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ANTENTOP- 01- 2017, # 021

The Three L-Antennas

It has 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

Antenna modeling software can optimize a vertical dipole  into the tall OCF form.

Using that 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).

Standard conditions are: 2/3-1/3 ratio, #14 AWG, ½ wavelength feedpoint elevation, over “real ground”.

• The lowest SWR occurs with the variable arm at 105°… 15° above horizontal.
• The gain of the Tall-L Antenna is over twice that of the OCF vertical dipole at 0°.

Figure 4

SWR and Gain at Tal OCF Dipole when Shorter Lower Arm is Swung Upwards

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Model Example (Figure 5)

Conditions: 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.

Predicted dimensions: Vertical arm: 3.147 meters tall; Horizontal arm: 2.08 meters long.

Total length: 5.227 meters at 60-40 ratio.

Impedance: 50.5 –j0.43; SWR: 1.01; Gain: 2.69 dBi

At page 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.

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