GMRS Antenna Wire Length and Position Question

[nwoods]

I vaguely remember from my HAM training a decade or so ago that the antenna CABLE length was a very specific thing, based on frequency and so forth. If I recall, a 2M HAM set up used an 18 foot long cable to connect to the antenna.

I recently purchased and installed a Midland MXT115 GMRS mobile radio, and it came with a rather lengthy antenna cable ( I didn't measure it). My install was fairly short and efficient, leaving about 8 or 9 feet of cable left over. I coiled the excess cable into a loop and stuffed the coil under the center console. Did I do the right thing? What should I do differently? Is there an ideal diameter for the coil? Does it matter?

I haven't had it out on the trail yet, so I have not done any real testing, but I would prefer to get it set up as correctly as possible before heading out into the desert.

 

[JimBiram]

Hi Nathan,

No problem with coiling the excess wire as they factor that into the antenna design. In your case the body of the car becomes both the ground plane and counterpoise. If you really want to boost that radio in the future, you can always get a 36-38” antenna that gives half wave for more DB gain. Those short antennas that come from Midland are at best 1/4 wave but they are convenient and should give you enough performance for trail use.

Jim


Sent from my iPhone using Tapatalk

 

[Climber Rob]

No issue with the extra wire coiled up, in general. I generally just cut to length and re-solder the connector where it's needed.. Putting a Ferrite (Toroid) around the loop can help suppress RF. Mix 61 is good.

 

[Bill Ruttan]

I agree with Jim B that the entire length of the supplied coaxial cable is factored into the antenna/cable design, but have read, elsewhere, that the excess cable should NOT be coiled into a loop.

 

[jadmt]

I emailed Midland and asked and they said it was ok to just cut however much off I needed. This was for the antenna that comes with the MXT105. I asked if I would have to check the swr and the said no.

 

[nwoods]

Interesting range of responses. There is a helpful thread discussing this, with some links to FAQ’s from Firestik and other sources. The consensus is that Jim Biram is correct.

Thanks guys!

 

[DaveInDenver]

If your antenna whip and mount are done correctly there is no magic coax length.

If changing the length of your coax is making a significant impact to SWR then it has become part of the antenna, which you need to avoid. The most likely reason why is your antenna isn't finding it's ground and the coax shield is part of the ground.

Excess coax can be wound up but you should avoid making it circular coil unless you add a choke. It's better to squeeze it into a bow tie shape with a zip tie in the middle.

Putting a Ferrite (Toroid) around the loop can help suppress RF. Mix 31 is good.

Mix 31 (and 43) are going to be marginal on 70cm and GMRS. They work best over about 3 to 300 MHz. Mix 52 or 61 are good choices for UHF chokes, they work best over 200 MHz out to 1 or 2 GHz.

https://www.fair-rite.com/61-material-data-sheet/

https://palomar-engineers.com/ferrite-products/ferrite-cores/ferrite-mix-selection

https://palomar-engineers.com/ferrite-products/ferrite-cores

 

[jadmt]

I would love to find a how too on cutting and splicing the cable on these midland antennas. they come 20’ or so and 10’ would be plenty

 

[DaveInDenver]

I would love to find a how too on cutting and splicing the cable on these midland antennas. they come 20’ or so and 10’ would be plenty

There's any number of videos and tutorials you can watch. Search for things like terminating coax, crimping or soldering connectors, etc. Pretty sure they just using RG-58 coax and PL259 connectors, nothing fancy.

If you do try, get a few connectors to practice. PL259 isn't the most friendly connector to use, especially to solder.

 

[jadmt]

There's any number of videos and tutorials you can watch. Search for things like terminating coax, crimping or soldering connectors, etc. Pretty sure they just using RG-58 coax and PL259 connectors, nothing fancy.

If you do try, get a few connectors to practice. PL259 isn't the most friendly connector to use, especially to solder.

it is RG-174 and a little connector.

 

[DaveInDenver]

it is RG-174 and a little connector.
View attachment 698301View attachment 698302

That's actually easier since they are usually double crimps or at least the outer braid is but the center pin soldering is just quick touch. The downside is you have to get the right dies but OTOH you're not trying to properly solder the braid on a RG-58 or larger without melting the dielectric.

