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