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  #1 (permalink)  
Old 03-30-2018, 5:55 PM
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Default Suggestions for building a low duty cycle repeater

I need to build a repeater to cover a small area. Around 10km². Won't have a lot.of traffic going through. Is using two mobiles with a duplexer feasible? Or maybe a HT for the Rx end?

PS-need to get this done on a very tight budget.

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Old 03-30-2018, 7:11 PM
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Default Suggestions for building a low duty cycle repeater

Depends on the environment at site and height is going to be the primary limiting factor. Easiest way, find a Motorola R1225 module and add a duplexer and power supply.

Second best way would be using two mobiles (you likely won’t need but 10W for this project) but it’s somewhat bulkier. Using a portable for receive can work in some applications but you typically have a harder time acquiring COR and line level audio on a portable.


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Old 03-30-2018, 7:19 PM
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Height would be around 100ft, on top a 10-storied building. Already have another 15ft-is pillar installed for an antenna with a small "room" below to house the repeater setup. Also this is in sort of a rural area so no buildings above 6-7 floors around. Thinking of running a 25watt mobile. Should that be good enough?

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Old 03-30-2018, 10:05 PM
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You can search for some coverage maps. You can input height, location, and power to get coverage. Here are a couple. Coverage Prediction Radio Mobile Online
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Old 03-31-2018, 9:24 AM
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I'd turn the power down on that 25 watt mobile if you can. Assuming that you're going to have people using hand-held portables as the "in-field" subscriber units, then even a 10 watt mobile (repeater transmit radio) with a 3 dB gain omni antenna will out-talk the portables all day long. In other words, there's little benefit to over-driving the transmit, if people trying to talk back in won't be able to reach the repeater. You're better off to have a little balance in the system, so that it talks out about the same that it will receive. Otherwise, you're creating a one-way paging system for 50% of your coverage area.

Taking a little power off the transmit side allows the system to generate less heat (#1 killer of a low duty cycle repeater) - and it also means that the inexpensive compact duplexer that you're probably going to be using won't be suffering as badly on the isolation side of things. In other words, the residual transmit energy that's leaking past your duplexer's reject notch won't de-sense your receive side quite as badly.
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Old 03-31-2018, 10:22 AM
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Are there any other repeaters on this same said building or in close proximity? Mobiles and mobile duplexers can easily become overloaded at a high RF site or a site with a high noise floor. Out in the country with nothing else around you can usually get away with a lot more, but all it takes is a few other repeaters or other RF (high power paging/FM radio stations/etc) to cause major problems.

What frequency band is this repeater and whats it's intended uses?
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Old 03-31-2018, 11:22 AM
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Quote:
Originally Posted by 12dbsinad View Post
Are there any other repeaters on this same said building or in close proximity? Mobiles and mobile duplexers can easily become overloaded at a high RF site or a site with a high noise floor. Out in the country with nothing else around you can usually get away with a lot more, but all it takes is a few other repeaters or other RF (high power paging/FM radio stations/etc) to cause major problems.

What frequency band is this repeater and whats it's intended uses?
Nope. None in "close proximity" nearest is maybe another 5-10 kilometres away. It'll be used for communicating through the building and some land around it. Security, maintenance etc.

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Old 03-31-2018, 11:23 AM
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Quote:
Originally Posted by 12dbsinad View Post
Are there any other repeaters on this same said building or in close proximity? Mobiles and mobile duplexers can easily become overloaded at a high RF site or a site with a high noise floor. Out in the country with nothing else around you can usually get away with a lot more, but all it takes is a few other repeaters or other RF (high power paging/FM radio stations/etc) to cause major problems.

What frequency band is this repeater and whats it's intended uses?
And it'll be in the UHF band around 420mhz.

