another notable feature. It
develops a horizontal component from
the side arm. This gives some directionality and gain over
a simple vertical antenna. The directionality is a bit more broadside
to the side arm but generally semi-circular as seen in Figure 5. The angle of the side arm can be used for
tuning using ratios as tall as a 3/4 up-bend . Ratio and arm angle influence the direction
of maximum gain.
modeling software can optimize a vertical dipole into the tall OCF form.
capability, Figure 4 below shows what happens to a Tall OCF
dipole as the shorter lower arm is swung upwards in 15° increments from 0° (down),
to 90° (horizontal), to 150° (60° up).
conditions are: 2/3-1/3 ratio, #14 AWG, ½ wavelength feedpoint elevation, over “real ground”.
lowest SWR occurs with the variable arm at 105°… 15° above
gain of the Tall-L Antenna is over twice that of the OCF vertical
dipole at 0°.
Model Example (Figure 5)
4NEC2 software (Reference 10) model of a Tall-L Antenna, fed at ½
wavelength over real ground using #14 wire, optimized at 28.4 MHz.
dimensions: Vertical arm: 3.147 meters tall; Horizontal arm: 2.08
length: 5.227 meters at 60-40 ratio.
50.5 –j0.43; SWR:
1.01; Gain: 2.69 dBi
16 is the 3D view and horizontal/vertical polar
graph produced by the model.
Note: the radiation pattern is 2.69 dBi on the hemisphere towards the side arm. The opposite
side has 0.01 dBi gain therefore signals
are about half as loud from the back hemisphere. Compare this
with an omni-directional 1.5 dBi circle
which is the norm for a vertical antenna.
Observe the 10-degree low
angle radiation for DX and the stronger signal at a 40-degree
upward angle. No energy
is wasted skyward.
This is a good configuration
for general, all-around band scanning because it will hear polarized
signals that might otherwise be too weak.