If a second horizontal arm
is added at 180 degree opposite
and both are tuned by length to the desired frequency, the radiation
field balances. The radiation pattern becomes circular and the
antenna becomes a tall ground plane vertical.
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The MFJ-1790 10 meter antenna (Reference 7) is an example. It has
an 11-foot vertical and two 6-foot radials to give "Low Radiation
Angle for outstanding DX".
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The Wide L-Antenna
When the
vertical arm is short, the length of a 90 degree side arm can be adjusted to resonate the antenna to the desired
frequency. The SWR match can be better than the Equal-L Antenna.
Depending on elevation, ground and diameter of the wires or tubes
used, there will some ratio between the length of the vertical
and horizontal arms that will give a nearly perfect match to a
50 ohm coaxial cable.
This Wide-L
configuration has the characteristic of being shorter than a halfwave
dipole with nearly the same gain.
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Having
both horizontal and vertical polarization, it is less effected
by undulations in DX skip reflections (QSB) than a conventional
dipole.
Antenna modeling software can optimize a vertical dipole
for the best wide OFC ratio at 1/2-wavelength feedpoint
elevation over ground. Using that capability, Figure
6 below shows what happens with a 2/3-1/3 ratio as
the longer lower arm is swung upwards in 15 degree increments.
Note that the best SWR and
Gain combination occurs at around 105,
or 15 degree above horizontal.
This
compares favorably with the gain of a center-fed dipole using
the same wire and fed at the same elevation over the same ground.
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