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Old 03-13-2018, 10:06 PM
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Default Repeater Build Preferences

I am building a 450Mhz repeater. Nothing special just a basic repeater for now. One Freq in and one out with one antenna. This will be a secondary channel for public safety. I would like to ask for anyone that has some experience with equipment programming and maintaining a repeater to recommend what they would use and why. Example: I am somewhat familiar with 2 - Moto CDM750's with a pigtail and a Duplexer. But is this the way to go, or am I better off to use an all in one like a Yaseu 7000. Later on I will be adding an identifier and a Pager to key up the repeater when a message
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Old 03-14-2018, 9:26 AM
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Originally Posted by sdreast View Post
I am building a 450Mhz repeater. This will be a secondary channel for public safety. I would like to ask for anyone that has some experience with equipment programming and maintaining a repeater to recommend what they would use and why.
You will need a repeater that is capable of operating at 2.5KHz deviation(Narrow band)

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Originally Posted by sdreast View Post
Example: I am somewhat familiar with 2 - Moto CDM750's with a pigtail and a Duplexer.
I would not recommend just using a "Pig tail". You will have zero control over the repeater, and should it lock up, you will not be able to turn it off, with out going to the site and physically turning the machine off.

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But is this the way to go, or am I better off to use an all in one like a Yaseu 7000. Later on I will be adding an identifier and a Pager to key up the repeater when a message
Not a fan of Vertex repeaters, BUT that would be a better option than a pair of CDM mobiles. WAY better than a pair of CDM mobiles. A mobile radio has about a 10-20% duty cycle at best. You can set up fans to get it closer to 100%, but not at the mobiles full rated power.

Use a proper BpBr repeater(Sinclair Q3330C-2 Sinclair Technologies Online Product Catalog - Q3330C-2 as an example) a 4 cavity(Q3220C-2) would work as well, and be slightly cheaper. Avoid mobile band pass duplexers at all costs.
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Old 03-14-2018, 10:02 AM
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It's concerning that this is for public safety….

I'm happy to see people trying to save money and do things on their own, but setting up a public safety system wrong can have some dire consequences.

Getting advice is a good start, but you'll need to understand that you are going to get some harsh words or advice you may not agree with. Understand that there are some of us that do this stuff for a living and have seen bad systems set up. Often we get brought in after the fact to "fix" it.

I'll also say that since you didn't bring up using amateur radio gear, Cheap Chinese Radios, etc. you are off to a better start than many others.

A couple of things you'd need to look at:

1. Get your licensing sorted out first. A coordinator will find a pair for you, and help determine how much power to license for.

2. Mobile radios often will not stand up well in a repeater application since the duty cycle can be high. They often will not handle the heat. Turning down the RF power quite a bit can help, but that can create coverage issues.

3. Some pre-made repeater systems, specifically the BridgeCom and others, are just two mobile radios in a box with a controller and sometimes a duplexer. Fine for low duty cycle use, but not for a primary dispatch system.

4. Avoid the temptation to go with cheap mobile/notch type duplexers. They can have a lot of loss and don't always work well in high RF noise environments. As Kayn1n said, avoid them.

If I were going to do this, I'd put money into the antenna, feed line and duplexer systems. If you do those right, you can have a well perming system. Often people will leave the antenna/duplexer choices to the last when most of the budget is spent and hope for a miracle.

If this is a secondary channel that doesn't get much traffic, then a couple of mobile radios might work, but keeping the power low will be beneficial to long life. Also keeping long winded people off them will help. Setting up time out timers on the system would be wise.

Controllers would be necessary, as kayn1n said. You also need it to do CW ID, as the system will need to identify itself.

This isn't an impossible project, but it won't be cheap. You didn't mention your budget, so it's hard to tell, but hopefully you have a few grand set aside for this. If you were hoping to do this for a few hundred bucks, you might want to rethink.
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Old 03-14-2018, 10:08 AM
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What mmckenna said about cost, expect to pay $15,000 MINIMUM to do it right


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Old 03-14-2018, 11:35 AM
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And if we are doing it right...lets start talking more about application. Is this repeater going to be serving mobile subscribers primarily? Or will it be service portable subscribers primarily? Or both? What kind of site is this going to be installed at? What height will the antenna be at? Is this going to be the top of the structure where lightning strikes will be a huge concern?

