David wrote:
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?

I'd like to take a somewhat different tack on handling this question.
It is a wave versus photon perspective. It seems to be very easy to
slip into the thought mode where we view radio transmissions as a wave
phenomena in some sort of space media. After all, most of the math we
use for electromagnetic signal propagation work quite well assuming
that. Maxwells math still works wonderfully even today, a century after
Einstien described photons. Maxwell died more than a decade before
Einstien published his seminal papers though.

The real model of the radio operation is for there to be alternating
electrostatic and magnetic fields surounding an antenna when it is
driven by an RF power source. Through some yet poorly understood
physical mechanism, either envolving the acceleration of electrons in
the antenna's conductor or from the alternating E and H fields, photons
are flung off the antenna.

I interpret this to mean that what we call the "near field" around an
antenna is the volume around an antenna where the electrostatic and
magnetic fields are at an energy level significantly above natural
background levels. The "far field" I interpret to mean where RF energy
exists as a photon flux.

To describe an antenna as a transducer is probably correct
symantically. It does convert between electrical energy in the form of
RF current and voltage and photons. This is both for transmitting and
receiving.

The 377 ohm thing though is a function of the releative intensities of
the electrostatic and magnitic fields surrounding an antenna. It is a
constant like pi or e. It is an emperically measured characteristic of
our universe. It does not, however mean that an antenna transforms a
feed impedance to this impedance. It simply tells us what to expect for
when we feed an RF current into an antenna. It is much like knowing
that a 1 foot diameter wheel will travel 3.14159265... feet for every
revolution.

Now, a point worth noting is that while RF current in a conductor
produces photons, photons produce RF current in conductors. That, of
course, is why antennas are able to operate for both receiving and
transmitting. That is also why radio signals bounce. Photons are
absorbed by objects such as wires or dirt and RF currents are produced.
Those currents, in turn, generate new photons. The conductivity and
dielectric constant of the absorbing material determine the amplitude
and phasing of the current and thus the primary direction of emission
of the new photons.

So... Yes the antenna is a transducer. No, it does not transform 50
ohms into 377 ohms. 377 ohms refers to the eletrostatic and magnentic
fields as they exist in the near field of an antenna or conductor. It
does not refer to what is going on electrically in the antenna
conductors.

Anyway, that's my take on the subject.

Gary
N0GW