Coax length

[4x4mike]

I couldn't find a definitive answer by searching so I thought I'd throw it out there.
I want to install 2 NMO mounts on the roof of my 4Runner. I'm somewhat limited on location which will require at least 18 feet of coax, 20 would be best.

When searching for NMO's I found a lot of 15.5' and 17' lengths. In my old offroad.com days I seem to remember folks saying coax had to be a odd number in feet or be in multiples of 3. Is that even right?

One of my NMO's will be for my 2m and the other for my CB. I found a guy with an eBay store and he said he could make me one any length I wanted. That's cool but I wanted to know if it would work. He said it would but I remember the internet telling me more back in the day.

Any help would be appreciated, thanks.

 

[BigJimCruising]

No worries, you'll be fine! Even further if needed. Don't worry about odds or evens, just use what you need to get the job done.

 

[4x4mike]

Thanks. That's what I figured considering guys run antennas from the roof of their house to their shack.
I wonder why there are guys out there selling random lengths? Like to the half foot.

 

[UK4X4]

usually to get the best reception and power you need to match the cable length to the antenna

Everdardt super tiger - plus the tuned cable out of the box from them as a matched set

with a standard Cobra from a truck stop got 11 miles out of the box

No one else in the convoy could hear the guy that left the convoy, while we were having a conversation

Any cable plus antenna will need tuning for the best reception

I used to tune fiberglass boat antenna in a past life on the night shift ...

 

[4x4mike]

What do you mean by match the cable length to the antenna. What if you change antennas depending on the terrain?

 

[slomatt]

Mike,

I've seen a lot of places claiming you should run an 18' cable for CB use since it corresponds to a 1/2 wavelength at 27 megahertz. This doesn't make a lot of sense since the signal travels slower in cable (measured by the velocity factor) so it's not a 1:1 comparison.

There's a good writeup on this out there somewhere..... found it!

http://www.stu-offroad.com/misc/myth-1.htm

- Matt

 

[gary in ohio]

usually to get the best reception and power you need to match the cable length to the antenna

NO YOU DONT..... For some junk CB antennas they may apply this but for a NMO mounted VHF/UHF antenna this does not apply.

You will need to tune the antenna whip for best SWR but dont worry about the coax length

.[/QUOTE]

 

[1911]

NO YOU DONT..... For some junk CB antennas they may apply this but for a NMO mounted VHF/UHF antenna this does not apply.

You will need to tune the antenna whip for best SWR but dont worry about the coax length

+1, just use the minimum amount needed to reach. It's not that hard to solder on PL-259 connectors if you need to cut it; even easier if you have one of those little pencil butane torches.

Some cheap cb antennas can use the coax as a counterpoise in lieu of a proper ground plane, but even in that case you would be much better off to use a decent 1/2-wave antenna or better yet, mount the antenna over some sheet metal on your truck so that you have a ground plane.

 

[4x4mike]

Mike,

I've seen a lot of places claiming you should run an 18' cable for CB use since it corresponds to a 1/2 wavelength at 27 megahertz. This doesn't make a lot of sense since the signal travels slower in cable (measured by the velocity factor) so it's not a 1:1 comparison.

There's a good writeup on this out there somewhere..... found it!

http://www.stu-offroad.com/misc/myth-1.htm

- Matt

Yes, Stu's is where I read it. Thanks for the help everyone.

 

[ScoutII]

Hey Mike glad to see you over here. aka R290

I used USACoax.com
They will make anything and for cheap too. USA based company. They sell on e-bay too

Those premade MNO cable assy they sell everywhere have crap NMO mounts in my view. I got some solid brass/nickle plated ones off e-bay. After mine came loose more times then I will admit, I got the heavy duty one.

I find these are NOT very good for offroad/adventure use. That little wing that tightens up is just too small.

This one has a solid base to tighten up.

 

[4x4mike]

Hey there R290. I went and ordered some up in 20'. They don't have the HD tightener like you showed though. I wonder is a little loctite in the threads would help?

Now I just need to poke some holes in the roof. I'm a little excited.

 

[GlennA]

In my experience,a little blue locktight does the trick. I like the lighter duty style because they are "break-away". If your antenna gets caught in something, the fingers on the mount should break before it rips a hole in your roof. I've installed hundreds of NMO mounts and had to replace dozens that have been broken by low hanging limbs etc. Usually, there is no damage to the vehicle.

 

[ScoutII]

I sure you will be fine. I upgraded to the beef mount after hammer the roof flat.:Wow1: I was part of a Rallye and it was more like lets fine the most brush overgrown trails on the planet. I was dumb, as I should have just pulled over the removed the antenna. It just deform the roof metal a little bit, but messed up the NMO connector and needed to be replaced.

 

[1911]

That's the great thing about an NMO mount - changing out the 5/8-wave whip for a 1/4-wave flexi-whip or rubber ducky on the trail can be done in 10 seconds.

 

[ScoutII]

1911 Exactly! I bought a short antenna for my Scion. I've been swapping it over when needed and has worked great. I was very surprised at signal quality of my little NR72BNMO Dualband Mobile Antenna by Diamond too.
I just bought a Larson NMO-2/70 which has been the standard for a lot of years and will see how that works out.