Hello Everyone,
While this is my first post, I have been lurking in the background for some time assimilating as much information as possible. The conversations on this board have been invaluable to assisting my pursuits in this hobby.
A little background...I have been out of the radio game for some time. Back in the 90's I built my own LPFM station from scratch and broadcast for roughly 2 years. In that time, I experimented frequently with different antenna designs. The 10' wavelength for VHF is far easier to work with than the 10-meters or so we deal with at the top of the HF range. I used and/or built a 5/8 wave GP antenna, a 1/2 wave J-Pole, a crossed dipole array, a stacked vertical dipole array, a log-periodic beam, a 1/4 wave GP and various co-phased combinations of all of these.
In the VHF (~100MHz) band I dealt with, nothing beat the performance of the 5/8 wave GP antenna. It had the strongest signal over the broadest region. Living on a hill at the time, I had people listening up to 30 miles away that were line of site. I eventually moved onto college and packed everything away.
I found an old Cobra 25-LTD Classic while going through storage and the radio bug hit me again. I had to find an antenna solution that would work in my circ*mstances since I am a renter. It could not be a permanent part of the structure, nor could it be overly obtrusive as to draw the ire of the landlord. I eventually settled on the Solarcon A-99. The Antron-99 was the only CB antenna that I remembered by name and figured it would be a good starting point.
I mounted the A99 on an old portable basketball hoop pole, approximately 10ft above ground. I almost immediately made contact with a gentleman 20 miles away barefoot, which surprised me. However, even that low power level was causing bad TVI, bleeding through speakers and causing smoke detectors to beep. This was unacceptable. I installed a 6-turn choke plus snap-on ferrite chokes at the feedpoint. It only partially resolved the issue.
Through my research, I discovered that the ground plane kit for the A99 supposedly aided in the reduction of TVI. I also heard that the kit was useless. It was a little too expensive and obtrusive in my case, so I resolved to build my own GP kit and hopefully resolve the TVI issues. I should mention that I slowly added 5ft mast sections to the top of the basketball pole, eventually setting up 3 of them for a total antenna height above ground of ~ 24 feet.
Being that a low-profile ground plane was needed as a renter, I first tried to follow Yates' write-up where he modeled that an end-fed half-wave vertical like the A-99 only needs a .05 wavelength radial system to be effective. This was a nice hypothesis to work with since it means radials that are barely 2 feet long. I thought, "Why use just 3 or 4 radials when I can use an infinite radial plane?". So I used half sections of curtain rod to support a 22"x22" square of reflective double bubble insulation. You can see it in the pic above. In testing, it did manage to reduce receive noise, flatten out the SWR curve and reduce TVI.
However, Southern California's infamous Santa Ana winds destroyed my infinite ground plane prototype in mere minutes. So I elected to try something a little more conventional. I used the same curtain rod radials, but I insulated them so that they were isolated from the mast and used them as lower supports for a set of four wire radials (see below)
I tied 102 inch (1/4 wave) sections of 18-AWG speaker to a hose clamp right below the tuning rings where the regular GP kit goes. I also wrapped the A-99's mounting base with electrical tape to isolate it from the mast and prevent it from interfering with the ground plane. The angle is a bit steeper than the regular 45 degree angle of the GP kit. That's fine though...it is a lower profile and it makes the antenna look like some kind of beam weapon from a star wars x fighter.
Now, let's get to the performance. First, the TVI issue is gone. I am running 25 times more power now and the TVI is still gone, which is a bonus. I then set up an experiment. I chose a clear freeband frequency and operated in FM mode at 4 watts. I then got in my car and turned on my mobile to the same channel. I kept the radio keyed the whole time of the test. I then drove down each of the four cardinal directions (NWSE) and logged the precise geographical point in which the full-quieting full-bars signal started to swish around and drop to four bars out of five on the signal strength meter.
I performed the above test with the homemade GP radials installed and with the antenna in its stock radial-free condition. In each of the four directions, the A-99 with the GP radials won hands down. The GP version sent the signal out between 20% to 25% further. Not only that, but the signal on the outskirts was far more consistent, suggesting that the radials are indeed cleaning up the radiation pattern, making it more circular and casting the signal towards the ground. Also, the radials flattened out the SWR curve with a 1.1:1 match mid band and a 1.15:1 match on the edges.
I should mention that one of the strongest talkers I chat with on the band lives in an apartment where he hauls out a portable tripod with a 5' mast and the A-99 mounted on it -- so just 5 feet above ground! Despite this and the fact that he's only running 25 or so watts, his signal is one of the strongest I hear. I think I've reached a conclusion:
If your A-99 is mounted one half wavelength or more above ground, get the GP kit or build your own.
If your A-99 is mounted under half a wavelength in height (< 18'), don't bother with the GP kit.
Under 18 ft, I hypothesize that the mirror half of the wave is interacting with the ground in a way that helps the signal get out. Above 18 ft, the mirror half of the wave is in free space and benefits from the use of an equivalent counterpoise in place of the ground itself.
What do you guys think?
-Dr. Nick
