CB aerial

[NJV6]

Hey, does someone that knows about radios answer this?

I have a second hand unit with a cut aerial cable right at the mount. I want to take 2 inches out of the length of the cable and I know they are tuned but will 2 inches make a difference? It's for UHF.

[UBZ]

It's the antenna length that is tuned . You can cut the cable as much as you want , won't make any difference

[MihiT]

It's the antenna length that is tuned . You can cut the cable as much as you want , won't make any difference

In theory correct, as long as your antenna (probably a monopole) and your radio are both grounded, ie the shielding on the coax is connected to metal at both ends. This creates an RF shield around the centre conductor, otherwise your feedwire forms part of the antenna, this is where you come in to "it making a difference"

Personally, I run odd-multiple*wavelength feed wire between unit and antenna and seal the ends with glue/heatshrink to prevent oxidation. The difference is probably sfa, but it makes me feel better.
I also tie my sticks to a groundplane, bonnet or roof depending where they're mounted and this does have a notable effect on range.

Modern radios aren't nearly as susceptible to mismatched antennas as old ones and generally have enough electronic wizardy to make up for it.

TLDR: if your feedline is isolated from your antenna and not part of it: it's length is irrelevant.

[NJV6]

Thanks for the reply, even if it made minimal sense to me!! To many big words

odd-multiple*wavelength feed wire
tie my sticks to a groundplane
TLDR

I think you are saying, if the shield wire (I know what that is!) is grounded then the length will be ok to be changed? The antenna is wire so effectivly is the feedline? The shielding is not grounded at the aerial end as it is mounted onto plastic. It will be grounded at the unit end I imagine as the unit needs earth to go and it is using the factory screw co ax connections. Do I run an earth wire to the plastic mount to ground the shielding at the aerial end?

[MihiT]

I think you are saying, if the shield wire (I know what that is!) is grounded then the length will be ok to be changed? The antenna is wire so effectivly is the feedline?

Yes. if you change antenna length you run in to issues. so as long as the feed wire and antenna are two "electrically separate" things the radio will not "see" the feedline so it's length is irrelevant.

The shielding is not grounded at the aerial end as it is mounted onto plastic. It will be grounded at the unit end I imagine as the unit needs earth to go and it is using the factory screw co ax connections. Do I run an earth wire to the plastic mount to ground the shielding at the aerial end?

If you have a typical antenna base:

you will find that the hole up the middle for your center conductor is electrically isolated from the nut/lug/clamp that grips onto the vehicle/bullys/z bracket.

In the above picture your center conductor solders to the tab at the bottom and your braid/shield gets unwound and screwed to the sharp metal bit which bites into the vehicle's metal - this forms both the coax shield/ feedline ground, and your antenna's groundplane.

check all connections for continuity, if your shield is touching the center conductor you will get very poor performance, usually happens when a strand of braid falls out of the bunch and touches the center pin.

you also wont get good grounding if it doesn't bite into bare metal, which often happens with the Z brackets, a continuity check from the coax braid at the radio end (unplugged) and your negative terminal will tell you if you have good ground all the way.

[NJV6]

To bring this up again, i have to mount both 27 and 477 MHz aerials. I got the aerials with the unit, the 477mhz one has no continuity from the centre of the co ax to the aerial, so I pulled it apart for a look, the centre piece went up the middle of a coil and stopped and was not attached to anything. The entire aerial was then earthed. I think this must be an aerial for mounting on plastic and doesn't need an earth???

The AM aerial also had both the feed wire and the shielding of the co ax going to the same point (both touching brass mount) and the entire mount where you bolt it to bullbar was plastic.