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Re: [TowerTalk] Director vs reflector

To: John Geiger <aa5jg@yahoo.com>
Subject: Re: [TowerTalk] Director vs reflector
From: Terry Conboy <n6ry@arrl.net>
Date: Fri, 09 Oct 2009 12:35:46 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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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