A REAL LIFE EXPERIENCE
We decided to install a CB radio system on a Yamaha Big Bear ATV. From the get-go we expected to run into a ground plane problem. The radio was mounted on the left front fender and an SS-64A, 3-way mount (complete with K-4Aantenna stud) was bolted to the rear cargo rack. Because we were anticipating ground plane problems, we ran an 18-foot K-8A coax cable, even though the distance between the radio and the antenna only required about 7-8 feet of coax (the excess was tied up and stowed under the rear cargo rack.
We selected a black FS3 antenna because the final installed height seemed to be right for the way the ATV was to be used. It also made sense, at the time, to use a spring to protect the antenna and mount should overhead objects be encountered. With all of the components in place, an SWR meter went in-line and we prepared ourselves for the disappointing results. As a matter of fact, as we were installing the components we discussed the probability of needing to build a custom antenna to compensate for the lack of ground plane. Even then, we were hoping that we could pull that off (we did not have the no-ground-plane antennas in our product line at that time).
The initial SWR test caught us by surprise. The antenna was definitely absorbing the radio's energy and we could see the SWR raise (as it should) when we moved either up or down from the resonant point of the antenna. We were able to achieve the classic wide "U" shaped SWR curve between channels 1 and 40. It was not a perfect match (1.1:1 at 27.185MHz .. channel 19). The SWR at channels 1 and 40 was a needle width below 2.0:1 and at channel 19 the reading was 1.5:1. This is a classic set of numbers on a installation that is ground plane deficient.
The system, electrically speaking, performed as required. The only problem was that during use the driver would forget about the antenna behind him (out of sight … out of mind). He would duck going under low hanging limbs and then sit upright as soon as he cleared the obstacle. The forgotten antenna, when it finally cleared the object, would fly forward and beat the driver silly. After two good whippings the driver insisted that the antenna be mounted on the front rack where it could be watched. (Go figure!)
We yanked the mount and moved it to the left edge of the front rack (we left the cable in place just in case the new location failed us). This mount movement meant that the base of the antenna would be about 10 inches from the back of the radio. Still hanging unto the fear that a ground plane deficiency would create some coax resonance, we used 18 feet of coax to span the 10-inch spacing. We also decided to try a 3-foot Fire-Fly antenna this time around (for no particular reason other than to try it).
An SWR meter was used to check SWR on channels 1 and 40. The antenna was tuned until the SWR was the same on both ends of the CB bands. We were pleasantly surprised to end up with 1.6:1 on channels 1 and 40 and nearly flat (1.1:1) on channel 19. Hoping to get by with less coax, we changed the coax length to 9-feet. There was very little change compared to the 18-foot coax. Next we tried a 3-foot piece of coax. The SWR on channels 1 & 40 went up to 1.65:1 and the SWR on channel 19 went up to 1.2:1, still very acceptable. Based upon the favorable results, we decided to challenge the set up and made up a 12-inch coax cable. The SWR on channels 1 & 40 went up to 1.75:1 and the SWR on channel 19 went up to 1.35:1. We decided to leave the short cable in place and deal with the slight increase in SWR rather than the extra coax.
We learned (again) that nothing is written in stone when it comes to antenna installations. We were sure from the start that we were going to have a problem and it never came about. We have seen worse SWR on vehicles with twenty-times the amount of reflective surface. Generally speaking, theory is a good foundation to build knowledge upon but you need to be careful because it often ignores solutions to real-life problems. When you close your mind to possibilities and surrender to theory, you may be turning your back to many solutions to common problems.