RadioBanter - View Single Post

December 1st 05, 09:26 PM posted to rec.radio.amateur.antenna

On Thu, 1 Dec 2005 14:18:33 -0600, (Richard
Harrison) wrote:

Hi Richard,

If antenna current flows, no matter where it comes from, loss resistance
causes a voltge drop. That`s why the wire needs to be perfect.

For one, there is no such thing as a perfectly conducting antenna,
except where one might truncate precision and measure at D.C. Even
for a perfect conductor (absolutely no Ohmic loss), it still exhibits
radiative loss, and any current through this loss must exhibit a
voltage (the same one described by Terman). Perhaps you intended
this, but you fail to offer Rr, a significant component.

The electric field produces no voltage in the antenna because the wavefront
has the same voltage across its entire surface. That`s because it all
left the same point at the same time.

An electric Dipole exhibits a loci of points in space that has the
same voltage, broadside to the radiator. And this loci is orthogonal.
All paired points in 3-space (in the same polarization to the dipole)
exhibit a potential difference. True, at a great distance it may be
meager, but the common evidence of reception proves it is adequate for
detection and measurement. Your discussion above is for the
insignificance of phase difference at a distance.

From the above, it is seen that the electric field is not effective in
inducing current in a receiving antenna parallel to a wavefront. All the
energy intercepted by the antenna is induced by the magnetic field.

From your copy of Bailey, review the text, and reconcile his remarks.

For others following the original poster's query for a source of
discussion about the physics of reception:

I would suggest reading the chapter "The Theory of Signal
Interception" (all may be advised this chapter runs to 63 pages),
specifically the first two sections "How the Antenna Intercepts a
Signal," and "The Current Treatment" from which I will lightly quote
to amplify the comments above:

"This electrical resistance is not only due to electrical
conductivity of the metal of which the rod is composed but also
due to other factors.... the predominant resistance is, strangely
enough, largely due to the fact that no electrons can move on the
antenna surface without also sending radio energy back out into
space."

By the decimation of the problem of treating a large surface as many
small ones (segmenting the antenna) Bailey offers:

"At each point along this rod we can arbitrarily say that a small
but finite voltage acts."

Notice the "acts" which is an initiator or causative agent, not a
passive result. This is not to say such action is in isolation,
Bailey clearly observes that E/H are inseparable and he offers that
the wave bootstraps the antenna's response (the point of his emphasis
on immediacy from Roy's Chapter 4 quote).

73's
Richard Clark, KB7QHC