Art, Kb9MZ wrote:
"---the local radio station has a line to ground with a large gap which
regularly arcs because of static build up. Most stations go off the air
momentarily when lightning strikes.'

AM broadcasters use unbalanced vertical radiators driven against a
ground radial system.

The vertical radiator is nowdays the insulated tower irself. It sits on
a base insulator, held erect by insulated guy wires. An arc-gap is
fitted across the base insulator. This is either a pair of spheres or a
pair of boomerang forms which are adjusted for a close spacing. Though
galvanized, these gap fixtures get tower paint applications.

Towers often get direct lightning hits. The paint remains pristene in
all the gaps I`ve seen. The arc to ground is always to the Faraday
shield between the tower coupling coils. That picket fence between the
coils is pock marked like the face of the moon from tower strikes.
Splattered copper abounds.

You hear momentary disconnects during lightning strikes when listening
to an AM station during this kind of storm. This is a defense mechanism.
When lightning creates an arc, the conductive plasma path allows RF to
continue feeding the ionization. This allows an arc to keep alive that
the r-f is too feeble to strike for itself.

Transmitter output into the plasma short circuit is an overload capable
of transmitter damage.

To counter the arc problem, the coax is d-c isolated with capacitors at
the ends of the center conductor. The close-spaced coax usually gets an
arc when the antenna system is overloaded. The coax has a high
common-mode impedance.

A relay d-c power supply and a d-c relay coil are connected in series
and this series combination is connected between the center conductor
and coax shield.

An arc completes the d-c path for the relay coil. Relay activation is
used to momentarily kill the transmitter, extinguishing the arc.

Best regards, Richard Harrison, KB5WZI

(Richard Harrison) wrote in message ...
Art, Kb9MZ wrote:
"---the local radio station has a line to ground with a large gap which
regularly arcs because of static build up. Most stations go off the air
momentarily when lightning strikes.'

AM broadcasters use unbalanced vertical radiators driven against a
ground radial system.

snip
Towers often get direct lightning hits. The paint remains pristene in
all the gaps I`ve seen. The arc to ground is always to the Faraday
shield between the tower coupling coils.




Can I assume then that broadcast coupling coils are always apart
to accomodate a faraday shield between them ? Is this an F,C,C, requirement?
Can't see how a Faraday shield can be used if they are link coupled
i.e. interleaved. I was contemplating an interleaf coupling until
I realised that I would have to do away with the Faraday shield !
Regards
Art





That picket fence between the
coils is pock marked like the face of the moon from tower strikes.
Splattered copper abounds.

You hear momentary disconnects during lightning strikes when listening
to an AM station during this kind of storm. This is a defense mechanism.
When lightning creates an arc, the conductive plasma path allows RF to
continue feeding the ionization. This allows an arc to keep alive that
the r-f is too feeble to strike for itself.

Transmitter output into the plasma short circuit is an overload capable
of transmitter damage.

To counter the arc problem, the coax is d-c isolated with capacitors at
the ends of the center conductor. The close-spaced coax usually gets an
arc when the antenna system is overloaded. The coax has a high
common-mode impedance.

A relay d-c power supply and a d-c relay coil are connected in series
and this series combination is connected between the center conductor
and coax shield.

An arc completes the d-c path for the relay coil. Relay activation is
used to momentarily kill the transmitter, extinguishing the arc.

Best regards, Richard Harrison, KB5WZI

Link coupling is possible with a Faraday shield on the link only.

The main tuning coils have a 'gap' of sufficient size to accommodate the
link. The link is shielded.

Back in the 'olden days', 1955, I used a shielded link from B&W in a 40
meter home brew project [a pair of 807s in PP].

Deacon Dave, W1MCE
+ + +

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

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

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