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Old 03-31-2018, 12:17 PM
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Basically a repeater is a receiver, transmitter, power supply, and a controller. If you look inside a Yaesu DR-1X repeater, it has two mobile radios along with a power supply and controller. So yes, two mobiles will work in theory. How well it works depends on several factors--mostly, what radios you use. There are many amateur repeaters that use two Motorola Maxtrac radios. The transmit radios require a minor modification to the output RF power circuits. I've built several and know of several that have been in use for 10 years--using one on 2m right now. Mobile radios are also used to link repeaters together. There are lots of bells and whistles that radio clubs add to their repeaters...they all cost money and really have zero effect on RF performance--do you really need a $500 controller with voice ID vs a $90 CW model? Not for what you need. Your country's laws have to be taken into account--we don't know them.

Don't skimp on the feedline! No LMR-400--hardline/heliax if you can. The feedline can't have foil inside. ID-O-Matic, NHRC, and Elektra controllers are cheap and accomplish the job--that is if you country requires a repeater to ID--if not you may be able to direct wire them together. You HAVE to add an extra fan on the transmit radio because you are using beyond its duty cycle and it will fry without extra cooling (not a tiny fan, something that puts out as much air as a blow dryer). Fiberglass antennas are not made for repeater use...people do use them and you can, just factor in replacing it after a couple of years. The wind will bend them and over time cause micro-cracks internally. This is fine for simplex use, but in a repeater it will cause crackling in the signal. Proper repeater antennas are very expensive in comparison, thus you can use fiberglass over and over for many years before reaching the price of a DB antenna.

These two websites may be helpful:
The Repeater Builder's Technical Information Page™
Batwing Laboratories
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Old 03-31-2018, 12:22 PM
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Quote:
Originally Posted by I_am_Alpha1 View Post
Basically a repeater is a receiver, transmitter, power supply, and a controller. If you look inside a Yaesu DR-1X repeater, it has two mobile radios along with a power supply and controller. So yes, two mobiles will work in theory. How well it works depends on several factors--mostly, what radios you use. There are many amateur repeaters that use two Motorola Maxtrac radios. The transmit radios require a minor modification to the output RF power circuits. I've built several and know of several that have been in use for 10 years--using one on 2m right now. Mobile radios are also used to link repeaters together. There are lots of bells and whistles that radio clubs add to their repeaters...they all cost money and really have zero effect on RF performance--do you really need a $500 controller with voice ID vs a $90 CW model? Not for what you need. Your country's laws have to be taken into account--we don't know them.

Don't skimp on the feedline! No LMR-400--hardline/heliax if you can. The feedline can't have foil inside. ID-O-Matic, NHRC, and Elektra controllers are cheap and accomplish the job--that is if you country requires a repeater to ID--if not you may be able to direct wire them together. You HAVE to add an extra fan on the transmit radio because you are using beyond its duty cycle and it will fry without extra cooling (not a tiny fan, something that puts out as much air as a blow dryer). Fiberglass antennas are not made for repeater use...people do use them and you can, just factor in replacing it after a couple of years. The wind will bend them and over time cause micro-cracks internally. This is fine for simplex use, but in a repeater it will cause crackling in the signal. Proper repeater antennas are very expensive in comparison, thus you can use fiberglass over and over for many years before reaching the price of a DB antenna.

These two websites may be helpful:
The Repeater Builder's Technical Information Page
Batwing Laboratories
Don't need no bells and whistles. Actually there's little to no requirements/restrictions on repeater installation where I am. As long as you're licensed for the frequency, you're good to go! There's a power restriction of 50W, I don't even intend on using half that anyways.

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Old 03-31-2018, 7:10 PM
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First of all to keep costs down you would need to use UHF.
..............
You can easily use two 50 watt mobiles turned down to 10 watts that can cover that far.
..............
No duplexer will be needed if you use 30 feet of vertical spacing between two antennas.
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Old 04-01-2018, 1:38 AM
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Quote:
Originally Posted by MOTEX View Post
First of all to keep costs down you would need to use UHF.
..............
You can easily use two 50 watt mobiles turned down to 10 watts that can cover that far.
..............
No duplexer will be needed if you use 30 feet of vertical spacing between two antennas.
I was thinking of this. Got a bad history with duplexers. 30ft seems pretty manageable. So I'll go with that.
On another note, don't want to be using Motorola radios. Programming them is a huge hassle. Any suggestions? Something that won't be so expensive/painful to program?

