David:

[snip]
"David" nospam@nospam wrote in message
...
The impedance of free space / air is said to be 377 ohms. Impedance is
ratio
of E/H. The feedpoint impedance of an antenna is usually 50 or 75 ohms.
Can an antenna ever be regarded as a transducer that transforms a radio
wave
from 50 ohms to 377 ohms i.e. provides an impedance transformation? With a
[snip]

The answer is a "considered" yes!

Although the [so-called] term "characteristic impedance", often labelled as
Zo, and units of Ohms are often used to describe a certain characteristic of
a propagation media in field theory, that governs the ratio of the E to H
fields propagating in the media. This "characteristic impedance Zo" is not
the same thing as "driving point or feed point impedance Z" in circuit
theory. Although closely related, circuit theoretic concepts and field
theoretic concepts are different views of electromagnetic phenomena.

Characteristic impedance Zo and the units of Ohms are often used as the
"name" for the square root of the ratio of mu the magnetic permeability is
[u = 1.257E-7 for free space] to epsilon the electric permittivity of a
propagating media [e = 8.85E-12 for free space]

Maxwell's celebrated equations then result in the fact that...

Zo = E/H = sqrt[u/e] = sqrt[(1.257E-7)/(8.85E-12)] = 376.7 Ohms ~ 120pi Ohms

This Zo is not the same as a "feedpoint impedance" Z which is the ratio of
voltage V to current I.

Z = V/I Ohms.

That said it should be recognized that any radiating antenna is "immersed"
in a propagating media, usually free space, and the u and e of that media do
have an important affect on the "characteristic impedance, or surge
impedance" of the antenna which will in turn affect the driving point or
feedpoint impedance of the antenna.

For instance it is well known that the resonant feedpoint impedance (Ratio
of V to I) at the center of a half wave dipole in free space is 73 Ohms. If
that dipole were placed in another medium other than free space with
correspondingly different u and e, the driving point impedance of the dipole
would definitely be affected. So would it's resonant frequency, etc...

And so in that sense, an antenna may be considered to be a transducer and
not a transformer. Antennas may then be viewed as transducers that
transduce the circuit theoretic variables of electric currents and voltages
flowing in and between conductors into field theoretic variables of electric
and magnetic fields flowing through a propagation media. And... indeed
there is a "reaction" between the u and e of the media in which the antenna
is immersed and the currents and voltages flowing in and on the antenna.

The 73 Ohm driving point impedance of a free space half wave resonant dipole
[in the ideal case this is the radiation resistance] is a direct result of
the u and e of the free space in which the antenna is immersed.

If all other things were held constant and the values of u and e of the
medial were changed [i.e. move the antenna from free space where u/e=377 to
be under water where u/e=x??? the driving point impedance of the antenna
would most certainly change!

[snip]
long tapered antenna, the feedpoint is at 50 ohms. Is the end of the
antenna
at 377 ohms to launch the wave easily into free space? In this case,
antenna
is a travelling wave antenna e.g. broad bandwidth biconical. Does the
impedance gradually change from 50 ohms to 377 ohms over the length of the
antenna?
[snip]

No! Not really.

Surprisingly, the actual surge or characteristic impedance Zo of a single
wire antenna in free space, considered as a one wire transmission line
placed high over a ground plane [the earth] is actually in the neighbourhood
of several hundred Ohms... say 600Ohms or so.

The exact value of Zo is easily calculated by well known transmission line
formulas, that assume TEM mode propagation on the line, and this Zo
basically depends upon the height over ground and the diameter of the wire.
This is not a driving point impedance but is a "surge impedance". The
driving point impedance of the single wire transmission line depends upon
where and at what frequency it is "driven" by a source.

Of course because this single wire is quite distant from it's return path
[ground] this single wire transmission line is "leaky". That is it radiates
and loses, or dissipates, power to some extent, as opposed to what it might
do if it were placed very close to the ground where there was a nearby field
"cancelling" current flow. [a microstrip transmission line for instance].

We know that if this single wire transmission line high above the earth is
driven by a source it will exhibit a driving point impedance that depends
upon its length relative to the wavelength of the driving voltage or
current. [73 Ohms resistive if it is a 1/2 wave, some other in general
complex Z if it is not 1/2 wave.

[snip]
The impedance of the end of an antenna (open circuit), where it is a high
voltage point, is usually 5K or 10K ohms.
[snip]

I believe that you are referring to the driving point impedance of an end
fed half wave dipole which is certainly high and in that neighbourhood.
This is not a characteristic or a surge impedance.

And so in summary...

An antenna may be thought of as a transducer between a circuit theoretic
electro-magnetic venue and wave propagation in a propagating media, but the
relationship between the circuit driving point impedance and the
characteristic impedance of the media is quite complex and is certainly not
a simple linear relationship such as found in a transformer or other device.

As far as I understand there is no practical application that has ever
required anyone to quantitatively determine the exact relationship between
the Zo of a propagating media and the driving point impedance Z of an
antenna that is immersed in that media.

In my opinion such a determination would be a very difficult
experimental/engineering exercise. The experimenal problem is one of how
does one vary the Zo of a media while measuring the effect on the Z at the
driving point? Here's a thought experiment...

Immerse an antenna in a liquid media with a given u and e in an anechoic
tank then drive the antenna with a generator while measuring the driving
point impedance (V and I) and then pour or mix in some other liquid with
different u and e and observe the change in Z.

Would that work?

It could also be accomplished numerically on a computer by using a program
[like the NEC programs] based on solving Maxwell's partial differential
equations iteratively.

As far as I know no one has ever attempted to do this... and notwithstanding
the possibility for "invention" or "discovery" I might ask, why would one
want to do this?

Hey it might make a good Ph.D. or M.Sc. thesis... but what is the practical
application?

For all practical purposes, the characteristic impedance of the media in
which antennas are immersed never changes!

Who cares how Z varies when Zo varies?

Thoughts, comments?

--
Pete k1po