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GMRS coax length


Guest 8NannyFoe
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Hi guys, another newbie here.  I am an old CB'r from a few decades ago and a recent discovery of GMRS has awoke an old flame of alternate communication.   Anyway I am setting up new base station.   I have about a 33' run (absolute minimum).   Its a fairly long run for this frequency and I don't see any readily made antenna cable runs made from RG58.   Loss is most likely the reason I am assuming.  From what I am getting cable length can and will make a difference in the GMRS experience.  To long or to short can effect performance.   I found two articles that implied this however neither gave the lengths which probably means its not an easy answer.   From what I understand a 1/2 wave length cable vs a 5/8 vs a full wave length cable will impact transmit and receive performance.   Given my absolute minimum length 33 feet  ( a longer than usual run) can anyone please advise me a good length and better quality cable to make this job complete and very good.   RG58U is the cable a friend recommended but he didn't expand on why and suggested just cutting it at any length and length doesn't matter.  I'm fine with the cost of this cable, but would really like the best performance out of my purchase even if it means I need to purchase a longer run to make sure it is efficient as possible.  May be over kill or underkill, so feel free to comment on that as well.   What length is the proper length of cable I need to fit my job?   Thanks in advance!!

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Do not use RG-58 cable at these frequencies for anything other than short runs. You are much better off using RG-8, LMR-400, Beldin 9913 or some type of heliax, typically 1/2 inch or larger, for the run. Also it is strongly advised to use "N" type connectors. You may need an "N" type to a PL-239 adapter for the end going into the back of the radio unless you make a cable with it on the end.

 

You can check here for coax cables. They may custom manufacture a cable for you.

 

https://www.theantennafarm.com/catalog/index.php?main_page=index&cPath=344&sort=20a

 

http://www.cablexperts.com/cfdocs/index.cfm

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Do not use RG58U, loss is very significant on 460MHz. Just look at what Lscott listed. Another reputable company that sells cables with connectors is dxengineering.com They have 25', 50', 75' runs of their LMR400 equivalent with with N-connector or PL259. Coax loss charts: https://w4rp.com/ref/coax.html

 

If radio-cable-antenna-connectors system is well-matched through all the components, the length of the coax will not matter. The 1/2 wavelength cable allows you to tune your device (usually antenna) more precisely with some equipment. Once match achieved, coax can be any fraction of wavelength. So, don't worry about it unless you plan to invest into instruments and knowledge. And 1/2 wavelength on GMRS is roughly 325mm, so quarter-inch less or quarter-inch more and you are out of optimal length anyway. Velocity factor is also needed to be considered...  Do not worry about coax length. The shorter the better, but that's it.

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Thank you.   I went with LMR-400.   The antenna I bought has the N type connector and I had Times custom make me the cable with N type connector and the 259 on my radio side.   I jumped the gun a bit when buying the antenna though.  I bought a 6db gain antenna.   I didnt know what that meant.   It is now my understanding that Unity gain would actually have been a better choice for hilly areas?   

Do not use RG-58 cable at these frequencies for anything other than short runs. You are much better off using RG-8, LMR-400, Beldin 9913 or some type of heliax, typically 1/2 inch or larger, for the run. Also it is strongly advised to use "N" type connectors. You may need an "N" type to a PL-239 adapter for the end going into the back of the radio unless you make a cable with it on the end.

 

You can check here for coax cables. They may custom manufacture a cable for you.

 

https://www.theantennafarm.com/catalog/index.php?main_page=index&cPath=344&sort=20a

 

http://www.cablexperts.com/cfdocs/index.cfm

 

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... It is now my understanding that Unity gain would actually have been a better choice for hilly areas?   

Depending on how tall the hills are and how close you are to them. It's likely be fine, if they are hills and not El Capitan (https://en.wikipedia.org/wiki/El_Capitan) and you are not at the bottom of it.

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Hi guys, another newbie here.  I am an old CB'r from a few decades ago and a recent discovery of GMRS has awoke an old flame of alternate communication.   Anyway I am setting up new base station.   I have about a 33' run (absolute minimum).   Its a fairly long run for this frequency and I don't see any readily made antenna cable runs made from RG58.   Loss is most likely the reason I am assuming.  From what I am getting cable length can and will make a difference in the GMRS experience.  To long or to short can effect performance.   I found two articles that implied this however neither gave the lengths which probably means its not an easy answer.   From what I understand a 1/2 wave length cable vs a 5/8 vs a full wave length cable will impact transmit and receive performance.   Given my absolute minimum length 33 feet  ( a longer than usual run) can anyone please advise me a good length and better quality cable to make this job complete and very good.   RG58U is the cable a friend recommended but he didn't expand on why and suggested just cutting it at any length and length doesn't matter.  I'm fine with the cost of this cable, but would really like the best performance out of my purchase even if it means I need to purchase a longer run to make sure it is efficient as possible.  May be over kill or underkill, so feel free to comment on that as well.   What length is the proper length of cable I need to fit my job?   Thanks in advance!!

 

In my opinion RG 213 will be more then enough for your application assuming GMRS@50 watts.

Since your cable run is min 33 feet. Now if you were running 100 feet of cable especially underground then you would use LMR 400. GMRS is max of 50 watts of power@tt. I can see if you were running 100 watts.

 

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  • 2 weeks later...

Thank you.   I went with LMR-400.   The antenna I bought has the N type connector and I had Times custom make me the cable with N type connector and the 259 on my radio side.   I jumped the gun a bit when buying the antenna though.  I bought a 6db gain antenna.   I didnt know what that meant.   It is now my understanding that Unity gain would actually have been a better choice for hilly areas?   

 

Real Times LMR-400 is probably the ticket for that application. If you can solder coax connections and the radio side is a UHF connector (pl-259/so-239 I forget which is which) you might consider cutting the N connector off and putting the right one on to not have any adapters. N is the way for exterior for sure.

 

Depending on how hilly it is, yes, less gain could perform better since it's not as focused on the horizon - but unity might be a wee bit excessive. Gain is good if you can stay in the radiation pattern of the antenna. Its so flat where i'm at, a machinist might not be able to get it any flatter so I want as high of a gain as I can get.

 

If you want to do some horse trading or whatnot I might be interested in a 6db uhf antenna, then you could get something else.

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  • 2 weeks later...

The exact cable length does not matter.  Shorter is better.

The only exception would be if you were using a matching network on the radio end of the cable.

But still, it would have a random effect... as long as your radio is pretty close to nominal 50 ohms (non reactive Z)

I won't get into smith charts so just trust me.

 

Pasternack makes custom cables and Digikey might also do customs.

Not cheap but maybe worth it for the quality etc.

Digikey would be cheaper than Pasternack.

 

Keep in mind you could use a much heavier cable outside and run something smaller for the last few feet coming into the radio.

Mix and match can make sense.

 

Vince

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