Monday, January 21, 2013

IRLP/Echolink on the '76 repeater noise problem - "fixed" itself

Users and regular listeners of UARC's '76 repeater on Lake Mountain probably noticed that for a few days last week, the IRLP/Echolink node was unusable:  When the link came up, it would blast noise until it was taken down - either locally, or remotely.

Internet audio from the ground to the '76 repeater atop the mountain is conveyed using a UHF link and it was this link's receiver that was running "open squelch", causing the blasting of noise.  Since this receiver is active only when an IRLP/Echolink connection is brought up, there wasn't a problem otherwise.
Transmit antenna and UHF link antennas
of the 146.76 repeater.  The left-most of the
white antennas is the 146.76 transmit antenna
while the two UHF Yagi antennas are visible
near the bottom of the picture, just
to the left of center.
Click on the image for a larger version.

Analysis revealed that it wasn't really a problem with the UHF link receiver on the mountain, but that there was a signal present on its input.  In listening carefully to this noise, one could make out music and advertisements amongst the noise and by listening to the badly distorted audio, it was determined that there were two sources of audio:  That of FM stations on 107.5 MHz and 107.9 MHz.

As it turns out the 107.5 MHz transmitter is only a few hundred yards south of the repeater site, so it was understandable that audio from that radio station could appear in an intermod mix on the receiver:  This sort of thing can happen if anyone on this site - or another, nearby site - were to install a transmitter (or maybe even a receive system) without also installing the equipment necessary to "safely" operate in an "RF-rich" location such as Lake Mountain.  The sort of equipment necessary on these sites include isolators on the transmitters and bandpass and lowpass filtering on both transmitters and receivers.

But why was 107.9 MHz also in the mix as well when its main transmitter seems to be 10's of miles away?

We aren't sure, but here is our guess:
  • Perhaps the 107.5 MHz transmitter was switched to a different, backup configuration and maybe it didn't have as much filtering between the antenna and transmitter.  It also could have been that something such as a filter or RF connection had "gone bad."
  • With a large antenna system tuned to 107.5, it may have been that it intercepted a significant amount of energy from the more-distant but nearly frequency-adjacent 107.9 MHz signal, mixed within the final of the transmitter 107.5 MHz transmitter, and was re-radiated on many frequencies at a much lower level.
Years ago, a similar (but much more severe) problem was noticed with the 146.62 repeater's receiver on two separate occasions and in each case it was due to a malfunctioning or improperly configured transmitter!  In both of those cases the problem was identified - and corrected - in fairly short order!

In a valiant attempt to do a work-around of this problem, Gordon (K7HFV) and Brett (N7KG) drove to Utah County to gain access to the mountaintop so that they could turn the squelch up slightly, despite the noise problem.  In so-doing, they first attempted access via the south road (Mercer Canyon) but were immediately blocked by a gate so they went to the north side (Israel Canyon) and found a similar gate.

While they were prepared to park there and showshoe to the top, it was now about noon and a bit late in the day to make a start.  At about that time Clint, KA7OEI, did more testing and noticed that the interference problem was now gone and the system was working normally so Gordon and Brett were now off the hook and went home.

What "fixed" the problem, then?

We do not know at the present time, but it's suspected that the operators of the 107.5 MHz transmitter switched to their previous, "normal" configuration and the intermodulation source went away.  It may have also been that they discovered the malfunction and managed to fix it at that time.

While testing was being done on the UHF link receiver from the ground soon after the interference started, it was noted that it seemed to be much more "deaf" than expected - but this wasn't too surprising considering that there were spurious signals with FM radio station modulation on and near the receive frequency.  After the problem was resolved the apparent sensitivity of the UHF link receiver was re-checked and found to be 8-10dB better.

What this tells us is that it was very likely that we weren't the only users on Lake Mountain that were being badly affected by this problem and who knows if anyone else had complained and resulted in an expedited repair?

For now, we'll leave things alone and hold off on making another trip to the mountain until the spring unless some other pressing issue requires a trip sooner than that.  If this problem appears again, we should be able to do a work-around from the ground to restore IRLP/Echolink operation.

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