Help With Repeater

[ECFD]

Greetings. I have gradually been working on a project to set up my own repeater for private/business use. I went through the process to get an FCC business class license with a VHF repeater pair as I wanted more coverage than simplex would provide. VHF was chosen over UHF after some testing showed VHF performed better. For the mast I'm using a 33 foot telescoping carbon fibre mast appropriately guyed. The current antenna is a tram VHF antenna mounted about 30 feet up. I understand this is far from ideal height, however this is the best I can do currently. Terrain is fairly flat, environment is mostly suburban. LMR-400 coax throughout the system. Repeater is a Tait TB8100 purchased from a reliable vendor who tuned and programmed it for me. The duplexer is a VHF duplexer from Bridgecom, tuned by them for my frequency pair. Prior to receiving the Tait repeater I had done some testing with a Single Frequency Repeater on DMR, same antenna and coax, and this yielded decent results. Coverage range was easily 10 miles in most directions, minus one area where terrain is an issue. After receiving my Tait analog repeater and connecting the new Bridgecom duplexer, coverage is very poor. Approximately 3-4 miles in all directions before the signal begins to cut in and out. Due to this cutting in and out effect I suspect a duplexer issue. I understand the flatpack/mobile-type duplexers are meh at best for most applications and I'm considering switching to a more proper Sinclair 4-cavity duplexer. I'm also considering switching the antenna to something more substantial. I realize an increase in height would also help but as stated this isn't really an option at the moment. Any help from those with more experience is very greatly appreciated. I realize this system is far from commercial grade, nor does it need to be, I'm just looking to improve the system and get the best performance possible given the limitations. Thanks in advance,

Chris

 

[mmckenna]

Couple of concerns just reading through this really quickly.

"33 foot mast" "tram VHF antenna mounted about 30 feet up".
So, is there 3 feet of metal mast next to the antenna? That's going to do bad things if it is. Antenna needs to be either above the mast, or stood off a few feet.

"LMR400". LMR cable uses an aluminum foil shield with tinned copper braid. This dissimilar metal can create noise when running a duplex system. You really need to be using Heliax. Really.

" VHF duplexer from Bridgecom, tuned by them for my frequency pair."
Couple of issues here.
Bridgecom products are pretty low tier.
Having it tuned and then shipped often results in them detuning due to handling/shock/vibration in transit. It needs to be tuned on site, or as you suggested, replaced with a better duplexer. That better duplexer needs to be tuned on site.

Probably some other stuff that others will catch, but it looks like a couple of things you need to address.

 

[prcguy]

This can go several directions at once. In my experience I've never seen a duplexer tuned at one location, shipped to another and had it work out of the box. Duplexers are very sensitive to shock/vibration detuning them and most need final alignment at the site unless someone is going to carry it there in their lap.

LMR type coax is a no no in any full duplex system. Its great for a radio or receiver or just a transmitter but not a repeater.

What is your frequency split and is it within the duplexer specs? Most VHF flat packs will barely do 5MHz.

What kind of antenna? Some are known to be avoided in a full duplex system with the same passive IMD problems that LMR coax has.

Do you have any test equipment like a service monitor? The only real way to know if your repeater is operating at 100% is to perform full duplex full power desense testing on the repeater while in service with its duplexer, feedline and antenna connected.

 

[WB5UOM]

Everything others already said.
plus maybe share tx/rx frequencies although I understand you may not want to do that so maybe share the frequency difference between the two?

 

[tweiss3]

Doesn't a licensed coordinated pair list exact location, height and ERP? Calling "I'm not sure what antenna to use" after the fact isn't quite the correct order of operations.

 

[ECFD]

Thank you to everyone for the helpful replies. I can say I'm already looking into switching to heliax... Would 1/2 inch flexible be sufficient? The total length of the coax run from antenna to duplexer shouldn't exceed 45 feet or so.

My repeater pair has a 5mhz split, and the duplexer is rated for 5-8mhz of separation. Currently I don't have any sort of testing equipment so that's something to look into. Any suggestions on basic equipment for tuning the duplexer?

The antenna is a basic fiberglass base station antenna, I'm definitely open to recommendations for an improvement there.

 

[WB5UOM]

ok what model tram antenna?
the ones I see require tuning?

 

[prcguy]

Thank you to everyone for the helpful replies. I can say I'm already looking into switching to heliax... Would 1/2 inch flexible be sufficient? The total length of the coax run from antenna to duplexer shouldn't exceed 45 feet or so.

My repeater pair has a 5mhz split, and the duplexer is rated for 5-8mhz of separation. Currently I don't have any sort of testing equipment so that's something to look into. Any suggestions on basic equipment for tuning the duplexer?

The antenna is a basic fiberglass base station antenna, I'm definitely open to recommendations for an improvement there.

