CB power

I see that you are stating cut to length if not properly grounded.

tw

twaldr> If the impedances are matched on the antenna, coax and connectors, the

Please examine a typical metal-bumper mounting bracket for a CB antenna, such as by Firestick. It consists of three things:

1. A metal screw-on coax connector,
2. A metal nut that screws onto the bottom of the coax connector,
3. A nylon bushing that is placed on *top* of the coax connector to isolate the antenna portion (the socket sticking up above the metal body of the bracket) from the metal bracket itself.

The coax screws to the bottom. The metal shielding of the coax is attached to the metal screw-on portion, which in turn grounds via the metal nut to the bracket which in turn attaches to the bumper which (on a Jeep TJ) is a solid body ground. The center portion of the coax attaches to the socket that sticks up above the bracket, the portion of this apparatus that is electrically isolated from the bracket by that nylon bushing.

I suggest that you go to the Firestick site and read their installation directions. They should help you figure this out.

It's not mylar. it is a conductive plastic. Please read the description on the Firestick site of how their magnetic mounts work.

You don't need to look at me. Go to the Firestick site and read their installation directions and look at their photos of their CB radio mounts. Proper grounding of the coax shield at both ends is critical to reduce emitted noise in CB installations. A one-end-grounded shield can have a current induced into it by the signal passing through the center conductor, thus turning it into an antenna. A shield grounded on both ends can never have a current induced into it, since both ends are hard ground. Firestick sells a specially tuned antenna-coax apparatus which does not require two-end grounding, but this is because the coax is set up at the exact length needed to avoid adding distortion into the induced signal.

-Elron

Hi Mike As I said before a mag mount is capacitive coupled to ground A plate (on the base of the mag mount) separated from the vehicle metal acts like (actually is) a capacitor (You know what makes a cap, 2 plates or more seperated by a dielectric)

In DC it's isolated, but RF = AC, AC flows through a cap.

I don't pretend to be an expert in antenna theory but You both (L Ron Waddle) have certain assumptions wrong.

Once again: This site is very accurate in describing the theory of 11 meter (CB) antennas and such. I suggest everyone read it and try to understand it all. The author really knows what he's talking about. It is not BS.

Mike Roma> If the shielding on your coax is not grounded to a hard

The coax cable, has a 50ohms impedance, IF the RF circuit closes within the coax. That is the RF must travel through the center electrode, through the antenna and then return through the shield of the cable. In this case the RF currents in the core of the coax cable and in the shield have opposite directions, and they cancel each other's electromagnetic field, so that the cable doesn't radiate. That's why a coaxial cable is used, so that the electromagnetic field is contained within the shield of the cable (between the core and the shield).

If you create an alternative ground path, by hard grounding both ends of the coax on the chassis, part of the RF will leak and will not return through the coax shield, but through the car's body shell. This WILL change the coax cable's impedance increasing the SWR, and WILL make it radiate part of the signal causing interference to other cables parallel to it's route. You could tune your RF amplifier output or your cable length to match the impedances to reduce SWR, but you won't be able to reduce interference to the car's circuits.

For a proper installation, you have to ensure that the RF circuit ground path closes through the coax cable shield only.

A good way to achieve this, is by not grounding the CB end of the coax cable, i.e. don't ground the chassis of the transmitter unit directly on the car body. The GND power supply wire of the CB must be used, which might have an RF choke inline (otherwise one must be added) that prevents the RF circuit to close through it. And a relatively low frequency 27MHz RF is much more difficult to choke than a higher 100MHz or 1 GHz RF signal. A choke must be added inline to the positive power supply wire too, the RF can travel from the car's chassis through the battery to the positive terminal easily.

If the antenna end of the coax needs to be grounded on the car's body depends on the antenna used. But if you have grounded the CB unit properly ONLY from it's GND power supply wire, and have installed RF Chokes in both +12v and GND wires, then you will not have problems for either case.

Regards,

Bill Spiliotopoulos. '96 XJ, '06 TJ.

