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