[TowerTalk] Homebrew loading coils

[dan hearn]

Mark: If I understand your setup there, the same vertical is used on
both 40 and 80. If it is 1/8 wavelength on 40, it is only 1/16
wavelength on 80 so a great deal more inductance will be needed. The
inductance of your coil will be proportional to the square of the number
of turns (twice as many turns will be 4 times as much inductance). I
would not use vinyl electrical tape on the coil. It is very lossy in a
strong r.f. field. I have seen it melt when used around r.f. coils. GL
and 73, Dan, N5AR

Mark . wrote:
> 
> Hello All,
> I have been making some loading coils for shortened HF verticals, and I
> could use some advice from those more experienced. I am making multi-tapped
> coil assemblies with alligator-clip jumpers for achieving resonance on
> short, end-loaded wires> They are similar to the 'Texas Bugcatcher' mobile
> HF antenna style, except with capacity hats all the way at the top of the
> radiator.
> 
> I am using enameled or insulated wire (different versions) wrapped around
> PVC pipe coil forms, and covered over with electrical tape. One problem I 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?
> I plan to try switching to bare wire with more spacing between turns, and to
> use spacers in 4 places around the coil form so that the wire only touches
> the plastic in 4 places per turn.
> 
> Any suggestions or comments?
> 
> Thanks,
> 
> --...MARK_N1LO...--
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