I would NEVER fuse the electrical ground line because if that fuse happens to go...and fuses do go...your radio circuit WILL find an alternate ground path and you will not likely appreciate it's choice. The likelihood of a body ground strap going bad is far less likely than a simple inline fuse. If you're paranoid about body straps going bad, replace it during your radio installation.

tw

Bullshit!

I have tested 'many' mag mount antennas and have had to reposition dozens that allowed the metal cup's side to touch the van body creating an electrical ground path shorting out the coax causing massive transmitter packet loss.

I actually read the Friestick site and from there and I Quote:

Magnet mounts rely on high resistance, capacitance grounding

End Quote.

I do see lots than need a grounded mount implying the coax shield might be grounded at the outer end though, so I figure I must have seen NGP mounts with isolated COAX mostly for some strange reason.

That is still a good reason for the Automotive makers wanting 'both' the power and ground fused so no matter which way a short starts or a vehicle ground gets lost, the power surge will hit a fuse before letting the magic smoke out of the transmitter.

Oh, the booklets also do 'just' say 'transmitter', they do not care what 'type' of transmitter is being installed.

Mike

You know what, you can try to feed us all the BS you want, but when it comes right down to it and you go into the dealer with an ABS failure light on or an automatic that keeps on slipping into neutral and they unhook your improper transmitter install and the light goes off, they have you by the balls and 'will' void your warranty.

Simple eh.

I have seen that 'exact' scenario happen on more than one new GM vehicle and on one Ford vehicle.

Three GM owners alone got nailed in one week period, man were they pissed! Especially when I 'too' politely asked when they were going to take the time from their busy schedule for a 'proper' install....

Oh, the booklets also do 'just' say 'transmitter', they do not care what 'type' of transmitter is being installed.

Mike

'I' had to 'go by the book' or be liable for lost vehicle warranties.

Mike

Yep, I understand why you're doing it, but I don't understand why they're requiring it. Warranty aside, no one has been able to show me why the NEG lead fuse is necessary/warranted...or even safe.

tw

The definition of DC is a build up of negative electrons wanting to go to a positive source so technically if a short happens the power has to travel through the CB first, spiking it with power to get to the fuse to blow. When both are fused and something happens, the negative fuse normally blows first for some reason. (from 9 years of observations and repairs)

Mike

Once that NEG fuse blows, where does the energy then travel? That's my concern. It will backtrack and find the nearest pathway.

tw

Um, the energy comes 'from' the negative side and goes 'to' the positive....

If it has to go all the way through the transmitter to get to the fuse on the positive side, that means the high spike hits lots of components that really don't like high power before it gets 'to the positive fuse.

Mike

I guess it depends on which direction the problem comes from. Sidenote...About 7 years ago, I had a neighbor's tree fall on the power lines, effectively creating a 440v short. This energy came barreling down my power line and entered the back of my big screen HDTV, somehow skipped past the quick blow fuse and fried a main circuit board. I lost a bunch of equipment that day. The fuse eventually went, but not before the damage was done.

tw

In my mind it is simple, I just fuse both of the suckers and be done with it. That 2A fuse isn't going to let the starter or headlights draw through the CB, that's for sure...

Mike

Yeah, I'll have to rethink that. Thanks for hashing it out.

tw

Nowhere, it's safe, again. We use to see this problem all the time, where GM accessories were cooked when the type of battery cables (four connections) they used in the fifties would disconnect the their smaller leads. God Bless America, Bill O|||||||O mailto: snipped-for-privacy@aol.com

The Chrysler TSB dealing with installing radio communication equipment specifically states NOT to fuse the ground lead. Apparently the manufacturers can't agree on this either.

That is a serious pain in the butt isn't it?

I think of all the body ground straps I have seen go bad on Jeeps so things like the headlights 'could' pull a chassis ground to the CB then to the negative lead to the battery. Even the starter could pull the other way through the body ground strap and back through the transmitters negative, if the big ground goes bad at the block which I have 'also' seen happen on Jeeps more than once.

I figure fusing both with the light 2A fuse has it covered...

Mike