Alfred
While the Cushcraft devices DO work, the PolyPhasers are
demonstrably better protection. Both devices, in fact any brand
of protector, are only as good as the lightning ground system
provided. If you provide me a snail mail address, I shall send to
you our catalog and other material which provide an overview as
to what constitutes a good lightning ground system. In addition
to a proper lightning ground and a single point ground system, we
suggest several feet of coax be placed between the PolyPhaser and
the equipment. This adds some inductance so that the surge is
more likely to see the ground as a lower inductive path to earth.
The name of the game is inductance (or, adding in the frequency
component, impedance). For that reason, the ground system should
utilize solid copper strap. If cable must be used, #2 is the
minimum you can "get away with," although we would recommend at
least 2/0 and 4/0 is even better (albeit rather difficult to work
with even compared to strap). Note that 1.5"-wide copper strap
has less inductance per foot than 2/0! Suffice it to say that one
ground rod and some #6 wire does not a good lightning ground
make!
The Cushcraft gas tube fires at some voltage which has to be high
enough to prevent it from being turned on by the RF voltage
(known as glow mode). I don't know what value Cushcraft uses, but
for argument's sake let's say it's 600 Vdc. Any voltage below
that point (and actually somewhat above that value, but let's
keep it simple) will get through to your equipment. In addition,
the gas tube puts out its own slew voltage (dV/dt) when it fires,
and this gets into your equipment too. Granted the time factor is
very quick. Note that a strike several miles away may cause a
surge of less than this 600 V but greater than what your
equipment would like to see to come down the coaxial cable. Even
grounding the shield is not enough of a cure because we're
talking surge current on the center conductor.
The PolyPhaser is dc-blocked on the center conductor. This means
that almost none of the strike energy gets through. Realize that
the bulk of the energy is at dc, with a "spike" between 10 kHz
and about 1 MHz. Due to the fast rise time (microseconds), there
is energy, albeit low in level compared to the dc and 10-1000
kHz, into the GHz range. We publish our throughput energy
(usually in milli to micro Joules); does Cushcraft (or any of our
competitors)? Obviously, this design protects the equipment from
the gas tube's slew voltage too.
We also make a model where the shield is dc-blocked as well. This
is the best possible protection especially in situations where
the quality of the ground may be far less than what's desired.
Hope this helps!
Sincerely and 73,
Bob Wanderer AA0CY
Senior Applications Engineer
PolyPhaser Corporation
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