[time-nuts] GPS Antenna and Lightning (Bruce Hunter)
Brucekareen at aol.com
Brucekareen at aol.com
Sat Sep 26 18:53:59 UTC 2009
A 6 to 30 MHz Voice of America receiving site in North Carolina had an
interesting experience when vacuum-tube antenna RF distribution amplifiers were
replaced with solid-state units. At first, frequent damage occurred to
the input stages of the solid-state amplifiers, whereas few problems had
been experienced with the vacuum-tube amplifiers. The many, large, rhombic
antennas were all equipped with spark gaps, gas tube suppressors, and 3AG
fuses in the balanced, transmission lines at the antenna feedpoints. Fuses
frequently opened during lightning storms.
Interestingly, the manufacturer of the solid-state RF distribution
amplifiers had also sold some of the units to the US Coast Guard in southern
Florida. The Coast Guard found that adding 30 MHz low-pass filters at the input
to the amplifiers almost completely stopped the failures. The VOA
amplifiers were equipped with similar low-pass filters and the North Carolina
failures also almost completely stopped. We were never sure whether the "fix"
resulted from eliminating energy above 30 MHz or because the phase shift of
the filters "unstacked" the harmonics making up the steep wave front, thus
reducing the peak voltage. But it would seem that a suitable band-pass
filter might significantly reduce the likelihood of a GPS receiver failing
from energy induced by a nearby lightning strike.
Bruce Hunter, KG6OJI
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