"In fact, if
you plug in 1300 ohms you'll see a very major erosion of the F/B ratio
from 33 db to less than 17 db. In addition the sharp inward, deep null
goes away and the cardiod starts looking like a circle"
This is why I'm asking... that's what I was expecting but the 180
degree front to back ratio with the 940 ohm resistor was only 10dB on
my 1500kHz groundwave station.
This is the 940 ohm plot:
http://n3ox.net/files/WWWT_940ohms
(for reference, the 1280 ohm : http://n3ox.net/files/flagpattern_lg.JPG)
10-12db F/B with the lower resistance, though the nulls off to the
sides of the rear lobe might prove useful... but still, not the deep
central null and cardioid I was expecting. The 1280 ohm plot is
closer to a cardioid. There's still a little fill-in bump... but
looks closer to the "right" pattern and certainly not the flat-backed
low-F/B pattern you'd expect from EZNEC at low wave angle.
I do have nearby metal objects, a shed a few feet away, a chain link
fence right under the antenna and both the floating 1/8th wave wire
and my 20m/17m Moxon Rectangle's 30 foot mast are within 30 feet of
the flag, but they shouldn't be resonant. The house is only a few
feet away ... full of wires and so forth.
I measured some patterns on 160 last night and F/B is actually fine,
but there is a fair amount of distortion in the broad forward lobe at
least for a source from the East from something... could be any of the
above...
Maybe 1300 ohms is a very special case for nulling a special direction
in the presence of nearby metal?
Anyway, something about my installation makes EZNEC a poor predictor
of performance, as you can see.
On the other hand, the poor performance was at least partially in my
imagination... I do see nearly 20dB 180 degree F/B with rather deeper
nulls on either side of a small rear lobe using a 160m pinger a few
hundred feet away from the antenna to the East.
73,
Dan
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