how much does coax matter?

A problem with cable is how well the shielding works. Some best coax has two layers of woven shielding. Shielding is expressed as "%" sometimes. 100% means there is no gap between the shield wires at all. 75 Ohm TV antenna cable is made that way, and some 50 Ohm. The point is not that you need 99% or the like, but that the cable should not skimp on copper for the shield. Some suck in that department. The RF can get out, and into a vehicle's electronics.

Next, the closer to 1:1 the SWR is kept, generally, the lower the amount of RF that will be present on the shield. RF on the shield will radiate like a poor antenna, but maybe enough to get into the electronics.

The more power coming from the transmitter, the more these issues are made worse. But shielding is not all of it, the SWR is most important. If a 50 ohm cable is given a 50 ohm match on both ends, it won't radiate. This is why they always say to put the antenna matcher at the (supposedly 50? Ohm) antenna, not at the (50 Ohm) radio. if there is a mismatch at one end, it will radiate all along its length, in some spots more than others.

This said, have you put a 50 Ohm load on the antenna end of the stock cable and then checked the SWR to see if there really is a problem with the cable?


This cable has quite loose shielding:
RG58U-BEL
http://www.rfparts.com/rg58u-bel.html
It's very cheap and likely OK for stock use and many uses. 35 cents a foot. Hopefuly most "CB shop" cable is at least this decent. Who knows that is made by odd brands and in China and what is thrown into a big truck to save a few cents.

RG214 is a very good cable. $3.95 a FT and the large 0.405 diameter like RG-8, but you only need to buy and install it once, right?
http://www.rfparts.com/rg214-u.html

RG-8X is Miniature RG-8 $0.44 per ft. better shield than the RG-58U. look at the center cond. also.
http://www.rfparts.com/rg8x-bel.html

A smaller sized double shielded cable. 0.195" diameter. RG-142B. mil spec cable. $3.45 per ft.
http://www.rfparts.com/rg142b-u.html

Below 30MHz hardly anything measurable is going to leak from those, especially the last three. RF issues are most likely to come from a bad match or directly from the antenna to the electronics. 100W is nothing for any of these.

 

95% is actually very good for a single braid shield. I wish that the crap sold in truck stops was as good as this cable, but it's not. I've seen crappy coax with 40% shielding. I know of at least one drive-in theater that ran a length of the cheap crap from their projector booth (where the FM transmitter was) over to the fence, then down along the fence to a dummy load; about a 500' run. They could be heard a mile away.

That said, I wouldn't recommend this cable for mobile use. The solid center conductor will eventually break from the vibration in a truck. Use a stranded center conductor (RG58A/U). This cable is intended to be used in a fixed location to connect equipment that is rarely moved.

Again, not recommended for mobile installations. Aside from the sheer bulk of the cable (you need a 5/8" hole to run it), it has a minimum bend radius of over four inches. To do a 180 degree bend, it can't be any tighter than a dinner plate. Any tighter, and the cable will eventually be damaged. (With the vibration, the center conductor will eventually migrate through the dielectric [center insulation] and short out to the shield.) Don't even think about running this stuff through a door hinge.

This is what I use. Cheaper than the .405" stuff, a LOT easier to run, and nearly as low loss. If loss is really a concern, I use the LMR-240-UF version made by Times Microwave.

 

I have used RG8 in place of Radiax to intentionally have the signal radiate thru the shielding, affectedly the the coax becomes the antenna. I have used this in many of the High rise bldgs in Los Angeles is the portable radios will work with 90 percent coverage theu the bldgs, these cable runs terminate into a Motorola Base Station on one end and a 1/4 wave antenna on the other, some of the runs also required a inline rff amp . But the point is it does and can radiate thru the braiding.

On 27 mhz I believe tha center conductor is more important then the shielding. RF travels around the surface of the center so a cable with a larger center conductor is will tend to work out better, double shield, dont need them on 27 mhz.