(Important Note: I am NOT an antenna expert. It is very possible, even likely that I have oversimplified and made errors in the following text. If you spot any errors or incorrect assumptions, please post a comment to let me know!)
The “star” of the show here is obviously the antenna. Antennas for low band VHF don’t come much bigger than this. As I mentioned before, the antenna measures a little over 100 feet tall, weighs about 7 tons, has six stacked bays, and is commonly called a super-turnstile batwing antenna. I don’t know what the power handling capacity is; it certainly didn’t have any trouble with the 5,000 watts from the analog transmitter, and will barely notice the 75 watts from our repeater. It is built to withstand decades of wind, rain, ice, and direct lightning strikes.
The basic design of a turnstile antenna is two horizontal dipoles (in this case, “batwings” or “bowties”), crossed in the middle at a 90 degree angle. If you feed one dipole 90 degrees out of phase to the other, you get an omnidirectional pattern of horizontal
polarization (radiated to the side, or toward the horizon). At the same time, radiation in the vertical direction (that is, above and below the antenna) is circular polarization. By selecting how the elements are phased relative to each other, you can create either left hand circular, or right hand circular polarization.
Turnstile antennas are often used for satellite work because the circular polarization means that the satellite antenna’s orientation isn’t critical, and they have a very wide pattern that covers the whole sky pretty well. However, that wide pattern is rather wasteful for broadcasting or two-way radio use. That’s why this design has six stacked bays, to concentrate the signal in the horizontal direction. I don’t have the exact gain figure, but I believe it to be more than 6dB over a dipole. With 75 watts expected to reach the antenna from our duplexer, that means we can expect an ERP (effective radiated power) of at least 350 watts! That antenna gain will be a big help on received signals also. A small amount of beam downtilt is part of the design, to help keep the signal from overshooting the ground.
You might expect that this antenna would have a single feedline, with the power splitter and 90 degree phasing of the crossed elements done at the antenna. Fortunately for us, the two halves of the antenna are fed independently, with completely separate coax running all the way to the transmitter room. One feed is for the batwings that face north-south, and one is for the east-west elements. The coax is 3-1/8″ diameter rigid copper, which specs to about .7dB of loss for the 1000 feet it takes to reach the antenna! Not bad! With the antenna mounted about 850 feet off the ground, where the ground is 1400 feet, we have a height above average terrain (HAAT) of 1289 feet. Again, not bad! We’ll have to build a power splitter and phasing harness, but we’ll be able to do that down on the ground, in the comfort of the transmitter room. More on that in a later post.
OK, so we’ve established that it is a great antenna, bigger and better than most people will ever connect to a ham radio. But not EVERYTHING is perfect. One of the first, most obvious questions is: What about the frequency difference? TV channel 3 occupies a 6 MHz wide channel from 60-66 MHz. The proposed six meter repeater will be around 53 MHz. I did quite a bit of research online regarding this issue, and was pleased to find several mentions of the fact that batwing dipoles have very broadband response, and that operating away from the design frequency by 15% is not a big problem. Eighty-five percent of 63 MHz is 53.55 MHz, so I think we should be good! Experiments with a 6 meter rig should verify this (more on that later.) By the way, I also found mention of how the omnidirectional pattern can get distorted when moving away from the antenna’s design frequency. So, the pattern might be a little egg-shaped; I don’t think that’s going to be a big deal.
The second obvious question: Aren’t repeaters normally vertically polarized? Yes, that’s true, but I think that what we lose in polarity mismatch will be made up by the height and overall antenna gain. Besides, being primarily horizontally polarized could be a real advantage for DX hounds who have horizontal beams on 6 meters! So I don’t think that polarity is going to be a huge factor in the overall performance of this repeater. And again, some experiments with a 6 meter rig should verify this.
Bottom line: Using this antenna at 53 MHz instead of 63 MHz may alter the pattern, gain and downtilt, and the primarily horizontal polarization may affect users with vertical antennas. Still, it is one FABULOUS antenna! In spite of the imperfections for 6 meter use, how can you NOT give it a try?! Any time I’ve described this antenna, and my idea of using it for a repeater to another ham or broadcast engineer, there is typically some wide-eyed pondering for a moment, followed by giggles! This is the kind of stuff we hams love, experimenting with antennas and radios. Whatever the results, as this project unfolds it will be offered as a gift to the amateur community, for the use and enjoyment of everyone!