In reference to my 18.25' radius grid of 1/4" round rod (cattle panels),
welded together at the feedpoint and to an 8' ground rod. The panels are
54" x 16' with a 6" pattern of 1/4" rod welded at each junction. 4 large
panels welded to a 54" square panel form an X around the base of the tower.
Pete Smith N4ZR
wrote:
>The first thing I would do is check this prediction using a modeling
>program with a NEC-2 engine and the Sommerfield-Norton ground enabled. You
>may find that your present grounding scheme is better than you'd expect
>from what you know so far.
I modeled it tonight. The feedpoint prediction is the same under Mininec3
and NEC2, or at least within an ohm. I am puzzled as to how to interpret
that info as it relates to Pete's comment "better than you'd expect..."
An experiment I did perform tonight that was insightful is that Nec4winVM
provides a nifty tool to lift the 0,0,0 point above ground to a fixed
height. I raised it to 1000'. The feedpoint became quite low impedance and
the antenna had to be raised from 66' to 90 ft or so to obtain resonance.
At resonance, the feedpoint impedance was about the same--38 ohms whether
attached to ground or 1000' in the air. The pattern went from 24 degrees up
to 27 degrees at maximum.
I am reminded of an 80 meter mobile whip I use all the time. It uses a 7"
square piece of magnetic sign material and a 6.5" square piece of copper
foil with a large gauge stranded wire attached to various points on the
copper sheet. I slap this little gidgit to the roof of my van and obtain an
excellent ground for an 80 meter base loaded whip. When you look at the
magnetic strip with a near-DC capacitor tester, it measures almost .01uF(I
should do the same measure with the autek VA-1 but didn't own one when I
conducted the original tests). More than adequate to couple the base of the
antenna to ground--even on 80 meters. The vehicle paint acts as the
dialectric and is very thin so the space between the foil and the vehicle is
quite small, which accounts for the high capacitance. By the way, after
using the system on the van for some time at 100 watts, the paint begins to
develop minute cracks, fissures, and discoloration--I suspect corona-like
activity is damaging the paint, even at just 100 watts.
Using this analogy, isn't the large gauge wire grid acting as a capacitor
coupling the feedpoint to the apparent ground (4' - 8' deep rich loam on top
of a wet clay-sand-loam gumbo--water table at 25') around the vertical? In
the model, when I lowered the system down to 1/2', the feedpoint remained
about 38 ohms (back to 66' height for 3.6mHz), even though the radial system
never even touched 0,0,0 in the model.
Again, is the fact that my actual impedance 44 ohms suggesting a feed system
loss of about 8 ohms? What about far field? Is the shear mass of the grid
system providing good coupling to ground? By the way, the gain of the
antenna is .71 dbi in the model--a disappointment but perhaps a clue as to
the far field ground loss in the system.
Does the .71 dbi gain figure indicate far field loss and the 44 ohm
feedpoint impedance indicate feed system ground losses? Am I viewing this
correctly or am I missing something?
Thanks for the bandwidth.
Ford-N0FP
ford@cmgate.com
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