In article ,
"Al" wrote:

"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.

Yes you are right about the fact that I am talking about two different
things.
1. The "characteristic impedance" of a device.
2. The "impedance" to RF at some arbitrary frequency.

People are used to thinking of #2 as a complex equivalent number of AC
resistance to be calculated in circuit analysis problem.

Antennas are a little different as we must consider all the
ramifications of the actual physical construction of the device.

In basic circuit analysis capacitors, inductors and resistors that make
up a circuit are looked at as simplistic lumped elements. Calculating
impedance to an AC signal implicitly means the circuit resistance to
current flow must consider a frequency to compute a value.

A better simulation of a circuit will consider the parasitics of the
elements and the characteristics of the paths between them, including
the electrical distance between them and through them for a more
accurate answer at higher frequency operation but again the nature of
coming to an equivalent value of impedance to a RF signal numerically
requires the frequency be a part of the resistance to current flow.

Antennas are transducers in a physical sense taking a local RF current
loop and translating it into an EM wave through space. Other language
would call this a near field to far field conversion.

The antenna electrical and physical characteristics require that you
look at more than just taking the view of circuit analysis affecting
the local current loop or near field. The physical properties must also
address the near field becoming the far field so the description of the
antenna as an RF circuit must also be more complex. The antenna can not
be seen as just a impedance number at some frequency because that would
not address its implicit purpose or utility.

Since an antenna is designed to be operated at some frequency the
calculation of impedance is known or considered as integral to the
meaning of the appearance of the value an antenna would present as a
resistive load with no reactive component. This value is a combination
of the conductor loses in the antenna itself, local current loop or near
field and is in combination with the radiation resistance, EM field
around the antenna or far field.

Many paragraphs to come to the fact that antenna impedance implicitly
uses a known frequency to compute the resistive value of the antenna as
a load. A corollary here that is that when you mean to use the antenna
at some frequency you adjust the electrical lengths accordingly so the
antenna ends up being that characteristic impedance value where you
intend to operate it so now the impedance value becomes a number
irrespective of frequency because it is implicitly considered. Circular
definitions at their best.

This is all part of actually using an antenna. You start with a design,
adjust the elements to be resonant in frequency you intend to use it.
Changing the operating frequency means that you change the electrical
length again to what is appropriate so you can see the frequency of
operation does not matter and that the antenna has a "characteristic
impedance value" when speaking about that design in a generic sense
because you make the adjustments to it in actuality.

Answering your question above if the antenna was a balance type where
the coax is inherently unbalanced then you would use the BALUN to
transform the 50 ohm impedance of the transmission line path to the 600
ohm resistive load of the antenna. BAL-UN is a term meaning BALanced to
UNbalanced for an impedance transforming device. If you used an
inherently unbalanced single wire to the coax then you would use an
UNUN. UN-UN means UNbalanced to UNbalanced.

--
Telamon
Ventura, California