On 19 Dec 2003 19:51:26 -0800, (Art Unwin KB9MZ)
wrote:
Can't see how a Faraday shield can be used if they are link coupled

Hi Art,

Nearly every power transformer on this planet uses a faraday shield
between link coupled circuits. Those that don't (and they got to be
dirt cheap from a garage shop operation) suffer from it too (as does
the user).

This is an old, old topic that Richard Harrison, KB5WZI, has
described, explained, discussed to considerable bandwidth that should
serve as a basis for your research in Google.

73's
Richard Clark, KB7QHC

Art, KB9MZ wrote:
"Can I assume then that broadcast coupling coils are always apart to
accommodate a Faraday shield between them?"

This is not an FCC requirement, I Believe. The FCC sets a low allowable
harmonic content level for broadcast signals. A Faraday shield between
coils eliminates capacitive coupling between them.

Capacitive coupling between coils favors harmonics, as capacitive
reactance is inversely proportional to frequency. Killing capacitive
coupling is effective in eliminating harmonic radiation. Putting the
Faraday shield in the tower coupling makes a powerful lightning
deterrent, too.

The usual shield construction is a metal picket fence with the coils on
either side sharing an axis.

Best regards, Richard Harrison, KB5WZI

Art, KB9MZ wrote:
"Do any commercials use a rotating coil to vary the coupling?"

Lots of ingenuity and variety have been used, and I`ve not seen it all.
But, I haven`t seen a swinging link out at the tower where the Faraday
shield is used in medium wave broadcast stations.

MW BC stations don`t ordinarily change frequencies or antennas except
for some day/night changes done by remote switching. For initial power
division among several towers, there are power and phasing networks back
at the station house. There is a "dog house" at each tower to house
matching and coupling networks. Coupling includes the Faraday shield
between primary and secondary coils.

The shield hides one coil from the other for electrostatic lines of
force. The shield is often a grounded metal plate. It is ineffective in
blocking magnetic coupling because it is only grounded on one end, and
has parallel slots perpendicular to the grounded end. These slots
prevent circulating current in the plate. Circulating current in the
plate would produce a counter-EMF which would neutralize the magnetic
field of the primary coil and its coupled energy in the secondary. The
slots make the shield plate pemeable to the magnetic field, but not
permeable to the electrostatic field.

Sometimes, individuual wires are used as a Faraday shield in place of a
slotted plate. The wires may work better at reducing circulating
current, but the slotted plate obviously works well enough for many
transmitting stations.

Best regards, Richard Harrison, KB5WZI

Richard,
Thank you for that extensive reply. I now see that a
floating coax shield would be ineffective as a screen
since it would not discriminate
I initially looked at a picket fence as having
two hortizontal members as well but now I see it as more
of a fork design with the tines at a 90 degree
axis to the axis of the secondary inductance and parallel
to the actual coils. We will see if works out for both
lightning and static as my tower is grounded together
with a heavy separate aluminum cable connected to my
ground grid. It has yet to be hit by lightning however

Best regards


(Richard Harrison) wrote in message ...
Art, KB9MZ wrote:
"Do any commercials use a rotating coil to vary the coupling?"

Lots of ingenuity and variety have been used, and I`ve not seen it all.
But, I haven`t seen a swinging link out at the tower where the Faraday
shield is used in medium wave broadcast stations.

MW BC stations don`t ordinarily change frequencies or antennas except
for some day/night changes done by remote switching. For initial power
division among several towers, there are power and phasing networks back
at the station house. There is a "dog house" at each tower to house
matching and coupling networks. Coupling includes the Faraday shield
between primary and secondary coils.

The shield hides one coil from the other for electrostatic lines of
force. The shield is often a grounded metal plate. It is ineffective in
blocking magnetic coupling because it is only grounded on one end, and
has parallel slots perpendicular to the grounded end. These slots
prevent circulating current in the plate. Circulating current in the
plate would produce a counter-EMF which would neutralize the magnetic
field of the primary coil and its coupled energy in the secondary. The
slots make the shield plate pemeable to the magnetic field, but not
permeable to the electrostatic field.

Sometimes, individuual wires are used as a Faraday shield in place of a
slotted plate. The wires may work better at reducing circulating
current, but the slotted plate obviously works well enough for many
transmitting stations.

