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Multiple repeaters

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G8CIX View Drop Down
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    Posted: 12/January/2016 at 23:11
Wondering what the implications are for setting up multiple repeaters on the same frequency.?

The application is to provide a repeater using a PMR allocated Ofcom area defined licence. Having purchased a square I want to improve coverage by using three repeaters. Could separate them by different CTCSS codes but wondering what the effect will be if all are the same. I guess the capture effect of FM will provided what one might call transmission diversity...
 
Any thoughts?

Martin
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Post Options Post Options   Thanks (0) Thanks(0)   Quote G1HUL Quote  Post ReplyReply Direct Link To This Post Posted: 13/January/2016 at 06:54
Leicestershire Police (and probably others) used to use a quasi-synced system on VHF FM with four transmitters on the same frequency, worked OK but there was some controlling technology behind the repeaters, not sure exactly what.

Some years ago I trialled a pair of cross-band TT's on the same 2m frequency (split 70cm so they didn't lock up). Worked reasonably well, but with a bit of audio degradation and produced some interesting effects when mobile as the flutter was different from each transmitter.

With separate CTCSS you could get different conversations on two repeaters which would end in disaster!
Jim, G1HUL
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Post Options Post Options   Thanks (0) Thanks(0)   Quote G0TLG-1 Quote  Post ReplyReply Direct Link To This Post Posted: 13/January/2016 at 08:24
In Hampshire we have some permanent talkthrough sites (3 at the moment, we are about to add a 4th), which we have sometimes used on separate 2M frequencies but a common 70 cms frequency, the effect of which is reliable solid mobile - mobile comms between any two points in the county - and pretty good handheld - handheld over most of it.
 
Obviously practical applications need careful thought, with one huge net only one station can talk at a time and it's often better to use two or more smaller nets, but it has proved useful for example when the Touir de France came to Hampshire several years back and we wanted to cover the whole course and ensure that every point was hearing other points reporting.
 
As Jim hints, I wouldn't go the separate CTCSS route - the only reason for having everything on the same frequency would be if you want one net. Unless you're thinking something I haven't thought of which is quite possible!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote G8CIX Quote  Post ReplyReply Direct Link To This Post Posted: 13/January/2016 at 09:02
Thank you for your responses. The object is to get a single system to provide coverage across Merseyside. This covers two Ofcom area defined squares. Merseyside is a strip running from the Wirral up to Southport. Operating on PMR to get away from licence restrictions we could drop off a base station or handhelds to any location to be left indefinitely without having to provide an operator. We could also have a perminante area wide event control room.

Was wondering if each repeater could hear the others so the signal is passed up the chain. Some sorted of gated input that listens to the output of the other repeaters unless overridden by a local incoming signal. Can't immediately see how this could be done without the whole lot going into feedback.

Used analog PMR kit is very cheap on ebay so think a good system could be implemented quite cheaply.

Regards

Martin
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Post Options Post Options   Thanks (0) Thanks(0)   Quote g6rib Quote  Post ReplyReply Direct Link To This Post Posted: 13/January/2016 at 09:41
We've done a lot of work with hybrid VHF repeaters - amateur 2m input and PBR for the output,  We then link multiple repeaters using 70cm amateur band.  The same could be done entirely using PBR frequencies.

We've used a mixture of Area Defined and PMSE short term licences.  A word of caution over Area Defined licences.  The terms of the licence include:

"7. Power Spectral Density at the geographical boundary

The Licencee must operate the Radio Equipment with a power spectral density of not more than -116dbM/12.5KHz from a single transmitter at and beyond the geographical boundary(ies) specified in Schedule 2 of this document."

This requires that there is a very low signal level beyond the area of your licence.  In order to cover West and North Yorkshire we had to licence 17 squares and there are certain hills where we know that a 12.5W ERP repeater would breach this condition.

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Andy
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Post Options Post Options   Thanks (0) Thanks(0)   Quote G8CIX Quote  Post ReplyReply Direct Link To This Post Posted: 13/January/2016 at 09:53
Thank you Andy. Any chance of a block diagram of you scheme?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote g7gmn Quote  Post ReplyReply Direct Link To This Post Posted: 29/January/2016 at 05:23
Hello Martin, Jim, Andy and Paul,

More than one TX on the same frequency would require them to be "quasi-synchronous" so that they transmitted at exactly the same frequency, phase and audio-polarity and level.

Hampshire and London FRS that I know-of (and many other police and fire) used to use quasi-sync TX from their AM base-stations (and others using a lesser number of FM base-stations) to gain a large area coverage using just one base-station TX frequency per channel.

