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Best upgrade for two repeater system

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pboyd

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Jun 5, 2015
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I run the radio system for an event. We recently moved from having to have a small portable rack for the repeaters to a trailer with built in rack space. We've had a few minor issues in years past, but mostly our radio system works pretty well... but I'm still not fully satisfied with the range we are getting on it.

What I have now:
2 Motorola XPR8400 repeaters (Capacity Plus)
2 Off-Brand Chinese mobile duplexers
2 Unity Gain Fiberglass Antennas
2x 50' lengths of LMR400 cable
4x LMR400 "jumper cables" between repeaters and duplexers
1 50' antenna mast

What is the weakest link in this system and what would be the ideal replacement and the more budget conscious replacement?
 

ts548

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Start with getting a good set of duplexers. Then I would look at antennas that aren't unity gain. If you want to stick to fiberglass then thats ok but look at some with db gains. You could also look at dipole antenna's. Height is also going to play a part, 50' is good but if you can get higher then that would be best. How big of an area does this event cover?
 

prcguy

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What is the frequency separation of the repeaters? Mobile duplexers are useless for close spaced repeaters and frankly any two repeaters within the same band with close spaced antennas. If your frequencies are very close together like less than 1 MHz, I would consider a hybrid type transmitter combiner and receiver combiner with adequate receiver preselector and dedicate one antenna for transmit and one for receive.

If frequencies are a MHz or more apart I would consider band pass/notch type duplexers to help keep the transmitter of one repeater from desensing the other. You should also have isolators on each transmitter to reduce the chance of creating Intermod when both transmitters are keyed up. Use good quality double shielded coax on all RF paths from the repeaters to any duplexer, combiner, etc. RG-142, RG-400 or RG-214 is standard.

Loose the LMR-400 or any LMR type cable in the RF path. It can create low level Intermod problems due to the dissimilar metals used inside the coax. Go with RG-213 or RG-214 or Superflex Heliax, anything but LMR coax.

Finally, the overall range of the repeater will depend heavily on the antenna(s). Get the biggest, longest, highest gain commercial quality antennas you can carry and put up.
 

RFI-EMI-GUY

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I concur with PRC .

The cheap duplexers have to go and a coax upgrade is a small price to ensure performance. Superflex might be a bit brittle for a trailer mounted installation.

Be prepared to measure receiver desense on the finished installation to make sure that neither receiver is blocked when either or both transmitters are keyed.

Sent from my SM-T350 using Tapatalk
 

pboyd

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We're limited to 50' by the tower. We have one of these Telescoping Fiberglass Push-Up Masts - Max-Gain Systems Inc and like it a lot better than our old metal tower that was really really hard to put up and take down by hand with a bunch of volunteers.

We're at 13MHz difference between the two repeaters (I have a third frequency pair assigned that is only 50kHz off another pair that I don't use).

And yes, I've considered a transmitter combiner, but the cost of that + the cost of the amplifiers needed to overcome the 3dB insertion loss are a bit out of my price range.
 

radioman2001

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If you have a separation of more than 1mhz between the 2 channels I would go with one 5db gain antenna on the top of your tower for RX on both radios using a splitter and a single pass can, or you can even use those CCR duplxers as rejects to the transmit frequency. Your RX antenna is the most important since it deals with the lower signals portables transmit. Using the CCR duplexers you are probaly desensing your own equipment, and don't use them. Mobile duplexers are not made for close separation, so find a good repeater 4 can (with 13mhz separation you might be able to get away with just 2) minimum duplexer that does close spacing of the band pass type, and 2 circulators to isolate the 2 transmitters. What I am quoting is basically a poor mans combiner. I had one at a tower site for over 30 years running 2 250watt stations this way at 16mhz separation. Loss was less than 1db. Add another 5db antenna located 10 ft from the top for isolation of the RXer's, that 10 ft should give a few more db. Coax should be RG-213 or similar, as stated LMR-400 uses aluminum as part of the shield ( It's ok for simplex, but I never use it for duplex systems), and can create noise once it starts corroding. Make sure what ever coax you use that the center and shield are copper you will pay more, but it works better.

Quote"
I have a third frequency pair assigned that is only 50kHz off another pair that I don't use.

Use it in simplex as a Tac-2 type of frequency.

I don't know how big an area you need to cover, but I had a 13watt MX UHF portable repeater on T-band that I used in Dayton every year and it covered the whole complex, and even inside with a 5DB stick at 20 ft.
 
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RFI-EMI-GUY

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We're limited to 50' by the tower. We have one of these Telescoping Fiberglass Push-Up Masts - Max-Gain Systems Inc and like it a lot better than our old metal tower that was really really hard to put up and take down by hand with a bunch of volunteers.

We're at 13MHz difference between the two repeaters (I have a third frequency pair assigned that is only 50kHz off another pair that I don't use).

And yes, I've considered a transmitter combiner, but the cost of that + the cost of the amplifiers needed to overcome the 3dB insertion loss are a bit out of my price range.
If this system is intended primarily to support portable radios, the insertion loss of a TX combiner is not an issue. If you ever go to a third repeater you will be spending that money on another antenna system as well.

