CB Radio or PRS

In my opinion, you would do well to keep the CB antenna away from the engine, as that is generally a noisy RF environment. Ignition impulse noise, computer hash, electric motors, fuel pump noise, and on and on can really put a dent in your radio's usefulness. On my elderly CJ5's I mounted 102 inch whips on the driver's side rear end of the body, mostly to help keep from clanging into overhanging tree limbs on country roads. On my XJ I mounted an L bracket NMO mount at the left rear hatch point, on the side that was away from the exhaust system (driver's side). Make sure you establish a solid mechanical and RF ground at the feedpoint of the antenna. This helps keep the coax from picking up noise and inducing it into the antenna, as well as establishing a counterpoise for the antenna.

Power the radio by running twisted pair back to the positive + terminal of the battery, and then to the body groundpoint where the negative - terminal of the battery is connected. Fuse both of these at the battery end! Ground the chassis of the radio at the mounting point as well. Hopefully you will not run into the classic RF noise issues that many have experienced! As someone else suggested, a great place to get mobile radio information is at:

It's great fun to use the radio to rank on your buddies as you roll past them in the mud.

Rob wb1aex

"The only thing necessary for the triumph of evil is for good men to do nothing." Edmund Burke

nothing." Edmund Burke

Mike, I don't mean to be an ass but I've gotta disagree with you on the coax length thing :) That's a very common misconception but it just ain't so, unless you're running a really non-standard antenna the coax length doesn't make any major difference.

In theory, shorter is better but for the difference in length thats possible in a vehicle I don't think the ol' mk.1 "ear" test will notice any difference.

The only concern I'd have with shortening the coax run would be getting the connector properly installed, may take a try or two for a first timer but not out of the question.

-Howard.

TW, another old wive's tale about the coiled coax thing. If everything is working the way it should all the signal travels through the coax between the center conductor & the inside of the braid, not along the outside where coiling it would have any effect. Coiling it, tying it up or crocheting it for that matter will not make any differencet :). There are actually some situations where coiling it up would make a system work better, but you won't run into them in a jeep using a commercially available antenna.

Howard.

I guess it depends on the individual situation, in some cases coiling it up may make sense, in some cases it may be easier to leave it uncoiled. Personally I usually cut it to length. On my last install (magnatopper) I had about 1' extra so I just rotated the antenna around to coil the extra length around the base.

Makes no difference one way or the other.

Howard.

Exactly... the 18' coax length recommendation for CB radios is an old-wive's tale... a complete myth.

Jerry

PL-238??? I think you mean PL-259....

I'd think you'd need a *big* amp to push enough watts to magnetize a screwdriver with the CB coax. Say, 4 or 500 watts maybe? Then you'd probably start to melt the soft top every time you keyed up the mic. I know for a fact that pushing 100 watts through a co-phased setup will light a 4' fluorescent tube held between the antennas.

-- Old Crow '82 Shovelhead FLT 92" 'Pearl' '95 Jeep YJ Rio Grande ASE Certified Master Auto Tech + L1 TOMKAT, BS#133, SENS, MAMBM, DOF#51

You are mistaken.

The coil can act as a pickup for noise and it can put out RF.

You should really read the TSB out on how to install a transmitter into a new modern vehicle or you will void your vehicle warranty.

The antenna cable can and will interfere with the vehicle's onboard electronics. It 'Will' cut out anti-lock brakes. It 'will' cut out a Jeep automatic tranny module so every time you key up you go into neutral. It also will interfere with the fuel injectors.

GM, Chrysler and all the rest have booklets out now on how to properly wire a transmitter in order to avoid voiding the vehicle warranty.

GM insists on minimum 10 ga. power and ground run right to the battery and double fused there on the opposite side of the vehicle from the wiring harness. The antenna must run on the opposite side from the harness too.

