Two repeaters, one frequency (part 2)

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.

 In the first part only the implementation of the receive portion of the system was discussed in which multiple receivers were used in a voting scheme - that is, a system in which the signals from the various receivers in the system were analyzed and the best one at that instant was sent on all transmitter.

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:

Figure 1:
Inside the frequency control/crossband repeater unit at Scott's.  There is
an identical unit at the other site at Farnsworth Peak.
The 10 MHz oven-controlled oscillator is in the upper-left corner
while the standard GE "EC" channel element is in the upper-
right corner.  This unit - like its twin - is hand-wired on glass-
epoxy prototyping board.
Click on the image for a larger version.

• 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.

As it turns out if there are two transmitters that are both held to very tight frequency standards (within a few 10's of Hz at most) and they carry exactly the same audio, they tend not to clobber each other to nearly the same degree if they are of similar signal strength. What's more is that these "similar" transmissions seem to bother each other less as the difference in their respective signal strengths become greater.

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.

What this means is that at Scott's, there's a box called the "Disciplined Oscillator" that contains a precision, oven-controlled 10 MHz oscillator that is capable of holding the VHF transmit frequency to within 1-2 Hz of where it is intended to be.  As it so-happens, this same box also contains the intelligence to function as a controller for a pair of crossband repeaters that goes from VHF to UHF for signals that are received and then again from UHF to VHF as the master audio from Farnsworth is transmitted.  This box also provides a few other basic functions such as timeout timer (in the event a link gets "stuck") as well as providing a Morse ID on the UHF link from Scott's to Farnsworth - just to keep things legal.  This same box also has an RS-485 serial interface to allow it to be connected on a bus with other devices so that it may be remotely controlled, configured and polled as needed.

 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.

Ultimately, we settled for the 3-6 Hz offset as it was deemed to be the most "user friendly" overall - especially when one considered that one was by far more likely to be traveling mobile through the overlap areas than stationary and that the Doppler shift of a moving vehicle might not only exceed the amount of frequency offset anyway (if it was only 3-6 Hz, at least) but that the "dwell" time in a precise null where the signals of multiple transmitters canceled each other out was going to be extremely short.

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.

It wasn't.

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 Part 3, a bit of "nerdy" technical information about how the various parts work...

[End]

This page stolen from ka7oei.blogspot.com
 

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.

[End]

This page stolen from ka7oei.blogspot.com