Antenna reception theory
 

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.

--
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Antenna reception theory
 

"Paul Taylor" wrote in message
...
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.

===================================

It is impossible for an E-field to exist without an H-field.

Therefore, antennas of all sorts receive signals in the same way as a
simple loop. Calculations can begin using either the E-field or the
H-field but they both give the same answer.
----
Reg.

Antenna reception theory
 

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.

Transmission and reception work essentially the same way -- if an
antenna induces a certain field pattern in space, then that same field
pattern will induce the same voltages going the other way.

Most books spend about that much space telling you about the principal,
then use the rest of the time telling you how antennas transmit, leaving
it to you to figure out how they receive.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Antenna reception theory
 

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

This is another example of quantum physics being easier
to understand than Maxwell's equations. RF photons are
absorbed by free electrons in the copper antenna causing
RF currents to flow in the antenna wire.
--
73, Cecil http://www.qsl.net/w5dxp

Antenna reception theory
 

"Cecil Moore" wrote
This is another example of quantum physics being easier
to understand than Maxwell's equations. RF photons are
absorbed by free electrons in the copper antenna causing
RF currents to flow in the antenna wire.
========================================

So how does a dielectric antenna work?
----
Reg.

Antenna reception theory
 

Reg Edwards wrote:
So how does a dielectric antenna work?

I'm not sure since I don't find it in any of my
references including ARRL, Kraus, Balanis, and
the IEEE Dictionary. Is it a waveguide where the
inside air is replaced by a dielectric?
--
73, Cecil http://www.qsl.net/w5dxp

Antenna reception theory
 

Cecil Moore wrote:
Reg Edwards wrote:

So how does a dielectric antenna work?


I'm not sure since I don't find it in any of my
references including ARRL, Kraus, Balanis, and
the IEEE Dictionary. Is it a waveguide where the
inside air is replaced by a dielectric?

In the third edition of the _Antenna Engineering
Handbook_ there is an article on surface-wave antennas
which includes dielectric antennas starting on page
12-8.
73,
Tom Donaly, KA6RUH

Antenna reception theory
 

Tom Donaly wrote:
In the third edition of the _Antenna Engineering
Handbook_ there is an article on surface-wave antennas
which includes dielectric antennas starting on page
12-8.

I'll take a look the next time I'm over at Texas A&M.
In a nutshell, where does the radiation come from?
--
73, Cecil http://www.qsl.net/w5dxp

Antenna reception theory
 

"Tim Wescott" bravely wrote to "All" (24 Nov 05 11:49:57)
--- on the heady topic of " Antenna reception theory"

TW From: Tim Wescott
TW Xref: core-easynews rec.radio.amateur.antenna:220333

TW 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.

TW Transmission and reception work essentially the same way -- if an
TW antenna induces a certain field pattern in space, then that same field
TW pattern will induce the same voltages going the other way.

TW Most books spend about that much space telling you about the
TW principal, then use the rest of the time telling you how antennas
TW transmit, leaving it to you to figure out how they receive.


For antenna to receive it must also transmit part of the signal it
intercepts. Now that confuses this discussion a little more, doesn't
it?!

A*s*i*m*o*v

.... I like the word `indolence.' It makes my laziness seem classy.

Antenna reception theory
 

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.

Antenna reception theory
 

Hi,

Thanks for the replies. I also found a sentence in Joseph Carr's
book, Practical Antenna Handbook second edition. On page 297: 'Large
loop antennas are sensitive primarily to the electric field
of the electromagnetic radio wave, but small loop antenna are primarily
sensitive to the magnetic field of an EM wave.'

I understand that the receiver antenna works in a reciprocal way to
transmit, but if there are other descriptions out there concentrating on
how antennas work at the receiving end, I would be interested to know.

Regards,

Paul.

--
Remove _rem_ before replying by email.

Antenna reception theory
 

Paul:
Try this site: http://www.qsl.net/vk5br/EHAntenna20_40.htm
Respectfully,
art in Reno

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.

--
Remove _rem_ before replying by email.

Antenna reception theory
 

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.

Antenna reception theory
 

Reg Edwards wrote:

It is impossible for an E-field to exist without an H-field.

Must have been before electrostatics was invented. :-)

ac6xg

Antenna reception theory
 

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

Antenna reception theory
 

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

Antenna reception theory
 

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

Antenna reception theory
 

Jim Kelley wrote:
Reg Edwards wrote:

It is impossible for an E-field to exist without an H-field.


