FAQ - So *you* are thinking about a synchronous repeater

"So, you want to put up a synchronous repeater system?"
or
Is a synchronous repeater right for you?

The Utah Amateur Radio Club (UARC) has embarked on a major repeater project: A synchronous/voting repeater system. More specific information on this repeater system may be found at http://www.utaharc.org/rptr/synrpt1.html and its links. I strongly suggest you read these pages to get a general idea of what this system is, and what it will do, as well as to get a handle on the approaches we are taking to accomplish our goals.

I have received a number of emails with questions and comments about the system which have prompted this page. At the risk of repeating myself, I'll attempt to answer some of the common questions.

"What is a synchronous repeater system?" A synchronous repeater is a linked system of several repeaters. Unlike a conventional linked system, all transmitters are on the same frequency.

"What is a voting receiver system?" A voting receiver system is where there are several receivers with different antennas at the same site (or different sites.) The advantage of a voting receiver system is that is it much more likely that at least one of the receivers will be able to receive the input signal. Having a voting system can go a long ways toward reducing multipath, improving repeater's receive coverage in fringe areas and/or to handie-talkies.

"Why go through the trouble/expense of putting up a synchronous/voting repeater?" Linked repeater systems are quite common these days. These systems offer the users wider coverage than a single-site repeater. However, when you go from the coverage area of one repeater into another, you have to switch to another repeater. This can be tedious and, as is often in the case in fringe areas, there are zones where you have to constantly switch among the various repeaters in the system in order to find the repeater that is good at that particular instant. Having a synchronous/voting system will allow this "site handoff" to become transparent, eliminating the need to constantly switch between sites.
An additional benefit is the ability to re-use frequencies. Instead of every repeater in a linked system occupying its very own pair, it is possible for every repeater in the link to be on the same frequency. The ability to do this would, of course, depend on the availability of frequencies in the areas to be covered and might require "realignment" of some other repeaters in the area.

"I have several repeaters in a (large/small) area that are linked. How far apart do they need to be from each other?" The decision of if/how to implement a system of synchronous transmitters takes a bit of study. The most ideal situation for synchronous repeaters is where the transmitter sites have only fairly small areas of overlap, such as that which would occur in a mountainous region (such as the case of the Farnsworth/Scott's Hill system in Utah.) Having very large overlaps would not only be a waste of resources, but it would allow for the inevitable degradation to an otherwise perfectly good signal in that overlap area: No matter what scheme of transmitter synchronization you use, the result in the overlap areas is always somewhat of a comprimise.
One of the primary considerations is to make sure the effect of the signals in the overlap areas causes less degradation than having having no overlapping coverage in the first place. If you were to allow a very large overlap area, you might end up with worse signals in that overlap area than if there was just one transmitter.

It is important that one doesn't go overboard with trying to put together such a system and cause more problems than are solved: You might find that with a synchronous system intact that one or more sites are no longer needed and it would be prudent to not include those sites in the system.

"What can be done to limit/reduce the amount of overlap between sites?" One of the most obvious ways to minimize unnecessary overlap is by careful site selection. Having too much overlap can/may result in actual degradation in those areas. Another way is to carefully select transmit power.
The coverage areas may be "sculpted" somewhat by the use of directional antennas. The possiblities include:

Yagi

Side-mounted verticals (1/4 wave spacing from, say, a tower, to create a cardiod null)

Corner/Trough reflectors

Phased verticals

Since the system is likely to employ a voting receiver scheme as well, one should consider seperate transmit/receive antennas: The directionality of the antenna will affect transmit as well as receive, and multi-element antennas such as yagis may be subject to creating their own intermod products in one or more of their many electrical/mechanical joints.

"Where can I buy a synchronous repeater system?" I know of no off-the-shelf solutions that are directed at the amateur market. If you have deep enough pockets, you can buy commercial equipment that can be configured for amateur service.

"Can existing equipment be modified to operate in synchronous service?" One of the most important parameters to control is transmit center frequency. If you are using crystal-controlled transmitters, it will be necessary to frequency lock them together. If synthesized transmitters are used, then the reference oscillators should all be clocked from the same source.
Almost as important is that the audio characteristics of the transmitters match. The most importat of these is that the modulation sense is the same for all transmitters. That is, all transmitter swing up and down in frequency in unison. The frequency/phase response should also be as uniform as possible everywhere in the system.

This implies that all radios should be identical throughout the system. This not only includes the repeater transmitters themselves, but the link transmitters and receivers that tie all sites together. This may even mean that at each site, you have a a receiver listening to the link transmitter in order to obtain transmit audio so that audio that is local to the repeater is affected in precisely the same way as the audio received from other sites.

"I have heard that you need to put an audio delay in the system. Is this true?" If there are large areas of overlap that are, for the most part, of consistant time-of-flight difference between the two sites, then it might be a good idea to consider adding an audio delay to make sure the modulations from each site occur simultaneously in that area. Remember: it takes 1 millisecond for the signals to go 186 miles and 1 millisecond is the period of 1 KHz, and only a few 10's of miles difference could put the delays well into the audio range and cause significant distortion. Don't forget that the audio/signal processing in the system will add a measurable amount of delay.
Since our overlap areas occur in areas with different timing differentials, we plan to evaluate performance in those areas before deciding if a delay is necessary.

