electric & magnetic fields ??
 

As I understand it a shielded loop (non-magnetic shield) favors the
magnetic field. If I wanted to measure the difference between the 2 fields
how would I measure the electric field? A loop shielded with magnetic
material would probably reject both fields. The 3.5 foot loop for my old HP
comparator for WWVB is totally non-magnetic.
Can I generate and transmit each field separately? If so how would I do
it?
tnx

--

73
Hank WD5JFR

"Henry Kolesnik" wrote in message
...
As I understand it a shielded loop (non-magnetic shield) favors the
magnetic field.
Correct.

If I wanted to measure the difference between the 2 fields how would I
measure the electric field?
Any antennna can measure the electric field, you simply have to know the
antenna factor.
Antenna factors can be calculated for various structures; for example the
aperture of a half wave dipole is given by: 0.13*lambda^2. Simple
calculations can then provide the antenna factor, and relate the E field (in
V/m) to the received signal. The electric and magnetic fields are related
by a constant -- the impedance of free space, 377 ohms. i.e. E/H = 377.
In the vicinty of an antenna (the near field) the impedance of free space
becomes a complex number.

A loop shielded with magnetic material would probably reject both fields.
Probably true, but have never experimented with such antennas.

The 3.5 foot loop for my old HP comparator for WWVB is totally
non-magnetic.

Can I generate and transmit each field separately? If so how would I do
it?
No. The E field cannot exist without the H field. See the relationship
above. Some people claim to have invented antennas that seperately generate
E and H fields. Such antennas are known as "EH" and "Crossed-field", and
have largely been rejected by the engineering comunity as bogus. The
designers claim that they do not conform to Maxwell's Equations, but some
other indefinable mathematics.

Regards,

Frank

tnx

--

73
Hank WD5JFR

Frank
If I have two parallel plates seperated by an air space and the plates are
connect to a batttery I have a capacitor with an E field between the plates
and squeezing out the sides. In steady state there's no current flow so I
have no H field.
If I have a coil or a solenoid and connect it to a battery I have a current
flow with a strong H field but I'm not certainabout the E field.
Can this be taken to the next step to answer my original question?
tnx
--

73
Hank WD5JFR

"Frank" wrote in message
news:4Pyie.68625$tg1.4151@edtnps84...
"Henry Kolesnik" wrote in message
...
As I understand it a shielded loop (non-magnetic shield) favors the
magnetic field.
Correct.

If I wanted to measure the difference between the 2 fields how would I
measure the electric field?
Any antennna can measure the electric field, you simply have to know the
antenna factor.
Antenna factors can be calculated for various structures; for example the
aperture of a half wave dipole is given by: 0.13*lambda^2. Simple
calculations can then provide the antenna factor, and relate the E field
(in V/m) to the received signal. The electric and magnetic fields are
related by a constant -- the impedance of free space, 377 ohms. i.e. E/H
= 377. In the vicinty of an antenna (the near field) the impedance of free
space becomes a complex number.

A loop shielded with magnetic material would probably reject both fields.
Probably true, but have never experimented with such antennas.

The 3.5 foot loop for my old HP comparator for WWVB is totally
non-magnetic.

Can I generate and transmit each field separately? If so how would I do
it?
No. The E field cannot exist without the H field. See the relationship
above. Some people claim to have invented antennas that seperately
generate E and H fields. Such antennas are known as "EH" and
"Crossed-field", and have largely been rejected by the engineering
comunity as bogus. The designers claim that they do not conform to
Maxwell's Equations, but some other indefinable mathematics.

Regards,

Frank

tnx

--

73
Hank WD5JFR

Henry Kolesnik wrote:
"If I wanted to measure the difference between the two fields how would
I measure the electric field?"

If this is the radiation field, not the reactive field, it would make no
difference if you measured the electric field or the magnetic field, as
they contain the same quantity of energy. In fact, the energy is
identical as one field begets the other field. That`s "The Secret of
Propagation".

That does not mean the fieldfs can`t be separated. It is easy. Enclose
your loop in an effective Faraday screen. This screen prohibits
electrostatic coupling to the loop, but freely allows magnetic coupling.

Faraday screens are not rare. Nearly every medium wave broadcast station
uses a Faraday screen at every tower between the primary and secondary
of an air-core coupling transformer. Because, without the screen,
capacitive coupling to the tower would favor harmonics of the broadcast
frequency over its fundamental frequency and make compliance with FCC
rules difficult. The Faraday screen is also an excellent lightning
protector.

