"Jack Painter" wrote in message news:r4_Rc.12079$Yf6.1279@lakeread03...
"Harry Conover" wrote
(Private) wrote in message
. com...


What I did note missing was the mention of 'lightning chokes' wound in
the coax lines decending from the tower. These are basic to lightning
protection for broadcasting towers, but I've rarely seen them
implemented by hams.

The idea of a lightning choke is to add a small amount of inductance
to the coax so that if a direct lighting strike happens, the
instantaneous current flowing though the outer jacked of the coax into
your lightning arresters will at least have some amount of impedance
limiting the current magnitude, thus reducing the probability of
destruction of both the coax and the the arrester itself.

These chokes are more often than not implemented by winding a dozen or
more turns of coax around a form (say a 4" diameter phenolic tube)
prior to the arrester or spark gap.

Harry C.

Harry, that winding of coax may be useful as an RF choke, but it is most
certainly not a lightning choke, and will act more like an air-wound
transformer than anything else. Not only is this not specified for any
lightning protection systems, it is specifically warned against in many.

Jack Painter
Virginia Beach VA

Jack, while I'm sure that you believe this, I really can't guess where
you obtained such IMHO such massive misinformation.

First of all, it is the purpose the the coiled coax to act as an
inductor (r.f. choke) because this is how it resists the very rapid
di/dt common mode characteristic associated with a lightning hit, thus
limiting the peak discharge energy that the following spark-gaps and
lightning suppessors must absorb and reducing the overall peak impulse
power damage potential.

It's basically a brute-force version of the common computer technique
of placing a ferrite torroid around cables to attenuate their common
mode impulse transmission/conduction ability. Realize that with
sufficiently low SWR, currents through both the inner conductor and
outer shield of a coax are equal, summing to zero, hence there is no
net electromagnetic field produced. The same is not true for a common
mode impulse traversing the transmission line. The bottom line is that
only the common mode impulse resulting from a lightning strike will
experience the results of the "choke".

I'm not sure why you would believe that a coil of coax would act like
a 'transformer' of any type, unless the SWR is truly enormous, a
probem in itself. A case in point is that in both proton synchrotrons
(typically operating in a swept frequency range of roughly 3-30 Mhz),
and in large phased array radars (some of which operate in the UHF
range) employ multiple feeder coax transmission lines that are cut the
the same electrical length, with the cable lengths in excess of the
required physical run length being coiled up somewhere in the system.
There is no net significant electrical effect on the cable's
transmission characterists in either case.

Prior to completing my degree work at Drexel, I spent 8-years
installing and maintaining broadcast transmission systems ranging from
5-Kw to 50-Kw. This included 5 years as chief engineer in one station
(WBUD in Trenton, NJ) plus part-time work for WFIL, WCAU, and KYW in
Philly. (Additional work in 4 or 5 smaller stations.)

Quite honestly, I can't remember a single one of these that did not
protect their very costly antenna installations, transmitters, and
on-air reliability without lightning chokes employed in their
transmission lines. Still, these broadcasters use equipment that, in
general, nothing more sophisticated than your average ham station, but
on steroids! True, the average AM broadcasting tower usually exceeds
200-feet, but in an intense lightning storm, a 40-foot ham tower is
fully capable of experiencing the same energy lightning hit!

I purchased my first ham receiver, an SX-71 from a ham named Bob
Rutkowski (sp?) in Trenton, NJ. A day or so before I picked it up from
him he took an evidently direct lightning hit on his 40-some foot
crank up tower holding a 2M beam that was attached to his house. He
had grounded the outer shield of the RG-8U coax at the based of the
tower, but without a lightning choke in the coax, the hit simply
vaporized the majority of his RG-8U, his grounding connection, and
most of the final tank circuit in his rig in the basement! Not a
pretty sight!

Realize that a commercial radio broadcaster has to survive episodes
like this without disruption of their operations. Hams don't. Still
the emulation of the broadcaster's time proven protection techniques
involves only a small additional cost to an otherwise excellent
installation.

For more information, see:

Edmund LaPort, "Radio Antenna Engineering", McGraw-Hill Book Company,
New York. (My issue carries a 1952 copyright, still it's an 'oldie but
a goodie' with many subsequent editions -- and AFAIK is still the
bible of the broadcasting industry.)

IIRC, early editions of the ARRL Handbook also described this
protection technique (likely pre-1970) in the days when most hams
built their own rigs.

Harry C.

p.s., Jack, I'd love to hear a citation where "it is specifically
warned against", and why.

Thank-you for the advice to this point. I think I will invest in the
rotator cable ground. Not sure what to do about the lightning choke.

I don't mind replacing the antenna/mast if I receive a direct
lightning hit, I just dont want to turn be fried to a crisp.....

