I have a shortened vertical dipole antenna that uses loading coils and large capacity hats. It uses a shunt coil (beta match) at the feedpoint to match to 50 ohms. The antenna looks like this:
I was considering an experiment to remove the loading coils and replace the capacity hats with spiral coils (also called pancake coils), similar to a Petlowany dipole or a TAK-tenna. I would connect the end of each aluminum radiator to a wander lead (alligator clip) that would then be used to select a tap point on the pancake coil. I would tap the coils symmetrically so each side of the dipole is a mirror image of the other.
My hope is that this might simplify the mechanical construction as the spiral coil might provide both inductive loading and capacitive end loading at the same time. Note that I want to use the dipole as a dipole, meaning that unlike the TAK-tenna I am using a balun and do not want the coax to radiate.
1. The current antenna can tune up from 40m to 20m just by tapping the coils symmetrically about the feedpoint. If I switch to using pancake coils, is it realistic to expect to be able to tune up on several bands just by selecting taps? The complicated thing is that I'm altering both the end capacitance and the end inductance with the tapping, there also seems to be a "transmission line stub inductor" (http://www.eham.net/ehamforum/smf/index.php/topic,66072.msg430844.html#msg430844
) within the spiral that needs to be taken into account. Basically, I'm not really sure how these pancake coils will behave when incrementally tapped at different positions.
2. With pancake coils, is a beta match (shunt coil) still appropriate to use for matching this antenna to 50 ohms?
3. Is it possible for the pancake coils to be as efficient as the current solution of the lumped inductor + capacity hat?
4. Any recommendations on the spiral coil geometry? (Wire spacing, etc.)
Thanks for any advice.