How do I ground my mobile CB?

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Grant:
I'm totally new to this stuff and I don't understand exactly what I'm supposed to do. People on eHAM have given me much appreciated advice, which sounds simple enough, such as "run a coax braid from the chassis to the antenna" but as a newbie, I don't know what to do without a step by-step guide. I have been searching the internet and eHam and I haven't been able to find simple explanation.

For those that haven't read my recent posts, a little background on the way I've mounted my antenna: I just installed a mobile CB radio in my Jeep Cherokee by bolting a small metal plate about 8 inches long and  2 inches wide to the underside of the chassis. The plate sticks out a few inches from the left rear side of my vehicle and few inches in front of my rear bumper. I put an antenna mount in the plate, such that that the mount is a few inches inches from the side of the vehicle. The mount holds a fiberglass antenna. Everyone on eHam keeps telling me I need to ground my system by running a coax cable braid from my antenna to the my chassis, and that I also need to ground the CB radio box itself.

I understand that I'm supposed to attach one end of the braid to the chassis of my car, that is simple. But when you say, "attach one end to the ground side of the antenna mount" or "run a coax braid from your antenna to your car chassis" I have no idea what that means?
Where and how do I attach the antenna-end of the braid? The antenna does not have a bolt or anything sticking out with a big label that says "attach coax braid here."


On the radio box itself how do I ground it? One guy said he ran a wire from the radio box to a self tapping screw that he put through his floor, but he didn't say where on the radio box I'm supposed to attach the wire. Again, I don't see anything on the box asking for a ground wire. I think I'm supposed to hook the ground wire to a screw in the radio case, or maybe the mounting screw?

Also, as for the location of the "self-tapping screw" to attach it to, the bolts that hold my driver's side seat to the floor are definitely hooked to the chassis, I checked them for continuity. Instead of a self tapping screw can I just slip the wire under the bolts that hold the chair to the floor?

Why do I have to make any grounds at all? I thought the antenna mount being in contact with the the chassis via the bolts holding the metal plate created a ground, and the radio itself was grounded through the antenna cable, which goes to the antenna mount, which is grounded to the chassis? What am I not understanding?

Can I have too many grounds?

And please, when you say "ground this to that" explain exactly how to do it, I'm sure for a seasoned HAM saying "run a coax braid from the chassis to the antenna" is self explanatory, but with my current knowlege I get lost right after the "run from chassis to...." Again the only metal objects visible on my antenna are the screw at the bottom, which screws into the antenna mount, and the tuning tip at the top. There is no wire/bolt/screw that says "attach ground here."

Also, I've looked at quite a few CB setups on different vehicles (owned by people who "had it done at the truck stop" and are otherwise as lost as me). None of their setups have ground wires running anywhere.
I assume this is beacuse the people at the truck stop installing stuff are creating inferior installations by not running ground wires?

Also, the thing I've been using to check SWR is a "MFJ Deluxe Versa Tuner II" it does have a screw on the back that says ground, is it neccessary for me to ground it to get accurate SWR readings? I'm only using it to check SWR, I know it has a bunch of other features, which I don't use and don't plan on using. If I do need to ground it do I ground it to the chassis? Or to the real dirt ground?


As always, thanks in advance, your comments have always been greatly appreciated and have gotten me to the point I am at currently.

Dee D. Flint:
Some mobile installations need additional grounding and some do not.  It just depends on how everything in a particular installation interacts.

On an antenna/coax combination, the ground side is the outside of the coupling not the center pin.  If you do not have a coupling, coax is made up of several layers.  The layer just inside the plastic coating is a copper braid and that is the ground side.

The "ground screw" on the MFJ tuner would be connected to the chassis.  However if you are using it only as an SWR meter, you do not need to ground it.  That is only needed if you use it as a tuner (the other features you mention).

Most ham radios have a screw on the back specifically for the purpose of connecting a ground wire between the radio and chassis.  I do not know about CBs.

tom lish II:
use  a screw on the back of the radio or one of the screw on the mount for the radio as a ground point.  run a piece of braid to a near by bolt that is grounded to the body, ( some go into plastic, no help) or use a self tapping screw to the floor boards through the carpet. this is easy and convienent. you could also put it under the bolt on the seat track, that is grounded.

some times the grounds don't seem to help, but most times they get rid of noise. to ground the antenna, usuall there is part that goes to the stick, which is the hot side and the other part is attached to the shield of the coax, that is the ground side. if there is a bolt or lug or even sometimes I have wrapped the braid under the setscrew on a trunk lip mount, to give the antenna a good ground at the antenna.

