Topband: 200pF HV-capacitor for shunt feeding

[brianboschma]

>Coax is a transmission line, it has both series inductance
>and shunt capacitance and it has standing waves.
>Ignoring that effect is the same thing that causes trap
>calculators to have error.
>
>  
>
One method I hav  used to create capacitors for end  fed 1/2 waves and 
traps on 40 and below is to construct capacitors from brass hobby 
tubing. By using two telescoping tubes  of a foot or less I make a 
tuneable capacitor. In general I have used  plumbers teflon tape as a 
dielectric wrapped as thick as desired on the inner tubing. The 
resulting C is easy to calculate from an old physics text. I use these 
presently in a traped 40/80 dipole with transmission line traps and at 
the base of a 1/2 - 1/4 wave vertical with good results.  I don't know 
the voltage breakdown if the dielectric I have used but it works well  
for by barefoot installation.

The only challenge is trying to get the guy at the hardware store to 
tell be the dielectric constant of the plumbers tape.

brian, n6iz