It`s just as easy to allow only capacitive coupling. Simply put a
circuit to be kept from magnetic coupling in an enclosure which is
completely enclosed in a metal structure (sealed like an expensive
signal generator except for one small hole). Use a capacitor through the
small hole to couple to the outside world. Only the electric field via
the capacitor will influence the circuit in the box.

R-F will not penetrate a metal shield, unless it`s special like the
sliced-up Faraday screen. Then, it`s only the magnetic field which
penetrates. Skin effect requires r-f to flow only on the surface of good
conductors to any appreciable depth.

Best regards, Richard Harrison, KB5WZI

Henry Kolesnik wrote:
"If I have a coil or solenoid and connect it to a battery I have a
current flow with a strong H field but I`m not certain about the E
field."

Resistance somewhere is limiting the current. The E field accompanies
the resistive voltage drop. Static fields don`t make waves. Only the
rate of change makes a disturbance which propagates in waves, E&M, which
generate each other.

Best regards, Richard Harrison, KB5WZI

Hank, and Richard, For a good explanation of this subject I always liked the
book: "Introduction to Electromagnetic Fields" by Paul and Nasar. The first
two chapters of mathematical review are excellent. I see barnesandnoble.com
has the 3rd edition, used, for as low as $66. John D. Kraus' book;
"Electromagnetics" is also a very good text.

73,

Frank

Isn't Kraus "Electromagnetics" a little heavy on the math for the
average Ham ??

Frank wrote:

Hank, and Richard, For a good explanation of this subject I always liked the
book: "Introduction to Electromagnetic Fields" by Paul and Nasar. The first
two chapters of mathematical review are excellent. I see barnesandnoble.com
has the 3rd edition, used, for as low as $66. John D. Kraus' book;
"Electromagnetics" is also a very good text.

73,

Frank

"Richard Harrison" wrote
Nearly every medium wave broadcast station uses a
Faraday screen at every tower between the primary and
secondary of an air-core coupling transformer... Because,
without the screen, capacitive coupling to the tower would
favor harmonics of the broadcast frequency over its
fundamental frequency and make compliance with
FCC rules difficult.
____________

Must politely disagree with that last part -- and probably the first part as
well, at least for modern ACU (Antenna Coupling Unit) designs used in MW
broadcasting.

As a condition of the FCC or other qualification needed legally to offer the
tx for sale, MW broadcast transmitters must meet their harmonic suppression
specs by themselves. They are not permitted to use ACU Faraday screens or
other external means in doing so.

A MW ACU is optimised for maximum power transfer between the transmission
line and the tower for the carrier and its sidebands, however it almost
never uses coupled coils to do that. It uses T, L or Pi networks.

But with or without a Faraday screen, an ACU can couple no more harmonic
energy to the radiator than exists, with respect to the carrier, at the
input of the ACU -- which already meets harmonic suppression specs.

RF

Richard Fry wrote:
"As a condition of the FCC or other qualification needed to legally
offer the tx for sale, MW broadcast transmitters must meet their
harmonic suppression specs by themselves."

It`s true. They are type accepted and don`t produce excess harmonics at
their outputs. The coupling system does not suppress the fundamental but
may further suppress the harmonics.

I`m sorry to misspeak. Never the less, a capacitor`s impedance is
inversely proportional to frequency. Its elimination as a coupling to
the antenna eliminates a preference for higher frequencies in the
coupling system. A pi or T network with shunt capacitance and series
inductance favors the fundamental frequency over its harmonics. These
aren`t required to meet specs but they further reduce harmonic radiation
from the radio station.

The Faraday screen is common in radio stations. It was put there not to
affect the antenna match but only to eliminate capacitive coupling to
the antenna. It also serves as a path to earth for many antenna
lightning strikes as evidenced by numerous pits and metal hrom them
splashed about the enclosure.

Best regards, Richard Harrison, KB5WZI

"Ham op" wrote in message
...
Isn't Kraus "Electromagnetics" a little heavy on the math for the average
Ham ??

I guess it depends on how interested they are. A good grounding in advanced
calculus is certainly a prequesit for either of those texts. Even
elementary calculus, combined with chapters 1 and 2, of Paul and Nasar,
should be sufficient.

73,

Frank


Frank wrote:

Hank, and Richard, For a good explanation of this subject I always liked
the book: "Introduction to Electromagnetic Fields" by Paul and Nasar.
The first two chapters of mathematical review are excellent. I see
barnesandnoble.com has the 3rd edition, used, for as low as $66. John D.
Kraus' book; "Electromagnetics" is also a very good text.

73,

Frank