HEY! Why is 146.94 so noisy?
Happy Monday, everyone! If you are a regular 146.94 user, or of you catch one of the many nets called on my 146.94 repeater, you may have noticed it is a little “noisy, “ or that portables and some mobile have trouble getting in to the repeater. It’s not the fault of the noisy stations. Here’s what’s happening:
The basic function of a repeater is to be able to receive and transmit signals at the same time without degradation. One of the performance functions that repeater operators check is that of receiver desensitization, or “desense” as we call it. “Desense” happens when the repeater transmitter masks the input sensitivity of the repeater receiver. This can be caused either directly or indirecty, but it is usually a direct effect. Most repeaters employ a duplexer to allow the repeater to function properly, i.e., to transmit and receive at the same time and filter the transmit and receive frequencies from interfering with each other. A duplexer usually consists of either four or six tuned cavities, tuned to “pass” the desired frequency, and “notch” (filter out) the frequency(s) that are undesired. It is possible to operate a repeater without a duplexer, but it is very difficult on two meters to keep two signals that are only 600 Khz apart from affecting each other in a duplex situation.
The 146.94 repeater has a six-cavity “pass-reject” duplexer, built by Phelps-Dodge in the 1980’s. At the time of its manufacture, it was most likely the best duplexer made. I have owned four of them, three are still in service (146.94, 147.075, and 145.47). I also have a similar Phelps-Dodge duplexer on 444.3. Recently, I had noticed .94 being a little more “noisy” than usual. On a recent repeater maintenance trip, I decided to check the .94 duplexer. While attempting to “touch up” the duplexer tuning, I discovered that one of the transmit filters had “locked up” and would not tune! I called Kevin, W4KEV, who has the preferred test equipment and knowledge to see if he could help. I took the duplexer to his home, taking .94 off the air for the first time in YEARS. Kevin tried to “free up” the tuning shaft of the affected cavity to no avail. However, he did loan me a Wacom pass/notch cavity to substitute for the bad one, and I returned .94 to service the same night (at 3 AM to be exact). Still, while .94 is on the air, the substitute cavity is not matched to the others, and .94 still has a considerable amount of “desense.” In addition, Kevin found another cavity that did not want to tune.
In the meantime, Kevin has been working on the defective cavity with success! He has replaced the tuning rod after getting the old one removed. He also has the parts necessary to repair the one still in service with the rest of the duplexer, but in order for this to happen, 146.94 will need to go OFF THE AIR. I will be working with the METERS group and other net controls to make sure that this scheduled “downtime” affects as little of .94’s activities as possible. If you or your net is affected by this temporary “off-air” situation, you are more than welcome to use one of my other repeaters until .94 is back on the air. The repair/retuning process may take a couple of days. I hope this works and restores .94 to its original great performance standard! If the duplexer continues to struggle, Kevin and I have agreed that the next step will be to replace it. That is not a cheap proposition. I have been shopping for duplexers that would be suitable for .94’s needs. A suitable replacement would need to be a six-cavity duplexer, due to the fact that .94 is in a very high RF environment. After all, it lives in the same room as WIVK’s 40 kilowatt transmitter! Plus, there are many other two-way and broadcast transmitters at this site. I want to make sure that .94 is not affected by their normal operation. Even with discounts, I am still looking at over $1600 for a new duplexer. Operating repeaters is not cheap, and I recently spent $1700 for a duplexer for 146.73. So, say a prayer that Kevin’s efforts will succeed and 146.94 will return to its pervious high standard of service. I’ll keep everyone apprised of our progress. Thank you for reading, and PLEASE thank Kevin when you talk to him for his efforts!
73 de Tim WB4GBI
Tim Berry WB4GBI (check out my website at www.wb4gbi.com)
Chief Engineer
Citadel Broadcasting of
WIVK-FM/HD1 WNOX-FM WNML AM/FM WNRX-FM WOKI-FM
Chief Engineer
Vol Radio Network
"Do you really want to know what's wrong...or do you just want me to fix it?
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