[TowerTalk] 1) Lightning & 2) Trylon towers
Wes Attaway
wes@attawayinterests.com
Mon, 7 Sep 1998 00:33:31 -0500
Re the question of Peter Shaw's recent strike and other questions ....
Based on reading, real life experience, and building a fairly effective
lightning protection system at my own shack, I can offer the following
comments (none of which are original):
There are only a few electrical rules to keep in mind when dealing with
lightning protection, the main ones being as follows:
1. The energy from the bolt(s) only wants to do one thing, and that is to go
to ground. It will always take the paths of least resistance, with current
dividing according to Ohm's Law. This seems simple enough, but many factors
are often overlooked. The most overlooked factor is the path to ground.
There are many possible paths to ground and most hams who are not careful
overlook probably 90% of them.
2. The expanding and contracting magnetic field created by the strike
induces voltages of various degrees in every conductor it cuts across, the
distance of major influence varying by the strength of the field. This
particular effect is what causes many of the most "surprising" detrimental
effects of a nerby strike, or "near hit". Telephone wiring, alarm system
wiring, electrical service wiring, computer network cables, etc etc offer
many feet of conductors to these magnetic fields.
In general, the way to minimize damage is to address the electrical
principals mentioned above:
1. Make sure that every path from antenna feedline to radio shack is less
appealing (i.e. higher impedance) than the alternative paths outside the
shack. This is actually fairly easy to achieve. Every feedline should have
a DC ground from both the RF carrying portion and the shiled (of coax). The
Polyphasor workbook, the ICE tech papers, and many other texts offer
numerous examples of how to do this. The ICE arrestors in particular do a
good job, at reasonable prices, of providing a low impedance path to ground
outside the shack. The common practice of using a #12 or #14 ground wire
from the radio to an 8' ground rod outside, coupled with an Alpha-Delta or
other "grounding type" antenna switch inside the shack is tantamount to
having no protection at all. It is actually an invitation to disaster.
With this arrangement, I can guarantee that your shack will be blown away if
you have a direct hit. The grounding switch is not the problem ..... the
problem is everything else that was and wasn't done.
It is interesting to note that one of the best things you can do when
dealing with a tower is to make sure your guy wires are grounded (see the
Polyphasor calculations, and other texts) .... that is, they are not broken
up by insulators. This flies in the face of many theories about the adverse
effects of guy wires on antennas. I've used uninsulated guys for years,
with multiple split-beads on them. Frankly, I can't tell any difference
between this approach and the insulated approach as far as antenna
performance is concerned. I don't want to start a thread on this because I
have never done any real research on it and I only have two beams on my
tower.
2. For protection against the "near hits" you have to do a lot more work.
Single point, even multiple point, grounding of electrical service,
telephone service, cable TV service, etc is mandatory. Grounds that are
many feet apart can be problematic. Currents can travel between the
different ground points and then through the associated wiring, possibly
negating some of your other efforts. Thus, it is imperative to tie the
grounds together. A simple $35 surge protector from Office Depot is, by
itself, not enough if the actual service ground is faulty. You should also
put surge, or peak voltage, protectors in your breaker boxes.
The foregoing is just an overview, but the central point is that the effects
of lightning are not mysterious if you really think about them. Ohm's Law
and other well known electrical principles are hard at work, doing just what
the books tell you they will do. Polyphasor and many other companies preach
this over and over, and make a lot of money producing devices that
capitalize on these facts. Lightning does not create some kind of new
system of electrical laws. A good lightning protection scheme is not
horribly expensive, but it does take planning, a lot of elbow grease, and
some time. Also, be sure to use first-class materials and workmanship, so
your work will last for a long time.
---------- Wes Attaway (N5WA) ---------
2048 Pepper Ridge, Shreveport, LA 71115
----(318) 797-3012; 797-4972 (Fax) ----
-----Original Message-----
From: owner-towertalk@contesting.com
[mailto:owner-towertalk@contesting.com]On Behalf Of Peter J. F. Shaw
Sent: Sunday, September 06, 1998 8:31 PM
To: jccrtv@jccrtv.com; towertalk@contesting.com
Subject: Re: [TowerTalk] 1) Lightning & 2) Trylon towers
Greetings Jim:
Regarding ur lighting question. I took a 4th lightning hit this
evening (Sun); all since June 29th. This time on the 160M inverted
"L". The feed line end was over 75 feet from the house and the
flashover cooked our AM/FM receiver/ intercom (2nd time for that) and
I'm still looking for more stuff. The lightning "found" earth ground
at the base of the vertical portion of the 175 ft wire but cooked the
matching network but didn't significantly travel into the coax since
the network presented a higher impedance than the 1.5 inch gap from
ant wire to ground rod and 1200 ft of radials.
This is the 2nd flashover damage we've had. I have concluded that no
matter what precautions you take, if you are in the path on the bolt,
look out. Yep, a re-engineering project is underway. We are in the
2nd highest lightning ground stike zone in the world; Florida's Gulf
coast. To boot we are 135' AMSL on a noticable rise with no pine
trees close to the house (pine trees are great lightning rods).
PolyPhaser has a comprehensive lightning protection techniques
publication. $22 I believe now. I have it. We have a Ufer ground
system in house. What am I doing wrong? Living in an excessively
high lightning area, where a 130 foot ground rod can't get to 25 ohms;
that's what.
Gud luck
73 Sparky in Citrus County, FL
(Pete K4LDR )
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