Best regards, Richard Harrison, KB5WZI

On 22 Dec 2003 07:07:20 -0800, (Art Unwin KB9MZ)
wrote:

Richard,
Thank you for that extensive reply. I now see that a
floating coax shield would be ineffective as a screen

A floating coax is as good as any shield to be used for a Faraday
Shield. It is used extensively in balanced loop constructions
everyday.

since it would not discriminate

You still don't get the purpose of the shield. It discriminates very
effectively - its whole purpose in life. Research Richard's comments
in Google.

I initially looked at a picket fence as having
two hortizontal members as well but now I see it as more
of a fork design with the tines at a 90 degree
axis to the axis of the secondary inductance and parallel
to the actual coils.

No such requirement exists - there are NO alignment issues. The
reason they fork is to eliminate current flow. Every power
transformer on this planet employing a Faraday Shield does it with a
solid sheet of copper. The copper does not conduct nor support
current loops because it is connected to ground at one point only.
Hence, such a design is consistent with the "fork" metaphor, if you
only think of it as having one, extremely wide tine.

Art, you have Terman, why don't you consult him?

We will see if works out for both
lightning and static as my tower is grounded together
with a heavy separate aluminum cable connected to my
ground grid. It has yet to be hit by lightning however

Best regards


Hi Art,

You should consult the Code as to grounding methods.

73's
Richard Clark, KB7QHC

Art, KB9MZ wrote:
"I now see that a floating coax shield would be ineffective as a screen
since it would not discriminate.."

Art is in agreement with King, Mimno, and Wing on this. They say on page
235 of "Transmission Lines, Antennas, and Wave Guides:

"The operation of the shielded loop is explained popularly by first
stating that the desired loop current is due to the magnetic field, the
undesired up-and-down current to the electric field, and then
maintaining that the metal shield cannot be penetrated by the electric
field but can be penetrated by the magnetic field. All these statements
are incorrect in light of fundamental electromagnetic principles."

Ever have an open coax shield connection only at one end of the cable?
Wasn`t the screening vitiated? I don`t want to argue with (3) Ph.D.`s.

Best regards, Richard Harrison, KB5WZI

Richard,
At the moment I am interested in Richards comment that the
tines that he has SEEN commercially were pock marked because
of flashovers. Hearing that encourages me that my choice of
doing the same thing was a good one where I was persueing
the hope that it will reduce local noise and static crashes.
I am not a long time user of the top band but it doesn't
take a rocket scientist to see that any noise reduction on
receive will help! Maybe at a later date I will try my coax
coil idea, but for now I am useing existing wheels before
thinking about inventing a new wheel.
Ofcourse, any comments on the subject are very welcome in
the event I decide to change my aproach as this is new to
me with regards to prior personal construction.
Have you personally constructed an antenna with a coupling
interface and how did it work out.
By the way the tines are already on my antenna
but it will take time to determine any advantages.
Tho it is a horizontaly polarised antenna I assume there are
vertical components that could be erased BEFORE it gets
to my radio.
You may recall that my antenna is very narrow banded so as
to prevent extranious noise getting to the radio, in that the
signal is filtered before it is amplified and removed by
the radio's filter.This is the phillosophy behind
my design and it is hoped that tines will help here also.
Best regards
Art






Richard Clark wrote in message . ..
On 22 Dec 2003 07:07:20 -0800, (Art Unwin KB9MZ)
wrote:

Richard,
Thank you for that extensive reply. I now see that a
floating coax shield would be ineffective as a screen

A floating coax is as good as any shield to be used for a Faraday
Shield. It is used extensively in balanced loop constructions
everyday.

since it would not discriminate

You still don't get the purpose of the shield. It discriminates very
effectively - its whole purpose in life. Research Richard's comments
in Google.

I initially looked at a picket fence as having
two hortizontal members as well but now I see it as more
of a fork design with the tines at a 90 degree
axis to the axis of the secondary inductance and parallel
to the actual coils.

No such requirement exists - there are NO alignment issues. The
reason they fork is to eliminate current flow. Every power
transformer on this planet employing a Faraday Shield does it with a
solid sheet of copper. The copper does not conduct nor support
current loops because it is connected to ground at one point only.
Hence, such a design is consistent with the "fork" metaphor, if you
only think of it as having one, extremely wide tine.

Art, you have Terman, why don't you consult him?

We will see if works out for both
lightning and static as my tower is grounded together
with a heavy separate aluminum cable connected to my
ground grid. It has yet to be hit by lightning however

Best regards


Hi Art,

You should consult the Code as to grounding methods.

