wrote:
Thanks, everyone. But you guys still made things complicated. I said it
is just wire antenna, only for a very small output power, 10mW. In
order to eliminate the confusion or misunderstanding, let me put some
pictures to illustrate:

(1) This transmitter, 10mW, at 450MHz, wire length(lambda/4 for
450MHz)=16cm. What is the wire antenna's impedance?
http://www.charto.info/antq1.jpg
(2) This transmitter, 10MW, at 450MHz, Rod Extendable Antenna is
extended exactly as 16cm(lambda/4 for 450MHz). What is its antenna's
impedance?
http://www.charto.info/antq2.jpg

Given that such specific conditions are as shown in the pictures, is it
still difficult to tell, even for a rough number within a range such as
50 to 80 ohms, or 500-800 ohms?

Absolutely.

You could get a pretty good idea of the impedance of the antenna with a
carefully constructed model and a modeling program if the device case is
metal and you assume that the device won't be anywhere near a person or
other conducting object. If it is, the person or object has to be
included in the model. There's no nifty and simple equation or rule of
thumb which will tell you.

You don't seem to understand that the "antenna" is only part of the
antenna whose impedance you're asking. The other part is the box and
whatever it's attached to or whomever is holding it. The "antenna"'s
impact on the impedance is no greater or less than the impact of the
rest of the real antenna, i.e., the box and what it's connected to.

If the box is plastic, then internal wires and boards are the other part
of the real antenna, and have a major impact on the impedance.

Roy Lewallen, W7EL

"John - KD5YI" wrote in message
news:96bsg.3011$pB.2982@trnddc06...
wrote:
Output power has nothing to do with antenna impedance.

At least until the antenna starts to melt. :-)

I have a suspicion that many of the "random wire" antennas you see on
something like his video transmitter are actually roughly lambda/2 ("end-fed
zepps") -- seems to me their impedance wouldn't be as sensitive to the exact
positioning relative to the case, his hands, etc., and that the higher
impedance is often easier to match to for a low-power transmitter anyway.

Joel Kolstad wrote:
"John - KD5YI" wrote in message
news:96bsg.3011$pB.2982@trnddc06...

wrote:
Output power has nothing to do with antenna impedance.


At least until the antenna starts to melt. :-)

I have a suspicion that many of the "random wire" antennas you see on
something like his video transmitter are actually roughly lambda/2 ("end-fed
zepps") -- seems to me their impedance wouldn't be as sensitive to the exact
positioning relative to the case, his hands, etc., and that the higher
impedance is often easier to match to for a low-power transmitter anyway.


There was an article on 1/2 wave whips for handhelds a few months ago in
QEX. The author did quite a bit of experimentation, as I recall, and
specifically made a hand-held metal box on which to mount the antenna so
that his body, hands, etc would be included in his measurements. I remember
his comment about how far away from his mouth he held the box and antenna to
make his measurements.

But, I don't know how this would compare with a 1/4 wave whip for
sensitivities. It is interesting to insert a 1/4 wave whip into an MFJ269,
walk around, and watch the readings.