Hey guys. I'm running a broadcast warehouse 150w amp, with a 1 watt mono exciter, with a jpole connected with 50ft of lmr400.
I've run this setup before without much issue, but recently after trying to get all the stuff enclosed in a case I'm getting some weird issues. The amp runs on 28v, but the amp is only pulling half that, and outputting only half the wattage. Also, it is splattering some across adjacent channels. I noticed that changing the length on the power leads from the power supply to the box made some weird interference noises go away. I've got the transmitter and amp in separate boxes, and I've tried a new power supply.
Also, another symptom is when its doing this, running at half power, if i put a multimeter on it to read the voltage, the fan on the amp kicks back into full speed and and the output power jumps up to full power. I'm guessing because it's putting a load on the circuit?
I've never had these kind of issues before, any suggestions?
Amp and exciter.... never put in one case if you can help it
Also make sure it's well grounded and loop the power leads through a ferrite toroid a few times like you find filtering the leads in a CRT monitor or TV (you can scavenge those too).
I find the best source for power filtering toroids are found on lots of computer cables. Most monitor cables, some computer mice and old printer cables have them. They look like a tubular bulge in the cable. Takes a while to cut them out and get the outside plastic off but it's worth it. I had to do the same exact thing with my FM transmitter as RF was getting into the power wires. Also had to add them to my stereo system because the high RF powers were getting into the stereos amplifier causing loud buzzing in the speakers. If that doesn't work you may need to add metal shielding between the RF amp and the exciter boards that's well grounded.
Using a multimeter should make no difference at all. The input impedance is too high. Every symptom you've described points to a bad connection somewhere. Double check your cables. It sounds like you have either an intermittent short or open line somewhere. You're dealing with a lot of power. Use ohms law-, 150 watts into 50 ohm impedance,etc. Makes for a lot of amps. Also, you have RF everywhere. I agree about the grounding and shielding. Keep us posted.
Grounding made a bit of a difference. Drove a copper pipe into the ground and soldered some romex to it and ran it to most of the equipment.
Still had issues tho, tracked it to the mixer we were using. It couldn't handle the direct RF radiation. If we ran the mixer with just 10w, the problem went away, and we can run ~100w without the mixer hooked up.
Be careful with grounding. You should only ground one point, usually the last piece in the chain. Sometimes just the mixer needs grounding. I've worked as a 2nd class commercial engineer, (which isn't licensed any more, it's just 'General' so that a bunch of goobers with clear channel can get away with over-modulation), so I've dealt with RF problems on a large scale. Sorry about that rant- Anyway, I had a feeling that spurious RF might be part of your problem. Try to find a better way of bonding to the ground rod than soldering. Even a clamp with a screw would be better. Solder has a habit of deteriorating due to electrolysis caused by the elements and over time can actually cause more problems than it solves. Keep us posted.
RF is a funny thing. I recently had a problem just by moving my antenna. I put it in the attic near my equipment. Everything went wrong. My modulation meters pegged, the sound was noisy and distorted, and the power was about half. I moved the antenna back outside where it was before, and it works fine again. Sometimes it takes a lot of experimentation to solve the problem. It could be that RF is what's causing your symptoms.
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