"How is UARC putting their system together?" The UARC system is being put together from the ground-up.
Well, mostly... The 33cm link radios are retired STL (Studio Transmitter Link) units from the broadcast industry that have been modified to operate in the ham band and various other components like Duplexers, power amplifiers, etc. from the existing equipment will be used.
But the actual 2 meter transmit and receive systems will be built from the component level.

We are taking a somewhat extreme approach in implementing this system. In order to assure that all receivers and transmitters match each other as much as possible, the actual receive and transmit audio for the entire system appears only in one place: At the Farnsworth Peak transmitter site.

Although you can read about it in detail here, I'll review the operation of the system in brief.

The receive signal is converted down to 10.7 MHz, where it is filtered and limited. It is then converted down to 40 KHz, yielding a replica of the received signal at that frequency.

The 40 KHz received signal is modulated onto the baseband of the 33cm microwave link (in the case of Scott's Hill) or it is sent via twisted pair from the receive site to the transmit site (in the case of Farnsworth Peak, as it is a split site.)

At Farnsworth Peak, the 40 KHz signals from each site are upconverted to 10.7 MHz, filtered, and the audio from each site is recovered by identical demodulators.

Each demodulator output is monitored to provide squelch information and data about the signal quality so that the receiver with the best signal will be selected for retransmission.

The received/processed audio is modulated onto a 40 KHz carrier. In the case of Scott's Hill this signal is sent via 33cm microwave and in the case of Farnsworth, it is local, but in both cases, this signal is upconverted first to 10.7 MHz for filtering, and then to the transmit frequency.

The 33cm microwave link also contains a reference subcarrier to allow all oscillators to be locked to the same source.
The advantage of this scheme is that there is only one modulator in the system for all transmitters. This helps assure that the qualities of the signal from each transmitter as as close to being identical as possible. The use of this scheme for receive signals also helps convey squelch information and preserves the audio consistancy among the sites for seamless voting receiver operation. It does involve a lot of construction of equipment, though.

"Is there any other place on the web where I can find more information about synchronous transmitter systems?" I have looked for other places, but have found very few. One of these is a paper called Synchronous FM Booster: A General Overview by TFT Inc., a manufacturer of FM broadcast exciters and other related products. (This information is for reference only: I have no connections whatsoever with this company or their products.) This article explains some of the reasons for having synchronous transmitters and some of the challenges involved in multiple transmitters on the same frequency in potentially overlapping areas.

If you are interested in the more technical aspects of this system, go to the Technical Description of the Proposed UARC 146.620 Synchronous Repeater page. You may also be interested in the Predicted Coverage Area of the system. Maybe you or your club is interested in putting up a synchronous repeater? Additional comments/questions are welcome.

Any questions or comments? Send me some email!

Go to the Repeaters of the Utah Amateur Radio Club page.

