Batboard

I realize this is a long thread, but the problem was finally solved. Of course, he had to build a new house and erect a 55 ft tower first.

Turns out that the RF from the antenna was just too close to the equipment in the shack. It didn't matter if it was LMR400, 9913, or 1/2 inch Heliax. Once he got the antenna about 50 ft above the shack, his desense went completely away.

I know that some VHF installations have the antenna on the roof maybe 20 ft above the equipment, but in this case, I'm sure his horizontal distance was also 20 ft, so he didn't benefit from being directly underneath the antenna; until he moved!

Bob M.

Hello Bob,

Well as I read through this post about the Receiver Desense problem, it jogged my memory back to 1978~1979 time frame while I'll was working in one of /\/\'s R & D Labs. We were building special VHF & UHF repeaters for customers. I was working with a senior engineering technician and we began to notice a Desense problem on every unit we built up. Fortunately we had time to start evaluating this problem before it got out of hand.

Like all of trial and error solutions ... being duplexers, circulators, double shielded coaxial cables, extra mechanical shielding around the TX & RX assemblies, FM transmitter sideband noise, spurious emissions and varying the RF Power Output, nothing seemed to help. Back in those days 99% of /\/\'s products were channels elements so we thought there may be a strange I.F. mix happening and ordered up some special channel elements. Even with these special channel elements, the Desense problem existed! I think this is why I have so many gray hairs and occassionly I walk around talking to myself.

My friend and I had now spent 25 days chasing this problem trying to resolve it. We both sat there one day and said, "We've changed every thing on these units including the main PC boards and the problem still exist! This problem must be waving a big red flag at us!" One our peers had walked by asking us how we were progressing on the problem and my buddy replied, "We aren't any closer to solving it than 3 weeks ago." The other guy says, "You know I began to think about these receivers because 7 months ago I had a strange problem I was chasing and it turned out to be the receiver's injection level."

Talk about two guys with light bulbs above their heads that went to blinding brilliance, lighting up the entire Schaumburg facility ... Whoa! We fired everything back up and experimented with the receiver's injection output level bringing the Desense Level down 1~2 dB ... very useable for the repeater function! It sure pays to get a third unbiased opinion. It seems that the receiver's injection level adjustment had been peaked in the wrong position providing too much injection output which caused the Desense to appear.

I can remember that the MSF5000 has an injection filter assembly with 3 tuning slugs and perhaps these tuning slugs can be adjusted for best sensitivity and a minimum Desense level.

Dan

Yes, the MSF5000 does have a rather high injection level. The injection amp puts out +29dBm. But in this case, after the MSF5000 had desense, he switched to an MTR2000 and had just as much desense. I would hope the technology had improved on the MTR, but one never knows.

Initially his antenna was on the roof of a ranch-style house, and not very far away horizontally. But the equipment definitely was not directly under the antenna, where the most attenuation would occur. Of course, everything worked perfectly on a dummy load.

After the new house and tower went up, his problem disappeared. Same antenna, same feedline, same MTR2000 repeater, same duplexer, etc. Just more height and probably less horizontal distance.

Live and learn.

Bob M.