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Old 04-01-2018, 2:17 PM
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Are you required to use a 5 MHz spacing between TX and RX freqs? Depending on your IM calculations you might get better isolation without using a duplexer with wider spacing.
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Old 04-01-2018, 8:03 PM
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Motorola mobiles usually have better front ends which is very important in repeater service.
..............
Kenwood TK830s were the best but i don't know if narrow band versions are available.
The TK890s also have very good front ends
..............
Speedway also brings up a good point.
5mhz offset is a must for this to work without a duplexer
.............

Last edited by MOTEX; 04-01-2018 at 8:26 PM.. Reason: corrections
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Old 04-01-2018, 8:40 PM
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Kenwood TK 830 G models do narrowband.
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Old 04-02-2018, 4:04 PM
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Looking into the TK 830G, it seems like a fairly inexpensive good little radio. Probably gonna go with that and a duplexer free setup! Will post updates when done. Thanks everyone for the help.

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Old 04-02-2018, 7:47 PM
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Quote:
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Kenwood TK 830 G models do narrowband.
The OP says he is Bangladesh and using 420 MHz. I wonder if narrow band (12.5 KHz channel spacing) is even a concerns?
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Old 04-03-2018, 5:09 AM
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Quote:
Originally Posted by RFI-EMI-GUY View Post
The OP says he is Bangladesh and using 420 MHz. I wonder if narrow band (12.5 KHz channel spacing) is even a concerns?
It is not actually.

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Old 04-03-2018, 12:36 PM
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Be a little careful in just cranking down the power on the transmitter. Going too low and you can generate trash out of the transmitter. Most solid state Power Amps become unstable if you go near or below about half their normal output. They tend to start going into unstable operation and start generating white noise, spurs, and trash in general.

It would be wise to look at the transmitter with a spectrum analyzer to make sure it's clean. Any crap being generated will effect the ability to have your repeater to work correctly. It will effect the range due to the noise being generated and you probably not having enough isolation between the receiver and transmitter.

Take care on what type of coax cable you use. The use of LMR type coax may look good up front, but will bite you big time down the road as moisture will get into the cable. The moisture will cause diode type activity between the different materials used in the shield of the cable. The interaction of the aluminum foil shield and the copper braided shield laying on top of it is just asking for trouble in a duplex application. There have been many threads and articles wrote on just this topic.

The antenna you select is going to be important. If I understand what has been said about the use of this repeater, it is mostly for use inside the building. This will effect what you do for your antenna. With the antenna on the roof, you may find your not getting very good coverage down on the lower floors of the building.

You may want to consider use of some leaky coax run down a shaft in the building to put more signal inside it. Only a trial of how it works will tell if you get the coverage your looking for. It may take placing a number of antennas inside the building to provide the coverage your looking for. Leaky coax may not be the right selection. You may have to run normal coax and the use of power splitters, low gain antennas like 1/4 wave whips hung from the ceiling of hallways on multiple floors.

Your taking on a project that can easily become a real problem child if not thought out carefully.

The radios you select should have good shielding to help with the isolation between the receiver and transmitter. The better the shielding, the better your repeater will work. How much isolation your duplexer needs will depend on a number of variables. Transmitter power, coax cable losses, receiver isolation and shielding, how clean the transmitter signal is and how good your antenna is. Some antennas are really bad for repeater use due to their ability to generate intermod problems.

As you can see, there are a whole bunch of issues in building a repeater system. Just walk slowly through all this and do your homework.

Let the group on here know how your doing and what parts you have chose to use. There is some really good people on here that can supple some good technical feedback.
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