Must have been before electrostatics was invented. :-)

ac6xg



How do you make an electrostatic radio wave?
73,
Tom Donaly, KA6RUH

Antenna reception theory
 

"Jim Kelley" bravely wrote to "All" (28 Nov 05 11:52:53)
--- on the heady topic of " Antenna reception theory"

JK From: Jim Kelley
JK Xref: core-easynews rec.radio.amateur.antenna:220506

JK Reg Edwards wrote:

It is impossible for an E-field to exist without an H-field.

JK Must have been before electrostatics was invented. :-)


Yes, but you are changing the topic into static fields. We were
discussing changing electric fields, not statics but dynamics!

A*s*i*m*o*v

.... "Hey, I'm just this guy, see?" --Zaphod Beeblebrox

Antenna reception theory
 

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

Antenna reception theory
 

Asimov wrote:
Yes, but you are changing the topic into static fields. We were
discussing changing electric fields, not statics but dynamics!

When is someone going to come up with a context-free language?
--
73, Cecil http://www.qsl.net/w5dxp

Antenna reception theory
 

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

Antenna reception theory
 

Tom Donaly wrote:
Jim Kelley wrote:

Reg Edwards wrote:

It is impossible for an E-field to exist without an H-field.



Must have been before electrostatics was invented. :-)

ac6xg



How do you make an electrostatic radio wave?
73,
Tom Donaly, KA6RUH

Certainly you're aware that radio waves don't have a monopoly on E
fields, Tom.

73, jk

Antenna reception theory
 

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

Antenna reception theory
 

Jim Kelley wrote:
Certainly you're aware that radio waves don't have a monopoly on E
fields, Tom.

But they should have a monopoly on threads in this newsgroup. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Antenna reception theory
 

Asimov wrote:

"Jim Kelley" bravely wrote to "All" (28 Nov 05 11:52:53)
--- on the heady topic of " Antenna reception theory"

JK From: Jim Kelley
JK Xref: core-easynews rec.radio.amateur.antenna:220506

JK Reg Edwards wrote:

It is impossible for an E-field to exist without an H-field.

JK Must have been before electrostatics was invented. :-)


Yes, but you are changing the topic into static fields. We were
discussing changing electric fields, not statics but dynamics!

But do you agree that it's not impossible for an E field to exist
without an H field?

ac6xg

Antenna reception theory
 

Jim Kelley wrote:
But do you agree that it's not impossible for an E field to exist
without an H field?

Depends upon the context. I suspect he was talking within the
context of RF EM waves? Is it possible for an RF E-field to
exist without an RF H-field?
--
73, Cecil http://www.qsl.net/w5dxp

Antenna reception theory
 

Tom Donaly wrote:
Jim Kelley wrote:

Reg Edwards wrote:

It is impossible for an E-field to exist without an H-field.



Must have been before electrostatics was invented. :-)

ac6xg



How do you make an electrostatic radio wave?


Wave to it first?

- 73 de Mike KB3EIA -

Antenna reception theory
 

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

Antenna reception theory
 

Cecil Moore wrote:

Jim Kelley wrote:

But do you agree that it's not impossible for an E field to exist
without an H field?


Depends upon the context. I suspect he was talking within the
context of RF EM waves?

That's certainly a context where an E field is always accompanied by an
H field. But the statement as it was written is nevertheless untrue.
That was my only point.

Is it possible for an RF E-field to
exist without an RF H-field?

Seems to beg an obvious answer. But the question brings up a point that
people seem to be missing here. An E field is an E field - there are not
different 'kinds' of E fields. The field itself is the same, whether it
varies in time or not. A non-zero dE/dt allows for some of the more
interesting properties to have non-zero solutions, but the fields
themselves are not unique. I hope that concept isn't too controversial
for this group. If it is, I will strive to keep such ideas to myself in
the future.

ac6xg

Antenna reception theory
 

Jim Kelley wrote:
The field itself is the same, whether it varies in time or not.

I wonder if that's true when taken out of context? :-)

I'm no physicist but wouldn't a static electric field
be made up of virtual photons while a dynamic electric
field would be made up of non-virtual photons?
--
73, Cecil http://www.qsl.net/w5dxp

Antenna reception theory
 

Cecil Moore wrote:
Jim Kelley wrote:

The field itself is the same, whether it varies in time or not.


I wonder if that's true when taken out of context? :-)

I'm no physicist but wouldn't a static electric field
be made up of virtual photons while a dynamic electric
field would be made up of non-virtual photons?

Non-virtual photons, as opposed virtual non-photons I presume. I think
physicists know they're going to have to wait until they get to the
pearly gates before they can really learn what "electric fields are made
out of". ;-)

73, jk

Antenna reception theory
 

Cecil Moore wrote:

Jim Kelley wrote:

Certainly you're aware that radio waves don't have a monopoly on E
fields, Tom.


