Inspired by the recent article in QEX, I was just doing a bit of
experimenting with NEC4 simulating portable verticals with severely
restricted grounding systems. That is, say you put up a 30 odd foot high
(for 40m) monopole and laid out some wire on or near the ground for
grounding. Sure, it's not going to perform like a "real" grounding system,
but I was wondering just how much difference there is between, say, a
couple 10 foot lengths of wire laid out, and a bunch of longer wires. If
you're setting up a portable operation, there's a lot of difference between
a minute throwing out a couple 10 foot wires, and spending half an hour
stringing radials around.
At first, it looks like not a heck of a lot of difference. I tried 4
initial shots:
4 radials 3 meters long
4 radials 10 meters long
10 radials 3 meters long
10 radials 30 meters long
In all cases, the radials were modeled as AWG16 wire, 3 cm off the ground
(which NEC4 should handle appropriately, but I'm not sure). I used the not
necessarily valid average 13/.005 ground (Sommerfeld/Norton). I'll
systematically look at ground property effects later.
Interestingly, the gain tends to run around -1.1 dBi for all cases, with
maybe 0.2 dB difference between the cases.
Also interestingly, just like the authors of NEC claimed in their report,
adding a ground stake (or, at least, a connection to ground under the
antenna) makes things worse. This kind of thing is precisely one of the
things they wanted to model with NEC4 when it was written. I haven't
compared my modeling approach with theirs, though, so my results (ground
stake wise) shouldn't be taken seriously yet.
Anyone aware of any similar modeling or testing that has been done,
specifically to compare subobtimal or marginal grounding systems.
Jim, W6RMK
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