Most authors explain how a wave is generated, then resort to reciprocity
to explain the reception process. But a clear and simple direct
explanation appears in Bailey, _TV and Other Receiving Antennas_ (pp.
141-2), of what happens when an electromagnetic wave strikes a conductor:

"The second, and equally important effect [the first being reflection of
much of the incident energy] is that some energy /does/ enter the outer
skin of the conductor. That part of the energy, which is not reflected,
must enter the conductor. The conditions at the surface of the
conductor, as we have already seen, give rise to a small resultant
electric vector and a large resultant magnetic vector. The presence of
these at the conductor is direct evidence that power is entering the
conductor. The small electric vector acts on the internal electrons of
the conductor and impresses a direction force, tending to drive the
electrons along the skin of the conductor in the direction of the
electric vector. But from experience we know that /no/ electrons can
ever be caused to move without gradually establishing their own magnetic
field, and this usually takes /time/. The motion of electrons (which is
electric current by definition) never takes place without the magnetic
field. How, then, is the electric vector from the electromagnetic wave
going to put these electrons in motion? It can only do so because the
electromagnetic wave /also supplies a magnetic vector/ as well as an
electric vector. And the value of this magnetic vector is exactly
proportioned to supply just the right amount of magnetic field energy
which the electrons require for immediate motion. Thus the electrons do
not have to establish their own magnetic field, since this field is
supplied by the electromagnetic wave. Hence, electromagnetic wave energy
entering the conductor establishes immediate motion of electrons /along/
the conductor, the direction of motion at any instant corresponding to
the direction of the electric vector. If the electric vector changes
direction, the electrons will follow suit."

Other posters have correctly pointed out that an antenna doesn't and
can't receive a signal solely due to the E field; a time-changing E
field can't exist without an accompanying time-changing H field.

Roy Lewallen, W7EL

Paul Taylor wrote:
Hi,

I am looking for an explanation of how an antenna receives a signal due
to the E-field of an electromagnetic wave.

I have looked in some books, and can understand transmission, but the
books I have looked in don't explain reception.

I have found an explanation of how the H-field induces a signal in a loop
antenna: a changing magnetic flux will induce a current.

But what about the E-field and a dipole antenna? I guess that the E-field
causes electrons to move in the antenna wire, because in a solid
conductor, electrons will move until the E-field inside the solid is
cancelled out?

I have googled but having difficulty finding a good explanation. Any
pointers?

Thanks & regards,

Paul.

Roy Lewallen wrote:

Most authors explain how a wave is generated, then resort to reciprocity
to explain the reception process. But a clear and simple direct
explanation appears in Bailey, _TV and Other Receiving Antennas_ (pp.
141-2), of what happens when an electromagnetic wave strikes a conductor:

"The second, and equally important effect [the first being reflection of
much of the incident energy] is that some energy /does/ enter the outer
skin of the conductor. That part of the energy, which is not reflected,
must enter the conductor. The conditions at the surface of the
conductor, as we have already seen, give rise to a small resultant
electric vector and a large resultant magnetic vector. The presence of
these at the conductor is direct evidence that power is entering the
conductor. The small electric vector acts on the internal electrons of
the conductor and impresses a direction force, tending to drive the
electrons along the skin of the conductor in the direction of the
electric vector. But from experience we know that /no/ electrons can
ever be caused to move without gradually establishing their own magnetic
field, and this usually takes /time/. The motion of electrons (which is
electric current by definition) never takes place without the magnetic
field. How, then, is the electric vector from the electromagnetic wave
going to put these electrons in motion? It can only do so because the
electromagnetic wave /also supplies a magnetic vector/ as well as an
electric vector. And the value of this magnetic vector is exactly
proportioned to supply just the right amount of magnetic field energy
which the electrons require for immediate motion. Thus the electrons do
not have to establish their own magnetic field, since this field is
supplied by the electromagnetic wave. Hence, electromagnetic wave energy
entering the conductor establishes immediate motion of electrons /along/
the conductor, the direction of motion at any instant corresponding to
the direction of the electric vector. If the electric vector changes
direction, the electrons will follow suit."

