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Old March 14th 06, 10:29 PM posted to rec.radio.shortwave
Al
 
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Default Transfer Impedance(LONG)

"Telamon" wrote in message
news:telamon_spamshield-
Whatever the characteristic impedance of the wire is if you stimulate
it with energy at some frequency it will react to that energy with a
combination of the characteristic impedance and added to that a value of
impedance based on the electrical length of the wire. The reactance of
the the wire would be the combination of the characteristic impedance
and the reflected energy together.
Telamon
Ventura, California


Telaman,
Perhaps the fault in my understanding is that there is more than one type of
impedance talked about here and in other posts. Perhaps the speaker is
talking about one and the listener is listening for the other. In the above
paragraph you mention characteristic impedance and impedance based on
electrical length. I conclude from your post here that the characteristic
impedance always remains the same unless some physical changes are made to
the components, antenna or feed-line. Whereas the other impedance (it would
be nice if it also had a unique name rather than just other impedance) which
is based on electrical length is therefore based on frequency. The
introduction of frequency introduces reactance which affects the other
impedance, but the characteristic impedance remains the same.

If that is the case (please correct if not) then the following should be
true:

If I have an antenna with a characteristic impedance of, say, 600-ohms, and
I have a coaxial feedline with a characteristic impedance of 50-ohms, the
two are missmatched and I should use a balun (unun?) with a ratio of
600-ohms to 50-ohms to properly connect the two components. At this time I
now have this antenna properly connected to this feedline, and as yet no
frequency issues have been addressed. Is this correct? If yes, then it could
be said that a balun (unun) matches these two physical devices without
frequency of operation considerations. True?

I'm not being argumentative, I'm asking. I have a loop antenna whose
characteristic impedance I do not know. I want to determine its
characteristic impedance. I also want to match it to my receiver (50-ohm
nominal input) the best that I can. I ask myself if I need a matching
device? These are the issues that I am working on and before I try to tackle
the answer, I first want to understand the theory.

Thank you.
Al KA5JGV


 
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