I finally took some time to review my modeling vs. measurement of the feed
impedance of my 160M vertical.
To review -- my original model showed 39.8 +j48 ohms at 1850 kHz, with 8"
diameter (yes, diameter) used for the 25G. This was a simplified EZNEC model
for a vertical, with the antenna at ground level, no radials and MININEC
ground. The modeled reactance is close to the capacitance value I ended up
with for matching. However, the measured (estimated) resistance was very
close to 50 ohms, which is a long way from 39.8 ohms.
Since I was unable to make the model match the measurement by adjusting
conductor sizes and lengths, I focused on my initial suspicion: the ground
slope. Although EZNEC does not have an easy way to change ground itself,
this is a system with radials that, hopefully, isolate most of the fields
from lossy ground, so I did the following:
Used 'High Accuracy' ground, and raised the entire antenna model 12 ft above
ground. I then let EZNEC create 32 radials based on a prototype 110-ft.
radial. This appoximates my actual radial system. With these horizontal
elevated radials, the modeled feed impedance was 40.2 +j37, a reasonable
match to the original model. Very promising!
I next re-created the radials with the prototype wire sloping from 12 ft.
down to 1 ft., representing the maximum slope of my terrain. This raised the
modeled feed resistance from 40.2 ohms to nearly 46 ohms -- more than I
expected from the fairly shallow slope angle, and much closer to the
measured impedance.
However, my ground is not uniformly sloped downhill from the tower, so I
modified the end heights of the radials to approximately match my ground
contours. About 1/4 of the modified radial system is horizontal, with the
rest at various downward slopes. But the resistance only dropped to 44.3
ohms. It appears that even a small amount of slope in the radial system will
significantly raise the resistance at the feedpoint.
While the method may not be perfect, I am now satisfied that the slope of
the ground is the primary reason for the difference in resistance.
73, Gary
K9AY
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