help......any AM tower guru's out there?

[fm-engineer]

I have an AM site with a tall 5/8 wave tower. Around 420' I believe. I just finished a new transmitter install and everything was fine until a thunderstorm approached. The guy wires are arcing across the insulators causing the transmitter to trip off. This obviously did not bother the old tube power rock 5. By the time I got there the transmitter had tripped over 200 times. I could hear and see the guy wires arcing. Each discharge probably runs the tower into a -J value. The first thing I checked was the static drain choke for the tower. No problem there. There was no arcing on the tower ball gap either. All of the static build-up was on the long sections of guy wires. Austin and LBA have static drain resistors for each insulator to take care of this. The problem is they are $700 a piece and there are over 240 insulators. There may be some bad insulators up there, but the ones near the ground look fine. No cracks or dirt, but you can see where static has jumped over two inches. That's a lot of voltage. The antenna engineering handbook says a 50,000 or 100,000 ohm resistor across each insulator can solve the problem. Can I get by with maybe an Ohmite 90 kV 100,000 ohm resistor, or do I need something such as the LBA or Austin resistors? The insulators will also be replaced since the guy wires have to be taken down. The ohmite high voltage resistors are much cheaper than the LBA or Austin specialty resistors. Any suggestions? Thanks

 

[sparks794]

Easrly in my ventures in RF I worked for a combo station. We put in a new solid state transmitter (replacing an old rock of an RCA tube transmitter) and had similar problems. Every time a storm approached the transmitter went nuts.

After several attempts with things we installed drain chokes on the top guy wire of each guy wire run at each johnny ball = problem totally solved. THe solid state did not like the static arc wheras the old tube transmitter didn't care. Until this was done we had to switch to the tube transmitter every time a storm approached. Once it rained and the guy wires got wet the problem was gone.

Hope this helpes. ;D

 

[LA_Guy]

You can use chokes (as mentioned above) or 100K 150 watt carbon power resistors. Both will stop the arcing around the johnny balls.
With the number you have to buy, you can probably have a mount for the resistors made at a local fabrication shop for a fraction of the 700 dollar cost. The resistors will probably cost about 25 dollars apiece at the quantity you need.