I can see from the way you have made your enquiry you havn't the foggiest
idea about what you are trying to accomplish. Do you know the frequency?
What transmitter power output do you have in mind - 10 milliwatts or 1KW?
For starters forget all about folding anything - you've been reading the
wrong books. However you now mention a short whip above a groundplane of
unknown construction.


The input radiation resistance at the base of a very short vertical antenna,
say less than 1/10th of a wavelength, is given by -


Rrad = Squareroot( 20 * Height in metres / Wavelength in metres ) ohms.


Rrad will be in the order of a few tenths of an ohm at 2 MHz but increases
fast as the square of frequency.


In series with this radiation resistance is a high value of capacitative
reactance which has to be tuned out somewhere by a lot of micro-henries.
Best located at or near the antenna base.


For a very crude guess the input reactance will be in the order of -


Xin = -300 * Cotangent( Angle ) ohms.


where Angle = 360 * Height / Wavelength degrees.


You will then have the task of winding the correct number of turns on a coil
former, of your chosen length and diameter, to provide an inductance of
similar value of reactance as presented by the whip. Download program
SOLNOID3 for coil design.

In series with Rrad and Xin there will be a loss resistance due to the
connection to the ground plane. If the ground plane is a vehicle then you
can expect a loss resistance between 3 and 15 ohms. If the ground plane
consists of a cigarette-pack size metal plate buried in your back yard then
expect a ground loss resistance between 500 and 5,000 ohms.


Overall antenna input resistance is then Rin = Rrad + Rcoil + Rground.


If it is your intention to connect the antenna directly to the transmitter,
or via a very, very, short length of coaxial line, then Rin is the
resistance which has to be matched to your 150-ohm transmitter by using an L
and C impedance matching network.


Frankly, it may be easier to redesign the transmitter to match the antenna
;o)


But you won't get very far without an impedance measuring device such as a
borrowed, begged or stolen antenna analyser.


As I have no idea of the purpose of the transmitter + antenna I suggest you
ask around for sombody who has already solved the problem and copy his.


It may be that a very short miniature centre-loaded dipole would do the job.
It doesn't need a groundplane and can be driven via a 150-ohm balanced,
twisted-pair line and, if needed at the transmitter end, a simple 150-ohm,
1-to-1 balun.


Download program MIDLOAD and amuse yourself. It also designs the loading
coil. I KNOW it works. Been there - done that!
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. .........


"Paul Burridge" wrote - .
"Reg Edwards"


[snip]

Hi Reg,

It so happens the uniformly distributed radiation resistance is exactly
twice the radiation resistance of a 1/2-wave dipole when concentrated at
its
centre. So the uniformly distributed radiation resistance along a
1/2-wave
dipole is about 140 ohms. It cannot be measured. It can be calculated
from
aerial dimensions. But best just to remember the approximate number 140.
It
does depend to small extent on wire diameter and 'end-effect'.

It's annoying, because the tx output Z I'm trying to match is (by a
strange coincidence) 140 ohms! So a folded dipole would be ideal, I
guess. However - and it's a big *however* - I can't use one. I'm stuck
with a telescopic whip and a ground plane the size of a box of Swan
Vestas. I imagine the radiation resistance of such a non-ideal antenna
is pretty low, but until someone can gimme a ballpark figure for it, I
can't even begin to think about how to go about matching it. :-(


--

"I expect history will be kind to me, since I intend to write it."
-
Winston Churchill