I originally wanted an antenna that would not hit everything overhead while driving, since it was to be installed on a van. I came up with an antenna that extended only 1.65 meters or about 5 feet 4 inches above the top of the vehicle. It had a calculated efficiency of 42% when compared to a half-wave dipole. It was my hope to be able to install a series of different loading coils to be able to operate on the other HF bands. However, calculations indicated that the antennas was simply too short to work well as the frequency was lowered. Something better was needed.
The Evolution of the Antenna Design
My first thought was to increase the length of the antenna below the loading coil. My reasoning was that since that portion of the antenna carries the highest RF current, then adding more length there would increase the radiation efficiency of the antenna more than would adding the same length to the antenna above the coil. Unfortunately, since the antenna is mounted on top of a van which is a rather high vehicle, that would place the coil at a dangerous height with regard to tree branches and the like. Since the loading coils were going to be rather heavy due to their sturdy construction, placing a heavy coil high up on a flexible support did not seem like a good idea. I decided to investigate what increasing the length of the upper portion of the antenna would do to the radiation efficiency.
It quickly became apparent that very good results could be obtained by increasing the top whip length. As it turned out, this was due to two factors. First, the antenna was center loaded, and second, increasing the top whip length greatly reduced the coil losses due to the smaller number of turns required. This also meant that I could use larger wire for the coils, which further reduced the losses. It was almost like getting something for nothing!
Since I had once a standard CB-length whip plus the magnetic mount and base spring installed on the van, I knew that a total height of 3 meters was workable, even though it did hit quite a number of overhead obstructions. I ran calculations to determine how it would work. The results were very encouraging, and I promptly set out to build the antenna. I still had the CB whip, and figured that I could simply cut the top end off of it to get the total length of 3 meters for the completed antenna.
While laying out all the parts on the ground to see how they would fit together, I looked at the CB whip and realized that I would only have to cut off about 17 inches. That seemed like a waste, and I really hated to cut that small amount off of a perfectly good antenna.
Just for fun, I decided to repeat the calculations to see what would happen to the gain if I left the extra 17 inches on the antenna. I was surprised to find that the antenna efficiency increased between 19 and 31 percent, depending on frequency. The trade off, of course, was that I knew the antenna was going to hit a lot more objects overhead than if I trimmed the 17 inches off of it. As a test, I assembled the antenna full length and drove around with it for a few weeks to determined how much of a problem it would be. I decided that I could live with it, as a necessary price for the increased signal strength…..Read More