On 2006-07-18, Owen Duffy wrote:
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?

When I use vertload.exe to model one HALF of the dipole, I get a
radiation resistance of about 12 ohms plus wire loss of about 1.5 for
an input resistance of 13.5 ohms. I am assuming that if I put two
of these shortened quarter-wave verticals back to back I get a dipole
with performance (but not radiation pattern) similar to the quarter
wave vertical over a near-perfect ground.

[and I'm going to stick with those numbers and ignore the other
program for this post]

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.

Ok, so the math would go like this:

At 10.149MHz, my input resistance would be 13.5*2 (two verticals back-
to-back) or 27 ohms. It's got no reactive component at all because
it's tuned perfectly.

So if I added -25j capacitive reactance, for example with about 620p
in series with feed point, then transformed the series impedance
Z=27-25j into parallel admittance Y=0.02+0.018j, then I see my
equivalent parallel resistive component is 50 ohms (ok!) and I am
left with a -54j parallel capacitive reactive component, which I can
cancel with a 54j parallel inductive component, which is my ~85uH
inductor across the feedpoint.

Now, to save money on capacitors, I could alternatively detune the
dipole by shortening it until it was Z=27-25j at my 10.149MHz center
frequency, and then shunt as before to get the same effect. Is
that right?

I guess I also don't understand what happened to my resonance point
when the changes were made to a tuned antenna to produce the desired
feedpoint impedance.

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

Well, I know relatively nothing about building antennas, and I have
no relevant test equipment (except an SWR meter, but no transmitter
as yet) and no antenna tuner. Getting the antenna built was a side
project while I wait for transmitter parts. Perhaps building a quick
and dirty L-match would be a better use of my time!

--
Ben Jackson AD7GD

http://www.ben.com/