By Peter Policani, K7PP
These questions and many more have been asked since we started simulcasting on the K7PP repeater system.
Well, let's talk about something you have heard. Have you ever heard two stations on two meters doubling with each other. It happens all the time and all that is heard is chaos.
You might think of a time when you were operating CW. You switch to calibrate mode and check the band edges. You zero beat the calibrator. What do you hear when you are zero beat? If you said nothing, you are right. But the fact is, there are two signals occupying the same rf frequency at the same time. When they zero beat, the beat note is zero and no tone is heard.
The same is true in simulcasting. In the K7PP repeater system, we have four transmitters on the same frequency. We zero beat all of the transmitters so that areas in between hear little or no zero beat tone.
This is tough to do, so we use hi stability oscillators that don't drift much. If they do it's only a couple of hertz. The beat note from a couple of hertz is almost inaudible. Most radios don't like to pass audio that's under 300 Hz and as a matter of fact the speakers themselves don't like to reproduce audio at that frequency either.
The one remaining thing is the phase of the audio. If we transmit the same audio from the same source but the transmitters in the system are located different distances from the audio source, the audio arriving at the closest transmitter gets there sooner than the audio from the farthest. If we could just slow the audio down a little or delay it's arrival time at the closest transmitter a little, the audio would then be in phase.
There are several manufacturers that make audio phase delay panels that are programmable. You just crank in the amount of delay you want.
You might ask, how do you know how much delay is needed?
The fact is, that radio signals travel at the speed of light. That works out to be about 5.4 microseconds per statute mile.
Let's say that your two simulcasting transmitters are located with the first one at 20 miles from the audio source and the other at say, five miles from the source, you would have a difference of 15 miles between your first and second simulcast transmitter. If you take 15 miles times 5.4 microseconds, you come up with 81 microseconds. Now all you have to do is delay the audio to your closest transmitter 81 microseconds and then your source audio arrives at the same instant at both simulcast transmitters.
With both signals zero beat and in phase, you have what amounts to one big transmitter.
I know of one paging system in Puget sound that has 47 transmitters on the same frequency.
This means that you could grow your existing system into coverage areas that you didn't have before. Many repeaters have gaps between them and their closest neighbors. Now you can move your coverage area out to your neighbors border where before nothing could be in operation because it was to close to either repeater. Since you won't be interfering with your self, you can pick and choose the right site to allow you to accomplish this.
The other big advantage is that you don't have to run high power or for that matter operate from a tall mountain to get the coverage you and your group want.
If it turns out to be too much, you could contact another group and have them install the equipment in their area while you maintain the original coordination. This way, they could be part of operating a repeater and be on the same frequency. The only concession would be the requirement to concede operational issues to the parent group.
A small price to pay for a two meter pair.
So, what is needed equipment wise??
Remember, I am talking about the transmitter part only. I will write a separate article about the receiver aspects in a follow up.
Let's say that you take the audio from your controller and feed your local transmitter and then send your audio also to your remote simulcast transmitter.
All you need to do is to get a map calibrated in statute miles and plot the distance from your local transmitter and the remote. Take the number of miles and multiply by 5.4 microseconds. Now you know the delay needed to insert between your controller and your local transmitter.
The audio for both will arrive in phase.
Netting transmit signals is a little more difficult. I am using high stab oscillators used with Motorola 800 MHz trunking radios. I have modified them to operate at 147.2 MHz and they are more stable than my service monitor.
Other crystal manufacturers make hi stab oscillators. You just have to ask about them.
I have also found that using any kind of clipping in the IDC audio makes for very raspy quality. I have gone to a non pre emphasized compression system which folds back the audio gain as you get to maximum deviation instead of whacking off the signal at 5 kHz. The clipping method creates unwanted harmonics and distortion, making any difference in the transmitters levels and frequency response a big issue.