MOTORCYCLES, ATV'S AND CB ANTENNAS
Compliments of Firestik® Antenna Company Technical Support Team
Copyright © 1996 Firestik® Antenna Company
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Do I need a no-ground-plane (NGP) antenna system or will a regular, ground plane (GP) dependent antenna system work better on a motorcycle or ATV? We know that the NGP system will work on any vehicle so long as it isn't mounted in a way that prevents it from radiating energy. But what about a regular GP system? The truth is, we don't know for certain. Read about an ATV installation.
GROUND-PLANE DEPENDENT ANTENNAS Transmitting antennas, unlike receive only antennas, need some sort of counterpoise in order to function properly. Lack of counterpoise (ground plane) will result in high SWR (standing wave ratio) across the bandwidth (usually at least 2.0:1 or higher). In almost every case, the antenna will appear to be electrically short. If you have a GP antenna mounted and you're are experiencing high SWR you need to be certain that your antenna mount has a good chassis ground. DO NOT test for mount grounding with the coax connected to the radio. Low resistance grounding must be in existence between the mount and the vehicle chassis directly, not through the radio. If the high SWR condition exist after verifying the ground at the mount, chances are there is an insufficient supply of reflective material available for the antenna to use. There is one exception that you may want to look at too. It involves the coax cable. We cannot stress enough that you use really good coax. Because of vibrations, you should only use coax with a stranded center conductor (RG-58A/U, RG-8X). And, because of the constant exposure to the elements, choose coax with polyvinyl insulation (hard) around the center conductor (versus soft polyfoam). Lastly, you want a very high shielding percentage. We only recommend coaxial cables that have a minimum of 90% shielding. Another anomaly occurs within the coax when the antenna and the available ground plane do not cooperate with one another. When the antenna and ground plane are compatible the length of the coax cable will have little affect on the performance of the antenna. However, it has been our experience, with our antennas, that incompatibility sets up a condition whereas the cable starts resonating as if it were an antenna. While this defies theory, we cannot tell you how many times that we have corrected SWR problems simply by having the users use an 18-foot (5.5m) coax cable. Again, if you have a good mount ground and still have high SWR we suggest that you give this a try. Before you route the cable, just connect it to the mount and the radio then retest. If the SWR improves, you'll know that the other cable is resonating and needs to be changed. If you find it necessary to use a long cable you will need to deal with a lot of extra cable. Whatever you do, do not coil it up into a nice pretty package because it will probably start acting like a RF choke and re-mess up the installation. Circular coils should be at least 10 inches (25cm) in diameter. We prefer the "bow tie" wrap whereas the coil is brought together in the middle and held there with a twist or cable tie. Finally, the location of the antenna mount can be of equal importance. Transmitting antennas radiate an energy field along their entire length. The design of the Firestik/FireFly antennas creates stronger energy fields the closer you get to the top of the antenna. Accordingly, it is best to mount the antenna high on the vehicle to eliminate side interference within the antenna's strongest energy field. It is better to use a three-foot antenna on the highest part of the vehicle then it is to use a four-foot antenna mounted one foot lower. Nonetheless, for any given location, longer antennas are usually better than shorter ones.
NO-GROUND-PLANE ANTENNAS Before any discussion of the NGP system, it is important to understand why NGP systems were developed. NGP antenna systems WERE NOT designed to replace standard ground plane dependent antennas. If you have sufficient ground plane to use a GP antenna ... then you should. The decision to choose an NGP system gets down to the need or want to have communications on a vehicle that otherwise could not have communications. That is, if you cannot make a standard GP system function on the vehicle you have two remaining choices; 1) chjoose not to have communications or 2) use a NGP antenna system. Insofar as motorcycles and ATV's are concerned, the primary issue to be dealt with when using the NGP system is the coaxial cable. The coax cable is 17 feet long and CANNOT be altered as it is matched to the NGP antenna. Accordingly, you must have the space available to stow it on the vehicle. Beyond that, they are easy to install and tune. SUMMARY We prefer to use ground-plane dependent antennas if there is sufficient reflective material. We try all of the things that we mentioned in this document. If and when we cannot make a GP system work, we go straight to the NGP system. If you can a install the antenna mount to a metal, grounded component of the vehicle there is at least a chance that you could make a normal ground plane dependent system work. But, if you are planning to attach the mount to a fiberglass storage compartment, you had better give a serious look at the NGP kits. We realize that we have a great advantage simply because we have everything at hand. If we are unable to make the GP antenna work we don't have to take it all apart and ship it back someplace. It is easy to take the quick way out and tell everybody to use the NGP system because they really are a no-brainer insofar as achieving satisfactory SWR. In every case, you must thrive for low SWR as it indicates how well the antenna is radiating the energy coming from and going to the citizen band radio. High SWR is an efficiency eater and it can get so bad that your radio can be damaged. Unlike many people, we are not obsessed with SWR to the point that we need it to always be below 1.5:1 but, we sure get uncomfortable when it gets over 2.0:1 (and panic over 3.0:1). Due to the numerous variables, mobile antenna installation is not an exact science. At the same time, regardless of whether you install or have someone else install a CB antenna on your motorcycle or ATV, make darn sure that the SWR is below 2.0:1 on no less than the primary channel you talk on, and preferably all of them. |
