DO I NEED A NO-GROUND-PLANE ANTENNA?
http://www.firestik.com/Tech_Docs/GP-or-NGP.htm
DO I NEED A NO-GROUND-PLANE ANTENNA?
Compliments of Firestik® Antenna Company Technical Support Team
"No-Ground-Plane antenna systems have a very specific purpose. They should be the system of choice when you have no other choice.
When your vehicle has little or no metallic surface area for the antenna to use as its NEEDED counterpoise your decision process should be;
1. I just won't have a CB in my vehicle.
2. I will use a ground plane (GP) system and suffer the possible consequences.
3. I will use a no-ground-plane (NGP) system and be able to use my CB.
In short, the NGP system (we call it a system because the antenna and coaxial cable are a matched set that may not be interchanged with other non-Firestik NGP antennas and coax assemblies) is a problem solver. If your vehicle does not provide sufficient ground plane for a regular GP antenna, the NGP system will solve the problem. Who should use an NGP system? Here are some probabilities. Those with ...
* Fiberglass or plastic vehicles
* Hot air balloons
* Wilderness back pack frames
* Fiberglass pick-up bed caps
* Aluminum and/or fiberglass boats and canoes
* Aluminum and/or fiberglass cab-over campers (antenna on camper)
* Aluminum and/or fiberglass travel trailers
The NGP systems are not "required" on metal vehicles. They will work but in most cases you are better off using a GP set-up if you have the reflective metal surface available. The ground wave field strength of a GP antenna on a metallic surface is about 15% stronger than a NGP system on the same vehicle in the same location. This is directly due to the way that the radio's power is delivered to the antenna via the special cable. There is some energy absorption within the cable assembly. However, and again we stress the specific purpose of the NGP system, it is better to have some energy absorbed in the cable assembly than it is to have no communications, or very poor performance with a GP set-up.
If your vehicle fits one of the above mentioned profiles and you are doing the first install, you should think of using an NGP system. Furthermore, if your vehicle fits the profile and you already have a disappointing performing antenna, you are a prime candidate for a change. Do keep in mind that the NGP antenna system will not fix the problems that are due to poor installation locations. That is, if you've mounted a GP antenna in a manner that prevents it from radiating energy into free space (usually shows up as an unmanageable SWR problem), the NGP antenna will fail as well.
As with most matters involving communications, we try to lay down some basic rules to help you before you get into too much trouble. While theory is okay for grasping a basic concept, if you let it give you tunnel vision you will probably run into a problem from time to time. We like a foundation of theory but cannot ignore over 30 years of actual experience in matters involving CB communications. Accordingly, we know there are exceptions to everything we write and say. But, when we write we need to aim the content towards the majority. It is beyond our capacity to write installation guidelines for every vehicle on the road and every possible location that an antenna could be mounted on the vehicle. For instance, on some fiberglass motorhomes with a steel substructure it is possible to tap into the underlying structure and get a GP antenna to work just fine. Likewise, many Corvette owners have attached a long mounting bracket to the right-rear frame and the good chassis ground allows the use of a regular GP antenna set-up. But, how do you explain to someone who just spent half a day routing cables and installing mounts that their problem is due to insufficient ground plane and they need to tear the whole thing apart and start all over.
There are some things you can do before you get too involved in the installation. First of all, find the metal. A one foot square chunk of metal on the roof of a motorhome falls under the catagory of "lack of sufficient ground plane mass". Every antenna has a somewhat different requirement. But if you don't have at least 9 square feet of metal (3ft x 3ft), don't even fool with a GP set-up. And if you do have sufficient metal mass, make ABSOLUTELY sure that your antenna mount is grounded to the vehicle in some manner, if not directly then with a short grounding braid or wire (minimum of 12 gauge). The fact that the coax cable may be grounded at the radio connection is NOT sufficient and does not exempt you from having a good chassis ground on the mount of a GP set-up. If you mount a GP set-up on an insulated roof rack, ladder, or spare tire rack (most of which have no or intermittent grounding), metal vehicle or not, you must run a ground from the vehicle to the mount. If you aren't sure what to do, you should find someone who can give you some help. Worse case, if you have a GP set-up and are wondering if it will work, than do a temporary installation. That is, put the mount in the area that you plan to make a permanent installation, ground the mount, route the coax from the mount to the radio through a window or door and do a SWR check. This could save you a lot of trouble and keep you from yanking us from your Christmas card list.
And last but not least, regardless of the antenna system selected, ALL transmitting antennas MUST be TUNED in their final mounting location. We thought that we would be able to stop mentioning this fact around 1978, but there isn't a week that goes by that someone doesn't says "Huh!" when we ask them if the antenna was tuned.
Not tuning your antenna is the same as not putting air in your tires after they are installed. A tire without air is a flat … an antenna without tuning is a stick."
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