Lloyd


(Harry Conover) wrote in message om...
"Jack Painter" wrote in message news:r4_Rc.12079$Yf6.1279@lakeread03...
"Harry Conover" wrote
(Private) wrote in message
. com...


What I did note missing was the mention of 'lightning chokes' wound in
the coax lines decending from the tower. These are basic to lightning
protection for broadcasting towers, but I've rarely seen them
implemented by hams.

The idea of a lightning choke is to add a small amount of inductance
to the coax so that if a direct lighting strike happens, the
instantaneous current flowing though the outer jacked of the coax into
your lightning arresters will at least have some amount of impedance
limiting the current magnitude, thus reducing the probability of
destruction of both the coax and the the arrester itself.

These chokes are more often than not implemented by winding a dozen or
more turns of coax around a form (say a 4" diameter phenolic tube)
prior to the arrester or spark gap.

Harry C.

Harry, that winding of coax may be useful as an RF choke, but it is most
certainly not a lightning choke, and will act more like an air-wound
transformer than anything else. Not only is this not specified for any
lightning protection systems, it is specifically warned against in many.

Jack Painter
Virginia Beach VA

Jack, while I'm sure that you believe this, I really can't guess where
you obtained such IMHO such massive misinformation.

First of all, it is the purpose the the coiled coax to act as an
inductor (r.f. choke) because this is how it resists the very rapid
di/dt common mode characteristic associated with a lightning hit, thus
limiting the peak discharge energy that the following spark-gaps and
lightning suppessors must absorb and reducing the overall peak impulse
power damage potential.

It's basically a brute-force version of the common computer technique
of placing a ferrite torroid around cables to attenuate their common
mode impulse transmission/conduction ability. Realize that with
sufficiently low SWR, currents through both the inner conductor and
outer shield of a coax are equal, summing to zero, hence there is no
net electromagnetic field produced. The same is not true for a common
mode impulse traversing the transmission line. The bottom line is that
only the common mode impulse resulting from a lightning strike will
experience the results of the "choke".

I'm not sure why you would believe that a coil of coax would act like
a 'transformer' of any type, unless the SWR is truly enormous, a
probem in itself. A case in point is that in both proton synchrotrons
(typically operating in a swept frequency range of roughly 3-30 Mhz),
and in large phased array radars (some of which operate in the UHF
range) employ multiple feeder coax transmission lines that are cut the
the same electrical length, with the cable lengths in excess of the
required physical run length being coiled up somewhere in the system.
There is no net significant electrical effect on the cable's
transmission characterists in either case.

Prior to completing my degree work at Drexel, I spent 8-years
installing and maintaining broadcast transmission systems ranging from
5-Kw to 50-Kw. This included 5 years as chief engineer in one station
(WBUD in Trenton, NJ) plus part-time work for WFIL, WCAU, and KYW in
Philly. (Additional work in 4 or 5 smaller stations.)

Quite honestly, I can't remember a single one of these that did not
protect their very costly antenna installations, transmitters, and
on-air reliability without lightning chokes employed in their
transmission lines. Still, these broadcasters use equipment that, in
general, nothing more sophisticated than your average ham station, but
on steroids! True, the average AM broadcasting tower usually exceeds
200-feet, but in an intense lightning storm, a 40-foot ham tower is
fully capable of experiencing the same energy lightning hit!

I purchased my first ham receiver, an SX-71 from a ham named Bob
Rutkowski (sp?) in Trenton, NJ. A day or so before I picked it up from
him he took an evidently direct lightning hit on his 40-some foot
crank up tower holding a 2M beam that was attached to his house. He
had grounded the outer shield of the RG-8U coax at the based of the
tower, but without a lightning choke in the coax, the hit simply
vaporized the majority of his RG-8U, his grounding connection, and
most of the final tank circuit in his rig in the basement! Not a
pretty sight!

Realize that a commercial radio broadcaster has to survive episodes
like this without disruption of their operations. Hams don't. Still
the emulation of the broadcaster's time proven protection techniques
involves only a small additional cost to an otherwise excellent
installation.

For more information, see:

Edmund LaPort, "Radio Antenna Engineering", McGraw-Hill Book Company,
New York. (My issue carries a 1952 copyright, still it's an 'oldie but
a goodie' with many subsequent editions -- and AFAIK is still the
bible of the broadcasting industry.)

IIRC, early editions of the ARRL Handbook also described this
protection technique (likely pre-1970) in the days when most hams
built their own rigs.

Harry C.

p.s., Jack, I'd love to hear a citation where "it is specifically
warned against", and why.

On 10 Aug 2004 21:20:29 -0700, (Private)
wrote:

Thank-you for the advice to this point. I think I will invest in the
rotator cable ground. Not sure what to do about the lightning choke.

I don't mind replacing the antenna/mast if I receive a direct
lightning hit, I just dont want to turn be fried to a crisp.....

