I've been reading the N0AX/K7LXC tribander test booklet with great interest
since its arrival Saturday. There are a lot of caveats that deserve
attention.
That being said, I have one big question. A horizontally-polarized antenna
over real (but ideally flat) ground has no radiation at zero degrees
elevation angle. The report says that the two antennas were, "to all
intents and purposes," at the same elevation. So what was the nature of
the received energy in the test setup? It clearly was not the
nicely-formed, x-degrees-above horizontal lobe we see in the modeling
programs. Even the TA trace shows a deep null at zero elevation angle.
I've always heard that yagis focus the beam more in the vertical plane than
dipoles, and that long-boom yagis do so more effectively than
short-boomers. So just for fun, I used EZNEC to model a C-3, a 5-element
39-foot-boom 20m yagi, and a dipole, all at 70 feet over "average"
high-accuracy ground. Height was 70 feet, and the frequency 14.2 mHz.
Interestingly, the take-off angle of the main lobe of the 5-el was 13
degrees, vs 14 degrees for the dipole and the C-3. Vertical 3-dB bandwidth
was 13 degrees for the 5-el, and 14 degrees for both the dipole and the
C-3. Then I modeled the field at 3 degrees above the horizon. The 5-el
was 8.86 dB down, while the dipole was 9.2 dB down, and the C-3 was 9.19 dB
down. Presumably, these values are representative of the relative
attenuation of the main lobe as it approaches zero elevation.
Tentatively, I'd conclude that the relative field strength on the very
lower edge of the main lobe is about the same for all three antennas
(relative to the main lobe, that is). This may mean that the measurements
in the report are a valid representation of the relative gains of the
antennas tested, at least within the precision of the technique. But I
sure wish I knew what was really being measured.
73, Pete Smith N4ZR
n4zr@contesting.com
"That's WEST Virginia. Thanks and 73"
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