Sent from my SM-T350 using Tapatalk
 

pboyd

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If this system is intended primarily to support portable radios, the insertion loss of a TX combiner is not an issue. If you ever go to a third repeater you will be spending that money on another antenna system as well.

Sent from my SM-T350 using Tapatalk

Yes, it is mostly serving portables, so I guess I can see your point that loosing 25W and still having ~25W of output power isn't the end of the world in that case.

Assuming I went that route, we're looking at:
1 multicoupler for the RX antenna (something like a Stridsberg MCA204M I guess?)
2 filters to prevent in-band feeding back to the receivers? Or would the ~55dB from 10 feet of vertical separation between rx and tx antennas be enough there?
1 TX combiner (something like a Sinclair TJ3 series?)

So somewhere in the $7000 range? Or am I mis-specing something?
 

Firebuff880

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You should talk with the folks at DBSpectra -- Home | dbSpectra

Not sure, but I seem to remember they gave a SAR group I was with previously a good price since we could produce 501(c)3 non-profit documentation. But it may have been some other vendor / component of what we were building..
 

prcguy

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There are a lot of good used repeater parts available in my town and I got a Telewave 2-ch UHF hybrid combiner with isolators for about $150 and band pass cavities run about $75 each used in perfect condition. I also got a 4 cavity band pass preselector for about $100 and an Angle Linear UHF preamp for a fair price. Those parts would build what you need to have a filtered and amplified master receive antenna with about a 2MHz wide receive window and combine two transmitters cleanly. This would work best using your two frequencies that are only 50KHz apart.

Your vertical antenna separation will add a little isolation but maybe only in the 10dB range or a little more. You want the TX/RX filtering to give you a big isolation number, maybe 90dB would be a goal, then a little vertical antenna separation will add to that.

Using your 13MHz separated frequencies you would be better off using good pass/reject type duplexers with isolators on the transmitters and a separate antenna for each repeater. With 13Mhz separation you probably have a transmitter and receiver freq that are about 8MHz apart without the ideal transmitter notching for that split.



Yes, it is mostly serving portables, so I guess I can see your point that loosing 25W and still having ~25W of output power isn't the end of the world in that case.

Assuming I went that route, we're looking at:
1 multicoupler for the RX antenna (something like a Stridsberg MCA204M I guess?)
2 filters to prevent in-band feeding back to the receivers? Or would the ~55dB from 10 feet of vertical separation between rx and tx antennas be enough there?
1 TX combiner (something like a Sinclair TJ3 series?)

So somewhere in the $7000 range? Or am I mis-specing something?
 

pboyd

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What kind of range are you getting, and in what kind of environment?

It's an outdoor environment, lots of trees, lots of various campsites and projects with lots of crazy EMF noise. Best case I get a few miles, but worst case I get about a mile and need the worst case to be about 2-3 miles. From what I can tell worst case occurs pretty randomly.
 

pboyd

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It looks like the higher gain antenna I get, the more beam width I loose. For example, a 10dBd gain antenna cuts the beam width to 6 degrees from 60 on my current unity gain antenna. Since beam width is the measure of where one gets half antenna power, in practice would I be in a worse spot in the 800 some odd feet difference in beam width coverage with that higher gain antenna?
 

radioman2001

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You are way overthinking this, we are not talking about a large city or county radio system. A 10db antenna is overkill and if you aren't putting it up some where high up it's not going to do anything for you. This whole scenario sounds more like you need more height, and if it's a campsite I would do a topographical survey using G earth and find the highest point and put you equipment there. 5db in what I assume is a UHF 450-470mhz system is the happy medium for antennas. A little gain with not a whole lot of tilt, think of a gain antenna as squashing a donut flatter, and that's basically what antenna propagation looks like.
By the sounds of it and by your listing of equipment that you own you are doing more to cause interference to yourself than anything. I have never found to be much outside interference on UHF to cause a breakdown in reception, more like co-channel operations not monitoring before talking, or your own poorly filtered equipment
 

pboyd

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You are way overthinking this...
By the sounds of it and by your listing of equipment that you own you are doing more to cause interference to yourself than anything. I have never found to be much outside interference on UHF to cause a breakdown in reception, more like co-channel operations not monitoring before talking, or your own poorly filtered equipment

Sounds like your vote would be better feedline and better duplexers then?
 

pboyd

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Any recommendations for good duplexer brands/models? I'm in the UHF 450-470 band if that makes a difference?
 

R8000

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I'd recommend consulting a local reputable two way radio shop for assistance. You would then have the local support for when you have issues.
 

12dbsinad

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I'd start with the duplexer, feedline and antenna. First and foremost, you're probably getting a couple of DB loss or more with the cheap china mobile duplexer and their inherently lossy. I've never had good luck with those duplexers, especially at more than 10 watts. I've had issues where if the unit was moved around, squeezed, etc, the thing would go haywire and really cause issues. So, that'd be the first thing I'd throw in the trash. Followed by the coax.

Just curious, have you ever done a PM check when you set this thing up? De-sense, etc? If not the thing could just be flat out lousy the way it sits and causing a degradation in range. Better components properly tuned could solve this.
 
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