On Fri, 25 Jun 2004 12:13:54 +0100, [EMAIL PROTECTED] wrote: >The lead has the normal >split ferrite on it and I had another one in the box so tried that but with >no difference. Is it worth me buying a large ferrite ring and trying to >wind a few turns on it.
YES! Virtually all of the attention to emissions is centered above 30 MHz, because that's where most regulations begin in the US and in Europe. Those "usual ferrites" start working at about that frequency -- in fact, they are chosen because they do. When you wind multiple turns around a ferrite, the frequency of maximum effectiveness of that ferrite material will move much lower in frequency. A wire passed through a ferrite will look like series R and L, but the values of both R and L vary radially with frequency. At low frequencies, it's pretty much all L, then R starts to show up while L drops and eventually it's all R. Important caveat -- the choke only works to suppress COMMON MODE radiation (that is, radiation from current flowing along the length of the wiring). It won't help with anything radiated by the device itself. >From a suppression point of view, R is much better than L, because the L can resonate with the C of a short radiator and increase noise current. Multiple turns around the ferrite moves this behavior down in frequency. Depending on the type of ferrite material and the number of turns, you can achieve between 1K and 2K ohms series impedance at any frequency between 500 kHz and 30 MHz, but also depending on the ferrite, the Q of the resulting choke can be fairly high. In general, the lower the design frequency of the ferrite, the lower the Q of the choking impedance at HF. The #78 material made by Fair-Rite is the lowest frequency of their torroids. The #31 material is a bit higher, then #43, and finally # 61 (the highest). #31 is what you want for HF suppression -- you can get a broad enough choke to be effective on several HF bands. #43, the most common material, has a bit higher Q, so chokes wound around #43 cores will be effective over only one or two HF bands. 8 turns around a 2.4" OD #78 toroid will get you > 1K ohms on 160 meters and the AM broadcast band! It takes a lot more turns to do that with #43 -- 8 turns on #43 will put the peak at about 5 MHz. I identified my Ethernet network as the source of some nasty birdies at 3511, 10106, 10121, 14129, 14130, and 21052 kHz (there are certainly others, but I'm a CW guy). I can achieve very good (more than 4 S-units) suppression of these birdies with multi-turn chokes wound around #31 and #43 toroids, cylinders, and clamp-ons. I believe that most of the ferrite toroids sold by Amidon and Palomar are Fair-Rite #43. Some are #61. I don't know of any retail vendors for #78 or #31 materials, but there are many such products in the Fair-Rite catalog. I've been testing a bunch of them for some work I'm doing with RFI to audio systems. Jim Brown K9YC _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net http://mailman.qth.net/mailman/listinfo/elecraft You must subscribe to post. Subscriber Info (Addr. Change, Unsub etc): http://mailman.qth.net/subscribers.htm Elecraft page: http://www.elecraft.com
