[TowerTalk] Homebrew loading coils

[Tom Rauch]

> Bugcatcher' mobile HF antenna style, except with capacity hats all the way
> at the top of the radiator.

Good. That's the spot where hats actually help instead of robbing 
current from the whip.
 
> I am using enameled or insulated wire (different versions) wrapped around
> PVC pipe coil forms, and covered over with electrical tape. 

Bad. Any dielectric around the conductor is bad, especially 
enamel, PVC and vinyl. 

> am running into is that the number of turns required to resonate the
> antenna lower and lower in frequency is not a linear relationship. For
> example, it takes significantly more than double the turns to go from 7.2
> MHz to 3.9 Mhz. It is more like triple. Is this normal, or am I getting
> inter-turn capacitance in my coil that is reducing its effectiveness?

Inter-turn (or shunt) capacitance increases inductance, and causes 
the inductor to have a reactance slope that changes with 
frequency...so it could be part of the problem.

Air-core solenoid wound inductors also rarely, if ever, have perfect 
flux coupling from end-to-end, so the reactance will not go up by 
the square of the turns increase. Especially when you have shorted 
turns at one end.

73, Tom W8JI
w8ji@contesting.com

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