multi-site repeater theories

[zerowatts]

How do engineers typically set up a multi-site repeater systems in regards to frequencies? I would like if the radios did not have to change channels based on their geographical locations if at all possible. Would prefer to keep all portable units simple set to one channel.

I do not like making posts long but feel the need to be clear in my question...

Site A - Northern Repeater
Site B - Central Repeater - 34 air miles to Site A & 32 air miles to Site C
Site C - Southern Repeater

I am trying to consider how to interconnect all 3 sites to give me over 60 miles of North to South coverage since I do not have access to a 300' tower. I currently have a pair of frequencies, will I need more? I am sure there is probably more than one way to do this, I am trying to figure out ALL the ways to do this? Please offer your thoughts as well as the major obstacles I will face.

 

[davidgcet]

you will have to license for that pair at all 3 sites, if this is VHF good luck on getting that too. in addition you will need to get a microwave or telco based link tieing the sites together. then simulcast them using voting receivers to determine which RX audio path to use. expect the costs for all this to be high.

or you could get 3 MotoTRBO( other vendors do IP connectivity too but i am not familiar with them) repeaters and tie them together over IP. cost would be lower than the above, but still not dirt cheap.

for dirt cheap, teach your people proper radio use and have them switch channels to talk on the correct repeater.

also, no matter which choice you go with asking for 16-17 miles of reliable comms on handhelds is stretching it for most locations. realistic is more like HALF that, so if your towers are in a N-S line you would probably need 5-6 towers depending on terrain/obstructions/freq band in use.

 

[zerowatts]

Thank you for the reply and wisdom.

telco based link

As in Echolink or ROIP type of system?

I am not a fan of MotoTRBO due to the proprietary lock downs by/of Moto. I will not even consider the system, nothing to do with cost though cost should be an issue lol. It is a shame to have a system so powerful and so capable, then restrict who can be a developer and what they can develop for the system. The hardware is fantastic though!

I am currently getting about 22-23 miles from the handhelds and about 50% more (30-34 miles) from the mobiles depending on weather conditions and fog. I think the key part/s I am clueless on is the voting receivers. Thank you for pointing out the proper research terms for me.

It is a UHF system, do you suppose getting the same frequency across so many miles would be difficult? I have one pair currently but I may be forced to change frequency I guess.

 

[davidgcet]

UHF is not so hard to get the same freq in multiple locations, depending on your area.

the biggest thing i can tell you is do not try to plan this yourself, you will fail. get a competent radio shop to do it for you. heck, make sure they actually know what they are doing before you commit to them, cause there are many techs who will say it can be done this way, and not have a clue about timing a voice simulcast system.

unless you have a large number of units if you have nextel or some other cellular PTT system or commercial wide area trunk system in your area, it honestly would be cheaper to go that route for wide area coverage like this. you are looking at a 1000.00 or more a month in tower rent alone, more if you need to install microwave/rf links on them, plus whatever telco costs should you go that route instead of microwave/rf links.

don't let me sound discouraging, it can be done 3 ways- quick or good or cheap. you can pick up to 2 of these and forget the 3rd one. in order to be happy with the system once it is in place you will need to spend bucks up front to do it right.

 

[bldavis]

I'll echo David somewhat. I would suggest, that you get in touch with a competent vendor (or three, depending on your purchasing rules). You might put out a fairly simple RFP and see what comes back. Using a single pair, is going to make this difficult on you in some ways. I don't think simulcast is an option, because of the linear conformation, and the distances between towers, but there are a couple of multi-site options that will be substantially cheaper anyway.

My guess, would be transmit from middle tower, voted receive from the other towers. Talk-in is going to be your problem, regardless. If IP connectivity is your desire, then you are talking about the larger vendors. Take a hard look at Tait gear. I've been playing with their stuff lately, and I have to say I'm impressed. And they are an order of magnitude cheaper than the big M or Middle H.

 

[flecom]

you will have to license for that pair at all 3 sites, if this is VHF good luck on getting that too. in addition you will need to get a microwave or telco based link tieing the sites together. then simulcast them using voting receivers to determine which RX audio path to use. expect the costs for all this to be high.

if you are doing simulcast you also need to delay the audio so all three xmitters are sending out the exact same audio at the same time, and your xmitters will need to gps disciplined to minimize intermod in the overlap areas

 

[davidgcet]

yep, try setting the simulcast on 10 sites where some locations get 3 or more sites in on overlap. it gets REAL fun then. and if you go telco based expect to redo the simulcast at least once a year because they will move your circuits and change the delay in the lines. i'm glad all my remaining simulcast sites are RF linked and GPS synced now, cause i absolutely hated the old telco links we used to maintain.

 

[stevelton]

What about going with IDAS or Nexedge?
Ten you could keep the system conventional, and have all 3 sites on different frequencies, and program the handheld or mobile units to vote scan, and the radio does all the work of finding the repeater with the best signal to affiliate with. The you can keep the license business/industrial conventional, and not have to do anything with trunking. Doing this, the sites connect with very basic internet set up, and cost about the same as a new analog system.
Steven

 

[bldavis]

To the original poster - You didn't specify whether this was a vanilla commercial system, or a Public Safety system, and that makes a large difference in terms of budget, and needs. Again, seek expert help. Although there may be a few pros on this forum (and a ton of scanner enthusiasts who really know very little about what goes into the planning and requirements of some of these systems), you need to work with someone right there, with a track record of building out successful systems.

Regardless, there's simply not enough information presented to give you an answer to your question.

if you are doing simulcast you also need to delay the audio so all three xmitters are sending out the exact same audio at the same time, and your xmitters will need to gps disciplined to minimize intermod in the overlap areas

Yeah, that's not really the story. The time source is more about ensuring consistent frequency and phasing at the base stations, and timestamping packets on digital, or timebasing the comparators on old analog gear. There are a number of schemes for doing simulcast, and they vary by manufacturer and system configuration. The idea isn't to make the transmitters fire simultaneously, it's to adjust phase, carrier, etc to deal with phase distortion or carriers far enough off to destructively interfere. Good system designs also balance transmitter output looking for that magic 3dB receive signal differential.

yep, try setting the simulcast on 10 sites where some locations get 3 or more sites in on overlap. it gets REAL fun then. and if you go telco based expect to redo the simulcast at least once a year because they will move your circuits and change the delay in the lines. i'm glad all my remaining simulcast sites are RF linked and GPS synced now, cause i absolutely hated the old telco links we used to maintain.

It's a good deal easier with modern digital systems, because you get some new metrics to use to help balance things out. Some of them do a form of auto-balancing, most of them have new and far easier methods. The best of them have done away with prime sites altogether, and comparators as well.

And as with all things, it depends on the kind of telco links. In urban areas, you can usually buy fractional fiber optic links at prices comparable to the old DS-x V.35 stuff. And Fiber systems are very stable, and can have performance metrics that make microwave backhauls look like two tin cans on a string. In my case, I constructed my own fiber system between my sites, and in the long term, I saved a fair sum of money over RF backhauls.