The Cabbary CFA Design is very primative to say the least! They sure are a very weird bit of gear and very touchy to get tuned correctly and the measured performance is different for each installation. I used to build antennas out of cat food cans on wooden sticks when I was a kid and had fun working the world on 27 mhz AM CB when the DX was in (1978). I soon realised if I played with the spacing between the cans and added an inductor to the top one the VSWR would increase and the radiation measured on a field strength meter would increase as well!
I noticed that when the coil value was varied the radiation would have a peak and this lead me to believe that the E and H fileds were correctly phased, when I measured the feed point with a bridge I borrowed from a friend that was a Ham it measured between 350 - 400 ohms! My trick was to peak the coil on the top can for the highest radiation on the FSM then use a cheap Radio Shack CB antenna tuner between the VSWR meter and feedline. I managed to get the match close (1:2:1) and the radiation increased to the point that I could light up a small floresent tube about 12 inches from the gap between the cans and I was only using 4 watts carrier @ 27 Mhz
This is totally the reverse of what is wriiten in books and against the theory we are tought about antenna systems. I have spent the last 30 years of my life building and testing these things with good results and there a 4 rules that must be followed to make a CFA or EH antenna work:
1. The radiation gap must be elevated 1/8 wave length mimimum above ground on an insulated mast to reduce ground loss and coupling to nearby metal objects like fencing or buildings and the feed line. (1/4 wave above ground is about optimum)
2. The shape of the elements have a huge effect on radiation angles
3. The lenght, diameter and spacing between elements controls bandwith
4. When the system is tuned correctly (phased) the Z in = 377 ohms approx this must be transformed to match the feed line correcly for maximum power transfer and efficiency.
I am hoping that some of the members here on the forum show some interst and I will post basic theory, circuit diagrams, designs and projects on my CFA topic page. so they can build and test there own and compare results. I will post some pictures of my experimental 6955 Khz antenna and I am hoping to have it back on the air again soon.http://www.broadcastengineering.info/viewtopic.php?f=27&t=2969