BUMPER MOUNTS
Compliments of Firestik® Antenna Company Technical Support Team
Copyright © 1996 Firestik® Antenna Company
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Installing CB antennas on the rear bumper of a vehicle has its place, it also has its problems. For the sake of this discussion, we will be referring to standard, ground-plane (GP) reliant antennas. Nonetheless, many of the problems involving GP antennas on bumper mounts will also be problems with NGP (no-ground-plane) antenna installations. Many people mount their antenna on the bumper because they are not totally familiar with all of the possible mounting brackets available. They figure it is better to drill a hole in the bumper than it is to do the same on the body of one of today's expensive vehicles. Another reason that we often hear is "it keeps the antenna very low so I can park in my garage without hitting the antenna or having to get out to remove it." Antennas can work from the bumper location on some vehicles but you should be aware of potential problems and performance limitations. When the GP antenna is mounted at the corner of your vehicle the wave pattern becomes distorted. Inasmuch as GP antennas use the vehicles metal to reflect its energy, you can imagine the shortage of reflective surface to the rear and to the side where the antenna is mounted. Energy in those directions will be less than in the directions where reflective surface area is available. Nonetheless, it is noted that not everybody wants or needs perfect long-range communications. For some users, antenna location can be more important than perfect performance. It isn't always possible or practical to mount the antenna in the perfect place and so it becomes necessary to be flexible. There is, however, one real serious problem that we see with bumper mounted antennas. You should avoid this pitfall. The problem involves interference/reflection within the antennas near field of radiation. When you key down on the microphone button the antenna needs to absorb the energy and radiate it outwards through free space. The radiation field on wire wound antennas is relatively weak at the bottom but as it moves upward, the field strength increases dramatically. This characteristic is the primary reason why wire-wound antennas are such efficient radiators, specifically because the highest point of the antenna generates the highest concentration of energy. There in lies the problem when bumper mounts are used on SUV's and other vehicles with tall parallel surfaces (trucks with campers, etc.). The highly reactive portion of the antenna is not in free space. We see it all of the time and it almost always results in a high SWR condition. Not because the antenna won't work, but because it simply can't under those circumstances. We can only imagine that the people with these installations have no clue as to what SWR is, let alone have their antenna tested and/or set. Solid items in the near field of radiation block the antenna's ability to radiate its energy. When the antenna is unable to absorb the energy because it is out of tune that problem can be fixed by tuning the antenna. However, when the tuned antenna (capable of absorbing energy) is mounted too close to other objects the energy is reflected back into the antenna. The only way to correct that problem is to move the antenna or take a chain saw and chop off all parts of the vehicle that are within a few feet of the upper two-thirds of the antenna. There are a minimum of four high-priority issues with every GP antenna installation.
The first item has near-zero tolerance. If you have a short in your coax you have a serious problem that must be fixed immediately. The second and third item can withstand very little tolerance (resistance) before performance problems become inevitable. As for high SWR, what are you willing to live with? The higher it is, the more power will be wasted And some people say "So what!" "It works good enough for me!" At 3.0:1 you loose 25% of the radios power. Maybe that is okay because you are only talking to your buddy in the car right in front of you. And maybe it is all right so long as you understand that the 25% loss is due to reflected power that is trying to go into your radio where it is suppose to be coming out. This action generates heat. If the heat accumulates and turns into "hot", the transmitters output circuits will be damaged, thus requiring replacement by a technician. But, high SWR and radio damage also has the element of time involved. If you key up a radio with a high SWR antenna system (not shorted … just way out of tune and/or poorly installed), you might slide by if every conversation from your end is "very" brief. Brief conversations allow your circuits to cool down between transmissions. But, if you decide to read the Declaration of Independence in one long winded broadcast, you will probably need to stop at the CB shop a couple of times before you get through the entire thing. |
Updated: 2023.12.05
Last Revised: June 6, 2006