Coax Length Question

There is nothing magical about 18' of coax for CB, other than by repeating that urban legend, dealers have been able to sell people more coax than they need.

This link explains it more eloquently than I can...
http://www.stu-offroad.com/misc/myth-1.htm

 

Geez, I just answered this yesterday; I even thought you were the OP. Oh well, answer is that as long as the antenna is resonant ("tuned") for the frequency your radio is putting out, and you're using 50 ohm coax, then the shortest coax that will reach from the radio to the antenna mount is the right length. Plus a fudge factor of a foot or so that I add at each length to allow for the inevitable "fiddlers" who will keep taking things off and swapping in meters and stuff, and "trying other things out", which will eventually wear out the coax where it makes the bend into the connector. By having a little extra coax at both ends, there will be enough extra to allow for installing new connectors a couple of times. It will also make it easier for a pro to get a somewhat bulkier piece of test gear in after the radio, or inline between the radio and coax, etc.
There are formulas for using fractional wavelength sections of coax as phase delay lines, or for impedance transformations, but they need very specific measurements and are for very limited frequency selections. You can't blindly put up, say,. 18 feet of 50 ohm coax and decide it's going to do *anything* specific for the entire CB range, *except* have twice as much signal loss as 9 feet of the same coax.

Finding a fractional wavelength in coax entails knowing both the exact operating frequency AND the velocity factor of the cable.

But for any single 50 ohm CB antenna, just use the shortest coax that will reach, with the little bit of spare as noted above.
Hope that clears it up some for you.
73

Edit: As usual, MsJamie beat me to it, and with a more succinct answer. So..... what she said

 

I recently also posed a problem in radio where one operates a multiband HF mobile antenna. If the coax length thing were true, then a ham, for example, would have to have about 10 different coaxes, ranging from 60 feet to 17 feet in a mobile! Yet ham and commercial operators successfully cover from 29 MHZ down to 3.8 (and below) with ONE coax at no particular length. WHERE would you, for example, put up to 60 feet of coax in an S-10 truck? The simple answer is, for but the most extreme cases, TUNE THE *ANTENNA*, NOT the coax. It is the ANTENNA that radiates the signal, NOT the coax. Don't sweat the details; go to the source--the ANTENNA!!!!

Only on CB does this question continue to crop up----over and over and over and over again! Few operators in any other service even give coax a second thought!

GF

 

The scientific answer for those running roger beeps with over-modulated boat anchors is easy. Always use coax a few inches too short to reach the antenna.

OK but it sounded funny in my head a while ago.

 

When I put my slip-seat rig in a truck, I have good low SWR with 18 foot of coax, and a high SWR with 9ft of coax. Is that not a "difference"?

 

How high of an SWR with the 9 ft. as opposed to the 18 ft. are we talking?

The difference in SWR could be a faulty coax in itself i.e. a break in the wires on the inside. Or even if someone used a coax with a different impedance (i.e. 75 ohms as opposed to 50 ohms)

Yes sometimes longer coax can "fool" an SWR meter into thinking there's a lower SWR. So can an antenna tuner. But the bottom line is CB radio coax, is nothing more than a piece of shielded wire, that connects the antenna to the radio.

Different lengths of coax should make little to no difference in an SWR meter reading, and if they do, I'd start by investigating the coax to ensure the coax is the right impedance, with no breaks in the wires, corroded connectors, etc.

Especiallly if the coax in question is the crap that truck stops sell. I have come to find, that stuff is always hit or miss.

 

Yes, that is a difference. The reason for that is your antenna is not tuned and matched properly, and the excess coax is acting as an impedance transformer.

If you would check the SWR directly at the antenna, you'll find that your SWR is quite high.

Using the feedline to match the antenna to the radio is like putting a brick on the gas pedal, and regulating your speed by changing how hard you press on the brake. It works, but your gas mileage (transmitted power) takes a hit.