Rick wrote:
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?

Hi Rick,

As Owen mentioned, a single coaxial stub would likely be physically
unwieldy. IIRC, I used a short stub as a padder across a doorknob capacitor
(tuned to 40m). Certainly, coaxial cable used as a capacitor won't be as
good as a ceramic or porcelain capacitor but, my trap dipole worked fine for
me. Another common idea is to use a coaxial trap. It's worth
investigating, and inexpensive to try.

The company I work for uses a lot of porcelain dielectric capacitors (a VERY
good hi-Q/low ESR dielectric) from American Technical Ceramics
(http://www.atceramics.com/). Look at ATC's "multilayer capacitor" line.
They're physically small and, if chosen properly, will handle higher
voltages and currents.

For ceramic and mica transmitting capacitors, you might try the surplus
vendors. A few a
Fair Radio Sales (http://www.fairradio.com/)
Surplus Sales of Nebraska (http://www.surplussales.com/)
All-Tronics (http://www.alltronics.com/)
Economy Electronics (http://www.economyelectronics.com/)
Ocean State Electronics (http://www.oselectronics.com/)

Another possible resource is FaradNet (http://www.faradnet.com/).

73,
Bryan WA7PRC

On Thu, 05 Apr 2007 20:17:59 -0700, Bryan wrote:

Another common idea is to use a coaxial trap. It's worth
investigating, and inexpensive to try.

Good morning, Bryan.

I have done some reading on coax traps, and consulted with some who, as
the old saying goes, have probably forgotten more than I will ever know
about antennas, and I'm told they tend to be fairly lossy and inefficient.
They are certainly easier and cheaper to build than the coil-and-capacitor
type. The big thing that concerns me about coax traps is that they need to
be fairly small and portable, meaning I'd probably make them out of RG-58,
and I wondered if RG-58 or even RG-8X would handle the voltage.

For ceramic and mica transmitting capacitors, you might try the surplus
vendors.

Thanks. Fair Radio looks like they have a pretty good selection.