On Feb 21, 2006, at 9:57 AM, Clark Macaulay wrote:
Sooooo.....how much impact on the take-off angle would you think
this configuration would have? Should I replace the V with a flat-
top doublet at the same 40' height which would result in most of
the antenna being at 40' but still below the roof level?
The key question is -- what band? The antenna will act very
differently on 80m than on higher bands.
The single most important dimension of a horizontal antenna is the
height above ground in wavelengths. At heights less than 1/4
wavelength, the horizontal antenna basically sends all the energy
straight up. At heights above 1/2 wavelength, broadside pattern
starts to appear. As the height increases above 1 wavelength, the
broadside pattern approaches the horizon -- low angles that favor
distant DX.
Lots of hams fret about the length of their doublets, but fail to
account for the height above ground. While the length may affect
resonance, the height affects the incidence angle of the antenna. The
incidence angle makes all the difference for DX. A dipole may be
(near) 0 dBd at one angle, but what is its gain at the angle the DX
signals arrive (and depart)?
At 40 feet, that's about 12m tall. That's a reasonably effective
height for 10 and 12m. At 20m, it's barely 1/2 wavelength, so the
broadside pattern starts to disappear. Below that, the antenna
becomes an omni-directional cloud-warmer. The inverted-V installation
will also tend to round out the pattern, since it reduces the nulls
off the ends.
Installing effective antennas for DX is tough, especially with
limited-height supports. If you want to work DX, and your support is
less than 1/2 wavelength high, consider installing a vertical
antenna. Ground-mounted verticals with 30 or more 1/2 wave radials
are very effective DX antennas.
At my station, I have a 15m (almost 50 foot) tower with a small
tribander. The tribander has a 40m dipole kit, too. I feed the whole
tower as a vertical for 80m and 160m. It's not perfect, no. The 40m
dipole is too low to show much of a pattern (I do see a 2 S-unit (12
dB) null off the ends), but it works better than anything else I've
put up.
The shunt-fed tower is only 1/10 wavelength long on 160m, but I've
gotten 43 states confirmed and 10 countries using 100 watts, mostly
in the last year.
Bill Coleman, AA4LR, PP-ASEL Mail: [EMAIL PROTECTED]
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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