73's
Richard Clark, KB7QHC

On 22 Dec 2003 17:17:02 -0800, (Art Unwin KB9MZ)
wrote:

Richard,
At the moment I am interested in Richards comment that the
tines that he has SEEN commercially were pock marked because
of flashovers. Hearing that encourages me that my choice of
doing the same thing was a good one where I was persueing
the hope that it will reduce local noise and static crashes.
I am not a long time user of the top band but it doesn't
take a rocket scientist to see that any noise reduction on
receive will help! Maybe at a later date I will try my coax
coil idea, but for now I am useing existing wheels before
thinking about inventing a new wheel.
Ofcourse, any comments on the subject are very welcome in
the event I decide to change my aproach as this is new to
me with regards to prior personal construction.
Have you personally constructed an antenna with a coupling
interface and how did it work out.
By the way the tines are already on my antenna
but it will take time to determine any advantages.
Tho it is a horizontaly polarised antenna I assume there are
vertical components that could be erased BEFORE it gets
to my radio.
You may recall that my antenna is very narrow banded so as
to prevent extranious noise getting to the radio, in that the
signal is filtered before it is amplified and removed by
the radio's filter.This is the phillosophy behind
my design and it is hoped that tines will help here also.
Best regards
Art


Hi Art,

I've worked on many shielding problems, and they are not always
intuitive, nor simple. In fact, the inappropriate "addition" of a
ground is often a cure worse than the original ill. There are also
issues of where to ground when the circuit being shielded has a
floating termination (like the coax example). In one instance the
shield is perfect, in others it is a coupling capacitor. To all
outward appearances, the circuits are identical until you come to the
question of ground.

Here the variables are manifold, but in general (unless experience
demonstrates otherwise) you always ground at the load of a signal.
For two-way communication systems, this presents something of a
paradox (which end is the load? When?). And of course, this has
NOTHING to do with lightening strikes, and everything to do with
Common Mode (which are sometimes one and the same problem).

If you are worried about received noise, the load is the receiver, but
this is not necessarily the lightning protection answer. Putting a
ground at both ends can create nightmares, and it can solve
nightmares. The creation of nightmares often comes in the form of
using the coax shield as the ground common between two points (a
serious risk to the point of death). Such a quick fix, ad hoc
solution may remove noise and yet kill you when you disconnect it for
maintenance. A separate wire conforms to code, keeps you safe, and
generally takes care of the noise and lightning.

73's
Richard Clark, KB7QHC

Art, KB9MZ wrote:
"---he has seen commercially were pock marked because of flashovers."

True. It mystified me that lightning preferred the Faraday screen to the
made for the purpose arc-gap across the base insulator. It may result
from impedance discontinuity. The surge impedance of the tower depends
on how skinny it is. The rigid rod connecting a tower to the "dog house"
network is at a certain height above the earth which gives the rod a Zo
as a single conductor transmission line, then this line connects with an
air-core solenoid returned to the earth to complete the circuit.

The coil must have a high reactance for some of the lightning`s spectrum
of energy, so it arcs across the gap between the coil and the grounded
Faraday screen. Once the arc is established, it is a low impedance from
d-c to daylight. The pock marks are deep and numerous.

A horizontal antenna reduces local noise because local signals and local
noise arrive at an angle almost parallel to the surface of the earth.
Propagation of horizontally polarized waves along the surface of the
earth is almost zero if the earth is a good conductor in the local area.
With good conductivity, incident and reflected waves are nearly equal in
strength. The reflection of a horizontally polarized wave is 180-degrees
out-of-phase with the incident wave. When these two waves, incident and
reflected waves arrive at a distant point, they nearly completely
cancel, as their path lengths are nearly equal too.

The bandwidth of standing wave antennas may be small when compared with
the frequency range allocated for an amateur band, but as compared with
the intermediate frequency passband of the receiver, the antenna
bandwidth is likely large. The Q of the antenna is lowered by radiation
resistance which is the antenna`s end product.

Best regards, Richard Harrison, KB5WZI

Richard
I think you are confusing low frequency aplications with
high frequency aplications. A flat sheet allows formation of
ground loops that then form their own emissions.
This is not desirable in high frequency aplications and
thus a common short circuit to ground for discharge is required.
Best regards
Art


ichard Clark wrote in message . ..
On 19 Dec 2003 19:51:26 -0800, (Art Unwin KB9MZ)
wrote:
Can't see how a Faraday shield can be used if they are link coupled

Hi Art,

Nearly every power transformer on this planet uses a faraday shield
between link coupled circuits. Those that don't (and they got to be
dirt cheap from a garage shop operation) suffer from it too (as does
the user).

