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Coax length for single antenna setups

splitz

splitz

NAXJA Forum User
Location
Nola
After doing some research, I have found conflicting information as to the "proper" length of coax for CB radios in a single antenna setup. Some have said that ideally you should use coax that is 1/2 a wavelength long. Others have said that the shorter the run the better.

Firestik, and others, have recommended using coax in multiples of 18'. In this document where Firestik says that "You tell the guy with the $30,000 vehicle that he must drill a hole in his roof so he can use a 9' cable," does this mean 9' is an acceptable length? One thing that confused me about his was in the very next paragraph they state "On that note, when you have 18' of coax going to a radio that is only 8' away, what should you do with the other 10'? " If it is only 8' away, why not use 9' of coax and not worry about the extra 10'?

To make it even more confusing, this document says that the run should be as short as possible, and even if you where trying to achieve 1/2 wave length, it would be 14' 4" according to the velocity factor of RG-8.

So which is it? 14' 4", multiples of 18' (which I learned is 18, 36, 54 etc. 9 is a divisor), shortest distance, or something else?
 
I wouldn't think length of the cable matters...more the length of your antenna, and where it is mounted........
 
I have always gone by the 18" rule, thats what I was taught in the Navy in both basic electronics and every other receiver/transmitter school I ever went to. Now if you are using one of those am/fm/cb antennas be aware they are a compromise and usually a bad one at that.
 
I have heard both ways also. I bought 18 foot of the best coax I could find. I only needed about 9 foot but took the rest and looped it in a 12" circle. It works for me. Remember to buy a best antenna you can and then the radio. I use a preditor 10k and it works great. I have used cheap and expincive CB's and that make little different.
 
RichP said:
I have always gone by the 18" rule, thats what I was taught in the Navy in both basic electronics and every other receiver/transmitter school I ever went to.
Click to expand...

Did you mean 18' rule?

Or should I start the Navy jokes? :)

Semper Fi,

Jim
 
I'm still failing to understand why cable length matters....if it did, you'd think CAT5 and/or regular coax cable length would matter....but it doesn't....I think you are ok with any length of cable. its your antenna that gets tuned....you just make sure you do it from the CB end of the coax....
 
I have been told on several occasions from different people who sell CB's to Truckers and tune CB's that you should use as little cable as possible and NEVER loop it to take up the extra length.

Having said this, I believe that a decent antenna is the most important part of the total equation. Then the location of the decent antenna. I think that the length of the coax is probably not going to make any noticeable difference to the total equation as long as the routing of the cable is reasonable.

Just my opinion,
Michael
 
The length actually does matter, but not to the degree some companies make it sound to be. Keep it as short as possible while still allowing an acceptable amount of slack. Coiling up any extra will negatively affect performance.
 
I think everyone is in agreement about the coiling.

The question is if matching the length of the coax to the the 1/2 wavelength of the radio signal really impacts the impedance of the circuit to the point of noticeable signal degradation.
 
splitz said:
The question is if matching the length of the coax to the the 1/2 wavelength of the radio signal really impacts the impedance of the circuit to the point of noticeable signal degradation.
Click to expand...
Nope. Just go to your local library and pick up an AARL Antenna Handbook, or just read this http://www.stu-offroad.com/misc/myth-1.htm
EDIT: Actually, I should have written, "Sorta." The "nope" was me thinking specifically of 18 feet.
 
Last edited:
2xtreme said:
I have been told on several occasions from different people who sell CB's to Truckers and tune CB's that you should use as little cable as possible and NEVER loop it to take up the extra length.

Having said this, I believe that a decent antenna is the most important part of the total equation. Then the location of the decent antenna. I think that the length of the coax is probably not going to make any noticeable difference to the total equation as long as the routing of the cable is reasonable.

Just my opinion,
Michael
Click to expand...
This is what I have always been told. When you set your SWR you will be then matching the radio, cable, and antenna all together as one.
 
My setup is a 12' cable, cobra 19 ltdII, 4' tunable firestick mounted directly above the tail light. I reached out 20 miles last night with the cheapest radio cobra makes. So I believe if you've got a good thick cable and adjustable antenna you'll be alright with even a 12 footer.
 
scoobyxj said:
This is what I have always been told. When you set your SWR you will be then matching the radio, cable, and antenna all together as one.
Click to expand...

Good point to add. Tuneing the antenna after installation.

Michael
 
Being a long haul trucker and a ham operator I'll put in my 2 cents.

In my Peterbuilt I run two attennas. But your asking about a single set up here anyway...
SO on my XJ I run 18 foot. Pumping a Galaxy 33 into 18foot to a 4 foot K40 mounted off the rear bumper just above the rig. This set up works good. I have sat in my driveway (California) and talked to people on the east coast.
Any extra cable should not be coiled. Rather run back and forth. In the XJ I was able to hide all the cable behind the panels and had just the right wength to get the the attenna.

I am a support of the 18 foot rule. I had shorty cables made for the big truck and it didn't send or recieve vary well. I stuffed in the 18 and all was better.
 
Not one of you guys touched on the important factors involving feedline length.

Coax Impedance and rating.

RG-8U does not have the same impedance or ratings as -8X or -213, for example. Sure many different values will fall under the 50 ohm or 72 ohm heading, but impedance transformation does take place. In fact, a feedline is an impedance transformer.

You cannot simply "assign" a length to a coax run without taking into account the coax itself, and other factors, such as but not limited to, feedpoint impedance, and input impedance.

I've said it a million times here, I'll say it again. Your 1:1 match doesn't mean squat with an X 10% out of match. :D

The best thing to do if you do not have a grid dip or analyzer like a MFJ-268 is to stick to matched systems, out of the box.

If you want to know more, follow the advise above, get the ARRL handbook and the antenna book.

73 de K0IOA/6 /MM /M

--ron
 
Rev Den said:
It's 4 flipping watts.

It is not all that important.

Welcome to the real world.

Rev
Click to expand...

I can't hear you. :D

QRP is all the more reason to have an efficent system.

--ron
 
Timber said:
Nope. Just go to your local library and pick up an AARL Antenna Handbook, or just read this http://www.stu-offroad.com/misc/myth-1.htm
EDIT: Actually, I should have written, "Sorta." The "nope" was me thinking specifically of 18 feet.
Click to expand...

Ok, I can get behind that. But how do you account for the variation in impedance like Capt Ron pointed out?

Oh, and Rev Dan, it may only be 4" but I want to work it as best as I can ;)
 
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