In Part One I'd described why it might be advantageous to place multiple repeaters of a linked system on the same frequency. In short:
- A single frequency conserves spectrum.
- Being on the same frequency over the system's coverage area is more convenient to the user as it eliminates the need to try to figure out which frequency might works best for a given area.
- The whole system is greater than the sum of its parts because of the probability that brief periods of poor coverage may be augmented by another site.
How, then, does one implement multiple transmitters on the same frequency without their clobbering each other?
This comes again to one of the peculiar aspects of Frequency Modulation (FM) mentioned in the first part of this series: The Capture Effect. Briefly, this is the tendency for the stronger of two FM signals to override the weaker - and if they are of sufficiently different signal strength, there may not even be evidence of the weaker signal.
As it turns out, for a number of reasons this effect is more obvious on wideband FM as used in broadcast and you may have even observed a different FM station to suddenly "pop in" in an area where there was overlap. On the narrowband FM used on amateur radio this effect is somewhat less dramatic and "doubling" (two stations inadvertently transmitting at once) is typically detectable by there being a rather obvious squeal and distorted speech behind the stronger station transmitting or, in cases where the signals are almost exactly of equal strength, neither party wins as the two obliterate each other in an unintelligible mess of noise.
What is worth noting in the above example is that the two transmitters involved are:
- On different frequencies. It's likely that the two transmitters that operated at the same time were on slightly different frequencies - even several hundred Hertz apart. This frequency difference resulted in a heterodyne (squeal) that decreased intelligibility.
- The two transmitters were definitely not carrying the same audio.
Again, the system is laid out thusly:
- Farnsworth Peak is the "hub" and the audio for all transmitters in the system originates from there. The audio to the auxiliary sites (such as Scott's) is conveyed via a UHF link and retransmitted on VHF.
- All audio from all receivers ends up at Farnsworth and the "best" audio is what is transmitted to all sites.
- The auxiliary sites (such as Scott's) are essentially crossband repeaters: 2 meter audio is received and relayed to Farnworth on UHF where it is voted upon and this audio is transmitted from Farnsworth on UHF where it is repeated on VHF at the auxiliary sites.
When we originally designed the system we anticipated that we may need to adjust a few parameters in order to successfully have two transmitters operating on the same frequency without their causing objectionable mutual interference. The first - and most obvious - of these was frequency control.
Because we use independent oven-controlled crystal oscillators, we couldn't nail the frequencies of the transmitters down precisely to match each other as would be possible were we to have used a GPS or Rubidium-based reference, but we could count on their being within 1-2 Hz of where we had parked them. Once the system was put on the air we solicited the help of someone who happened to live in an area where the strength of the two transmitters was precisely equal and then tweaked the frequency offsets and then made a subjective analysis as to what was "least annoying."
As it turned out, there were two ranges that seemed to be reasonable in terms of frequency offset:
- 3-6 Hz offset. This caused a bit of a "whooshing" sound if the two signal strengths were fairly close and fairly weak. If the signals were exactly the same strength then the periodic nulls could cause it to drop out briefly and make the signal unintelligible, but even a slight reposition of the receive antenna could mitigate this, however.
- 40-60 Hz offset. This caused a buzzing somewhat akin to the sound of a subaudible tone as heard on a signal with severe multipath distortion.
Another factor often considered in multiple-transmitter systems is that of audio delay to match the time-of-arrival of the different distances between transmitters - plus additional delay in the audio links used to tie the disparate systems together. Before we were to go through any hassle of adding an audio delay somewhere, we first wanted to see if it was really going to be a problem in the overlap areas, anyway.
The only thing that we did do was observe the audio phase at and below 1 kHz and then, using the ability to select either a 0 or 180 degree audio source, set them as close as we could.
So, what does it sound like in the overlap areas?
First of all, the coverage of the sites and their geography meant that about the only significant overlap areas were in canyons to the east of the Salt Lake area where signals from either transmitter would already be subject to multipath, anyway. As it turns out, traversing these area it's rather difficult to tell where the coverage of one transmitter begins and the other ends - and it often goes both ways. In those area that do have severe overlap the contention between the two transmitters sounds little different than typical mobile flutter - perhaps slightly "faster" than typical 2-meter flutter but not as fast as what might be heard on a 70cm repeater in an area with severe multipath!
In the next part, a bit of "nerdy" technical information about how the various parts work...
This page stolen from ka7oei.blogspot.com