If it were me, I'd try and find an older Quantar or MTR2000 that is in working condition...but that's primarily because they have better than average receivers in them and power amplifiers rated to handle 100% duty cycle at full power and the price compared to building and adding the features requested (even over time) will be similar.

Don't get me wrong, I love building repeaters for fun but when you just need it to work and work well from the start...might as well start out with purpose built equipment.
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Old 03-14-2018, 12:57 PM
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Now that I have received some input, let me clarify a bit. First off there is a county system in place but access via volunteers is limited. We will not be replacing but supplementing. So the need is to allow us to checkin on our own channel so we will know who is coming, and to provide us with a fireground simplex channel. We run 100 or so calls a year so duty cycle is minimal. I have worked for a FD in the past that had a repeater just like what I am considering. 2 moto CDM1250s connected as a repeater. It was inservice for about 10 years. Flawless operation. However that doesn't mean that should be my solution. This is not necessarily our end solution either but a start, to introduce the concept to the membership so we can plan and grow. The freq pair has been applied for. Tower is currently specked to be on top of a 100' water tower. Yes I know lightening. Under consideration. Terrain is flat trees only 8 square miles service area at best with tower in the center. 30 or so mobile/handhelds.
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Old 03-14-2018, 4:23 PM
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OK, that's useful info.

Your FCC license will dictate power levels, antenna gain, antenna height, etc. So that's probably already in the application. Designing the system to fall in line with what the license says is more important than what we say. Without knowing what those values are, we'd be sort of guessing.

Fire ground should be simplex, so using the output frequency of the repeater in simplex mode would cover that.

A 100 foot water tower sounds like a good setup.

The comfort level with failure risk is what matters. A couple of mobile radios can work fine for low duty cycle stuff. If it's going to get used like a party line, then you really need to step up to a purpose built repeater. Duty cycle isn't the traffic spread out over a week, it's about what happens over 10-20 minutes. The heat build up is the concern. Mobile radios can get pretty hot if they are transmitting a lot.
A lot of that comes down to radio discipline.
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Old 03-14-2018, 5:56 PM
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You are always better off using a ACTUAL repeater since it was designed from the ground up to be a repeater.
..........
Bridgecom sells a UHF repeater for $995 that is a good low cost repeater as well.
And they have excellent support.
.............
The Kenwood TKR-850 is also a low cost reliable repeater.
..,....,......
You want a 6 cavity BPBR base duplexer to insure adequate isolation and interference rejection as well.
...........
The VXR-9000 is another low cost solution
.........
And water towers are good if you are on top of the tank and the huge shadow created by the tank under the antenna doesn't block the pattern from going places you want to cover.
.............

Last edited by MOTEX; 03-14-2018 at 6:03 PM..
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Old 03-14-2018, 7:27 PM
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I also have access to a 30' Antenna tower if that would be better. The main plus is the Coax would be 50' instead of 150'
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Old 03-14-2018, 7:47 PM
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Bridgecom sells a UHF repeater for $995 that is a good low cost repeater as well.
And they have excellent support.
Bridgecoms are two TecNet/Maxon mobile radios in a box with a power supply, controller and a mobile notch duplexer. The only redeeming value is that they have fans to help with the heat

"Inherited" a few of these at one of our remote sites. They work, but I wouldn't recommend them for use in public safety. Good for amateur radio, GMRS, etc, though.
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Old 03-14-2018, 7:55 PM
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I also have access to a 30' Antenna tower if that would be better. The main plus is the Coax would be 50' instead of 150'
Not really. Especially on UHF, antenna height is key. You'd need to look at distance to horizon calculators.
30 foot high antenna to a 6 foot high portable radio on flat ground, 11 miles.
100 foot high antenna to 6 foot high portable radio on flat ground, 17 miles.

However, that's all a moot point since the FCC license tells you what you are allowed to do.
The FCC license will show you:
Location of repeater. It's fixed, unless you specifically licensed for a portable repeater.
Antenna height. You can't go higher than what you are licensed for.
ERP, which is power out of the transmitter, minus duplexer and feed line losses, plus antenna gain.
Area of operation, usually spelled out as X kilometer radius around the latitude/longitude of the repeater site.

