On Tue, 18 Jul 2006 17:37:29 -0500, Ben Jackson wrote:

I've used a couple programs (vertload.exe and I think LODI) to design
a shortened, single band dipole antenna for 30m. Specifically I'm
interested in 10.149MHz.

The result is:

--1m wire--[~34t 1.5" form 2" long, ~24uH]----2m wire----[center]--...

The center would be a 1:1 current balun, twisting the wires together
and then taking 10-12 turns on FT50-61 (or if that won't fit, fewer
turns on FT50-43).

The predicted resistive input impedance is 22 ohms. Vertload is just
simulating half the antenna (with 0 ohm ground loss) so its match info
is not useful. The other program called for a ".71uH matching coil"
for 50 feed, which I assume is to cancel a calculated -j45 reactive
component??

This is not very clear, you say 22 ohms then seem to say it is half
the antenna, is the input resistance to the dipole ~22 ohms or ~44
ohms?

Whichever, it is a relatively simple matter to calculate the
components of an L match, where you detune the dipole to get a small
capacitive reactance and shunt the feedpoint with a coil to match to
50 ohms.

If the R component is around 44 ohms, you need to detune the dipole to
about -18 ohms of reactance and shunt the feedpoint with about 2uH.

If the R component is around 22 ohms, you already know the answer, you
need to detune the dipole to about -25 ohms of reactance and shunt the
feedpoint with about 0.7uH.


I would like to pre-calculate an approximate match to Z=50ohms which
I can put directly at the feed point and shrink-wrap with the balun.
Is there a way to do that, or is it too touchy to do without measuring
the actual antenna?

An adventurous approach. You seem uncertain about the calculated
design, doesn't that suggest trying it before committing it
permanently?

Owen
--