help. And lots of people use these snap-on beads to good effect even in situations where multiple turns (which of course would be better) are not practical.
Oliver W6ODJ On 25 Jan 2010, at 9:31 AM, Jim Brown wrote: > On Mon, 25 Jan 2010 09:14:21 -0800, Oliver Johns wrote: > >> Try putting a snap-on ferrite bead ON the actual boom of the >> electret headset microphone. Put it right at the end, just before >> the microphone bulb. Maybe the RF is getting into the electret or >> its FET and is being rectified there. > > Ferrite beads are tuned to about 150 MHz. Clamping on onto a cable > simply adds that 150 MHz resonance to the common mode circuit. > That's great if the RFI is on 2M, but next to useless if it's on > 20M. To make a choke work on 20M, we must wind multiple turns > through the ferrite to more the resonance down to 20M. For a typical > ferrite clamp-on, that's about 5 turns. You can't do that on the > boom, but you can do it on the cable. > > The original poster of this question emailed me this morning telling > me he had solved his problem by re-cabling the coax between his rig, > amp, and antenna switching, so the source of his RF was probably a > badly terminated coax connector. > > 73, > > Jim K9YC > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
