Regarding spark gaps and shunting static and lightning transients from
I refer you to "Lightning: Physics and Effects," by Vladimir Rakov and
Martin Uman, 2003, Chapter 18, Deleterious effects of lightning and
An air gap and a gas discharge tube operate on the same physical
principles. Both transition from a non-conductive state in which they
do not interfere with the RF power on the antenna or transmission line
to a highly conductive state with a non-linear voltage-current
relationship when the voltage limit is exceeded. In that regard, they
are better than MOVs, which clamp at a voltage higher than the operating
voltage. (Of course, the capacitance of MOVs also makes them
unsatisfactory for use on transmission lines and antennas.)
The advantage of a gas discharge tube over a simple air gap is that the
operating voltage and the voltage current relationship are controlled by
the choice of gas, the pressure of the gas, the design of the
electrodes, and the susceptibility of the simple air gap to
contamination and atmospheric variations.
The Mouser catalog lists gas discharge tubes from four manufacturers and
gives the specifications regarding maximum current and limiting number
of operations at different currents for various models. Mouser sells
GDTs with ratings up to 20,000 Amp, one time.
Paralleling GDTs won't work unless some form of voltage equalization is
employed, or the GDTs will operate one at a time.
ICE parallels the GDT in their feedline lightning arrestors with an
inductor. The GDT responds quickly to a transient to conducts high
current while the current shunted by the inductor must increase in
accordance with the voltage-current-time relationship of an inductor.
However, the inductor does relieve the GDT of having to pass the entire
current for the duration of the event. Thus, the inductor is thought to
prevent failure of the GDT.
I haven't found authoritative information on GDT failure modes. I hope
to find that in one of the many references listed by Rakov and Uman,
including "Protection of Electronic Circuits from Overvoltages," by R.
B. Standler, 1989. I have requested a copy of that via inter-library loan.
Meanwhile, an inductor from a vertical antenna element to ground is a
good idea. It must be a low impedance ground, preferably better than a
single ground rod. I believe an inductor across the elements should
also work on a dipole.
A high quality lightning arrestor should be installed in the feedline
from any antenna. No other part of the ham station is more likely to be
struck or to experience a really strong induced transient from lightning
than the antenna. I recommend one of the arrestors that employs a
combination of GDT and a high voltage blocking capacitor, such as the
PolyPhaser or ICE.
In any case, the lightning arrestor or other shunt device is no better
than the ground system to which it is attached. The voltage developed
at the ground terminal is a function of ground impedance (resistance and
reactance, usually inductive at lightning frequencies). Earth is not a
particularly good conductor; it is more like a lossy capacitor. For
that reason, paralleling a set of radials and grounding each radial with
two or three ground rods, spaced at twice their length, will further
reduce the potential at ground level to make it manageable by lightning
73 de WOØW
Mike Bragassa, K5UO wrote:
>...speaking of lightning:
>What is a simple and effective ground for my base-loaded 80 meter vertical
>(Rohn 25g)? "Spark gap": How can one be made and what gap is correct?
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