GMRS Network Design

[duckaholic]

Looking for a little insight….

I’m in the process of trying to determine how to build a GMRS network with a range up to 20 miles to open the doors of the radio world to at least 14 friends who are not currently interested in testing for the Tech license. They are however interested in putting funds together for a GMRS capability. In addition, I have access to a bunch of old but working motarola GMRS two ways to build repeaters.

Been around the 2 meter band a bunch and it provides everything needed minus a large cohort of amateurs. Thus, I’m going to use this project as learning endeavor and try to pull off something similar with GMRS for myself and hopefully turn a few friends onto the Tech path. I realize there will be limitations ( likely range) that 2 meter won’t have.

At this point Im still researching GMRS and it’s limitations so bear with me….

Ive got access to a property that is about the half way point- and the highest peak in the area- about 8 miles out. Has power and an barn that I’m thinking a repeater with an antenna @40’ should do. Not sure if antenna limitations so I’ll work with what I can..still looking this up. Guessing temps; extreme hot/cold; will be something I’ll have to combat with repeater placement.

For base units- it appears some of these units push up to 50 watts. That with a yaggi beam towards repeater should work fine at both ends of the line?

For true mobile(vehicle) I really don’t know what to expect with GMRS mobile. We live in somewhat hilly terrain (Midwest). On 2 meter we experience issues in areas out to 30-40 miles with mobiles. Guessing same here at leas range?

Handhelds are of interest but I they appear to be very low power and I have a hard time thinking they reach the repeater 8 miles out. Not something I’m going to start trying make capable but if I can pull off the base/mobile units I might expand the capability to Handheld. I assume these handhelds would require multiple linked repeaters to boost the limitation on ranges of those units?

Again- not been around GMRS much so shoot hole in my thoughts!

 

[jeepsandradios]

GMRS is the same as 440 in the ham bands. If your already a ham and use 2MTR try some 440 gear and it would be similar in range performance. With a good site and proper install 20 miles should be fine with mobiles and portables. Buy a quality antenna (DB404 is good base antenna), Hardline and duplexer. Height is king for the repeater.

 

[prcguy]

I've built some very high performance commercial, amateur and GMRS repeaters that lived on the busiest RF hell mountain tops in So Cal. First off, ditch the idea of building a repeater using old mobiles, erase that from your head. I have a lot of experience doing that and have built some great systems over the years using mobile radios and separate TX/RX modules, but it takes years of experience and a lot of build time to pull that off and have it work well. Instead, buy a used Motorola UHF range 2 110 watt Quantar repeater. I've bought many off Ebay in the $600 range and they are available from many surplus dealers.

A 110 watt Quantar turned down to 50 or 60 watts in GMRS service will run very cool and the fans in the amplifier will probably never come on. The Quantar has a built in community tone panel so you can issue different tones or DPLs to different people or groups, or just use one for everybody. Quantars are extremely reliable and should never break. Buying a used one simply means the last owner ran it to flush out any infant mortality issues over the last 20yrs and now you have access to it cheap. A new Quantar was in the $15k range, so you could never begin to build anything close to that for its current used prices.

Some advantages of buying a Quantar over using mobile radios is the Quantar has a built in power supply and you can get them with battery revert that will charge a pair of 12v gel cells (28 volts) and automatically switch over during a power failure. The most power you can run a mobile in repeater mode is about 15 to 20 watts, otherwise the chassis and heatsink are too small for continuous operation and you will need an external power amp for more power. The Quantar is rated 110 watts continuous forever keydown and you will be turning it down to about 50 watts for legal GMRS use. Or not if you choose . The Quantar has an isolator on the transmitter, another cost that is not only recommended but required at all commercial repeater sites. The Quantar has a community tone panel, another costly option to buy and interface with mobiles. The savings in build time frustration alone pays for the Quantar. How much is YOUR time worth assembling all the stuff to make a repeater then the time to troubleshoot all the mistakes you will make then tuning everything up? If this is your first repeater project I give it a full week minimum and more like two weeks of your time to assemble a repeater. A Quantar arrives ready to go on the air.

You will also need a good BpBr duplexer with at least 90dB TX/RX isolation. The Motorola 1500 series duplexers are ok and the later versions with the black wrap around chassis sold with some Quantars is also ok. These can be had in the range of $150 to $200 used. The best duplexer out there is the PD/Celwave 526 series with upwards of 120dB isolation but they run more at around $400 to $500 used.

Then shop for a single or dual 1/4 wave tunable cavity filter and for a dual you need a critical length of tuned cable between them. This will be used as a preselector for a preamp. Then comes the preamp and there are many to choose from since the duplexer and preselector in front of it will filter out lots of strong out of band stuff reducing the need for a super high level model. Recently I've used the Minicircuits ZX60-P103LN, which has a great low noise figure but way too much gain. Its output will be attenuated to limit it to about 8dB gain, maybe 10dB max. The basic rule for using a preamp on a repeater is the preselector in front of the preamp needs to have more isolation at the repeater TX/RX spacing of 5MHz in this case. The preamp will perk up the receiver slightly and make hand helds easier to hear.

Finally, for a low level repeater the antenna will make all the difference so aim for something in the 9 to 10dBd gain range, not dBi but dBd. These will be at least 20ft long but will run circles around something 6ft long or 12ft long, etc. Try to keep the feedline loss to under .5dB and if you can get the antenna higher using more feedline, maybe .75dB max. Do not use any kind of LMR coax, only copper Heliax. 1/2" Heliax (LDF4-50) is fine for short runs up to maybe 35ft then I would consider 7/8" (LDF5-50) for longer runs up to 75ft.

The end result will be you can't buy or build any better performance, and since the repeater is at the heart of the system you want the best performance at the repeater and its antenna. The Quantar is is all there and all you have to do is program it and maybe do a 5 minute simple alignment on it and its nearly plug and play. You will need a service monitor and something with an RF tracking generator to align the duplexer and preselector cavity(s).