But they should have a monopoly on threads in this newsgroup. :-)

Methinks you ridicule the optically disinclined, Cecil.

ac6xg

Antenna reception theory
 

"Jim Kelley" bravely wrote to "All" (29 Nov 05 10:26:08)
--- on the heady topic of " Antenna reception theory"

JK From: Jim Kelley
JK Xref: core-easynews rec.radio.amateur.antenna:220548

JK Asimov wrote:

"Jim Kelley" bravely wrote to "All" (28 Nov 05 11:52:53)
--- on the heady topic of " Antenna reception theory"

JK From: Jim Kelley
JK Xref: core-easynews rec.radio.amateur.antenna:220506

JK Reg Edwards wrote:

It is impossible for an E-field to exist without an H-field.

JK Must have been before electrostatics was invented. :-)


Yes, but you are changing the topic into static fields. We were
discussing changing electric fields, not statics but dynamics!

JK But do you agree that it's not impossible for an E field to exist
JK without an H field?


A static E field can exist alone but to detect it requires something
like a field-mill which basically converts it into a changing EM field
that can be readily detected. A simple field-mill is basically a
rapidly spinning antenna. Relativity at work.

A*s*i*m*o*v

.... The truth is WAY out there!

Antenna reception theory
 

Reg, G4FGQ wrote:
"It is impossible for an E-field to exist without an H-field."

Agreed.

By definition an electromagnetic wave includes an electric component and
a magnetic component. That does not mean the components are inseparable.

The purpose of a Faraday screen is to eliminate capacitive coupling
while permitting magnetic coupling.

I`ve worked in several medium wave broadcast plants. In these, each
tower was coupled through a 1:1 air-core transformer to its transmission
line. The transformer consisted of two identical coils, one on either
side of a Faraday screen. The coils shared a cmmon axis.

Electrically, the transformer was transparent at the transmitting
frequency. It coupled the transmitting frequency as if the transformer
did not exist to impede. Its purpose was to eliminate capacitive
coupling, The Faraday screen provided a place where electric field lines
are shunted to ground.
The problem with capacitive coupling between a transmitter and a tower
is that the higher the frequency, the less the reactance or opposition.
The coupling is better through a capacitance to the harmonics of a
frequency than it is for the fundamental. The Faraday screen removes
this unwanted bias for imroved harmonic propagation.

A side effect of the Faraday screen is that it removes lightning strokes
before they reach the transmission line from the tower.

The Faraday screen looks like a metal rake. Its back where the teeth or
tines join is firmly grounded. The teeth are open-circuited. Current
cannot circulate between and through the teeth, so no counter
electromotive force can be generated to oppose magnetic coupling between
primary and secondary coils. The rake is transparent for magnetic
coupling but it is a stopper for electric coupling.

By complete shielding, that is metalllically enclosihng one or both
coils of an impedance coupling pair, magneric coupling between them can
be practically eliminated. A coupling capacitor between the coils allows
only the electric field to be effective. There`s no magnetic field
involved.

I`m no advocate of the E-H antenna, but the electric and magnetic
components of a wave are easily separated.

Best regards, Richard Harrison, KB5WZI

Antenna reception theory
 

I'm afraid your Faraday screen might not work quite like you think it does.

In the vicinity of the screen, the E field is indeed reduced. However,
you haven't stripped off the E field from the EM wave, or separated it.
The E field is largely reflected from the screen, but out of phase with
the original wave. So the E/H ratio is smaller on *both* sides of the
screen. Close to the screen, much of the energy formerly in the E field
has been transferred to the H field. But as you go beyond the screen in
either direction, you'll find the E field increasing and the H field
decreasing as the energy redistributes itself. Within a short distance
(typically considerably less than a wavelength, but depending on the
size of the screen), the ratio of E/H will again be close to 377 ohms,
assuming air is the surrounding medium. The Faraday screen works in the
broadcast application only because the "shielded" component is close to
the screen, where the E/H ratio is low.

In other words, you can modify the E/H ratio in a small region of space
by moving the energy from one to the other. But you can't separate the
two components or eliminate one or the other.

This is of course referring to time-varying, not static, fields. Reg's
statement is technically false, since he didn't say whether the fields
are time-varying -- static E and H fields can independently exist. But
time-varying E and H fields, which I'm sure is what he meant, can't.

Roy Lewallen, W7EL


Richard Harrison wrote:
Reg, G4FGQ wrote:
"It is impossible for an E-field to exist without an H-field."

Agreed.

By definition an electromagnetic wave includes an electric component and
a magnetic component. That does not mean the components are inseparable.

The purpose of a Faraday screen is to eliminate capacitive coupling
while permitting magnetic coupling.

