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.