If you already have the license in process, you are too late to redesign the system without putting in for a change. Changing the specs of the system -might- require re-coordination.

If you haven't applied for the license yet, then you'll need to know the answers to those questions, as well as have the frequency pair signed off by a frequency coordination service.
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Old 03-14-2018, 7:57 PM
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I also have access to a 30' Antenna tower if that would be better. The main plus is the Coax would be 50' instead of 150'
And the type, condition, quality, of coaxial cable is important too. If the cable is already on the tower, it's suspect. Without sweeping/inspecting it, it should be considered no good. Wet/damaged/corroded coax will give you no end of trouble.
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Old 03-14-2018, 9:10 PM
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Let me add a few comments that haven't cropped up yet.

The choice of using CDM radios is not what my first choice would be. The reason is you want good shielding of both the RX and TX side of the picture here. The CDM radios are not that great for shielding. they make fine mobile radios, but again, they would not be my first choice for repeater operation.

The coax between the duplexor and the radios needs to be double shielded or of the heliax type coax. Don't plan on using any LMR coax for duplex operation. Yes the LMR coax is double shielded, but it uses different type of material for the 2 shields. Over time this type of cable absorbs moisture and the aluminum and copper shields start acting like diode junctions and it creates all sorts of noise you will never be able to eliminate. So start off not even trying to use it. If you don't believe me, start searching around and see all the posts about it.

The next area to look at is the antenna. Don't skimp here either. A cheap antenna will be just that and the problems that come with it. I have seen the cheap fiberglass sticks cause all sorts of problems after they have been up in the air for a while with the wind cranking them all around. The joints tend to break and now you have a noisy antenna that won't work worth a crap.

Repeaters are bad enough to keep running on their own. You don't want to throw the dice in the wrong direction and shoot yourself in the foot. Been working around a good number of public safety radios systems over the years. It just isn't worth the effort to skimp and try to save a few dollars on low cost materials and equipment.

Don't forget to install good grounding on everything. That grounding is going to protect your investment. The Motorola grounding spec is your best friend. Pay attention to it and you won't go wrong.
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Old 03-14-2018, 9:55 PM
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jim202 Thanks for the great input. A few follow up ?s on your comment. Is LMR good from duplexer to antenna? What UHF 450Mhz antenna would you recommend?
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Old 03-14-2018, 10:15 PM
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jim202 Thanks for the great input. A few follow up ?s on your comment. Is LMR good from duplexer to antenna? What UHF 450Mhz antenna would you recommend?
Absolutely NOT. NO LMR cable. Use genuine Heliax cable from the duplexer to antenna.

READ what Jim posted:

" . Don't plan on using any LMR coax for duplex operation. Yes the LMR coax is double shielded, but it uses different type of material for the 2 shields. Over time this type of cable absorbs moisture and the aluminum and copper shields start acting like diode junctions and it creates all sorts of noise you will never be able to eliminate. So start off not even trying to use it. If you don't believe me, start searching around and see all the posts about it.
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Old 03-14-2018, 10:22 PM
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Quote:
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Absolutely NOT. NO LMR cable. Use genuine Heliax cable from the duplexer to antenna.



READ what Jim posted:



" . Don't plan on using any LMR coax for duplex operation. Yes the LMR coax is double shielded, but it uses different type of material for the 2 shields. Over time this type of cable absorbs moisture and the aluminum and copper shields start acting like diode junctions and it creates all sorts of noise you will never be able to eliminate. So start off not even trying to use it. If you don't believe me, start searching around and see all the posts about it.

Yea, going 150’ up a water tower, you will want 7/8” or 1-5/8” hard line. Bigger is better, but more expensive.

Stay far away from fibreglass antennas as well.




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Old 03-18-2018, 2:40 PM
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What information was sent into for coordination? Which location, power output, antenna gain etc. Also is there any other Radio Equipment at the repeater site?

I have set something up like this exact thing and doing another, however on VHF as a support channel. Unless he finds a great deal on some Motorola Equipment, going Kenwood, Icom or even a Vertex will be fine. They all have there pros/cons but I am big believer in good cabling, duplexer and antenna makes the repeater and is key. Spend the money on good heliax, bandpass-band reject duplexer preferred and folded dipole antennas.

Last edited by brushfire21; 03-18-2018 at 2:48 PM..
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