Monday, July 2, 2012

Two repeaters, one frequency (part 1)

These days, finding a frequency to expand ones repeater system can be a challenge - even in "rural" parts of the country such as Utah where the Salt Lake area is about the only large population center for hundreds of miles.
Figure 1:
The Scott's Hill site, part of the UARC 146.620 system
Click on the image for a larger version.

Typically, a linked repeater system consists of several repeaters tied together on a backbone frequency and each of these individual repeaters is usually on its very own frequency:  About the only time that frequency re-use is implemented is if several of these individual repeaters are located far enough apart that they won't bother each other and it is often the case that different subaudible tones are used to prevent mutual interference should a user be in an area with potential overlap.

More than a decade ago the Utah Amateur Radio Club decided to expand the coverage of its 146.620 repeater and a mountaintop site was secured - a story in and of itself to be told another day, perhaps.  As things often happen the project lay fallow for several years until a set of circumstances provided the ambition and impetus to push it along farther.

From the beginning, the intent was to have a "Synchronous" and "Voting" repeater on this other site, Scott's Hill, that was to share the same frequency as the original repeater on Farnsworth Peak, but putting together such a system was understandably more involved than the typical linked (but each site using a different frequency) repeater system.

The original repeater on Farnsworth Peak provides impressive coverage, from north of the Utah/Idaho border, west beyond the Utah/Nevada border, to the south into parts of central Utah but pretty much stopping at the Wasatch range to the east of the Salt Lake metro area.  For the most part, the coverage of Salt Lake area repeaters is limited eastward by the abrupt rise of an 11,000 foot mountain range along the east side of the populated areas and unless a repeater is located atop those mountains, coverage to the east is minimal.  Unfortunately - or fortunately - repeaters located in the Wasatch intended to provide coverage to the high valley areas east of the Salt Lake Valley tend not to provide good coverage into the Salt Lake valley itself owing to the shielding effects of the mountains themselves - that is, the taller peaks on which repeaters are placed are generally set back a bit and the somewhat lower "front" peaks to their west tend to block the view of the valley.

Scott's Hill is such a site:  It sees well from the East through the Northwest but it can actually see none of the Salt Lake valley to the south and west.  It does, however, have a good, line-of-sight view of Farnworth Peak, so the linking between the two sites is pretty easy.  This general exclusivity of coverage also means that having the two repeaters effectively sharing the same frequency would be simplified as there were relatively few places where the two would overlap with comparable signal levels.

Figure 2:
Voting controller for the 146.620 system.
Click on the image for a larger version.
Now, how does one go about putting two repeaters on the air, on the same frequency, without their clobbering each other?

Multiple receivers on the same frequency:

For receive, the answer is pretty easy:  Voting receivers.

On a "Voting" system, one typically brings the audio from all of the separate receivers to one central location and there, they are all analyzed for signal quality and the best of the lot is selected and used as the audio source for the entire system.

Compared to the typical linked system where the user selects which repeater/frequency is to be used, there are advantages to having ONE frequency with multiple (voting) receivers:
  • Easier to use.  If there is only ONE frequency, the users don't have to constantly change to the best frequency for the area from which they are transmitting - assuming that they know which is the best for their specific location!
  • Frequency re-use.  With a voting system, only ONE frequency is required which can save a bit of spectrum.
  • The whole is greater than sum of the parts.  On a multi-receiver system, it's typical that while one particular receiver works best for a specific area, it's also likely that the less-optimal receivers will also provide a degree of coverage in that same area.  If one enters an area where coverage is a bit "spotty" on the primary-coverage receiver, there's a reasonable chance that one of the other receivers may be able to still hear the mobile and "fill in" - all of this without the user having to worry about it!
  • The addition of even more receivers.  Once the "base" voting system is installed, it's practical to install additional "fill" receivers for those areas where better coverage might be desired:  These extra receivers need only be a simple receiver and link transmitter rather than a full-blown repeater requiring a lot of expensive filters.
While there are a number of ways that voting systems can work, pretty much all of them exploiting a "feature" of the frequency modulation (FM) that we use on our VHF and UHF bands:  Quieting.

You have probably noticed that as an FM signal sounds the same whether it is very strong or weak - at least until the signal gets to be really weak - at which point it starts to sound noisy but NOT quieter!  If one were to listen carefully, it might also be observed that the noise tends to start out at the higher frequencies first - and this is how a radio's squelch works:  It listens for the high-pitched hiss that starts to show up as the signal gets weak.

Most voters listen for this "hiss."  On a typical system, since all of the receivers are listening to the same audio being transmitted, the one with the least amount of hiss is, in fact, the one receiving the best signal.  If you think about it, all one really needs to determine is which one has the least amount of "audio plus hiss" as the only thing that will be different among the receivers with different-quality signals will be the amount of hiss on them.

Ideally, one would do this comparison at the receiver itself where one has access to the "guts" of the receiver and can look at the "discriminator audio" where the spectral content can go into the 10's of kHz.  Practically speaking, however, we have to link these individual receivers back to one site for the voting and conventional FM link radios can't pass the 10's of kHz of audio necessary to do this so the "audio plus hiss" scheme is used.  The voter on the 146.620 system works this way, mostly looking at the higher-frequency audio (e.g. above 2.5-3 kHz) to determine which of the inputs has the "best" signal (e.g. least "audio plus hiss.")

There are other ways to do this - including digital means where precise signal quality measurements are telemetered to the main controller - but our intent was to construct the entire system using "off the shelf" radio modules that were available on the surplus market so that there would be a reasonable hope of it being maintained in the future.

This article - including more details on two transmitters sharing the same frequency - continues in Part Two.


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