CB Antenna Primer

 
This is a copy of a post I made to the XJ list in April of '99.
There was a discussion on mobile antennas and ground planes,
and some folks were under the impression that the radiated signal
from the radio was reflected away from the ground plane.
I responded with this reply, and when I was recently reminded of it,
I realized it could still be of value to others, so I have posted it here:

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"Happy Easter all,
 

I think it was Paul Greig that invited "someone to speak with authority", and David Bond that was asking for someone with "credentials" to clarify this discussion. Well I feel qualified to respond.

I have been involved with transmitting RF signals into the air waves for over 30 years,  and I hold a degree in Electronics and Telecommunications.  Antenna theory was my major area of study, and I have built and tested many antennas over the years. I currently manage a 2-way radio business, and part of my daily work includes (but is not limited to) installing radios and antennas in vehicles for government agencies, law enforcement & public safety organizations, and commercial fleets, so I have literally installed hundreds of antennas, of all kinds. Oh yes, I also hold an Amateur Radio license (N7UTB).

The biggest problem facing the average CB owner, is separating the truth from all the miss-information, misnomers, myths and rumors, that have been passed along through the CB community for generations. One of these rumors is that RF travels away from the ground plane. It does not. If it did, then an antenna in the middle of the roof  would not work at all,   ...unless you were talking to aircraft!

RF will not pass through the ground plane however, so this rumor could come from the idea that if you mount the antenna too low, like on the bumper (a bad location), and to close to the body, the body will block the transmitting RF, and *seem* to give to poor performance in that direction. In this case, it is somewhat true because too much of the body (ground plane), is too close to the radiating element, and this will skew the RF pattern in undesirable, and unpredictable directions.

The truth is, the ground plane is the "counterpoise" for the radiating element (think of it as the other half of the antenna), and in the case of the ground plane antenna, where the counterpoise is 90º from the radiating element, the electromagnetic energy WILL be drawn in  the direction of the mass of the ground plane.... an antenna on the front, will radiate mostly to the rear, an antenna on the rear, will radiate mostly to the front. The front may work better
than the rear, due to the fact that the antenna is further away from the roof, which could distort the radiating pattern. Mounting it on a "Bull Bar" however, might seem to work okay, but is not necessarily the best location. Here's why. In order for a ground plane antenna to work properly, the radiating element needs to be at least 1/4 of a wave length, and so does the ground plane. For a CB, the wave length is about 36 feet, so 1/4 of a CB's wave length is about 9 feet for the antenna, and  4.5' in all directions (or at least the equivalent mass), for the ground plane.  A brush guard, roof
rack, or tire carrier, cannot provide this much mass... you need the body of the vehicle to do this job efficiently. Any attached accessory (bull bar, roof rack, tire carrier), even though it is grounded to the vehicle, it is not necessarily "RF coupled" to it. If it is not, the accessory could try to act as the ground plane, and it does not have the mass necessary for the CB's frequency range, in which case it might even try to act as part of the antenna. This is not so important for VHF, UHF, or cellular,  since their wave length is so much smaller than CB, and they can get away with a much smaller ground plane mass.

The results of mounting an antenna to an accessory cannot be predicted. It might seem to work okay, but the pattern will be distorted, and performance will suffer. How much it suffers, depends on many factors. An improperly coupled antenna can cause interference on other channels, or on every TV in your neighborhood, even your neighbor's telephone. In most cases this is not going to be noticeable when you are talking to a vehicle in close proximity to yours, like on a trail ride, or on the highway with line of site. So in these circumstances, just about any antenna
location will work okay ...(if it wouldn't hurt your CB, you could even talk to your trail buddies with no antenna at all!).

So for many folks on this list, a good performing antenna is not real important, and I understand this thinking. I believe however, that the CB can be good for more than trail chat and "Smoky" reports. I have used mine several times to successfully call for help (for me and others) from sources that were many miles away, and when you are way out in the boonies with a break down, or a "worst stuck", it is good to be able to talk more than a few thousand yards.

If all your ever going to do is talk on trail rides, you shouldn't bother with a big or heavy antenna like a  Firestick or a 102" stainless whip that will be awkward to mount.  Just about any antenna, in any location (as long as the SWR is less than 1.5:1), will do that just fine.  But if you are serious about getting good performance, take a little time and examine your options. Decide what you're willing to do to achieve it, and install it right. Just as a stereo sounds only as good as it's speakers, your radio will only perform as good as your antenna system.

You don't have to fear depreciating the value of your Jeep by installing a commercial NMO style mount with cable, in your roof. Properly installed, it will not leak, can be capped when not in use, and will accept any antenna from CB, to Cell phone, to GPS... and beyond...   and will probably outlast the vehicle.

If you want the ultimate in performance, then get a 102" whip. Mounted on a ball mount with spring, located high on the rear quarter panel, at the base of the "D" pillar (http://olypen.com/craigh/images/cxj14.jpg), is the next best thing
to the middle of the roof. Even though this will make your radiating pattern "directional", the superior performance of a full 1/4 wave antenna, will still be effective in all directions, but up to 3 times stronger toward the front, than any other type antenna mounted anywhere else. Long whips have their drawbacks, even though most of which can be overcome, they're not for everybody. But they have no equal in performance, ...even when improperly mounted,
they can still out perform most other antennas. (Spotters beware!)

Although I do not recommend the use of fiberglass antennas for off-road use (reasons on request), Firestick has an excellent web site, with a lot of good information in their FAQs, and Tech sections, that would helpful to you no matter what type of antenna you choose.

So do your homework everybody... there *will* be a test. :o)

Sorry for the length, I guess my 2 cents turned into 2 bucks!
I just want help folks avoid some of the pitfalls that lead to
interference, and poor performance.

Much more information will be covered in an article, coming soon.

Craig."

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I don't have anything to add to this other than, ...I am still planning on finishing the article, someday.
In the meantime I'll be happy to answer any questions this brings up but doesn't answer.