Over
the years, the question of what constituted a good technical grounding has been
answered in very different ways. Still, just about anyone
would concede that an AM transmitter site has about the best ground system
there is, with its thousands of feet of buried ground radials. Couple that excellent ground with one or more
great big lightning rods (that’d be “towers” to you) pointing skyward, and you have the classic elements for nature’s
electrical fireworks.
The
classic modern paper on lightning abatement (perhaps “redirection” is a better
word) was written years ago by Nautel*, and is
included in their transmitter service manuals.
When Larcan wanted to cover this topic in
their manuals, they asked for and received permission to reprint the Nautel stuff in their books, too. Both manufacturers are trying to describe
installation procedures that will minimize damage to their products, should
nature come calling with a stroke or two of
enlightenment. There’s a lot of good
information there, but two facts that have stuck in my mind, and that are
capitalized upon to reduce damage, are (a) lightning has a very, very fast rise
time; (b) coaxial transmission lines have both a differential and a common mode,
just like twisted pair wires. And there
can be other applications of this information, too.
Since
a lightning pulse has a very sharp rise time, its forward pulse acts as if it
has a very high frequency component. At
last we have some evidence that those lazy loops placed in the base connection
to a series-fed AM tower actually do some good – I remember when they were kind
of controversial, with some saying they helped redirect lightning across the
ball gaps at the tower base, and others stating that they did no good
whatsoever. The slight inductance caused
by the loops should look like a brick wall to the steep lightning pulse looking
for a quick way to ground.
I
confess that prior to reading that paper, I never thought about differential
mode in coaxial lines. The shield was at
ground, and just “there.” The placing of
large ferrite toroids over top of the transmission
line seemed like heresy, and makes you stop and think about differential vs.
common modes: after you’ve reflected on
it for a while, you realize that to a common mode pulse, that toroid represents a great big choke, but it is invisible to
a differential signal (like the transmitter output).
Incidentally,
those smaller snap-together ferrites are available from Digi-key
and others (sometimes including Radio Shack), and I’ve developed the habit of
carrying a couple in my toolbox—they’re great problem solvers for RFI at
transmitter sites and elsewhere. I sure
have been seeing them used a lot by manufacturers in newer computer and telecom
equipment. Quick and easy to use, they
can be surprisingly effective. They come
in several flavours:
round, square, and there’s even a rectangular version for ribbon
cables. The argument in their favour would be that RFI tends to come into equipment common-mode,
same as the lightning in the last paragraph.
For really stubborn problems, don’t forget that you can loop your cables
through the toroid more than once…
SNEAKY TRICK
OF THE MONTH:
Let’s
say you’re in the field and have a quick need for a logarithmic-taper
potentiometer, but all you can find in your junk bin
is a dual linear pot of the right resistance.
There’s a really simple circuit to make up a log taper from it – can you
figure it out? The solution, next time….
*It’s
called “Lightning Protection for Radio Transmitter Stations, © 1985 NAUTEL.