Frank wrote:
"Wes Stewart" wrote in message
...
On Sat, 19 Mar 2005 12:45:53 -0800, "RST Engineering"
wrote:

My bag is VHF, so forgive me if I've bumbled up the calculations.
My
numbers show that a standard 108" whip has an input impedance of
somewhere
around 0.04 ohms in series with a 22 pf capacitor.


Not quite. Over perfect ground the Z is ~ 0.37 -j8170 @ 1.9 MHz
for a
12mm diameter radiator.

So you add an inductor of +j8170 with say Q=250. That gives Z =
(0.37+32.68) +j0 = 33.05 +j0. Then you add 16.95 ohm of ground
loss
and Z = 50 +j0.

A perfect match with a gain of -17 dBi. Easy huh?


These numbers are so far from what I normally deal with that I'm
not sure
that I'm right, and I'd appreciate somebody who actually works down
in
this
area giving my numbers a reality check.

If they ARE right, how in heaven do most people match to 0.04 ohms?
The
22
pf I can resonate out with a 1 millihenry choke (or thereabouts),
but how
do
most people match 50 ohms to fractional ohms in the homebrew arena
-- that
is, without just going out and buying a "magical matching box" of
some
sort?

Jim



The following code produces Zin = 0.117 - j2717 ohms at 1.9 MHz.
What did
I do wrong? Note the high segmentation to place the feed-point near
the
base of the antenna.

Frank

CM 9 ft monopole
CE
GW 1 108 0 0 0 0 0 108 0.25
GS 0 0 0.025400
GE 1
GN 1
EX 0 1 1 00 1 0
LD 5 1 1 108 5.8001E7
FR 0 9 0 0 1.8 0.025
RP 0 181 1 1000 -90 0 1.00000 1.00000
EN

Hi Frank, Well, I got different numbers than both of you. Must depend
on the ground type of the models, as well as the # of segments.
If you are trying to match an antenna with fractions of an ohm
resistive and thousands reactive, the differences we are all observing
will not make a whole lot of difference in the matching network you
decide to use. It will be big and bulky, and very inefficient.
Gary N4AST