Hi Roy -

It's certainly true that a moving charge generates a magnetic field, so
perhaps I'm reading it wrong. But it appears to me that Mr. Bailey is
arguing here that an electron cannot be compelled to move simply by the
application of an electric field. Do you think that is what he is
saying? Do you agree?

Other posters have correctly pointed out that an antenna doesn't and
can't receive a signal solely due to the E field;

Given the statement below, I would be interested to know how anyone
could have tested the claim. ;-)

a time-changing E
field can't exist without an accompanying time-changing H field.


Roy Lewallen, W7EL

Jim Kelley, AC6XG

Paul Taylor wrote:

Hi,

I am looking for an explanation of how an antenna receives a signal due
to the E-field of an electromagnetic wave.
I have looked in some books, and can understand transmission, but the
books I have looked in don't explain reception.
I have found an explanation of how the H-field induces a signal in a loop
antenna: a changing magnetic flux will induce a current.
But what about the E-field and a dipole antenna? I guess that the E-field
causes electrons to move in the antenna wire, because in a solid
conductor, electrons will move until the E-field inside the solid is
cancelled out?

I have googled but having difficulty finding a good explanation. Any
pointers?

Thanks & regards,

Paul.

Jim Kelley wrote:

Hi Roy -

It's certainly true that a moving charge generates a magnetic field, so
perhaps I'm reading it wrong. But it appears to me that Mr. Bailey is
arguing here that an electron cannot be compelled to move simply by the
application of an electric field. Do you think that is what he is
saying? Do you agree?

No, I don't believe he's saying that. He says,

The small electric vector acts on the internal electrons of
the conductor and impresses a direction force, tending to drive the
electrons along the skin of the conductor in the direction of the
electric vector. . .

Roy Lewallen, W7EL

Roy Lewallen wrote:
Jim Kelley wrote:


Hi Roy -

It's certainly true that a moving charge generates a magnetic field,
so perhaps I'm reading it wrong. But it appears to me that Mr. Bailey
is arguing here that an electron cannot be compelled to move simply by
the application of an electric field. Do you think that is what he is
saying? Do you agree?


No, I don't believe he's saying that. He says,

The small electric vector acts on the internal electrons of
the conductor and impresses a direction force, tending to drive the
electrons along the skin of the conductor in the direction of the
electric vector. . .

Yes. But then he goes on to say,

How, then, is the electric vector from the electromagnetic wave going to put these electrons in motion?

That's what I was referring to. Do you understand why he would pose
this question if he believed he had already given the answer in the
paragraph you quoted? He shoulda quit while he was ahead maybe? ;-)

Thanks,

Jim Kelley, AC6XG

On Mon, 28 Nov 2005 12:15:30 -0800, Jim Kelley
wrote:

How, then, is the electric vector from the electromagnetic wave going to put these electrons in motion?

That's what I was referring to. Do you understand why he would pose
this question

As already stated:
But from experience we know that /no/ electrons can
ever be caused to move without gradually establishing their own magnetic
field, and this usually takes /time/.
The need for time (impedance) is accommodated by the wave:
It can only do so because the electromagnetic wave
/also supplies a magnetic vector/ as well as an electric vector.

The phase of the re-radiated signal is a function of the path length.
If the path signal required the electric potential to sustain movement
(no other motive force available), that would add an additional phase
retardation that is not observed.

Observation of what does occur is other wise described by Bailey as
from experience we know....

Roy's quote comes from a nascent discussion of the topic of Reception
that has a complete, later chapter devoted to it.

73's
Richard Clark, KB7QHC

Jim Kelley wrote:

Roy Lewallen wrote:

Jim Kelley wrote:


Hi Roy -

It's certainly true that a moving charge generates a magnetic field,
so perhaps I'm reading it wrong. But it appears to me that Mr.
Bailey is arguing here that an electron cannot be compelled to move
simply by the application of an electric field. Do you think that is
what he is saying? Do you agree?



