Magnets are our friends!

KD6MSI writes "And now from the "aint that slick as a whistle" department ......



I was having some really terrible interference problems that I found a very simple solution to, and I thought I would share it with you all.




Long, long ago in an ITU region far, far away, lived a kit builder, (me), that was only marginally good at building. Sure I have lots of fun doing it, but I wish I understood a bit more of the technical stuff. Anyway, as the story goes, a long time ago, I built a Ramsey FM stereo transmitter which has been one of the neatest things I ever soldered together. I have used it in so many applications, I can't even begin to list them. I recently pulled it out of the bit bin and dusted it off, wondering if it still worked. Lo and behold, I gave the tuning inductor a few turns, and it was working good as new.



To make a long story short, I had to run some RG-8X in the attic one sunday afternoon, and I wanted some tunes while I crawled around in the dust up there, banging my head and crunching my knees. I popped my favorite CD in the stereo downstairs and hooked up the ramsey transmitter. After turning the setup on, I took the portable radio up to the attic with me and turned it on.



Well, It was bearable. I hadn't noticed in my tests earlier, that there was a background noise suspiciously reminicent of an obnoxious AC hum in the FM transmission. "This will never do", I thought, and I vowed to hunt down the source as soon as I finished running the coax.



Well, by the time I finished in the attic, I was too pooped to pop. I didn't get to do any detective work that day, and in fact didn't get back to it until just today (a couple of weeks later).



I was at radio shack and a tidbit of info came to the forefront of my brain. I remembered that an inductive field will cancel out an ac hum, and I also remembered that RS carried a decent snap on choke (snap together magnet in a plastic housing, part #273-104). I promptly bought two (be prepared I always say!).



I was pretty sure that the culprit causing the hum was the cheap made-in-the-peoples-republic-of-china wall transformer power supply that supplied the Ramsey transmitter. I got the choke(s) home and popped one out of the package. They actually come with some pretty good instructions, and in no time I had measured the power cord and wrapped it around the outsides of the core. I wrapped it in such a way that there were seven wraps around each side, and it was as close to the transmitter as possible. I slipped the other side of the magnet in place and snapped the plastic case closed. I fired up the stereo and transmitter and took the portable out to the garage.



It was a thing of beauty. I could turn up the volume on the radio all the way, and all I heard was music. No hum; in fact, no noise at all.



I grabbed the duct tape from the garage and went inside. To make it (more or less) permanent, I neatly wrapped the whole package in the duct tape. Shrink wrap would have been better, but I didn't have the right size.



I thought I would share this experience with my ham friends in case anyone was struggling with RFI or line interference such as my AC hum or interference from dirty power. It is cheap and works like a charm. Just remember that power cords act like antennas, and you should snap the choke in place as close to either the offending appliance, or the affected gear, as possible.



Now, not being terribly technically savvy, would anyone care to elmer me on why inductance works so well in a situation like this?



73 de Mathew, kd6msi

dit dah dit dat dit"