Grant:
I've run a wire from the radio case to the bolt on the back of my seat (which I am positive is grounded) and it doesn't seem to make any difference. I haven't done extensive testing on audio quality but it doens't sound any different and I'm sure the SWR isn't affected.

I'm getting a consistent 1.5:1 SWR on all channels (wire or no wire). Does this mean I'm set? Or can I get it down more? Yes, I have tuned the antenna to optimum performance.

Is the main benefit of a well grounded system low SWR or is it better sound quality (less static)? Are SWR and sound quality joined at the hip? Does a low SWR always have less static than high SWR?

Also, I noticed something strange, on some frequencies (channels) my radio puts out 4 watts. On others it only puts out 3. I have tested it many times and the power readings are consistent. It seems that channel 1 has the highest power output, at exactly 4 watts. But as the frequencies (channels) get higher it slowly loses power. Channel 40 puts out a little under 3 watts. I highly doubt it, but could this somehow be related to it being plugged into the cigarette lighter? (I haven't run a wire to the battery yet.)

If I'm getting good SWR using the cigarette lighter is there any real reason to run a wire to the battery? I'd  rather not snip the cigarette ligher end off unless it will make a noticable difference in quality.

Dale Hunt:
Hmmm...  let me see if I remember all the questions...

Grounding:  the important thing is that the shield of your
coax cable be connected to the frame of the car at the
antenna mount.  If your mount is designed to take a coax
plug, you shouldn't have to do anything other than to
ensure that the mount itself is securely bolted to the
car frame.  If the antenna mount is the type where the
coax center conductor attached to a ring lug on the
antenna mounting bolt, then there should be a smaller
lug on the coax shield that goes under one of the bolts
that holds the mounting plate on the car.  Because of
the wide range of mounts available, it is difficult for
us to tell you exactly how to do this, but the closest
grounded bolt you can find to the antenna feedpoint is
often a good place to start.

Sometimes folks mount antennas on the bumper, only to
discover that this isn't securely fastened to the
frame.  Similarly with doors, trunk lids, etc.  If the
mount isn't fastened directly to the frame, you may have
to add some jumpers either from the mounting bracket to
the frame, or from the metal object it is mounted on to
the frame.  The antenna will be more efficient (that is,
your signal will be stronger) the more of the metal of
the car that you can bond together.  For example, the
trunk lid may not be well grounded through the hinges,
so you can take a short piece of braid and connect one
side to the frame and other to the trunk lid.  You can
use existing bolts if they are convenient, otherwise
try sheet metal screws through the braces.  Make sure
you leave enough slack that the trunk lid can open, but
not much more than needed.  (If the antenna is mounted
on the trunk lid, this step would be even more important,
but it does help even when the antenna is mounted elsewhere.)

You can add a ground strap to the CB if you wish if it
isn't grounded through the mounting bracket, though it
may or may not make any difference.

An SWR of 1.5 : 1 is not unreasonable: that means the
input impedance is probably around 35 ohms.  If you
improve the ground connections you may find that the
SWR INCREASES, which is not a bad thing.  The SWR usually
can be corrected by connecting a small coil of wire from
the antenna to ground at the same place where the coax
is connected, but I wouldn't bother with it as long as
the antenna seems to work well enough without it.

If the antenna is adjustable, you can shorten it a bit
to increase the power output on the higher channels.  I
suspect the nearness of the metal car body to the antenna
has lowered the resonant frequency a bit.  However, if
you are just looking at the forward power meter, that
will only give you a reasonable indication when the
SWR is 1 : 1.  You really have to subtract the reflected
power (indicated on the SWR scale) to get an accurate
reading.  But, again, get the antenna mounted and well
grounded first.  Then you can experiment with other
adjustments later.  If it seems to work OK, just use it.

Power:  You're probably OK getting power from the lighter
socket, especially at the relatvively low power of a CB.
The general recommendation (especially for ham radios
that can draw 10 to 20 amps on transmit) is to run a
fat cable directly to the battery posts:  you can mount
a second lighter jack for this if you wish.  This tends
to minimize the noise picked up from the electrical
system and some glitches that may be caused by the
transmitter.

I hope this gives you a good start.

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