Repeater Updates

Repeater Update 12/14/99

As most of you are probably aware, we have been going through some growing pains with the new repeater. The text that follows is kind of long, but it will give you a run down of the problems and solutions up to this point. If anybody has any questions, you can email me at jlabrec1@twcny.rr.com.

Our first major problem was with the exciter of the MASTR 2 radio. When this exciter was designed, provisions were made so that it could operate on the VHF Hi band. This band was further broken down into two sub-bands; one from 138 to 150 MHz and the other from 150 to 174 MHz. GE was rather smart in their implementation of the exciter in that the same exciter could be used for either sub-band by simply changing a small bandpass filter module. We had a 50/50 chance of having the correct bandpass filter installed, and of course, we were wrong. With the wrong bandpass filter, the exciter would generate several moving spurs, as well as the 146.97 MHz signal that we wanted. These spurs would key up the 146.895 repeater as they moved through its pass band, and eventually key up our own repeater. As you can probably guess, having your repeater's exciter generating signals that your repeater's receiver can hear, is not a good thing. Changing the bandpass filter resolved this issue.

Our second major problem was that the receiver of the MASTR 2 radio appeared to be quite deaf and was subject to a lot of interference. By that I mean it's sensitivity was down as compared to the receiver of the old repeater and at times you could hear paging signals on weak signals. If any of you were in any of the fringe areas, you probably noticed the new repeater had a much more difficult time hearing you than the old repeater.

These are really two separate problems, so I'll talk about the deafness first. At first we though that maybe we were getting some desense from our exciter. We tried adjusting the output power levels of the exciter with no obvious effect. At about this same time, I received a call from W2QYT, who maintains the 147.00 repeater near Auburn. (W2QYT worked as a two way radio tech for many years and still has the GE test sets). He offered to tune up our MASTR 2 as long as we delivered it to him. Since tuning the radio seemed like a good first choice, we took the repeater off line and delivered the receiver and preamp to him. He had everything tweaked up that night, we picked it up the following morning and had it reinstalled that afternoon. Now we had a receiver that was not deaf. In fact, with the preamp, it was a rather 'hot' receiver.

Now onto the interference issues. As many of you may be aware, Connecticut Hill, which is where our repeater is located, is a very RF rich environment. On or in close proximity to our tower is Q103.7, State Police Repeaters, County Repeaters, Mobile Comm paging repeater, NYSEG repeaters, 146.895 repeater, ... the list goes on. Trying to get another piece of RF gear to work in this environment is nothing short of impossible. Intermod, the byproduct of having lots of strong RF signals, produces signals on frequencies mathematically related to signals that are already present but on different frequencies.

To try to reduce the chances of intermod from the many RF sources, cavity filters are used to bandpass certain frequencies and notch others. Hopefully the combination of bandpass and notch filters will reduce out-of-band signals to a point where they are not a problem. For the past week or so we have been trying various configurations of these filters to try to reduce the intermod issues. Some configurations worked better than others, but we couldn't find anything that solved all the issues. In fact, last Friday, we tried a new combination of cavity filters which seemed to work OK, until we started hearing the 146.895 repeater through our repeater. This problem was caused when our repeater was up and somebody transmitted a strong signal into the 146.895 machine. The mixing of these various signals produced a signal that was very close to our repeater's input frequency resulting in our repeater keying up and transmitting a distorted copy of the strong signal on the 146.895 input frequency.

Back up to the hill we went to try and diagnose this new problem. While we were up on the hill, I received a call from WA2ZSC, who runs the 146.82 machine near Binghamton. He suggested that we try removing the preamp because in his experience, when there is a lot of RF fields, preamps do not behave very well and can be a source of intermod problems. So, on his recommendation we removed the preamp. We then had a couple of people run high power into the 146.895 machine while we keyed up our repeater. There was no interference. In fact, things sounded really good. The only disappointment was that without the preamp we'd lost some sensitivity. WA2ZSC also suggested that we remove one of the cavity filters on the receiver side. (Although cavity filters help reduce the effect of interfering signals, you pay a price in that they introduce some loss). He thought that if the preamp was the source of our intermod problem, then perhaps we didn't need two cavity filters on the receiver side. So, we removed one of the cavity filters. Things still sounded good.

We buttoned things up and headed down the hill. For the next couple of days, things sounded pretty good. We still appeared to be getting hit by the paging repeater keying up and down but nowhere near to the degree we had been. We then headed back up to the hill on Sunday and installed a notch filter (1/4 wave open ended stub) tuned to 72.74 MHz, which is the offending paging repeaters output frequency. Things seem to have improved even more. The repeater seems relatively quiet except when it is being used and the sensitivity is not too bad.

Over the weekend, I had a chance to work the repeater from Elmira. I was using my DJ-582, running 2.5 watts, and a rubber duck antenna from inside my car in Dick's Sporting Goods parking lot. My signal was a little noisy, but I was being heard. I have also had W2QYT work us from his driveway up near Auburn using an HT, running 1.5 watts with a rubber duck from his driveway. Again, his signal was a little noisy but he was into the machine solidly.

So, at the present time, the repeater seems pretty stable and hopefully we won't be making near as many trips to the repeater site. We are also going to try to take some pictures of the repeater site -- towers, shack and equipment -- and post them here on the web site.

On a side note: We have recently reacquired the receiver from the old repeater. (this receiver did not belong to us and was on loan from some hams in Binghamton). In the next couple of weeks, we'll probably be doing some experiments with this receiver to determine whether we would be better served by purchasing a new receiver of this type and replacing the current receiver or just keeping things the way they are.

Jim, KC1RM

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