Magic length for coax in a CB installation

Is there a magic length for coax in a CB installation ?

Some say 18 feet is the magic length, what do you think ?

 

[jimbo74]

i say 18 is good. coil up the extra though

 

[JamminG4Jim]

18ft is correct. 1/2 Wavelengh of cable between the transmitter and antenna at CB Frequencies = close to 18 ft.

Don't coil the extra coax too tightly, cuz it will act like a electrical choke and cut down your output power, along with making it do other strange things....

Jim

 

[Ray Hutchinson]

Coax length

Sorry guys, but the "magic length" or some multiple of the wavelength is an old wives tale or myth. The shortest run of coax you can have, providing enough length so that you have gentle radiius turns, not tight kinks is the best. The only time the length of coax is important is when you want to run two "phased" antennas (like the truckers do with antennas on each mirror). then the length connecting the two needs to be electrically equal so that the antennas transmit equally, or "in Phase".

Jammin is correct about coiling extra coax tightly though, It can create a "choke" effect, also, bending tight turns in the coax cable can change the distance between the center conductor and the grounded shield. This creates a change in the impedance of the cable (nominally 50 Ohms) at the tight bend. Although this isn't a completely correct analogy, think of it as a similar effect as a kink or tight bend in your garden hose interfering with the flow of water.

 

[Locksmith]

I concur with Ray. It is an old wives tale and does not stand up to examination.

 

[JamminG4Jim]

Whatever.

I know what I know. Unbalanced feedline needs to be cut at lenghts resonant to the operating frequency of the transmitter to obtain optimum output / Minimum SWR's

I WILL cave and say i've saw it done with less than 18ft, but I think the ERP (effective Radiated Power) of the rig was less than optimal.

Some times "old wives" aren't TOO far off the money!

IMHO

 

[Sandy]

Sorry Guys

I put these treads in to get conversation started...
This one always starts people talking.

I agree with Ray, run only the length necessary to reach your antenna and do not create any sharp bends.
Also, the coax is 50 ohm no matter how long it is, and the only thing that can cause an "unmatched" condition is the antenna not being tuned properly or if someone decided to install an "11 meter" amp, none of which are any good.

Of course this is my two cents, for what it is worth.

Tnx for the replies.

 

[Ray Hutchinson]

No Worries!

Sandy,

Not a problem! There's nothing wrong with good healthy discussion, and disagreement just as long as it doesn't get vindictive, personal and insulting. None of these things has happened here!

Keep up the conversation starters... that keeps us all interested and thinking.

73,

 

[Ray Lobato]

Sandy,
Good topic, I also thought that there was a "magic length, 18ft", but guess I was wrong, however for the sake of argument, dose anybody have facts to back up their position?

 

[Carolina Express]

Back when I was deeply into the CB thing, we used to use an SWR meter. Using a cable of the shortest length necessary to reach the antenna, we'd then use small made-up cables of different lengths and a female/female connector to try the different cables in order to fine-tune and get the lowest SWR reading possible. Then when final length was determined, made up a single cable of that length.

I don't know if this was any great benefit, but it showed up on the SWR meters. Now, I just run whatever it takes to reach the antenna...

 

[Ray Hutchinson]

Coac Length "facts?

Ray L, and all:

Well, Ray L, after extensive searching of two "Bibles" of Amateur Radio: "The ARRL Handbook" 1998 Ed. (1191 pages long), and "The ARRL Antenna Book" 18th Ed. (728 pages) both published by the Amateur Radio Relay League (ARRL) the only references to the length of coaxial feed lines for fixed station or mobile radio antennas is that they should be kept as short as possible to minimize the rf loss in the cable run. (The typical loss of good quality RG58U 50 Ohm coax is about 2.5 dB per 100 feet @ 28 mHz- very close to CB @27 mHz).

The only exception listed is, as I mentioned before, when using multiple "phased" antennas, and another is when an impedance matching section of single or parallel coax of some other impedance, say 72 Ohm TV coax(RG59) is used to achieve some different value, say 35 Ohms (2 1/4 wavelengths of parallel 72 Ohm coax) to bring the transmission line into closer match with an antenna that has a low feedpoint impedance. then they switch to 50 Ohm to finish the run to the transmitter.

CB 1/4 wave whips, and base loaded mobile antennas are virtually all engineered to have a 50 Ohm feedpoint impedance and have no need for these impedance matching sections, and therefore use a straight length of 50 Ohm RG58.

Since both of these texts delve pretty deeply into the mathematics and thoery of antenna design and installation, as well as lots of practical suggestions, I firmly believe that if there were any "magic length" of 50 Ohm coax to feed mobile antennas, there would be a definite technical description included. The very lack of any mention tells me that it isn't a factor.

 

[JamminG4Jim]

Well, that's that! Who could argue with the Bible's????

I guess all this time i was just following a "Rule of Thumb" guideline of 18Ft.

Silly me.

73's de Jim AA0UP

 

[Sandy]

Never Silly for Opinion !

Everyone has an opinion, and it is always good to state it in my book.

I have alway enjoyed the "Magic Length" discussions on CB forums. There are many people that believe in the magic length, I guess if it works for you then do it.

As Ray has stated longer length mean more loss, all though at the mobile lengths and the 11 meter frequencies the losses are low.

Keep it on all four and your antenna up...

 

[Ray Lobato]

Carolina Express, Ray
Thanks for the answers. That's what I love about this board, you ask for facts. You get facts, and I learn alittle more everyday.
Thanks again guys.