Andy Cowley wrote in message
As I understand it, there is nothing that can work if a direct lightening strike occurs. We are talking megavolts and thousands of amps.
Way beyond anything an amateur could build.

Sure, but for only a short duration. There is plenty the average ham
can do to reduce damage.

The best we can hope for is to dissipate charge build up on the
aerial

Useless....Not much hope in that tactic.

and ensure that there is no more than a kilovolt or so at
the input to the rig caused by the (field) effects of a nearby
strike.

Actually, I think just a run of coax itself will reduce the potential
to a few hundred volts before it gets to the rig.

A spark gap that can conduct a lightening strike would
be the size of a small truck.

A 10 gauge wire can safely conduct a lightning strike to ground. But
you must have a good low resistance connection to ground. If not, the
wire will be burnt toast.

The only way to make sure you have a working station after a
lightening strike on your aerials is to take out good insurance.

Insurance won't do any good for the existing gear. Proper setup to
avoid damage, or disconnecting is a better idea. I take strikes around
here all the time. Two were direct strikes to my mast in the last 4
years. I had no damage at all to anything, and I was sitting 15 ft
from the base of the mast both times at this puter. Didn't flinch at
all. MK

On 15 Dec 2003 14:33:27 -0800, (Mark Keith) wrote:

Andy Cowley wrote in message
As I understand it, there is nothing that can work if a direct lightening strike occurs. We are talking megavolts and thousands of amps.
Way beyond anything an amateur could build.

Sure, but for only a short duration. There is plenty the average ham
can do to reduce damage.

The best we can hope for is to dissipate charge build up on the
aerial

Useless....Not much hope in that tactic.

and ensure that there is no more than a kilovolt or so at
the input to the rig caused by the (field) effects of a nearby
strike.

Actually, I think just a run of coax itself will reduce the potential
to a few hundred volts before it gets to the rig.

A spark gap that can conduct a lightening strike would
be the size of a small truck.

A 10 gauge wire can safely conduct a lightning strike to ground. But
you must have a good low resistance connection to ground. If not, the
wire will be burnt toast.

The only way to make sure you have a working station after a
lightening strike on your aerials is to take out good insurance.

Insurance won't do any good for the existing gear. Proper setup to
avoid damage, or disconnecting is a better idea. I take strikes around
here all the time. Two were direct strikes to my mast in the last 4
years. I had no damage at all to anything, and I was sitting 15 ft
from the base of the mast both times at this puter. Didn't flinch at

All my antenna systems ground to the tower. The tower is thouroughly
grounded into a network of ground rods and #2 bare copper.

The cables come into the house through underground conduit where they
are again grounded and run through PolyPhasers.
http://www.rogerhalstead.com/ham_files/cablebox.htm

I need to add some photos that show the grounding at the tower base.
Each leg is grounded through an 8' ground rod and the bare #2 radiates
out from there for a minimum of 80 feet with ground rods spaced about
8 to 16 feet, depending on what's in the way.

There are crossing cables that also bond the cables and one run that
parallels the conduit into the house with at least 5 ground rods along
its length. It also ties into the old ground system for the original
90 foot tower. All joints are Cad Welded except the tie to the tower
legs which use cable clamps to attach the cable to the leg. Then the
cable is gracefully bent at the base to curve out to the first ground
rod in the series.

It took a direct hit late this past summer with no harm to any
equipment. I say direct hit as my neighbor happened to be looking at
the tower when the strike hit. He was impressed. :-))
I just tell the neighbors it's the neighborhood lightening rod and
after that I think they believe me.

http://www.rogerhalstead.com/ham_files/tower.htm The view (third row
from the bottom) is from the back yard of the above neighbor near our
lot line.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair?)
www.rogerhalstead.com
Return address modified due to dumb virus checkers


all. MK

Mark Keith wrote:
"There is plenty the average ham can do to reduce damage."

True, and the ham needs a good ground anyway. Most commercial radio
installations operate 24-7 and are nearly unaffected by lightning.
Protection comes from common-sense lay out and usually does not include
many expensive arresters.

One arrester salesman said his business was exemplified by the story of
a bar patron who had a pipe on a lanyard about his neck.

Bartender asked about the thing pending from his neck. Client said it
was an elephant whistle. Bar tender asked why? as no elephants were to
be found in the environs.

Bar patron says: See, it works doesn`t it?

Best regards, Richard Harrison, KB5WZI

Richard, the local radio station has a line to ground with a large gap
which regularly arcs because of static build up. Most radio stations
go off the air momentarily when lightning strikes. Is it the static
arc that drives a disconnect relay for a few milli seconds and is this
used as a primary protector for lightning?

Regards
Art




(Richard Harrison) wrote in message ...
Mark Keith wrote:
"There is plenty the average ham can do to reduce damage."

True, and the ham needs a good ground anyway. Most commercial radio
installations operate 24-7 and are nearly unaffected by lightning.
Protection comes from common-sense lay out and usually does not include
many expensive arresters.

One arrester salesman said his business was exemplified by the story of
a bar patron who had a pipe on a lanyard about his neck.

Bartender asked about the thing pending from his neck. Client said it
was an elephant whistle. Bar tender asked why? as no elephants were to
be found in the environs.

Bar patron says: See, it works doesn`t it?

Best regards, Richard Harrison, KB5WZI

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

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