On Tue, 09 Dec 2003 00:04:00 +0000, Paul Burridge wrote:

On Mon, 8 Dec 2003 21:32:52 +0000 (UTC), "Reg Edwards"
wrote:

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.

Perhaps you didn't see the earlier posts on the subject. The frequency
is 40Mhz (radio control band) and the tx output stage as it stands
puts out maximum power of 475mW with a 140 ohm resistor as load. I
only *need* 50mW ERP., however, so can stand to see quite a bit of
loss from an inefficient antenna. The antenna I will be using is a
telescopic whip - exactly the same set-up as you see with model
vehicle radio control transmitters- which is what it is, in fact.
It ain't rocket science.
Someone out there must know the radiation resistance of such a
telescopic whip (which has a ground plane of just around 16 square
inches contained within the remote control handset) and the best way
to couple it to a PA with a 140 ohm output impedance?

You have had quite a few answers which seem pretty good to me.

I have a few comments.

Stop saying "radiation resistance" and start saying "input impedance" or
"feed impedance." Once you know the input impedance, getting maximum power
transfer is a straightforward matching problem that can be solved in a
variety of ways. True, you won't know for sure how much power is "getting
out," but some of the posts in this thread have given you some estimates.
If that isn't good enough, then you will have to measure by putting a
receive antenna in the far field.

Anyway, if you want to measure the input impedance, you should be able to
do it with a two-channel oscilloscope and a 40 MHz function generator.
Drive the antenna with the function generator and simultaneously measure
the input voltage and current, including the phase relationship between
them. The input impedance is V/I. To measure the current, put a suitably
sized resistor in series with the antenna and measure the voltage across
it. The voltage across the resistor should be small compared to the
voltage into the antenna. If it is too small to see, use a bigger resistor
until you can see it. Make sure both oscilloscope probes are grounded in
the same place. Unless you have a differential probe (in which case, use
it to measure the voltage across the current sensing resistor).

From what others have already said, it sounds like the input will be
extremely capacitive with a small series resistance. Once you know the
details, however, it will be easy to tell you how to match this to your
amplifier.

That's my $0.02.

Mac