Rick wrote in
news
On Wed, 04 Apr 2007 21:48:08 -0700, Bryan wrote:

A stub of coaxial cable will work... approximately 30pF/foot for 50
ohm types... approximately 20pF/foot for 75 ohm types.

Good evening, Bryan.

Thanks, but there are a couple of problems... the antenna will be for
160 through 40 and so two of the traps will be for 80 meters... I
forget what the capacitance will be but I think it was somewhere in
the 200 pf range. Also, the antenna is meant to be portable, and so
the traps need to be as physically small as practical. Also the power
will usually be low... as low as 5 watts, usually no more than 50 and
never more than 100 watts... so I guess I need to make the antenna as
efficient as I can... kind of a contradiction in terms for trapped
antennas especially where the traps need to be physically small.

If I could find the doorknob capacitors for a reasonable price that
would be ideal, but $15 each is way too much.

On the other hand, the ones from RF Parts are all at least 4 KV, and
up from there (40-50 KV). I don't know how to accurately calculate
the voltage rating I'll need (I'll post that as another question under
another topic), but my guess is that for 100 watts max, I don't need 5
KV, and may not need 1 KV.

Who else besides RF Parts has these things and has a decent website?


I would just use normal 500v silver mica capacitors in series if
necessary. At the 100 watt level the capacitors in a trap don’t have to
work too hard. Silver micas have good RF performance and easy to come by.
If you use EZNEC to model your antenna it can tell you the voltage across
your traps at your max power level making it easy to pick out your
capacitors. You can down load a trial version that can easily model your
antenna.

73
John Passaneau W3JXP