UHF Antenna cable shortening

Thicker Coax cable also have a larger bending radius, bend it too tightly and the cables impedance goes astray ( never, never kink it).
Thicker cables are harder to pull through small orifices in fire walls, etc.

If your antenna relays on a 'tuned' coaxial cable length then it is a poor design for its intended use. To improve the SWR, at a particular frequency, one alters the antenna length, NOT the cable to it???

Coaxial losses do not really warrant the thicker cable for the short distances involved. Now, if the antenna was mounted at the back of a 30' caravan and the radio was in the car then, perhaps, its worthwhile.

But a better QUALITY cable is always a good idea, it should be a well known brand (Belden, perhaps), have a tightly knit weave , tinned, shield rather than the crappy alfoil or loose weave plain copper stuff one sometimes sees and also have a tinned, stranded centre conductor - not a solid one.

The cable can be joined by using male/female coaxial connectors like the BNC type ( these are almost water proof) but their expense (and fiddly exercise to fit if you havn't done it before) would make a new cable a cheaper option. I take it you are using an antenna base where the cable plugs into and not an antenna with a fixed cable?
Klaus

The amount of cable changes the amount of resistance, ergo changing the length of cable can alter the amount of load the antenna has, therefore changing it's SWR.
Most cars have a rather large gromit on both passenger and driver sides of the firewall that you can easily fit 25mm conduit through it you had to. All conections BNC or otherwise will cause signal loss to a certain degree, you can get both crimp and solder type BNC connectors, solder is more fidly, but you don't require a $60-$70 tool to crimp them.

Jeff, radio frequency is an interesting topic, where many misconceptions, like the one you mention in your first sentence, that are passed amongst laymen.
FYI, resistance has little to do in that context, the correct word is impedance. (Look up the definition of 'impedance')
The antenna cable, having the correct impedance to propagate the RF signal, is just a means of getting it to the antenna. IF the antenna is tuned properly the RF signal will radiate. If its not tuned properly, some of the outgoing signal will be reflected back, setting up a standing wave and it is the ratio between the outgoing and reflected power that the SWR refers to.

So you see that the antenna, being properly tuned, will radiate the outgoing signal without having to worry about critical cable length - AS LONG AS THE CABLE & PLUGS HAVE THE CORRECT IMPEDANCE.
You do not want the coax cable to radiate the signal as well, as that what is happening if altering its length alters the SWR, indicating a problem elsewhere.

Yes, there is some attenuation along the cable, nothing runs loss free, so, the shorter the better, especially for higher frequencieslike UHF. But, this loss can easily be exceeded by connector mismatches, poor connections at the antenna, water in the coax jacket, etc.
BNC connectors are a much better choice since they DO match the cable's impedance (if they are 50 Ohm impedance). There are better connectors around, but at a price (N type). The cheap 239 style connectors are a waste of time with UHF frequencies, IMO.
I have used BNC connectors extensively in both the solder and crimp versions, I find the solder style more reliable where there could be a strain on the cable due the gland and clamp nut. The last two items also make the connector just about waterproof.
Klaus

If you take the top nut off the extension base, then unscrew the black part you can then see the end of the cable. Be careful not to lose the little nylon insulator. Pull the cable down through the middle. Obviously mine is not soldered & have never a problem with reception. You can then also unscrew the antenna base & just leave the spring part bolted on.

Cheers

Jackablue