For a 45ft run at VHF FSJ4-50A 1/2" Superflex Heliax would be fine with about .6dB loss. Its easy to work with and there are inexpensive connectors available.

 

[ECFD]

ok what model tram antenna?
the ones I see require tuning?

Model 6051

 

[mmckenna]

Model 6051

What does your license say?
I understand you may not want to share the call sign, but like was said above, the frequency coordination/license will specify very specific things like ERP, antenna height, power output, etc. Randomly picking an antenna may not jive with what your license allows.

So, on your license, it should show an ERP for the system. We'd need to know what that says, plus what your repeater is set to put out. That, minus the losses in your feedline, will dictate how much gain you can run. I would get an antenna that has more gain, if your license allows, something that's tuned closer to where you are licensed, and probably something better than Tram/Browning.

 

[ECFD]

Per the license the ERP is 50 watts. Repeater is also set to output 50 watts.

 

[mmckenna]

Per the license the ERP is 50 watts. Repeater is also set to output 50 watts.

OK, so you don't have a lot of room for antenna gain.

You may want to think about this carefully. The lower gain antenna that you'd be required to use to stay in compliance with your license is going to have a radiation pattern that may not be ideal for what you are trying to do.
A higher gain antenna, and the transmitter power turned down accordingly, will throw as much energy out, but more towards the horizon. That'll also help receiving from subscribers out there on the fringes. The extra gain on the receive side might help your coverage, in addition to the changes suggested above.

But local terrain may impact that. If you are really deep in the mountains, the lower gain might work better. If you are out on the flat lands, more antenna gain out towards the horizon may be better.

 

[freddaniel]

All good advice. Also consider the short jumper cable between the repeater receiver and the duplexer should be a double-shielded silver-plated cable with silver plated connectors. This cable is the most important when using a duplexer, as it reduces the likelihood of any RF from your transmitter leaking into the receiver and causing desense. Some quality RG cable types for jumpers are 214 ,142, 223, 400. You can also use Heliax, but it is stiff and may break a connector. DO NOT USE INEXPENSIVE CONNECTORS for this jumper cable.

Also, do not use coax adapters, should the cable not match the radio or duplexer.

 

[MTS2000des]

Repeaters work well when:
-Competent technician with proper tools (spectrum analyzer with tracking gen, ISO-T, jumper cables, etc) along with proper training and experience to use them
-Duplexer filters of good quality, tuned on site when connected to antenna and feedline. This is IMPORTANT as the antenna, feedline and grounding all play a part of the SYSTEM. The duplexer should offer 85db or better of rejection of the TX side. The better the rejection, the better the receiver will perform when the transmitter is keyed.
-Proper repeater sensitivity testing using an ISO-T method to determine if receiver is working in duplex mode.
-Quality feedline, proper grounding, surge arrest hardware, etc.

Repeaters work poorly when:
-Inexperienced "teknishuns" expect to buy low cost repeaters or cobble together cheap mobiles, flat pack duplexers "pretuned"
-No proper performance testing
-Poor quality coax, such as LMR400, for duplex operation
-Poor quality antenna, grounding, etc.

Transmitting into the same antenna one is receiving with at the same time is a black art. Many people can toss up repeaters, few can actually get a good performing duplex filter.

 

[davidgcet]

Per the license the ERP is 50 watts. Repeater is also set to output 50 watts.

yeah you need to turn down the repeater to offset the gain of the antenna. word of advice, whoever you use to coordinate this should be shot. there is very few times that a unity gain antenna is required to be used, and unless you fit one of those one should always try to license for a higher ERP. i've done hundreds of licenses in the past and at least in my area never been told i had to lower ERP from what i asked. i always figured on at least a 9db gain antenna even if we only planned to use a 3 or 6. you can radiate less than licensed, but not more.

i agree with others the combination of a flat pack that is poor to begin with before it was shipped "tuned" and the LMR that explains the huge difference in range between a simplex channel and the repeater. if you want this to work right spend a few hundred bucks and get a real tech to come on site and tune it and troubleshoot it. the hard part is getting them to come out on a homebrew install, you have to be willing to pay up front and that may get them out. but understand just because they tune it does not mean it will fix it. you may well have to scrap everything from the duplexer to the antenna and start over.

 

[speedway_navigator]

DMR has better range than analog, have you done SFR testing through the duplexer to verify that's the problem?
A 3 dB gain antenna would keep you close to your 50 W ERP limit if you have 1.5 dB insertion loss for the duplexer and 1 for the coax and connectors.
I don't recall any industrial business (IB to us tech types) ERP violations by the FCC, have seen them in the broadcast bands.
A 2.5 dB loss into a 3 dB gain antenna would give you about 56 Watts from the antenna if my math is right.