On Mon, 4 Sep 2006 20:36:00 +0100, "David" nospam@nospam wrote:

The impedance of free space / air is said to be 377 ohms. Impedance is ratio
of E/H.

Hi David,

True only in free space.

The feedpoint impedance of an antenna is usually 50 or 75 ohms.

Rarely true, but to the point of your posting, this is not
substantially wrong.

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?

A transducer changes energy between domains. That is from electrical
to acoustic, or back again. Or between electrical to photo-electric,
or back again. And so on. What you are describing is transforming
and the appropriate device name would be transformer, not transducer.

To the intent of your statement, yes, an antenna is a transformer.

With a 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?

The ends of an antenna are no more important than the middle.
Radiation occurs on the basis of the entire radiator, not simply
"some" parts.

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?

In fact it is quite the opposite. The biconical is a classic constant
impedance structure. Thin antennas such as the typical doublet or
dipole are non-linear.

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

This is a characteristic, not an explanation.

73's
Richard Clark, KB7QHC