I had posted a comment that I was going to move my antenna into the attic to kill two birds; hide the antenna and attenuate my signal. It was a DISASTER!!! RF is a funny thing. The only place to put the antenna was right above my equipment. I got everything hooked up and turned it on. The modulation meters and compressor indicators pegged. The signal was distorted and low. I figured for sure I'd blown the output transistor. So I spent 24 hours thinking the worst. The next day I got the antenna down and put it back outside where it worked before. It works fine now. It's not back on the chimney-It's on a pole out my back door, about 70' off the ground. I haven't had a chance to check the fringe signal yet, but I'm good for 2 miles except north. There's a large park in that direction that I think soaks up the signal. It might get better when the leaves fall off. Anyway- I just thought I'd share. There's another thread talking about signal problems, grounding, etc. I think this is an example of how RF can really mess everything up. If I could have seen it, my station would have probably looked like the Northern Lights. Gee it feels good to be back on the air. t.
RF feedback is always a pain to deal with when running the transmitter so close to the audio gear which almost all pirates are forced to do. It's not like most of us are lucky enough to have the transmitter located a mile or two away from the studio like licensed stations have I even know a few cases where Part 15 legal AM operators had RF feedback and that's only with 1/10 of a watt! With my setup I keep the transmitter in a separate room and pass the audio lines through toroidal cores to block RF from getting back into the studio room. With lower frequencies it becomes a bigger problem because the longer wavelengths travel along wires more easily. When I run 10 watts on AM it seems almost impossible to not get RF back into some equipment. One time a friend came over with his guitar and amplifier and was going to play live. That was a disaster lol. The amp picked up the RF no matter what we did. A lot of older stereo equipment seems to have that issue also, especially the ones that don't use ground prongs on their plug cord. Even professional licensed setups have this issue once in a while if their transmitter is "in house". I know our local 1kw AM towns station has got complains in the past from people who live near by claiming the station is turning on their touch lamps, causing interference to computer speakers and phone lines. I even had a mixer board once that when the mic fader was all the way up would pick up stray RF from other stations near by. Using RF traps and good quality cables helps a lot. Using the three wire balanced audio cables helps greatly which is why most professionals use XLR type cables for low level audio runs to mixers and mics. With FM it's no different but usually easier to deal with since AM gets its audio into equipment more easily than FM because of the difference in the type of modulation. With FM I usually hear hum as the artifact of feedback, along with the usual Vu meters showing ghostly readings. AM will get the modulated audio right back into the mixers causing squealing and emphasized volume that sounds bad. When all else fails you just have to get the antenna and transmission gear as far away from the audio equipment as possible. Lots of good grounding and shielding helps to.
Yeah, I have to run my audio cables very carefully too. I've noticed that the mic cable, which goes from balanced to unbalanced for the mixer, has to be moved around every once in awhile. Even my headphone cable will sometimes insert some hum. Usually, I turn on the transmitter with the 'phones plugged into my monitor receiver, (next to the equipment). If there's any hum that day I'll move stuff around until it goes away, then start my program. It changes around from day to day, which is weird. Of course, I have a cat who thinks he's an engineer and messes with stuff. He's learned how to press the 'cue' button on the CD changer, which is lots of fun. And he knows which adapter on the power strip is the mixer. He had me chasing around the other day before I discovered that's what he'd done. Fortunately my equipment is on a roll-around cabinet and everything is fairly easy to access. Then he gives me that-"Who, me? I'm just a cat". expression. Yeah, right! The two things I hate more than anything are hum and distortion. Of course I'm very sensitive to them. For instance I hate air conditioners, and some goober rolling down the street thinking a rattling trunk lid is a bass speaker drives me crazy. t.
My problem was RF over-loading my audio equipment, but it sounds like yours is something physically blocking the signal. Roofing tiles with some type of metal or other substance might be the culprit, especially if all of the other perimeters are the same.
I'm going to make some changes as soon as I get back on my feet, (broke a bone in my foot). I'm still having some minor RF leakage. My transmitter is still only a few feet from the audio chain, and the feedline goes into the attic just above it. So I'm going to move the transmitter across the room, closer to the antenna. I'm also going to enclose the feed wire in the attic in conduit from where the cable goes into the ceiling to where it exits the building. I might even use wiremold to come down the interior wall. I'm also going to invest in a decent SWR/FS/MOD meter. The problem with that of course is finding one that goes up to 108mc.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~