Mark Doiron

Not saying that TORN had cheap coax--I have no idea--but on the drive from OK to EJS I had a lot of difficulty hearing him. I had a new CB, so naturally my unit was suspect. We were on a lot of back roads through the panhandle and northern NM and AZ, so not any other CB traffic to figure out who really had the problem. Well, after arrival at Moab, TORN looks closely at his coax and where it bends (flexes, really) to go from the rear tub into the tailgate, it had split nearly clean through. Fixed that up and all of a sudden I could hear him from more than a couple hundred yards away. Get good coax and be careful about those bends (rule of thumb is no less than nine times the diameter of the coax for a bend).

mikeyfl

hxxp://www.gijoesradioelectronics.com/scripts/prodView.asp?idproduct=544

how bout this place? lengths with 3' increments.

scottrock

last time I set foot in a Radio Shack (about 6 months ago) they looked at me like I had three heads when I asked em for CB components, cable, connectors, etc.

I've hit up the closest interstate truck stop in a pinch.

Mark Doiron

Of course. But, I didn't do a very good job of making my point: Bending coax too sharp may cause failure of the coax. That failure may not be visible. That is, I've seen where the inner conductor pulled to the edge of the coax outer shield, shorting out to it. From the outside, the coax looked just fine. Albeit, in that situation it was a high power (1KW output) HF radio transmitter, so heat from the transmitter may have contributed to the problem.

The important point to remember isn't just that nothing will last if you keep bending it. It's also that bending coax sharper than nine times its diameter will eventually lead to trouble.

BMPJDS

my cb is mounted under the rearview and i used an 18ft coax and went all the way down the roll bar under the padding on my 4door and had no problems. i have atleast 2ft extra

TurkeyFreer

hxxp://www.compressionconnectors.com/

(replace xx with tt)

Mark Doiron

Good point. Heat can cause the center conductor to sag though the dielectric in long lengths of coax carrying high RF power. But, it was the bend and heat in combo that cause problems in the case I described. It's just like the wheels in your JK when turning: The outer part of the coax must run a longer length than the inner part, and sharp bends are going to pull the outer part of the radius in towards the inner part. Apply a little heat and the center conductor can actually "melt" its way through the dielectric.

Of course, in the case of CB radios, high power should not be an issue (assuming the rig is legal). But, if a bend is too sharp something must give. Either the center conductor will pull towards the inner part of the radius, or one of the center pins on either end will be pulled back (or, in the case of weak soldering, the center conductor will be pulled out of the center pin).

AK4Dave

Mark.....I also seem to remember hearing that if a coax is bent "just a bit too far" that it can creat an impedance mismatch as well. Maybe at high power levels? Does that sound logical to you?

Mark Doiron

It's the same issue I've been trying to describe: The center conductor can pull out of the center, creating the problem you describe. Heat (from operating environment or from high power emissions) just exacerbates the problem, as does age (gives time for the center conductor to work its way through the dielectric). Since impedance of RF cabling is dependent upon the size and spacing of the shield and the center conductor, any changes in that geometry will create an impedance lump--basically a location where the impedance is mismatched from what the cable is otherwise rated. This reflects RF energy, creating a higher than optimum SWR.

TNJKER

ok not to high jack the thread but how big of a difference is there between the dollar per foot radioshack brand r58* coax and say a firestick or competitor coax that is 2-3 times the price? I am simply setting up a cb for trail use only... maybe 2 mile range tops.