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What type of coax for portable repeater?

rural_radio

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I'm building a portable repeater setup for my RV to extend the range of my family's GMRS handhelds. What type of flexible coax would y'all recommend that would withstand the repeated up-and-down of the mast and antenna yet not cause PIM issues from the dissimilar metals in the foil and braid?

I've got a roll of RG213, but that seems like a lot of loss to deal with at UHF. Is there a flexible version of Heliax?
 

prcguy

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How long of a run do you need? There is a flexible Heliax and 1/2" FSJ4-50B is not bad, I have hundreds of feet in use here. You can coil in safely in about 2ft dia and its not that heavy. Loss at 460MHz is about 1.17dB for 50ft. I would try to limit the run to maybe 35ft and the loss would then be in the .82dB range.
 

rural_radio

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Thanks, @prcguy !

My mast is is 28 ft tall, so a total length of about 35 ft sounds right. I'll have a look at those. I think I've seen folks talking about using superflex for jumpers to connect antennas to hardline.
 

mmckenna

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Thanks, @prcguy !

My mast is is 28 ft tall, so a total length of about 35 ft sounds right. I'll have a look at those. I think I've seen folks talking about using superflex for jumpers to connect antennas to hardline.
I've used superflex on repeaters and it works just fine.
I have a couple of networked UHF repeaters at one of our remote sites. These are completely off the grid, solar powered, and mounted in cabinets on skids. There's a 10 foot pole that supports the antenna. With a run that short, it made no sense to transition back and forth between regular Heliax and superflex. At 10 feet or so, I used Heliax FSJ1-50A. It's a 1/4" coax and flexes very easily.
They also make a 3/8" version.
For the longer run, the 1/2" would be a better choice, like PRCGUY said.
 

12dbsinad

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Any type of hardline would be ideal as well as superflex, just be cautious that it can still kink so you'd need to be extra cautious during deployment and break-down. Once it kinks, you're done. If it's a really short run, you'd want RG-214, not RG-213.
 

rural_radio

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Thanks @mmckenna and @12dbsinad

I appreciate all of you sharing your experience and knowledge.

I know RG213 wasn't a good choice but other than that, I was really only familiar with the Times Microwave LMR series of cables.

I know the basics of duplex operation, but usually only as a user. I'll freely admit I'm not qualified to engineer a system from the ground up.
 

a417

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I've done mobile (onsite repeaters for casualty vehicles) repeaters and unless you really need the height afforded by a mast, you may just get away with using a repeater with good antennas mounted on the RV chassis - which would eliminate that run entirely.

Is the height afforded ( as well as the wear and tear of setup/breakdown) by the antenna something that makes or breaks this plan?
 

rural_radio

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You know @a417, I'm embarrassed to say I've never tried this setup without the mast. 🤔 There wasn't a convenient place to permanently mount an antenna directly, so we just put it up every time as part of our routine for setting up the camper.

But of course you're right, and since we're already setup for simplex operation using the RV as a base, it only makes sense to compare the range with the antenna at roof level. If the range is comparable, a permanent antenna would certainly simplify things, even if I had to pay for a professional installation.
 

vagrant

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A. What range do you need in miles? ( Terrain and buildings will of course interfere )
B. Have you already switched to an improved handheld antenna for GMRS like one of the Diamond 77CA versions? They work well for GMRS. Would this upgrade on each radio possibly solve your range need? Even if you have a repeater, a better antenna on the handhelds definitely helps. What antennas are you using now?

Some things to consider for your build...

Antenna Height: ( Many things can and will interfere with line of sight range )
1. Line of sight at 12' AGL is around 5 miles. ( Presuming your RV roof is that height )
2. Line of sight at 28' AGL is around 7 miles.
3. Line of sight at 5' AGL, a person holding a handheld, is around 3 miles. ( Add that distance to either #1 or #2 )

Antenna:
1. What would be the gain of the UHF/GMRS mobile antenna on your RV roof?
2. What would be the gain of the mast mounted antenna?
 

rural_radio

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Thanks for the input @vagrant ,

My goal isn't so much to achieve a certain range in miles, but to be able to achieve the same reliability of communications between handhelds that we currently enjoy between the base station and handhelds. We might currently get seven miles between the base and handhelds, but I know we don't currently get three miles between handhelds. That said, if someone is in the camper on the base station, we can usually maintain comms for any distance family members go on foot. But if the girls go kayaking in one direction and the guys go hiking in the other, the two groups often lose comms after the first hill if there's no one back at the base station to relay.

I've got a couple of the larger antennas for the handhelds, but we don't use them all the time. Like so many things, it's a compromise. We'll use the larger antennas if the radio will be clipped to a backpack, but for kayaking with the radio clipped to a life jacket, family members can only get smacked in the face so many times before they don't want to carry a radio at all. The larger antennas seem to make more of a difference when there are only woods or buildings in the way, but in my experience, they don't help a lot in overcoming terrain. If others have had different results, please let me know.

My current "base" antenna up on the mast is really a Laird mobile antenna with an NMO ground plane kit. The claimed gain is 5 dB, and I'm assuming that's dBi, since they didn't specify. It looks like a 5/8 over 5/8 collinear.