No, I don't believe he's saying that. He says,

The small electric vector acts on the internal electrons of
the conductor and impresses a direction force, tending to drive the
electrons along the skin of the conductor in the direction of the
electric vector. . .


Yes. But then he goes on to say,

How, then, is the electric vector from the electromagnetic wave going
to put these electrons in motion?


That's what I was referring to. Do you understand why he would pose
this question if he believed he had already given the answer in the
paragraph you quoted? He shoulda quit while he was ahead maybe? ;-)

Well, it's obvious that an electric field can move an electron. The
Lorentz force law tells us how much force results from a given E field,
and we can get the resulting acceleration from Newtonian physics. An
everyday example is an oscilloscope deflection system which uses an
electric field to deflect electrons. (Actually, modern digital scopes
typically use raster displays with magnetic deflection -- but many of
still have older analog types with electric field deflection.)

But if the antenna conductor were perfect, no E field at all could exist
at the wire surface regardless of the amplitude of the E field of the
oncoming wave. The wave's E field therefore couldn't directly influence
the electrons in the (perfect) conductor. Only the H field of the wave,
then, can induce a current in the perfect conductor. The direct
influence of the E field on an imperfect conductor would be highly
dependent on the conductivity of the wire, and I'd guess it would be
very small compared to the influence of the H field from a typical
oncoming wave on an electron in a good conductor. Maybe that's what he
was saying.

Roy Lewallen, W7EL

Roy, W7EL wrote:
"But, if the antenna conductor were perfect, no E field at all could
exist at the wire surface regardless of the magnitude of the E field of
the oncoming wave."

If we have a non-varying E field, a perfect conductor in the field would
have the same voltage everywhere due to the short-circuit connecting all
points.

But, an electromagnetic wave sweeping the wire has an alternating
electric field. Its phase is uniform (the same) across the wavefront
because all points are equidistant from the source. A wire parallel to
the E vector would simultaneously experience the same E field force
throughout its length. "No E field at all could exist at the wire
surface regardless of the magnitude of the E field of the oncoming
wave,"

Why must the wire be perfect?

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Roy, W7EL wrote:
"But, if the antenna conductor were perfect, no E field at all could
exist at the wire surface regardless of the magnitude of the E field of
the oncoming wave."

If we have a non-varying E field, a perfect conductor in the field would
have the same voltage everywhere due to the short-circuit connecting all
points.

But, an electromagnetic wave sweeping the wire has an alternating
electric field. Its phase is uniform (the same) across the wavefront
because all points are equidistant from the source. A wire parallel to
the E vector would simultaneously experience the same E field force
throughout its length. "No E field at all could exist at the wire
surface regardless of the magnitude of the E field of the oncoming
wave,"

Why must the wire be perfect?

A time-varying E field can exist in a non-perfect conductor; it cannot
exist in a perfect conductor. You can find the explanation for why this
is in any electromagnetics text.

Roy Lewallen, W7EL

Roy Lewallen wrote:

Well, it's obvious that an electric field can move an electron. The
Lorentz force law tells us how much force results from a given E field,
and we can get the resulting acceleration from Newtonian physics. An
everyday example is an oscilloscope deflection system which uses an
electric field to deflect electrons. (Actually, modern digital scopes
typically use raster displays with magnetic deflection -- but many of
still have older analog types with electric field deflection.)

Yes, I thought that much was obvious as well.

But if the antenna conductor were perfect, no E field at all could exist
at the wire surface regardless of the amplitude of the E field of the
oncoming wave. The wave's E field therefore couldn't directly influence
the electrons in the (perfect) conductor. Only the H field of the wave,
then, can induce a current in the perfect conductor. The direct
influence of the E field on an imperfect conductor would be highly
dependent on the conductivity of the wire, and I'd guess it would be
very small compared to the influence of the H field from a typical
oncoming wave on an electron in a good conductor. Maybe that's what he
was saying.

Roy Lewallen, W7EL

It could be what he was saying. But conductors are are called
conductors for a reason, and it's not necessarily because they conduct
magnetic fields well.

73, ac6xg