My thanks too to everyone involved.
Michael 805.886.8887
W7HUT
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On Aug 17, 2008, at 18:42, Bill Talanian w1uuq@cox.net wrote:
8/17/2008
To: SBARC List Server From: Bill Talanian, SBARC Telecommunications Services Subject: Interference on 146.79 system
Since last April the LaVigia Hill 146.790 system has suffered from a very low level inter-mod signal on the input frequency of 146.190 which created sporadic and intolerable interference. We have spent hundreds of man hours in determining that our system was free of any orders of signal mixing and spurious radiation. In addition, we went through a number of corrective methods to eliminate any inter-mod source such as antenna, corrosion, tower bolts, fences, rusty hardware, and very near field antennas. Through some very unique testing methods the test team was able to determine the offender signal was an extremely low level signal barely above the noise level. This occurred only when a signal appeared on 146.790. In repeat mode this would reverberate and trash itself regardless of PL. A unique DF arrangement indicated the signal was emanating from the on-site cellular tower which is about 50' away. The dilemma was how we could provide with 100% repeatable assurance to the cellular engineers that something in their system was related to our problem.
Meanwhile to reduce the interference problem we transferred the 146.79 system to an alternate and lower antenna which did not excite the source of interference. This antenna did not have the coverage of the main antenna but was adequate.
This past Monday while on site I ran into a contractor who was doing work for the cellular company. In our technical discussion I concluded, "well let me show you how it all happens." Much to my surprise the offender signal did not appear. So what changed? Without knowing I left everything connected to our original antenna and for the past several days asked for reports of interference. None came in. For 5 months we had interference and now it was gone.
Andy Seibold, W6AMS, made contact with engineering personnel at the cellular company and reports the following:
"I determined that they had, in the past few weeks, re-programmed their site on the 850 MHz GSM side and changed channels. GSM channels are 30 KHz wide and some are used for voice and some for data (GPRS and EDGE), they could not tell me if the changes were permanent but they claim that nothing else was done remotely to the tower and they do not believe that anything else has changed."
While not conclusive we shall now wait and see what happens. Cellular tower frequencies are way beyond our 2-meter communications spectrum, however, it should not be automatically concluded that interference of an inordinate order is not possible. Indeed we must look beyond harmonically related problems and normal frequency mixes for offenders. These may be just as problematic and much more difficult to find when it involves a cell site.
Our THANKS go to the Super Tech Team of Calli (KD6OVS), Bruce (N6OLT), Andy (W6AMS) with side assists from Ken (N6KTH), Mike (N6QD), and Bruce (KF6RAI) and myself.
Lastly our thanks go to the great folks at Telewave, Inc. (San Jose) who graciously exchanged our new collinear antenna at no cost for another factory new antenna to help eliminate one potential source.
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