I kind of assumed I would have to upgrade the antenna when switching from a regular base station to a repeater, but I think I'd have to stick with a mobile antenna if I permanently mounted the antenna. I'll try my current antenna lower to see if it's the height or just a more efficient antenna that's responsible for the better performance. I suspect a higher gain antenna would focus so much energy at the horizon that we wouldn't get any better performance over rough terrain, but I've got a big 8 or 10 foot base station antenna at home I could take down and try at camper-roof-height.

I say RV, but it's a trailer, not a motorhome. There are no mirrors or ladder for an easy permanent antenna mount. If we get pretty good results at the lower antenna height, and the big higher gain antennaI works better than the mobile antenna, I guess I could have the local radio shop install some sort of mount up high on one side of the RV, and just install the antenna on it instead of setting up the mast each time.

No one has asked, but I'll go ahead and mention that we almost always use low power. With the base/handheld setup, one end of the conversation is always a handheld, so it didn't make sense to pump out 50 watts. The vehicles have mobile radios, and higher power at the repeater might allow handhelds fairly close to camp to communicate with mobiles farther away, but that would be more of a bonus than a design goal.
 

a417

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<snip>

My goal isn't so much to achieve a certain range in miles, but to be able to achieve the same reliability of communications between handhelds that we currently enjoy between the base station and handhelds. We might currently get seven miles between the base and handhelds, but I know we don't currently get three miles between handhelds. That said, if someone is in the camper on the base station, we can usually maintain comms for any distance family members go on foot. But if the girls go kayaking in one direction and the guys go hiking in the other, the two groups often lose comms after the first hill if there's no one back at the base station to relay.
I highlighted some very important things that you said. Just based on your information, a repeater on your current base antenna will probably get you close to your 7 miles. If there's a repeater controller sitting there doing what it's supposed to do, you get that 7 miles range between handhelds, as long as they can hit the base station....and no one has to be there at the base station to relay.
 

rural_radio

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Yeah, that's what I was trying to get across. Functionally, the repeater will be extending the range of the handhelds, but what I'm really after is getting the reliability of the base station when no one stays behind. I'm satisfied with the base-to-handheld range we have now.

It sounds like my next step should be to figure out what specifically is contributing to that reliability, the height, the antenna's pattern/gain, or both, and optimize for that. Originally, I just put an extra mobile antenna I had up on a mast, stuck a trailer-hitch pole mount in the camper's hitch receiver, and hooked up a radio. It met our need for a base station at the time, and I didn't really think too much more about it.

But if I'm going to go to the trouble of installing a repeater, I owe it to myself to make the installation as durable as possible and easy enough that my family could set it up if I wasn't there. I think some practical testing of different antennas at different heights will be required. Theory is fine, but if this is going to be a "buy once, cry once" installation, I'll need to see some practical results.

A permanently installed antenna is appealing, but I suspect the added height of the mast is contributing a good bit to the performance in hilly terrain, so I'll have to test that out to be sure. I originally used an antenna with some gain because it's what I had in the garage, but now y'all have got me thinking about the effects of antenna gain and vertical beamwidth. I suppose I'll need to also compare a quarter-wave or something else with unity gain.

Thanks to everyone for all of your help and input.
 

a417

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Yeah, that's what I was trying to get across. Functionally, the repeater will be extending the range of the handhelds, but what I'm really after is
<snip>
Thanks to everyone for all of your help and input.
you can get your hands on a used repeater controller for a pittance, and do a proof of concept with another mobile and your current base radio...then you can decide how much your money is worth.
 

paulears

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On UK OB vans, 213 is quite happily used on the pump up masts. Heliax or hardline really isn't an option because the stuff is not designed for movement and constant coiling and uncoiling - the copper fractures. Cable loss is NOT a be all and end all. The height is the most important factor - and sets the distance in practical terms, a few dB really doesn't matter. I have also never worried about the dissimilar metals issues and intermod. It's important at multi-user masts where these things can really cause bizarre and difficult to trace issues - but they really don't cause issues on a basic pump up or guyed mast in the context discussed here. We put H100 on one of the pump ups because it coiled itself rather nicely around the base when it was down. H100 was very stiff and it survived quite well.
 

prcguy

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Superflex Heliax is designed for movement and constant coiling and uncoiling. The 1/2" size FSJ4-50B is perfect for this application and has less loss than RG-213.

On UK OB vans, 213 is quite happily used on the pump up masts. Heliax or hardline really isn't an option because the stuff is not designed for movement and constant coiling and uncoiling - the copper fractures. Cable loss is NOT a be all and end all. The height is the most important factor - and sets the distance in practical terms, a few dB really doesn't matter. I have also never worried about the dissimilar metals issues and intermod. It's important at multi-user masts where these things can really cause bizarre and difficult to trace issues - but they really don't cause issues on a basic pump up or guyed mast in the context discussed here. We put H100 on one of the pump ups because it coiled itself rather nicely around the base when it was down. H100 was very stiff and it survived quite well.
 

paulears

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That's interesting - I've not seen this over here in use locally but it's available, I'll investigate for a project coming up. It's got a respectable minimum bend radius, but I wasn't aware it's rated for repeated flexing. Useful!
 
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