On 2009-10-03 1:53 PM, John Geiger AA5JG wrote:
> With a 2 element beam, you have the option of the parasitic element being
> either a director or a reflector. Is there a reason why all commercial 2
> element beams use a reflector? I seem to recall from the ARRL antenna book
> that you can get slightly more gain out of using a director instead, and it
> seems that the director would be cheapter to produce than the reflector since
> it uses less materials.
>
John,
The configuration with the director generally has better performance,
but only at closer spacings.
Unfortunately, the trade-off with close spacing is lower feed Z and a
similarly reduced bandwidth.
Here are some numbers from my EZNEC modeling.
Dir@ Gain F/B Z 2:1_SWR_BW
wl dBi dB ohms kHz
0.08 6.58 21.2 22.6 155
0.09 6.54 17.5 26.2 185
0.10 6.46 14.9 29.7 225
0.11 6.42 12.9 32.2 255
0.125 6.27 10.7 36.2 302
Refl@ Gain F/B Z 2:1_SWR_BW
wl dBi dB ohms kHz
0.08 6.39 11.5 17.1 130
0.10 6.33 11.4 24.5 193
0.11 6.32 11.3 28.1 225
0.125 6.32 11.2 33.1 275
0.15 6.19 11.0 43.0 375
0.1685 6.09 10.8 50.0 460
0.185 5.97 10.6 56.2 530
0.25 5.38 9.5 76.3 565
These are in free space at 7.15 MHz with 2" diameter zero-loss elements,
optimized for maximum F/B and feed-point resonance. It appears that for
spacings less than 1/8 wl, the director configuration has higher gain,
F/B, feed Z, and SWR bandwidth. If adjusted for max gain, the F/B will
degrade somewhat. Optimization with traps, loading coils, small
diameter wire elements, or over real earth (especially at low height)
may produce different relative results.
73, Terry N6RY
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