I believe Staffordshire police and fire (STARNET ?) did it a different way, using different base-station TX frequencies at each site per channel and letting the mobile radios "vote" over the base-station frequencies. The mobiles all transmitted on one frequency of that channel. That is probably why a large number of Cleartone CM7000 radios have a flavour of software that allows "voting" in the RX programming.

The modern way of doing the "quasi-sync" bit was (and still is) to have the various transmitters locked to GPS using mainly Dalman-SOLAR control equipment (other manufacturers make similar) at each site, which compensated for different propagation times and different levels from the central-control to each base-station. Not only did it keep the carrier and CTCSS frequencies exactly synchronized at each site but made sure the polarity and level was correct and also did the "voting" on the received signals from the base-receivers.

Earlier base-stations, back in the HO days, like PYE F400/4000 series of the 70's & 80's used very high stability external ovened-crystal-oscillators such as PYE HS400 for AM or FM TX carriers as GPS did not exist for Joe Public.
The different base-station TXs would never actually be totally "zero-beat" and because of this they would very slowly drift in-and-out of phase. That would cause you to hear a slow chuffing noise which was due to the very small frequency and phase difference if you were in an area which had good coverage from more than one base-station TX.
Both the Dalman-SOLAR and HS400 oscillator systems were present at our Alton hill-top site.

The very-old way before WARC when all HO networks were AM, was to have 50 KHz channels and 3 offset TX carriers within that channel. (Could also be done with 4 carriers but cannot remember how). Of the 3 carriers I think that one was "on-frequency" and the other two were +9 and - 9 KHz from that centre-frequency, so that if more than one base-station was received by a mobile, any heterodyne tone caused between any two of the carriers would be well outside the audio-range produced in the mobile radio, ie: above the normal upper audio limit of about 3 to 4 KHz.

The mobile radios were 25 KHz channel bandwidth so all 3 carriers would be within the approx 15 KHz bandwidth of the mobile's first IF filter.
The received audio would be fully readable without any heterodyne tone. The mobile radios would receive the whole 50 KHz channel.

I might be a bit adrift with my actual figures above, but the general scenario is there. Further general details are on a very interesting site on the interweb describing and showing diagrams of the older systems.
 
http://www.ringbell.co.uk/ukwmo/index.htm

Best wishes,

Ian.
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Controller, NE Hampshire RAYNET.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote G1HUL Quote  Post ReplyReply Direct Link To This Post Posted: 29/January/2016 at 07:10
Originally posted by g7gmn g7gmn wrote:

I believe Staffordshire police and fire (STARNET ?) did it a different way, using different base-station TX frequencies at each site per channel and letting the mobile radios "vote" over the base-station frequencies. The mobiles all transmitted on one frequency of that channel. That is probably why a large number of Cleartone CM7000 radios have a flavour of software that allows "voting" in the RX programming.

Strathclyde Police had the same system in Western Scotland, very effective in that terrain. The base stations were all linked back to a voting controller (presume by land-line), which used the receiver with the best signal to re-transmit.

Would love to be able to do a system like that for RAYNET, but the infrastructure linking required is not feasible on our budgets and no amateur sets do voting.
Jim, G1HUL
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Post Options Post Options   Thanks (0) Thanks(0)   Quote G8CIX Quote  Post ReplyReply Direct Link To This Post Posted: 29/January/2016 at 09:20
Ian

Thank you for your excellent reply.

Jim, Agreed but wondering what we could do in terms of wide area link ups on a budget?

The pilots use a similar system on AM with a +- 7khz shift.

Regards

Martin


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Post Options Post Options   Thanks (0) Thanks(0)   Quote G1HUL Quote  Post ReplyReply Direct Link To This Post Posted: 29/January/2016 at 22:39
Originally posted by G8CIX G8CIX wrote:

wondering what we could do in terms of wide area link ups on a budget?

Assuming all your intended repeater sites can communicate with each other, the simplest would be in-band repeaters with a common link frequency (the system Andy has developed in Yorkshire).

Benefits are that each site is independent and extra units can be added (i.e. have a fixed system as a starting point, and have some mobile repeaters that can be deployed needed by the operation).

The Yorkshire system uses in-band on VHF and links on UHF, however that isn't easy with the 2m band and they use event-based PBR spots, plus having to use commercial-grade repeaters.

But if you put the in-band on 70cm (plenty of space for wide-shift) and link somewhere on VHF (6m/4m/2m) it gets very easy. Given that it will only get occasional use, providing you get the cooling right mobile radios can be used and will keep the cost down. Might be an issue if you have a pitch on a commercial site due to lack of filtering, but not insurmountable.
Jim, G1HUL
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