At 07:59 AM 3/26/2006, Red wrote:
>Greetings, TowerTalkers;
>
>Regarding spark gaps and shunting static and lightning transients from
>vertical antennas:
>
>I refer you to "Lightning: Physics and Effects," by Vladimir Rakov and
>Martin Uman, 2003, Chapter 18, Deleterious effects of lightning and
>protective techniques.
>
>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.)
Gas discharge tubes and open gaps also have some capacitance. (several pF
or more wouldn't be unusual, for the little 300V tubes)
>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.
You can also use a gas with a lower "minimum sparking voltage" than air.
>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.
I have some anecdotal knowledge of failure modes (albeit not from
lightning, but from HV equipment).. One failure mode is the catastrophic
destruction from gross overcurrent (i.e. don't try hooking one across a 10
uF capacitor charged to 20 kV).
I've also seen them fail due to temperature cycling, but that was in a
terrible mechanical mounting, and the daily temperature cycle and thermal
expansion and contraction ripped it apart.
>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.
Which brings up an interesting question... How does one send DC power up
the coax, safely, while still providing transient protection, etc.
Jim
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