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

(Richard Harrison) wrote in message ...
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

Just to be sure I understand this. Is one coil made from coax with the
braid grounded or is the Faraday shield of a corugated sheeting form
that provides a clearance groove in one or more turns to allow
entrance of the other
coil turns? Do any commercials use a rotating coil to vary the
coupling
with a fixed coil thus making the adition of a Faraday shield pretty
straight forward ? The latter is the form that I favour.
Regards
Art

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