By Peter Policani, K7PP
In my last article about simulcasting, I promised another about the method we use to bring signals into the repeater.
Although we have many transmitters in the area, all acting as one, we also need a receiver to accompany them for after all, a repeater is a unit that receives on one frequency and transmits what it hears on another.
All of our transmitters have a receiver that listens for a signal on our input frequency and then transmits what it hears to our controller.
There are many ways to do this and we have taken the most simple and the most inexpensive way. We may upgrade to a better system in the future but for now, here is what we have.
Each site has it's own unique subaudible tone assignment. That means that if you are not running the proper tone for that site, nothing is received and nothing is relayed to the controller.
All the remote receivers use one link frequency except for the main receiver on Buck mountain which has it's own.
If you are in the South part of the State and wish to access the repeater, you would select our South tone, the South receiver would pick you up and relay you to Renton where you would be picked up, sent through the controller and then sent up by link to the simulcast transmitters where your signal would be sent out to all corners of the State on two meters.
The same feature works on all of the other receivers in the system.
The important thing to note is where you are verses the receiver you are trying to talk to.
This is where an amateur can showcase his operating skills and knowledge of the system he is operating.
How many times have we heard people say that they didn't know what tone they were on or for that matter heard me telling someone that they needed to run a tone.
This is the biggest failing of my system design. I had thought for sure that people would be able to put in the main tone for Puget Sound and the South tone for Southwest Washington and so on. We publish these tones for all to have FREE.
We seem to have good luck with folks using the autopatch, so it puzzles me to no end as to why people don't run the proper tone.
We also have the ability to have one tone and vote between the receivers. I have been preparing for this day which is one of the reasons that we have stopped all system expansion.
I am sure that the only way to solve the tone problem is take the choice out of the hands of those operating the repeater and make it an automatic function.
No matter where you go in the repeater coverage area, you run the same tone for access. The repeater will look at the quality of the signals from each site, compare them and then select the best to be repeated.
This comparison is made on the basis of signal to noise. The big disadvantage being the need for separate frequencies for each link receiver and a separate link receiver for each site.
The other problems include the fact that the repeater may select a site with terrible multipath only because of the quieting being superior to another site. There would be no recourse since the system is automatic. Other problems include that it is possible for a link path to fade and become noisy. In this case, the voter would make it's decision based on the over all noise it was seeing on a particular link receiver. Your signal could be full quieting into that receiver but you could be selected into a worse receiver based on signal to noise. The voter adds the noise algebraically.
Some systems use a time domain type of voting which means that signal to noise is not used. The repeater voter is looking at which link signal arrives first. The disadvantage is that a signal arriving first may be very noisy and a signal from a very high site that is full quieting may be discarded only because it didn't arrive first. This is particularly bad when a base station is communicating. It selects the wrong site every time.
I hope this will allow those who are building systems to consider the methods mentioned here and make a better decision on which way to go.
I might also mention in the way of history, that I have run a noise comparison system before. I found myself spending every Saturday running to each site to adjust the balance of the voting because I didn't understand that the link path was changing every day and some times by the hour.
The subaudible system was selected on this basis because it was simple, allowed maximum sensitivity, allowed the operator to select his receiver of choice and used only one link frequency.
During a time when UHF frequencies are becoming a premium, I thought this might be a good choice. I have considered an automatic announcement every five minutes or so of repeater operation explaining the different tones and operating areas but this would leave us open for jamming and interference.