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?

wrote:
Thanks, everyone. But you guys still made things complicated.

No, they didn't. Antennas themselves are complicated. Formulas exist only
for specific well-defined environments, none of which fits your
configuration close enough to be meaningful.

I said it
is just wire antenna, only for a very small output power, 10mW.

Output power has nothing to do with antenna impedance.

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?

Yes, it is. And, whatever number you are given will not necessarily be
correct from one time to another. The impedance will change if you pick up
the radio or the camera. The impedance will change if you change position to
be near another object.

Suppose you are told that the impedance will be 20-j35? What will you do
about it? Since you feel that the expert advice you received simply
complicated the issue, one can only assume that you do not have the
knowledge to cope with an answer.

Go to your local ham club and find somebody with an antenna analyzer. Put a
quarter-wave wire into the connector and wander around while recording the
maximum resistance and reactance. You can see it change all over the place.
No, it doesn't go from zero to infinity. The impedance will be different
from that when the antenna is installed on your actual object.

Here is a number. Do with it what you will.... Z ~ 35-j35. If you don't like
this number, get EZNEC and model your own antenna. Or, get your own analyzer
and measure your wire. Be sure to include hand and body capacitance in your
model. Also be sure to model your tripod, soil conditions, nearby trees, the
object enclosure, etc.

Good luck.

John

"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.

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