To: <towertalk@contesting.com>
> Date: Thu, 07 May 1998 20:06:48 -0700
> From: Bob Wanderer <aa0cy@nwrain.com>
Hi Bob,
Thanks. I'm trying to make sense of this stuff, but customary
reliable sources have little data. I hope this is taken in the spirt
that I write it, because I would like to understand exactly what the
basis for adding a blocking capacitor is.
I have a few "hard" questions.
> Most of the energy in a lightning strike is at dc and that's what causes the
> bulk of the damage.
I understand a leader stroke is followed by one or more reversing
polarity strokes, all of which have very high slew rates and unstable
current levels. If that is true, it means the strike contains a lot
of ac energy, even if it is superimposed on a dc offset.
Is that true or not?
Here's my puzzlement:
I can hear strong continuous noise during a strike (even one
hundreds and thousands of miles away) that lasts the duration of the
strike on all frequencies from audio to UHF. That fact alone
absolutely convinces me there is a lot of time-varying energy
continuously in the discharge.
As a matter of fact, the signal level from a strike is often stronger
than high power transmitters on just one given frequency, and the
strike has much wider bandwidth! For example, the strikes in Atlanta
override the video carrier level of a VHF TV transmission from
Atlanta, and that transmitter is running multi-kW at the lowest
levels of video modulation. They also override amateur transmissions
from kilowatt rigs, and kilowatt AM BCB transmitters, during the
FULL duration of the strike.
Doesn't the above information indicate the strike contains more RF
energy in the entire duration of the strike than the transmitter at
just that one particular frequency, and so would contain much more
energy over a broadband spectrum?
> literature the statement to the effect that if your equipment can tolerate up
> to 400
> volts of surge voltage, their device will protect your equipment. Can ham gear
> handle that? Doubtful.
I think I can answer that for you. I do some CE testing at my
lab, and part of the test is to discharge a few kilovolts into the
ports on various devices, as well as chassis and cabinet parts. Many
devices we use every day happily live with many kilovolts of arcs and
sparks, even directly into the antenna port. They won't tolerate a
lot of current, however.
Unless an FET gate (that punches through with almost no current and
high voltage) is tied from some port to ground, most if not all,
devices fail from excess current...even if at only a a few volts.
> they don't have any learning curve to go through. Unfortunately, however,
> Poly's
> management are now totally uninterested in the ham market although one
> distributor,
> W.R. Block in southern NJ (800 421 7170), will assist hams with product and
> technical
> help. Ron Block is a ham and also Roger Block's brother (Roger founded Poly).
Not that I doubt anyone's integrity, but I never allow the source of
esoteric data to verify his own claims, unless I know exactly how the
claims were verified.
There are so many wild techno-rumors and so much folklore
circulating, I had to develop a K7LXC directive of my own...
The W8JI prime directive....
"Never trust one source for critical data, no matter how much you
hear the same data repeated."
With that in mind, are there sources or references that provide
measured or statistical data supporting the advantages of dc
blocking, other than the people who are associated with the sellers
or inventors of such devices?
What is the voltage rating and capacitance value of the capacitor
typically used to "block dc", and is there any shunting impedance
between the capacitor and the port nearest the receiver or
transceiver?
73, Tom W8JI
w8ji.tom@MCIONE.com
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