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
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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.