Roy Lewallen wrote:
"You can find the explanation for why this is in any electromagnetic
text."

I found it in Terman.

As we all know, we place correctly polarized dipoles, for example,
parallel to the wavefront for maximum response. Terman confirms the
electric field in this instance induces no energy in the antenna. It all
comes from the magnetic field.

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. 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. So, a wire parallel to the front
has no difference of potential induced by the wavefront`s electric
field. It all must come from the mgnetic field.

On page 2 of his 1955 edition, Terman says:
"The strength of the wave measured in terms of microvolts per meter of
stress in space is also exactly the same voltage that the MAGNETIC FLUX
(my emphasis) of the wave induces in a conductor 1 m long when sweeping
across this conductor with the velocity of light."

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.

Best regards, Richard Harrison, KB5WZI

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

It looks like time to remind readers that charge isn't the same as
electrons. On a wire, charge moves at nearly the speed of light, while
electrons only go a few miles per hour. Most of the relevant theory
actually deals with the interaction of fields and charge, not fields and
electrons.

Roy Lewallen, W7EL

Roy Lewallen wrote:

It looks like time to remind readers that charge isn't the same as
electrons. On a wire, charge moves at nearly the speed of light, while
electrons only go a few miles per hour. Most of the relevant theory
actually deals with the interaction of fields and charge, not fields and
electrons.

Roy Lewallen, W7EL

Good point. Charge can be holes, or electrons, or even ions. It is the
fields which move at the speed of light. Charge tends to have to hang
around with the charge carriers. But once a field arrives someplace, it
will immediately influence the motion of charges that happen to be
hanging around there locally.

ac6xg

Richard Clark, KB7QHC wrote:
"From your copy of Bailey, review the text, and reconcile his remarks."

Richard Clarks advice is good. Arnold B. Bailey in "TV and Other
Receiving Antennas" does a more extensive job of explaining how antennas
work than most other authors. Wish everybody interested in antennas
could read Bailey.His catalog of antenna types is convenient too.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote in message
...
Richard Clarks advice is good. Arnold B. Bailey in "TV and Other
Receiving Antennas" does a more extensive job of explaining how antennas
work than most other authors. Wish everybody interested in antennas
could read Bailey.His catalog of antenna types is convenient too.

It's out of print, but I've requested a copy through interlibrary loan. I'm
curious to see what he has to say on the standard V-shaped rabbit ears that
have been in use for decades... although one can readily simulate them and see
exactly how they perform, I've yet to find anyone who had a good idea as to
why rabbit ears haven't traditionally been oriented purely horizontally!

"Joel Kolstad" bravely wrote to "All" (06 Dec 05 09:28:55)
--- on the heady topic of " Antenna reception theory"

JK From: "Joel Kolstad"
JK Xref: core-easynews rec.radio.amateur.antenna:220898

JK "Richard Harrison" wrote in message
JK ...
Richard Clarks advice is good. Arnold B. Bailey in "TV and Other
Receiving Antennas" [,,,]

JK It's out of print, but I've requested a copy through interlibrary
JK loan. I'm curious to see what he has to say on the standard V-shaped
JK rabbit ears that have been in use for decades... although one can
JK readily simulate them and see exactly how they perform, I've yet to
JK find anyone who had a good idea as to why rabbit ears haven't
JK traditionally been oriented purely horizontally!


I think the reason why you don't see rabbit ears oriented horizontally
is that they don't seem to work well as dipoles. When standing up they
aren't even a V antenna and at first one would think they are
vertically polarized but they are somewhat directive with a bipolar
pattern. Rabbit ears are a *******ized form of a couple antenna types.

Sometimes mainly one element is responsible for most of the signal
while the other behaves as a reference or ground. For example on low
frequency channels like Ch-3, reception is best if one of the elements
is oriented straight up and the other horizontally pointing in the
direction of the transmitter. This won't work well with Ch-12 and the
standing V is best then.

A*s*i*m*o*v

.... Men are men and needs must err. - Euripides

"Richard Harrison" bravely wrote to "All" (01 Dec 05 14:18:33)
--- on the heady topic of " Antenna reception theory"

RH From: (Richard Harrison)
RH Xref: core-easynews rec.radio.amateur.antenna:220709

RH Roy Lewallen wrote:
RH "You can find the explanation for why this is in any electromagnetic
RH text."

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


That is outright false. Because I can very easily demonstrate
detecting a static E-field by waving a sensitive probe across it.
An antenna is just a stationary probe with a moving E-field. It is
equivalent. Terman sucks.

A*s*i*m*o*v

.... Horse sense is the result of stable thinking.

Asimov wrote:
"Terman sucks."

Termn`s writings have been exposed for anyone to criticize for most of a
century. His 1955 edition has been out there for 50 years. No
retractions or corrections are necessary.

Detection of static E-fields is not relevant.
Charles Coulomb in 1785 showed electric charges exert forces on each
other that are inversely proportional to the square of the distance
between them. This was the birth of the "inverse square law" as
Coulomb`s discovery applies to magnetic attraction and repulsion, too.

In an electromagnetic field, propagation depends upon the electric field
begetting a magnetic field and vice versa. On average, each field
contains 50% of the total energy.

The electromagnetic field of an antenna could be calculated from the
distribution of voltage on the conductors. Problem is voltmeter leads
would be in the r-f field and this would tend to make measured voltages
inaccurate. R-F current is conveniently and accurately measured with a
thermocouple ammeter.

Strength of an electromagnetic wave is usually measured and quoted in
terms of its electric field in volts per meter. This is the number of
volts which would be induced in a one-meter length of wire placed in the
field parallel to the electric lines of force. Volts in the wire are
produced by movement of magnetic flux across the wire.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
. . .
Strength of an electromagnetic wave is usually measured and quoted in
terms of its electric field in volts per meter. This is the number of
volts which would be induced in a one-meter length of wire placed in the
field parallel to the electric lines of force. . .

If the wavelength is 1 m, the voltage induced in the center of an
open-circuited 1 m diple by a 1 V/m field is 0.5 volt, not 1 volt.

Roy Lewallen, W7EL