"What is a synchronous repeater system?"	A synchronous repeater is a linked system of several repeaters. Unlike a conventional linked system, all transmitters are on the same frequency.
"What is a voting receiver system?"	A voting receiver system is where there are several receivers with different antennas at the same site (or different sites.) The advantage of a voting receiver system is that is it much more likely that at least one of the receivers will be able to receive the input signal. Having a voting system can go a long ways toward reducing multipath, improving repeater's receive coverage in fringe areas and/or to handie-talkies.
*"Why go through the trouble/expense of putting up a synchronous/voting repeater?"*	Linked repeater systems are quite common these days. These systems offer the users wider coverage than a single-site repeater. However, when you go from the coverage area of one repeater into another, you have to switch to another repeater. This can be tedious and, as is often in the case in fringe areas, there are zones where you have to constantly switch among the various repeaters in the system in order to find the repeater that is good at that particular instant. Having a synchronous/voting system will allow this "site handoff" to become transparent, eliminating the need to constantly switch between sites. An additional benefit is the ability to re-use frequencies. Instead of every repeater in a linked system occupying its very own pair, it is possible for every repeater in the link to be on the same frequency. The ability to do this would, of course, depend on the availability of frequencies in the areas to be covered and might require "realignment" of some other repeaters in the area.
*"I have several repeaters in a (large/small) area that are linked. How far apart do they need to be from each other?"*	The decision of if/how to implement a system of synchronous transmitters takes a bit of study. The most ideal situation for synchronous repeaters is where the transmitter sites have only fairly small areas of overlap, such as that which would occur in a mountainous region (such as the case of the Farnsworth/Scott's Hill system in Utah.) Having very large overlaps would not only be a waste of resources, but it would allow for the inevitable degradation to an otherwise perfectly good signal in that overlap area: No matter what scheme of transmitter synchronization you use, the result in the overlap areas is always somewhat of a comprimise. One of the primary considerations is to make sure the effect of the signals in the overlap areas causes less degradation than having having no overlapping coverage in the first place. If you were to allow a very large overlap area, you might end up with worse signals in that overlap area than if there was just one transmitter. It is important that one doesn't go overboard with trying to put together such a system and cause more problems than are solved: You might find that with a synchronous system intact that one or more sites are no longer needed and it would be prudent to not include those sites in the system.
*"What can be done to limit/reduce the amount of overlap between sites?"*	One of the most obvious ways to minimize unnecessary overlap is by careful site selection. Having too much overlap can/may result in actual degradation in those areas. Another way is to carefully select transmit power. The coverage areas may be "sculpted" somewhat by the use of directional antennas. The possiblities include: Yagi Side-mounted verticals (1/4 wave spacing from, say, a tower, to create a cardiod null) Corner/Trough reflectors Phased verticals Since the system is likely to employ a voting receiver scheme as well, one should consider seperate transmit/receive antennas: The directionality of the antenna will affect transmit as well as receive, and multi-element antennas such as yagis may be subject to creating their own intermod products in one or more of their many electrical/mechanical joints.
*"Where can I buy a synchronous repeater system?"*	I know of no off-the-shelf solutions that are directed at the amateur market. If you have deep enough pockets, you can buy commercial equipment that can be configured for amateur service.
*"Can existing equipment be modified to operate in synchronous service?"*	One of the most important parameters to control is transmit center frequency. If you are using crystal-controlled transmitters, it will be necessary to frequency lock them together. If synthesized transmitters are used, then the reference oscillators should all be clocked from the same source. Almost as important is that the audio characteristics of the transmitters match. The most importat of these is that the modulation sense is the same for all transmitters. That is, all transmitter swing up and down in frequency in unison. The frequency/phase response should also be as uniform as possible everywhere in the system. This implies that all radios should be identical throughout the system. This not only includes the repeater transmitters themselves, but the link transmitters and receivers that tie all sites together. This may even mean that at each site, you have a a receiver listening to the link transmitter in order to obtain transmit audio so that audio that is local to the repeater is affected in precisely the same way as the audio received from other sites.
*"I have heard that you need to put an audio delay in the system. Is this true?"*	If there are large areas of overlap that are, for the most part, of consistant time-of-flight difference between the two sites, then it might be a good idea to consider adding an audio delay to make sure the modulations from each site occur simultaneously in that area. Remember: it takes 1 millisecond for the signals to go 186 miles and 1 millisecond is the period of 1 KHz, and only a few 10's of miles difference could put the delays well into the audio range and cause significant distortion. Don't forget that the audio/signal processing in the system will add a measurable amount of delay. Since our overlap areas occur in areas with different timing differentials, we plan to evaluate performance in those areas before deciding if a delay is necessary.
*"How is UARC putting their system together?"*	The UARC system is being put together from the ground-up. Well, mostly... The 33cm link radios are retired STL (Studio Transmitter Link) units from the broadcast industry that have been modified to operate in the ham band and various other components like Duplexers, power amplifiers, etc. from the existing equipment will be used. But the actual 2 meter transmit and receive systems will be built from the component level. We are taking a somewhat extreme approach in implementing this system. In order to assure that all receivers and transmitters match each other as much as possible, the actual receive and transmit audio for the entire system appears only in one place: At the Farnsworth Peak transmitter site. Although you can read about it in detail here, I'll review the operation of the system in brief. The receive signal is converted down to 10.7 MHz, where it is filtered and limited. It is then converted down to 40 KHz, yielding a replica of the received signal at that frequency. The 40 KHz received signal is modulated onto the baseband of the 33cm microwave link (in the case of Scott's Hill) or it is sent via twisted pair from the receive site to the transmit site (in the case of Farnsworth Peak, as it is a split site.) At Farnsworth Peak, the 40 KHz signals from each site are upconverted to 10.7 MHz, filtered, and the audio from each site is recovered by identical demodulators. Each demodulator output is monitored to provide squelch information and data about the signal quality so that the receiver with the best signal will be selected for retransmission. The received/processed audio is modulated onto a 40 KHz carrier. In the case of Scott's Hill this signal is sent via 33cm microwave and in the case of Farnsworth, it is local, but in both cases, this signal is upconverted first to 10.7 MHz for filtering, and then to the transmit frequency. The 33cm microwave link also contains a reference subcarrier to allow all oscillators to be locked to the same source. The advantage of this scheme is that there is only one modulator in the system for all transmitters. This helps assure that the qualities of the signal from each transmitter as as close to being identical as possible. The use of this scheme for receive signals also helps convey squelch information and preserves the audio consistancy among the sites for seamless voting receiver operation. It does involve a lot of construction of equipment, though.
*"Is there any other place on the web where I can find more information about synchronous transmitter systems?"*	I have looked for other places, but have found very few. One of these is a paper called Synchronous FM Booster: A General Overview by TFT Inc., a manufacturer of FM broadcast exciters and other related products. (This information is for reference only: I have no connections whatsoever with this company or their products.) This article explains some of the reasons for having synchronous transmitters and some of the challenges involved in multiple transmitters on the same frequency in potentially overlapping areas.