Now and then I have used Jim K9YC's "A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing" as a reference but very recently I finally read the whole thing straight through, and it finally dawned on me that Jim advocates putting a common-mode choke right at the feedpoint of the antenna. The reason he gives is that most 'balanced' antennas are not in perfect balance due to the antenna's surroundings, and this imbalance makes the feedline pick up more noise. Much of his paper consists of measured data of many kinds of chokes, but I built what seemed the simplest effective choke -- 15 turns of solid THHN electrical wire bifilar wound on a #31 toroid-- and put it near the feedpoint of my dipole.
I say 'near' because for mechanical reasons the closest I could put the choke was about 12 transmission line feet from the feedpoint. I use all-homebrew 500 ohm open-wire line. I finally got the antenna up Saturday and have been tuning around the low bands and it appears that my noise floor has indeed dropped. Whereas in this noisy location I have had the REF LVL of the P3 set to -114 dBm on 40 meters for some time, I now have to drop the REF LVL to -118 dBm to move the noise floor to the same location on the bottom of the display. There is a similar outcome on 80 meters. But man, I just picked up another half S-unit of noise floor last weekend. This K9YC is a smart man. Another change I've made since reading his entire paper was replacing my 'shielded' speaker cables (which nonetheless were always bothered with RF buzz whenever I transmitted on 40) with simple unshielded twisted pair, and it looks like my buzz problem is pretty much gone. Two changes which have improved the station by a lot. Thanks, Jim. Al W6LX ______________________________________________________________ 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