Lloyd

Hi Lloyd,

Of the advice offered, out of the dozen or so comments, only the
addition of radials and grounding of peripheral equipment made any
sense to your already extensive installation. Adding "more" rods
sounds like hail Mary solutions. I would bet almost every house for
10 miles around you survives quite well with one.

Sure, few if any sport towers, but lightning in the vicinity is not so
choosy as to miss every house, building, or power pole simply because
they are not radio amateurs.

I also note the complete absence of discussion about the Code. All
grounds must be bonded (clamped, not screw attachment nor
solder/brazed) with a continuous wire (no breaks or splices). I also
note some rather bizarre descriptions of how chokes work (and to add
that they are not used by Hams is simply ignorance or the choice to
illustrate with poor examples).

I would suggest you mine the archives of rec.radio.amateur.antenna for
two correspondents: Richard Harrison, KB5WZI for thousands of
commercial and amateur tower installations over a career spanning 50
years (you will discover half to two thirds of suggested
embellishments are immaterial fluff); and Reg Edwards, G4FGQ for the
topic of successful ground rod application (notable in that laying
them horizontal is just as good, if not better).

As to your last comment about getting fried. You should examine that
illusion and recall that it brings voltage to mind, not current.
Voltage comes from two mechanisms common here, impedance and
resistance (the lightning strike is considered to be from a constant
current generator willing to present any potential necessary to
preserve flow). A sharp bend in a conductor is one source of
impedance change for the bad - so graceful sweeps are preferred where
you want to change direction. I note the irony of discussion where
chokes are used to build voltage (power being equal to I·E and current
being constant guarantees a build up of power beyond what would have
been suffered) to guarantee spark discharge elements firing (I suppose
so, but this sounds like Advertising Copy to sell spark discharge
elements). Most successful solutions offered here for years and years
all have the common goal of burying the lightning stroke's current
into the soil as soon as possible without any impedance to its path.
If you research the archives for low resistance paths of lightning,
you will find out how little total power is suffered in a strike
(don't fall for the monster under the bed stories of a gazillion volts
at a bajillion amps).

73's
Richard Clark, KB7QHC

Richard Clark, KB7QHC wrote:
"(don`t fall for the monster under the bed stories of gazillion volts at
a bajillion amps)."

Good point! It`s akin to: "You can`t protect against a direct hit!"

Oh yeah? How about 10,000 medium-wave broadcast stations struck by
nearly every charged cloud passing overhead? Sometimes several times a
minute for a long time period. The listener is often unaware of the
instantaneous carrier drops to extinguish the arcs initiated by the
lightning strikes. And, one of the most important lightning opponents is
a large coil of large wire in each tower lighting wire at the base of
the tower. It keeps lightning as well as R-F out of the electrical
service to the station.

If tower lighting chokes stepped up the lightning, they would all be
replaced with Austin transformers or some other technique such as shunt
feed of the radio towers to eliminate the base insulator. Truth is,
lighting chokes are very effective at keeping lightning out of the power
supply.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Richard Clark, KB7QHC wrote:
"(don`t fall for the monster under the bed stories of gazillion volts at
a bajillion amps)."

It's not a monster under the bed. But it is a heaven of a blast!

The actual USAF specification for lightning strikes is based on a
probability model.

It's been 20 years since I read the actual wording, but the values are;

90% of all lightning strikes in the USA are described as falling within
the spectrum defined by a strike with the following characteristics:

1) Double strike. First peak 100,000 amperes with a second strike of
50,000 amperes. Full Width Half Maximum [approximately 50% pulsewidth]
of 100 useconds for each peak.

2) Rise time from 0 to peak is 1 usecond on both strikes.

3) Fall time is a decaying exponential from peak to approximately 500
amperes sustaining current in the lightning channel for 300 milliseconds.

4) Electric field intensity prior to prestrike is greater than 10,000
volts per meter.

Conclusion: Lightning has lots of energy. Systems have been designed to
not only survive a direct strike but to operate through the direct
strike. All it takes is $$$$$$.

Note: 50% of all strikes use the same pulse width model but with a
reduced amplitude of 20,000 and 10,000 amperes for the peak values.

W1MCE
Program Chief Engineer, retired
USAF MX MIssile RS/RV, WS-118

So to those that think a simple Transorb or air gap device will protect their 5
v CMOS circuitry...

On Sat, 14 Aug 2004 01:08:12 GMT, Dave Shrader
wrote:

Richard Harrison wrote:
Richard Clark, KB7QHC wrote:
"(don`t fall for the monster under the bed stories of gazillion volts at
a bajillion amps)."

It's not a monster under the bed. But it is a heaven of a blast!

Hi Dave,

The point of the monster is that it is NOT under the bed, but in the
heavens. Let's look at the numbers you provide:

First peak 100,000 amperes with a second strike of
50,000 amperes. Full Width Half Maximum [approximately 50% pulsewidth]
of 100 useconds for each peak.

