At 07:23 AM 04/26/10, you wrote: >Hello all, > > I am in the process of putting up a 2M repeater on what I would > consider a pretty vacant site. There is only one other machine > (70cm Repeater) currently out there. My concern is with an AC > (60Hz) buzz that comes across on the VHF band. It doesn't have a > signal to it that will key uip a radio, but when you receive an > actuall signal you can here it. We have heard this on both > handhelds and mobiles. This site is unique as it is a duel tower > with old (unused)Microwave panels and drums on the bridge at the > top of the two towers. The microwave equipment is no longer hooked > up. We have had the power company totally unhook the power to the > site and the buzz was still present. The nearest high voltage lines > are about 1.5 to 2 miles away. You can not hear the buzz on any AM > reciever. Does anybody have a clue as to what this could be and > what we could to do prevent it? We have some thoughts on the > grounding of the guy wire being a cause but we are unsure of that. > Any ideas would be greatly > appreciated! > >Thanks >Wade >KC0MLT
The way I'm reading this is that you are hearing an AC buzz (60hz) on a FM receiver listening to ANY VHF frequency at a site that has only one UHF repeater, and it's there even when the AC feeder to the site is totally dead, right? Look for a arcing insulator on a power pole. And it might be a rural cross-country high tension tower that is MILES away from any pavement. I had one of those a number of year ago, and it took using an AM receiver on the highest frequency we could monitor (initially the aircraft AM band. We got an initial bearing at the repeater site, and it pointed into a national forest !!! We went to other sites several miles away with the intent of getting cross bearings and couldn't hear it. So we fell back to plan B ... A friend was a pilot bit we could not figure out how to get a directional antenna onto an airplane, and we quickly figured out that the one airplane that had a loop antenna wasn't going to cut it... first, it was a twin and not only was the plane rental out of sight, but out pilot wasn't multi-rated, plus we couldn't afford to feed the twin radial engines. So we fell back to plan C. Look at this photo - it's somethign like what we used... <http://www.ultralightnews.ca/sunfun02/images/lilbreeze.jpg> We mounted a VHF beam under the fuselage.. The pilot (a ham) started at about 1500 feet over the repeater site and on an idle aircraft channel, then flew a circle to get an initial bearing, then "flew towards the noise". By the way, the "real" initial bearing was about 20 degrees off of what we had from ground level at the repeater site. The noise peaked over a power line over 7 miles from the site the source but could have been any one of two dozen towers. He then switched to a PRO-43 handheld scanner set to AM and pretuned to the top of it's coverage range) and shielded so it was directional. The scanner was in a piece of plastic pipe lined with foil and grounded to the chassis. The tube was duct-taped to the ultralight frame and pointed downwards at about 30 degrees. Crude, but you could point it like a bazooka and get a noise peak. He was able to ID the individual power tower by flying over the power line until the noise peaked, then turning away and coming back at it from right angles to it and flying over each individual tower until he found which tower had that same peak. He "bombed" the ground near the particular tower with a bag of flour, then flew the access road to where it met the highway and noted where that was (anybody remember Sky King flour bombing from the Songbird? that's where he got the idea). All during this time he was in contact with us, the chase crew. We drove to the maintenance road access gate, drove to the tower, and could HEAR the arcing and zapping above us - no electronics needed ! We copied the tower ID number - and didn't take any chances, we used both pencil and paper and a 35mm camera! (Southern California Edison has a 10-character ID number on each tower) and reported it. SCE had WA6FQG (now SK) as their in-house RFI guy since the 1960s... and for many years he had a tech session at every ARRL Southwestern Division convention... one of us had his business card from one of those sessions. A couple of weeks later we met FQG and his tower crew at the gate and followed them in, and watched while they changed the insulator. Problem solved. As I re-read what I've written above, it sounds like it took one flight - it didn't - it took three Saturdays across eight weeks as we had scheduling problems with the ultralight, the pilot, and we had to figure out the mounting for the 2m beam, and we had to build the bazooka tube for the scanner. And the overall attitude was SAFETY - we were not going to duct-tape a beam to the airplane, and we couldn't (and wouldn't) drill any holes. The keys to finding power line interference: 1) A directional AM receiver. It MUST be AM. 2) As high a frequency as possible - start out low (like the aircraft band) to get an initial bearing, and go up in frequency as you get close. Since the radiated power level goes down with increased frequency the higher the frequency the better the precision in narrowing it down to the power pole or power tower. 3) a clear shot for the directional antenna. This is where the ultralight came in handy. Mike WA6ILQ