But I'm kind of surprised they'd use RG-174 for GMRS. Even at mobile installation lengths the loss can start to be significant. At 465 MHz Belden 8216 (RG-174/U) will have 2.1 dB of loss at 10 feet. The specification is 19 dB/100 feet at 400 MHz.

https://catalog.belden.com/index.cfm?event=pd&p=PF_8216

That's means you'll have 38% loss at 10 feet or IOW 5 watts transmitted gets 3 watts at the antenna before any mismatch you might there.

At the original 20 feet the loss will be 4.2 dB, so 62%, so 5 watts in gets 1.9 watts at the antenna.

Compared to other options, for 20 feet lengths at 465 MHz.

Belden 8240 (RG-58): 1.83 dB (about 34%)
Belden 9258 (RG-8X): 1.69 dB
LMR-200 (similar to RG-58): 1.41 dB
Belden 8237 (RG-8): 0.92 dB
LMR-240 (similar to RG-8X): 1.08 dB
LMR-400 (similar to RG-8): 0.55 dB (about 12%)

It's of course easier to deal with RG-174 but the loss at UHF gets to be a problem so most of the time you have to balance reasonable performance of RG-58 against ease of routing. Even the losses for larger 0.405" coax (RG-8/LMR-400, etc) at 70cm can get to be a problem on long runs (roughly 3 dB/100'). But in a car trying to use RG-8 is beyond a PITA so you do the best you can.

 

[jadmt]

That's actually easier since they are usually double crimps or at least the outer braid is but the center pin soldering is just quick touch. The downside is you have to get the right dies but OTOH you're not trying to properly solder the braid on a RG-58 or larger without melting the dielectric.

But I'm kind of surprised they'd use RG-174 for GMRS. Even at mobile installation lengths the loss can start to be significant. At 465 MHz Belden 8216 (RG-174/U) will have 2.1 dB of loss at 10 feet. The specification is 19 dB/100 feet at 400 MHz.

https://catalog.belden.com/index.cfm?event=pd&p=PF_8216

That's means you'll have 38% loss at 10 feet or IOW 5 watts transmitted gets 3 watts at the antenna before any mismatch you might there.

At the original 20 feet the loss will be 4.2 dB, so 62%, so 5 watts in gets 1.9 watts at the antenna.

Compared to other options, for 20 feet lengths at 465 MHz.

Belden 8240 (RG-58): 1.83 dB (about 34%)
Belden 9258 (RG-8X): 1.69 dB
LMR-200 (similar to RG-58): 1.41 dB
Belden 8237 (RG-8): 0.92 dB
LMR-240 (similar to RG-8X): 1.08 dB
LMR-400 (similar to RG-8): 0.55 dB (about 12%)

It's of course easier to deal with RG-174 but the loss at UHF gets to be a problem so most of the time you have to balance reasonable performance of RG-58 against ease of routing. Even the losses for larger 0.405" coax (RG-8/LMR-400, etc) at 70cm can get to be a problem on long runs (roughly 3 dB/100'). But in a car trying to use RG-8 is beyond a PITA so you do the best you can.

this is way over my head lol. I do know with my little 5w MXT105 does really well. My buddy also has the same one and we can talk when we are both in our driveways and probably at least 6 miles as the crow flies with a city between us. I would love a shorter wire tho as the way I run my antenna the extra cable is just balled up on my dash.

 

[Climber Rob]

Mix 52 or 61 are good choices for UHF chokes, they work best over 200 MHz out to 1 or 2 GHz.

Agreed. My typo; good catch.

 

[Climber Rob]

this is way over my head lol. I do know with my little 5w MXT105 does really well. My buddy also has the same one and we can talk when we are both in our driveways and probably at least 6 miles as the crow flies with a city between us. I would love a shorter wire tho as the way I run my antenna the extra cable is just balled up on my dash.

He's just breaking down the losses on high-loss cable like RG-174 to show how much power the antenna is actually seeing. By using a lower-loss cable, you will more efficiently and effectively utilize the power that your transmitter/transceiver is capable of.

 

[jadmt]

He's just breaking down the losses on high-loss cable like RG-174 to show how much power the antenna is actually seeing. By using a lower-loss cable, you will more efficiently and effectively utilize the power that your transmitter/transceiver is capable of.

i like the convenience and ease of running the thin cable out my door with no issues of squishing or damaging it. I run it under the pillar door seal up to the top of my truck. I couldn’t do that with thicker cable. I need to find a comm shop to see if they can shorten it to 10’ probably would be cheaper than buying the crimpers.