So for the power lines, to have a CB on the center or passenger side of a GM van, the wires have to run up the passenger side fender to the front of the vehicle, across the front of the radiator supports and back to the battery. If you run them across the firewall, you void the warranty for the vehicle's electronics, all of them.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Howard Eisenhauer wrote:

Ya????

Ok, but I was the system op for a mobile fleet of 150 vehicles and man oh man I could sure tell when someone had been messing with a cable.

We were transmitting data over the system and with a messed up coax the retries went up exponentially. We needed an 18' or a 12' or a 6' cable to have good clean reception. If someone chopped 2' off in a door, I would need to fix it by cutting 4' more off.

Now for that 'ear' test, well I guess a crackle won't hurt sound like it does data.

And I guess having a 3' coax cable wouldn't mess up the SWR at all either eh?

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Jerry Bransford wrote:

Here's one nice writeup on the 18' cable myth you may enjoy reading Mike...

Jerry

That is one person's opinion Jerry and I don't know that person from a hole in the ground.

Though I did notice the reception seemed better at 12' than 18' and that having a cable 'not' in one of those multiples really messes up data packets. Transmitter makers like Ericsson also call for the 12' or 18' lines and warn not to coil them which can 'short' the RF out.

I haven't played with enough voice systems to try and set the SWR with unmatched cables. Have you?

Mike

Jerry Bransford wrote:

Just read it carefully Mike and perhaps you'll see the electronic theory behind what he writes. I have an electrical engineering background and what he says is right on the money. I personally know him very well too and he has a solid background in this "stuff". Plus we are both heavily into ham radio and this 18' myth was shot down in ham radio antenna circles many years ago.

Jerry

message

Mike, I hope I didn't come across the wrong way on this, I have no desire to start a flame war over this. You have, IMHO, a well deserved reputation on this group for knowing your stuff, but I still think you're wrong.

My position on this comes from being an electronics technician working in the radio field for somewheres on the upper side of 30 years. An awful lot of that time has been spent testing & troubleshooting antenna & transmission lines on everything from microwave relay links to CBs, with a bunch of cell tower & military stuff in between. I think by now I'm pretty well grounded in both the theory & the practice.

As far as CBs are concerned, if your antenna is properly designed & installed then the length of the coax line & whether or not the coax is coiled up is immaterial to the operation of the system. If there's a problem with the installation then you may well have problems with the coax radiating the reflected power that was supposed to go out the antenna. You may also experience the signal trying to get to ground through you power cabling instead of the coax shield. In this case you may well have interferance problems inside the vehicle & changing the length of the coax may affect your SWR meter's reading, but changing the length to make the meter read "right" is really only covering up the problem. The best way to check the SWR is with the meter inserted into the line right at the antenna, unfortunately the way most antennas are built makes this difficult. To get the real picture of whats happening from the radio end of the line requires an antenna line analyzer ($$$- google "Anritsu Sitemaster" for one model, did I mention "$$$"?).

I saw what you said about the problems you were having with your fleet vehicles & the data systems, I've seen it too. When the digital "TDMA" vehicle mount cell phones came out, we really had to crack down on the installers to get the VSWR down as far as possible, otherwise the bit error rate went crazy. We also made them trim the coax to "just long enough" in order to eliminate the loss from the extra length, which admittedly is a lot worse @ 850 Mhz than 27 ;>.

If your SWR is high & you can't do anything about the antenna then you can reduce to problems inside the vehicle by (gasp) looping the coax a few times at the antenna to form an rf choke, this prevents the reflected signal from travelling back down the outside of the coax. Another trick that works is to get one of those ferrite sleeves that you see showing up on power & telephone cables these days. Stick it over the coax at the base of the antenna & it'll do the same thing as the loops. Running the power leads through one of these things wouldn't hurt either.

Tell ya what- next time you're in Halifax drop me a note, I'll get some test equipment togeather & I'll try to show to prove to you in person what I'm saying is right. If I'm wrong I'll buy you a 24, If I'm right you do the buying, How's that?

-Howard.

p.s.- I drink Keith's :).