This is an old, old topic that Richard Harrison, KB5WZI, has
described, explained, discussed to considerable bandwidth that should
serve as a basis for your research in Google.

73's
Richard Clark, KB7QHC

On 22 Dec 2003 21:06:44 -0800, (Art Unwin KB9MZ)
wrote:

Richard
I think you are confusing low frequency aplications with
high frequency aplications. A flat sheet allows formation of
ground loops that then form their own emissions.
This is not desirable in high frequency aplications and
thus a common short circuit to ground for discharge is required.
Best regards
Art

Hi Art,

I am confusing nothing. A current cannot be induced without a path,
if you do not provide any more than one terminus, there is no path
EXCEPT TO GROUND and it is a common shared by both halves of the
magnetic linked circuits (read your Terman). This means the circuits
on both sides of the shield are electrically decoupled from each other
and are driving ground directly - capacitively (the electric field).
The sole purpose of the Power Transformer's Faraday Shield is EXACTLY
for snubbing VLF/MF/HF/VHF/UHF coupling (there's no purpose to killing
50/60 Hz fields with a Faraday Shield, that just doesn't make sense in
the first place).

The guff you are repeating is the ill-understood concepts of UHF/SHF
circulating currents that could only be developed if the wavelength
scale can support it. Are you talking about 900 MHz applications with
3 inch Faraday screens? I think not.

There's no mystery about this, dimension and frequency drive all such
issues.

73's
Richard Clark, KB7QHC

Richard you are a smart guy with respect to radio matters so you must
be misinterpreting things that are being said on this thread
That commercial radio stations use tines is evidence to me that they
serve a purpose i.e. electrostatic shielding, not everything, just
electrostatic stuff
( ignoring harmonics e.t.c. )
As for low frequency use, that would include audio, not just 50/60
which you refer to 'power . Now you say look at Terman and I have, not
only the section on transformers but on coupling circuitry...way back
to the days of spark gaps and ticklers, each of which are treated
differently by not only Terman but also by others. You will also note
that audio cables have sheet like covering just like books do with
TOTAL shielding. Pretty much all books on fields and waves have a
section of total shielding of nearby circuits with special reference
to TOTAL enclosure and the effects on circuits or inductances that are
so enclosed.
With all that being said and getting back to the initial area of
discussion
where Richard alluded to the picket fence. Why do you think that the
broadcast industry hung on to the ungainingly picket fence after all
these years and why would it not descriminate with regard to other
influences ?

Best regards
Art

Richard Clark wrote in message . ..
On 22 Dec 2003 21:06:44 -0800, (Art Unwin KB9MZ)
wrote:

Richard
I think you are confusing low frequency aplications with
high frequency aplications. A flat sheet allows formation of
ground loops that then form their own emissions.
This is not desirable in high frequency aplications and
thus a common short circuit to ground for discharge is required.
Best regards
Art

Hi Art,

I am confusing nothing. A current cannot be induced without a path,
if you do not provide any more than one terminus, there is no path
EXCEPT TO GROUND and it is a common shared by both halves of the
magnetic linked circuits (read your Terman). This means the circuits
on both sides of the shield are electrically decoupled from each other
and are driving ground directly - capacitively (the electric field).
The sole purpose of the Power Transformer's Faraday Shield is EXACTLY
for snubbing VLF/MF/HF/VHF/UHF coupling (there's no purpose to killing
50/60 Hz fields with a Faraday Shield, that just doesn't make sense in
the first place).

The guff you are repeating is the ill-understood concepts of UHF/SHF
circulating currents that could only be developed if the wavelength
scale can support it. Are you talking about 900 MHz applications with
3 inch Faraday screens? I think not.

There's no mystery about this, dimension and frequency drive all such
issues.

73's
Richard Clark, KB7QHC

On 23 Dec 2003 10:27:38 -0800, (Art Unwin KB9MZ)
wrote:

That commercial radio stations use tines is evidence to me that they
serve a purpose i.e. electrostatic shielding, not everything, just
electrostatic stuff

And what have I written that would suggest otherwise?

( ignoring harmonics e.t.c. )

Ignoring harmonics? Read your Terman. Read Richard Harrison's
postings.

As for low frequency use, that would include audio, not just 50/60
which you refer to 'power .

Faraday shields are used universally there for the same reasons they
are used in ELF/VLF/LF/MF/HF/VHF/UHF so there is absolutely no
argument to made for or against ANY band. As I said, wavelength and
physical size dominate ALL considerations and no one is talking about
a 6 inch shield used in the SHF.

