good news for 146.73!

Good News for 146.73!!

 

For those of you who remember 146.73 as Sam (WB4HAP, SK) and Matt (WB4IOB) Kirby’s repeater, and how well it worked from it’s English Mountain home, you know that I have not been able to duplicate their great success since I returned .73 back to English almost two years ago. It’s been a “work in progress,” and I’m happy to report that progress is being made!

 

If you have tried .73 lately, you have noticed that it has had a “squeal” in the audio on occasions when it has been keyed up, or when it has been up with a QSO in progress. The squeal is a result of an “intermod hit,” or two or more (in this case three) signals that mix together and produce a different signal that is the sum or difference of the others. Intermodulation, or “Intermod,” has long been a nemesis of Mountaintop repeater operations, but sometimes it is misdiagnosed, or confused with other problems. When Sam had 146.73 on English for all of those years, he never had to deal with this problem- probably because the offending transmitters in question were not on the mountain or on the air. In this case, while Todd (KA4OAK) and I were on a recent trip to English Mountain, we actually observed the problem in person for the first time. We were changing the LMR 400 cable on .73 for ½ inch heliax, which by the way eliminated all of the “desense” that the repeater had and really perked up the coverage. It allowed me to take the repeater back to full power, and re-connect the receiver preamp for weak signals. I learned the hard way to never use LMR400 for duplex repeater service. But the “squeal” continued. While we were there, we found one of the commercial UHF trunking transmitters in the building was “keyed up” every time that .73 had the problem squeal. The noise was consistent with this UHF transmitter being active.

 

“So,” you think- “it’s that fault of that commercial transmitter!!” Well, not so fast. The offending signal is actually produced when two UHF transmitters (there is another one involved) and the 146.73 transmitter are all “up” at the same time. How did I determine this? There several software programs out there in shareware and software land that will help you calculate all of the possibilities. I tabulated the frequency of EVERY transmitter on the tower and loaded them into a database. The software that I used showed a “fifth-order” intermodulation product caused by  three transmitters that would cause a signal to be generated on 146.13 Mhz. Recognize that frequency? It’s the INPUT to 146.73. The two UHF transmitters are on a combined antenna system, which means that they already have an isolator in place to keep one transmitter’s RF from going back into  the other transmitter(s)’ RF output section. However, 146.73 did not have one of these devices. RF from the outside world could make it back into the .73 PA, and cause the “mix” to happen. So, armed with an intermodulation report, and a clean bill of health from the owner of the UHF transmitters (who gladly checked them just to make sure), I ordered an isolator tuned for 146.73. Isolators are not cheap. They are precision-tuned devices that allow RF on the tuned frequency to pass in one direction, but nothing to pass back into the PA on any other frequency. Mine was almost $500. Now you know why repeaters and their upkeep are not cheap J. Most commercial tower sites now require the use of an isolator.

 

In just a few short days, we will find out if the RF “mix” was occurring in the .73 transmitter, or if I will have to spend more time and effort troubleshooting this perplexing problem. I’m betting that I have found it, and .73 will make another huge step in becoming the mountaintop high profile machine that it once was and deserves to be again. Stay tuned here on the blog…or better yet, come check 146.73 for yourself!

 

73 de Tim Berry WB4GBI