[Topband: Lightning Arrestor]

[Dave Bowker]

> Does anyone have any data on how an arrestor designed
> for use in 60hz service will perform or behave in 3.8mhz service?

I don't recommend use of power-frequency lightning arrestors (LA's) in RF
applications for the following reasons:

1.  A 5kV LA functions at 10-20 times the required sustaining voltage
necessary for an amateur 1/4-wave vertical antenna system operating at
maximum legal power and assuming a base impedance of 50+/-j0 Ohms.  For
example, the peak voltage to ground at the base of such a system is
approximately 300 volts, while the lowest available distribution-class LA
rated at 3 kV (pretty much obsolete these days) probably has a threshold of
around 5-6 kV.

2.  LA's designed for the power industry are complex devices (not simple
spark gaps), usually contain a ceramic "valve" (a non-linear device, like a
large varistor) to prevent  operation during non-transient over-voltage
conditions, and are sealed devices which are evacuated and filled with an
inert gas (helium, argon, etc.) to maintain long-term stability of the LA
under varying installed envionmental conditions.  The fill does not ionize
in normal power-frequency applications, however, performance at RF
frequencies and power levels is not a design specification nor a tested
parameter of the LA devices in the power industry.  Further, the LA's
specified for a specific power-frequency application are designed and tested
to protect apparatus with far higher basic insulation levels (BIL's) than
those commonly experienced and used in the amateur applications.  Refer to
some of the standard power industry handbooks available in your utility
company's engineering office for detailed info on the design, construction,
application, and performance of the standard power-frequency LA's.

3.  In the AM broadcast industry, a standard "lightning gap" is generally
used across the base insulator with the gap width set to twice the spacing
which will arc over on 100% AM modulation.  For a description of the
lightning protection devices used in the broadcast industry, refer to the
"NAB Engineering Handbook" or "Instructions for Installation of Radio
Broadcast Stations Ground Systems", Smith Electronics, Inc., 1973.

Thus to protect an antenna with a base impedance of 50 +/-j0 Ohms at maximum
legal amateur power levels a gap spacing which will sustain 600-1000 volts
under should permit normal operation while affording lightning protection
for a direct hit.  The ultimate "safe protection" is to also disconnect all
equipment interfacing to the antenna system during periods of high lightning
activity and "hard ground" the antenna(s) when not in use.  I use the
broadcast industry recommended method on each of my phased array elements
and 1/4-wave 160M vertical (with a 3/16" lightning gap/rain drip), have
sustained a direct hit on the vertical, and suffered no damage to the
vertical matching and array phasing network components (3 kV components) in
the "dog house" at the base of the vertical (the station was disconnected
and isolated).

It is generally considered poor engineering practice to utilize a device
designed and tested for a specific application in another application far
removed in operational parameters without first conducting extensive
analysis and controlled laboratory testing of the device under the expected
operational paranmeters.  This becomes especially critical where the device
is "expected" to provide a measured degree of protection for equipment and
human life!

73, Dave, K1FK
Fort Kent, ME


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