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.