Expressed as power into a section of tower where the cumulative
resistance is 1 mOhm (not unreasonable) and giving the stroke a full
second sustained current flow (I've never seen such a long one) so we
can round the numbers into watt-seconds (never mind KWH); and figuring
a duty cycle of 0.01% based on your pulse width, but let's get
extravagant and say 0.1%; then both strikes express all of 150
milli-watt-seconds of power.

500 amperes sustaining current in the lightning channel for 300 milliseconds.

Again, expressed in watt-seconds (sneering KWH) this probably doubles
the power burden another 150 milli-watt-seconds. Total power: less
than half a watt-second or as much heat as 1/10th of a Christmas tree
bulb held for the same time as the strike. For those who remember
NE-2 bulbs in their radio's front end, these are rated at 1/4 Watt.
The paranoid may wish to parallel several, but such devices exhibit
what is called current hogging - one will fire to destruction before
the others light up.

To demonstrate the catastrophe that is so often associated with a
strike, divert the same strokes to a nearby tree that shows all of 10
Ohms resistance in its sap: 3 KW-Seconds. Try holding three clothes
irons for 1 second. :-)

Even this is barely remarkable given the heat is spread over a
considerable bulk. What makes the difference so destructive? That
same time element. The heat does not have the leisure of dissipation
in 100µS and concentrates. This accounts for the scoring of a strike
on metal, or the steam explosion in a tree trunk.

4) Electric field intensity prior to prestrike is greater than 10,000
volts per meter.

Conclusion: Lightning has lots of energy.

Energy is a strange thing, sunlight has vastly more energy than radio
waves at HF (or VHF or UHF or SHF or....) No one worries about their
radio at the beach, but they put sun screen on their skin. Walking
across a wool carpet generates far more energy than a pre strike, but
hardly enough power for a pinwheel. Separating two sheets of typing
paper is about the same risk.

This does not diminish the liability to sensitive components. The
electric fields created by the casual separation of paper can destroy
a transistor IFF it is not in a circuit. The power absorbed by
common, resistive components in relation to that same transistor
protect it simply. There are some circuit designs that seek a high
input resistance that easily fail to this assault. I should note that
in this day and age of surface mount that there are also resistors
that can be destroyed by these casually generated potentials.

Systems have been designed to
not only survive a direct strike but to operate through the direct
strike. All it takes is $$$$$$.

Well, for the amateur (not working through a strike) perhaps $$.

The risk is: "Do you maintain 0.001 Ohm or better strike paths?"

73's
Richard Clark, KB7QHC

"Richard Harrison" wrote in message
...
Richard Clark, KB7QHC wrote:
"(don`t fall for the monster under the bed stories of gazillion volts at
a bajillion amps)."

Good point! It`s akin to: "You can`t protect against a direct hit!"

Oh yeah? How about 10,000 medium-wave broadcast stations struck by
nearly every charged cloud passing overhead? Sometimes several times a
minute for a long time period. The listener is often unaware of the
instantaneous carrier drops to extinguish the arcs initiated by the
lightning strikes. And, one of the most important lightning opponents is
a large coil of large wire in each tower lighting wire at the base of
the tower. It keeps lightning as well as R-F out of the electrical
service to the station.

If tower lighting chokes stepped up the lightning, they would all be
replaced with Austin transformers or some other technique such as shunt
feed of the radio towers to eliminate the base insulator. Truth is,
lighting chokes are very effective at keeping lightning out of the power
supply.

Best regards, Richard Harrison, KB5WZI


Hi Richard, really glad to see you chime in. Even ignoring the few examples
I found that argue against the choke concept, what was more relevant to me
was that in poring over hundreds of documents lately, I can find no modern
specification for coiling the coax at any point, high or low. Not in the NEC
or NFPA, not in the descriptions and specs to nationwide antenna tower
systems, and not in National Lightning Safety Institute, University of
Florida or other acedemia writings of such protection systems. So what seems
to remain, is its record of use, perhaps prominently at one time, without
evidence that the design was ever effective. Remember that for 230 years
science seemed to support the pointed lightning rod without really testing
it against other attachment points. Now it is fairly well agreed that
blunt-tip rods were sceintifically tested to do a much better job of
attracting the leader that was headed for a given area anyway. Perhaps the
colied coax chokes are just fading away due to no real evidence that they
work, and some theory and maybe even feeble arguments that they could do
harm. From an EMI standpoint, it's hard to argue the concerns. And from
direct attachment, only a massive winding of very heavy conductor could slow
down lightning (providing there was an arc-gap for it to take as an
alternate to that slowdown). Might be why the modern lighter cabling of
todays proliferant towers find little usefulness for the concept - just as a
possibility.

Best regards,

Jack Painter
Virginia Beach VA