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Commercial Radio Antennas - Please keep discussion related to professional, commercially used antennas and antenna systems for the two-way radio industry. Topics for the use of these antennas on amateur bands are accepted here.

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Old 11-02-2018, 5:19 PM
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Default best coax for portable uhf repeater

I'm in the process of building a uhf repeater for a local non profit. I have most of it figured out except the coax. We don't own the property we will be using the repeater on, so after the event is over we have to tear it down and take it with us. This means i need coax that can be rolled and unrolled without breaking, while keeping loses to a minimum. I will need at least 75 feet since my antenna is going on top of a 45 foot antenna mast.

I was originally going to use lmr-400uf/9913f7 but after some research i found that it isn't suitable for duplex. Superflex looks like it has a solid center conductor so i cant imagine that would last very long.

What kind of coax would be best?

I realize that this isn't amateur radio, i wasn't sure where to post my question.
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Old 11-02-2018, 5:49 PM
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How large an area are you looking to cover? If not a large area, the coax losses really aren't going to make that much of a difference. I have run portable repeaters like that using RG8 or even RG58 (gasp!) without any issue. Yes, the losses are large and add up, but over short distances, it really doesn't matter.

I know the purists will be irritated, but I don't care.
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Old 11-02-2018, 6:35 PM
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So lmr-400uf would work? I need to cover 1.5 - 2 miles, but with lots of trees.
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Old 11-02-2018, 8:40 PM
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Quote:
Originally Posted by demandzm View Post
I was originally going to use lmr-400uf/9913f7 but after some research i found that it isn't suitable for duplex. Superflex looks like it has a solid center conductor so i cant imagine that would last very long.
LMR400 will work as long as it stays completely dry and doesn't get any corrosion on the outer shield.

But, it's less than ideal.

LMR-400 has a solid center conductor, so does heliax.
1/2" Heliax superflex would work fine. Just make sure the guys rolling it up don't roll it too tight, and that would go for any coaxial cable.
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Old 11-02-2018, 9:27 PM
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I would also recommend 1/2" Superflex Heliax. I think its slightly more flexible than standard LMR400.

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Originally Posted by mmckenna View Post
LMR400 will work as long as it stays completely dry and doesn't get any corrosion on the outer shield.

But, it's less than ideal.

LMR-400 has a solid center conductor, so does heliax.
1/2" Heliax superflex would work fine. Just make sure the guys rolling it up don't roll it too tight, and that would go for any coaxial cable.
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Old 11-02-2018, 10:56 PM
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I will do some shopping around for a good price. I found some 3/8 superflex for a good price and the loss difference between the two is only .3db. I have 9 months before it needs to be done so i have plenty of time to shop. Thanks for your help.
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Old 11-03-2018, 1:37 AM
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I would second the superflex Heliax, but I did setup a VHF portable itinerant repeater for a friend with 75’ft of RG-214 double shielded copper braid cable and it has worked well for events. Easy to roll and unroll for moving around. For what it’s worth!
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Old 11-06-2018, 7:07 PM
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I would suggest using Belden 8267 which is a RG213 cable. It has a stranded center conductor, is very durable & inexpensive for a quality cable. As you noted, I also would avoid using LMR400 due to the poor duplex performance and a solid center conductor.

Newer Heliax cables, including Superflex types, have problems "kinking" the outer thin copper shield under the plastic when the cable is twisted. In other words, the cable outer shield will collapse in a small area resembling a hose kink, thereby causing a distortion in the cable impedance, and will sometimes cause a DC short. The kinks are often hidden by the heavy plastic outer jacket. So in repeated field use, you could have a dozen or more kinks, unknown to you without sweeping the line with an analyzer. Therefore, because of the solid center conductor and the kink issue, Heliax is poorly suited for any application requiring constant and frequent bending. This includes most test cables.
The ideal cable for your application would be a "flooded" version of RG213, which would add silicon gel around the shield, providing weather resistance. Unfortunately, the only 50 ohm flooded cables made are similar to LMR400, like the Belden 7810WB cable. The gel should also eliminate the duplex noise problem LMR400 type cables are plagued with. Only time will tell.
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Old 11-07-2018, 3:07 AM
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What about LMR400UF with stranded center conductor?
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Old 11-07-2018, 1:20 PM
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Its still LMR400 with potential IMD problems. I think 1/2" Superflex Heliax is the best solution and its less loss than LMR400, although a bit on the lossy side at UHF for a 75ft run. For a UHF repeater I would try to limit the run to less than 50ft. I have hundreds of feet of 1/2" Superflex Heliax around here and it coils safely into 24" or smaller rolls and is fairly indestructible.

My second choice might be RG-213 or RG-214 but only if you could limit the run to maybe 30ft or less.

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What about LMR400UF with stranded center conductor?

Last edited by prcguy; 11-07-2018 at 1:27 PM..
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Old 11-08-2018, 9:25 PM
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Heliax is the ONLY way to go for repeaters.
LMR and most coax will only cause desense isssues.
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