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Coax length

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Mrplow285

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Hi new to the forum. Have a question I can't seem to find a straight answer for. Running an icom 5021 with a 5/8 ant in my work truck and was considering changing over to RG-8 coax. My question is regarding coax length. I was always under the impression and I also remember reading and being told years ago that the coax needed to be 18' coil up the acces in a large loop but do not cut it shorter. Is there any truths to this? Most mobile kits come in that pre assembled amount. I only need 6 feet and would just order a custom made coax. Or do I need the full 18'? I just figured shorter may be better? thanks
 

WA0CBW

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The coax has to long enough to reach from the radio to the antenna. Unless you are using the coax to "tune" the antenna the length is not important other than the longer the coax the more loss in signal from the radio to the antenna. Most antenna mounts are designed for RG58 sized coax and may require some modification to change over to the larger RG8. In practice the difference between 18 feet of RG58 and RG8 will not be noticeable unless you are operating on the higher frequencies (400 to 800 MHz).

Just for information a 1/4 wavelength of coax (or multiples of 1/4 wavelength) inverts the impedance from one end to the other and a 1/2 wavelength of coax (or multiples of 1/2 wavelength) repeats the impedance from one end to the other. If you want to know the "true" SWR at the antenna use a 1/2 wavelength (or multiples of 1/2 wavelength) of coax between the antenna and the SWR meter.
 

prcguy

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You forgot to mention the velocity factor when using 1/2 wavelength or 1/4 wavelengths of coax to repeat impedance etc.
prcguy


The coax has to long enough to reach from the radio to the antenna. Unless you are using the coax to "tune" the antenna the length is not important other than the longer the coax the more loss in signal from the radio to the antenna. Most antenna mounts are designed for RG58 sized coax and may require some modification to change over to the larger RG8. In practice the difference between 18 feet of RG58 and RG8 will not be noticeable unless you are operating on the higher frequencies (400 to 800 MHz).

Just for information a 1/4 wavelength of coax (or multiples of 1/4 wavelength) inverts the impedance from one end to the other and a 1/2 wavelength of coax (or multiples of 1/2 wavelength) repeats the impedance from one end to the other. If you want to know the "true" SWR at the antenna use a 1/2 wavelength (or multiples of 1/2 wavelength) of coax between the antenna and the SWR meter.
 

Mrplow285

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Well thanks for the credit. Might have givin it to me to soon! I have a small grasp on wavelength. So correct me if I'm wrong. When you refer to 1/2 wavelength with VHF freq would that be 3ft? Or multiples of that. 3,6,9?
 

wa1nic

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That 18' thing is a CB radio wives tail that doesn't apply to VHF at all.

RG-8 for a mobile installation sucks. It is hard to work with.

RG-8X is the same size as RG-58 and almost as good as RG-8. For a mobile installation where the cable length is going to be under 20 feet, RG-8X will work just as fine as RG-8 and you will have much less hassles snaking it around.

Personally, I like to use the the marine grade RG-8X... it is super flexible. The outer jacket is white (absorbs less UV light).

Rick
 

teufler

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I go along with rg8x. rg8 andrg8x have loss, in 100 feet. the loss in 20 feet is so small that you cvan not tell the difference, plus rg8 wilol have a hard time bending in a car or truck. Most installations, you see rg58 so a jump to rg8x is am improvment.
 

prcguy

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RG-8X is bigger than RG-58 and its approximately the same OD as RG-6. White marine grade coax will degrade faster than the average black color coax mostly due to UV damage, the black color inhibits UV from penetrating the insulation compared to white.
prcguy

That 18' thing is a CB radio wives tail that doesn't apply to VHF at all.

RG-8 for a mobile installation sucks. It is hard to work with.

RG-8X is the same size as RG-58 and almost as good as RG-8. For a mobile installation where the cable length is going to be under 20 feet, RG-8X will work just as fine as RG-8 and you will have much less hassles snaking it around.

Personally, I like to use the the marine grade RG-8X... it is super flexible. The outer jacket is white (absorbs less UV light).

Rick
 

Voyager

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the loss in 20 feet is so small that you can not tell the difference
+1 on that. It's not worth the change when you are talking about 20 feet or so. You will do a lot of work for very very little (if any) improvement.
 

jeatock

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Performance powerboat builder Reggie Fountain once said "ALL of my boats will run 70 MPH. After that, its only $10,000 per additional MPH. How fast do you want to go?"

My point is that the difference between a quality brand foil-shielded RG-58 properly installed to a hole mount NMO antenna base and anything else is minimal. You can spend many days and many dollars for a very tiny improvement, which only a high end radio service monitor can detect. For mobile installations stick with the basics.
 

Gashliquor

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((462/frequency)*velocity factor)= 1/2 wavelength of coax



Two year old message, Your math is wrong


It's 492 x (vf) / Freq ( Assume .66 for RG58



492 x .66 / 146.000 = 2.22 Feet is 1/2 wave



492 * (vf) / freq = 1/2 wave
 
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wa1nic

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The white coating of RG-8X has UV inhibitors in it. Because it is white, it also reflects much of the light that black insulation absorbs. That is why they color it white. On a boat mast, the sun gets very intense.
 
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