The coils are just "plain old" coils. The wire used on both the Alpha Delta and mine is just solid wire. Theirs is insulated with a clear, thin film of some kind, mind just happens to have a little thicker jacket. I didn't build a coax coil trap because I was trying to copy what Alpha Delta did (and learn something).
Here's a quote from the link I attached earlier:
"It should also be noted that due to the high number of turns on each coil a trap like effect takes place. The inner A sections also work as a half wave dipole, for me that was a bonus as I gained a SWL antenna for the 25 - 27m bands (11 - 12MHz). You could experiment with different lengths for A and B to gain dual band operation or even tri-band operation on the 3rd Harmonic but that's up to you"
I took yesterday off from work and had a couple of hours to play with the antenna. I went over to my Dad's house since he has a nice big American Elm tree in the front yard to hang this from.
This is where I demonstrate how to screw up even the simplest projects. Remember the lengths that I measured and cut earlier for the 40m segment? Well I failed to plan for enough wire to run through the PVC and turn back on itself and then attach to the coil. So when I put the antenna up in the tree with wire attached to the PVC (but not connected to the coil) I now found that my dipole was resonant at 10mhz, instead of 7mhz. I had shortened the antenna because I didn't leave enough wire for the attachment. :headbang . Now I know how Homer Simpson feels.
Well, I wasn't going to let that stop me since I already had the thing in the tree, so I decided to experiment by putting the coil in play. Using the MFJ analyzer, I found that the coil did not change the resonant point significantly around 10mhz. Attaching the tag ends past the coil didn't affect the higher band, either. The higher band simply didn't "see" past the coil.
Unfortunatley I forgot my good camera, so all I had was my crappy cell phone camera. I adjusted the tag ends slightly to 65" each to get it in good shape on the lower side of 80m and called it good: I attached a fuzzy pic of the Analyzer showing how well it tuned up with a 1:1 current balun attached to the analyzer. as well as how the wire is attached to the coil on either side, as well as the tag end. I ran the ladder line into my jeep, used a small travel tuner to test out how difficult it would be to load up the higher bands. It was very, very easy to tune up 40m, 20m, & 10m. (Those were the only ones I tried). It was approaching time for the daily local MARS traffic net, so I retuned for the appropriate 75m frequency and successfully checked in using about 100 watts.
I intend to add some more length to the main wire to try and get it resonant on 7mhz. I would like to get it back up in the air and take some decent pics of the analyzer showing it with the coil attached and without at that frequency so you can see how little difference there is. I'll call the project successful, even though I screwed up the main length since the antenna really is useful and works fine anyway. I'm not sure how much I'll gain by having the antenna self resonant on 40m, except that it would be nice to be able to use it without the tuner.