I`ve worked in several medium wave broadcast plants. In these, each
tower was coupled through a 1:1 air-core transformer to its transmission
line. The transformer consisted of two identical coils, one on either
side of a Faraday screen. The coils shared a cmmon axis.

Electrically, the transformer was transparent at the transmitting
frequency. It coupled the transmitting frequency as if the transformer
did not exist to impede. Its purpose was to eliminate capacitive
coupling, The Faraday screen provided a place where electric field lines
are shunted to ground.
The problem with capacitive coupling between a transmitter and a tower
is that the higher the frequency, the less the reactance or opposition.
The coupling is better through a capacitance to the harmonics of a
frequency than it is for the fundamental. The Faraday screen removes
this unwanted bias for imroved harmonic propagation.

A side effect of the Faraday screen is that it removes lightning strokes
before they reach the transmission line from the tower.

The Faraday screen looks like a metal rake. Its back where the teeth or
tines join is firmly grounded. The teeth are open-circuited. Current
cannot circulate between and through the teeth, so no counter
electromotive force can be generated to oppose magnetic coupling between
primary and secondary coils. The rake is transparent for magnetic
coupling but it is a stopper for electric coupling.

By complete shielding, that is metalllically enclosihng one or both
coils of an impedance coupling pair, magneric coupling between them can
be practically eliminated. A coupling capacitor between the coils allows
only the electric field to be effective. There`s no magnetic field
involved.

I`m no advocate of the E-H antenna, but the electric and magnetic
components of a wave are easily separated.

Best regards, Richard Harrison, KB5WZI

Antenna reception theory
 

"Asimov" wrote:

A static E field can exist alone but to detect it requires something
like a field-mill which basically converts it into a changing EM field
that can be readily detected. A simple field-mill is basically a
rapidly spinning antenna. Relativity at work.

It's similar in some ways to a method for detecting magnetic fields used
prior to the advent of Hall effect devices. Not sure how it relates to
relativity.

Perhaps it's true that an electric field is simpler create than to
detect by direct means. But it isn't really any more difficult than,
for example, measuring power by direct means. I think Ben Franklin
measured the E field in a Leyden Jar by calibrating the leaf
displacement caused by the Coulomb force resulting from the electric
field between the two similarly charged surfaces.

jk

Antenna reception theory
 

"Jim Kelley" bravely wrote to "All" (29 Nov 05 14:46:41)
--- on the heady topic of " Antenna reception theory"

JK From: Jim Kelley
JK Xref: core-easynews rec.radio.amateur.antenna:220573

JK "Asimov" wrote:

A static E field can exist alone but to detect it requires something
like a field-mill

JK It's similar in some ways to a method for detecting magnetic fields
JK used prior to the advent of Hall effect devices. Not sure how it
JK relates to relativity.

I think a saturable core can be used to measure a static magnetic
field. Early computer magnetic core memories worked like this.

Relativity transforms static fields into dynamic fields by adding a
velocity component to the measurement.


JK Perhaps it's true that an electric field is simpler create than to
JK detect by direct means. But it isn't really any more difficult than,
JK for example, measuring power by direct means. I think Ben Franklin
JK measured the E field in a Leyden Jar by calibrating the leaf
JK displacement caused by the Coulomb force resulting from the electric
JK field between the two similarly charged surfaces.

That Leyden Jar experiment was measuring charges not the E field
itself. An E field doesn't require the exchange of charges.
I wonder if it is possible to directly measure an E field by the
effect of the virtual quanta in its close vicinity?

A*s*i*m*o*v

.... Quoting one is plagiarism. Quoting many is research.

Antenna reception theory
 

I wonder if it is possible to directly measure an E field by the
effect of the virtual quanta in its close vicinity?

If the effect of virtual quanta could be measured,
would they still be virtual?
--
73, Cecil http://www.qsl.net/w5dxp

Antenna reception theory
 

Asimov wrote:

I think a saturable core can be used to measure a static magnetic
field. Early computer magnetic core memories worked like this.

I was referring to the similarity to a rotating coil gaussmeter. I
think what you're describing now is something more akin to the fluxgate
magnetometer.

Relativity transforms static fields into dynamic fields by adding a
velocity component to the measurement.

I see. Is Omni magazine still in print by any chance?

That Leyden Jar experiment was measuring charges not the E field
itself.

Leyden jars store charge. As I said before, they produce an indication
by relying on the electric field between charged surfaces and the
resulting Coulomb force. The more charge stored in the jar, the greater
the electric field. Charge, E field, and Coulomb force all being in
proportion, the Leyden jar produces a response in proportion to all three.

jk