Or are you?

Now you say look at Terman and I have, not
only the section on transformers but on coupling circuitry...way back
to the days of spark gaps and ticklers, each of which are treated
differently by not only Terman but also by others.

Art, did you actually read Terman? You offer nothing in that regard
to support or negate any discussion here. I have references going
back to the spark gap days of 100 years ago and nothing there bears on
any of this additional discussion you are bringing to the mix.

For the mythical lurker, such a reference is found on page 39 of
"Electronic and Radio Engineering" (1955 ed.) and specifically to the
matter of tines in problem 2-45. The matter of its construction is
strictly a matter of convenience as solid conductors would work as
long as there is no complete loop introduced by grounding the sheet at
two (2) points. This, as I've pointed out in recent posts, is the
commonplace of neophytes going hog wild making multiple grounds and
"curing" problems where the cure kills the patient. This last comment
is the assignment to Terman's problem. He pointedly uses the one word
imperative: "Explain."

You will also note
that audio cables have sheet like covering just like books do with
TOTAL shielding. Pretty much all books on fields and waves have a
section of total shielding of nearby circuits with special reference
to TOTAL enclosure and the effects on circuits or inductances that are
so enclosed.

There is no such thing as total shielding from these rather ordinary
sources you have access to; except as an academic wet dream argued by
philosophers who have not actually had to perform a total shielded
solution. Some of the worst interference problems I've solved have
come from "totally shielded" designs by pHDs. Again, wavelength and
physical size will not be denied.

With all that being said and getting back to the initial area of
discussion
where Richard alluded to the picket fence. Why do you think that the
broadcast industry hung on to the ungainingly picket fence after all
these years and why would it not descriminate with regard to other
influences ?

Best regards
Art

Hi Art,

To answer your last question - there are far more issues considered in
the practical world of commercial design that require no proof through
cut-and-paste philosophies. In simple terms, Production Engineering
works and its success does not breed the need for a new science or
physics. Just as fractal antennas work (any hank of wire does too);
there was no need to proceed to invent a fractal electromagnetics to
tart up the "new" idea (those mantles of nobility didn't add even a
tenth dB more gain for all the pHd's efforts to publish).

73's
Richard Clark, KB7QHC

Art, KB9MZ wrote:
"That commercial stations use tines is evidence to me that they serve a
purpose, i.e. electrostatic shielding, not everything, just
electrostatic stuff."

That is exactly correct. The magnetically coupled coils have a rather
broadband response, but the coupling coefficient is less than 1. If you
took the advice of others in this thread and checked Cebik`s pages, he
advises that incomplete coupling (k1) does not insure inefficiency.
High efficiency is available with loose coupling though throughput may
be constricted.

Reduced magnetic coupling may make additional coupling from capacitance
bypassing the transformer throughput more significant. Impedance of the
gratitous capacitance, bypassing the coupling transformer, at the 2nd
harmonic frequency is only 1/2 the impedance of that at the fundamental
frequency. Likewise, at the 3rd harmonic frequency, impedance of the
capacitive reactance is only 1/3 that at the fundamental frequency. The
stray coupling capacitance amounts to a high-frequency boost circuit
which is thwarted by the Faraday screen.

Faraday screening to eliminate priority harmonic coupling to the antenna
is an important advantage in harmonic suppression.
I think the Faraday screen is an expected feature of coupling to the
antenna tower in medium wave broadcasting.

My copy of Terman isn`t at hand now, but I recall that he treats skin
effect, iron-core transformer Faraday screens, and air-core transformer
Faraday screens. Richard Clark is right. The implementations are
slightly different depending if it`s an iron-core low frequency
transformer or an air-core high-frequency transformer. The aim is the
same, eliminating capacitive coupling between the transformer coils.

At short wave frequencies there are other practical ways to rid antennas
of harmonic content. I recall King, Mimno, and Wing describing tuned
transmission line traps to expel harmonics.

Kraus says antenna useful bandwidth is generally a matter of both
pattern and impedance. In a thin dipole, Kraus says the pattern usually
changes slowly with frequency, so likely it`s impedance variation which
limits the useful bandwidth.

In a fat antenna, or in a conical, horn, lens, rhombic, and some others,
the impedance is so well behaved that the pattern variation may limit
the useful bandwidth.

Kraus` 1950 2nd edition of "Antennas" includes several transmission line
tricks for matching short wave antennas over a wider bandwidth. See item
14-24 (Matching Arrangements) on page 434.

Best regards, Richard Harrison, KB5WZI