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Old 27-Mar-2013, 8:52 PM   #19
ghz24
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Join Date: May 2011
Location: Central Illinois U.S.
Posts: 71
Scalas don't scale well

I set aside the scala (like) models temporarily due to strange (and interesting) results. I built them as an intermediate step to the chevron type due to the tendency to beam split/spread above a certain width. I had a dusty 10 foot tall model sitting forgotten in my parabolas folder (I don't remember what focal point was ) I polished it up to find the answer you asked.
According to how the models optimized.
All figures are in dBi of raw gain @ MHz.
Your 10 foot high example can get 19.46 @585 and 16.82 @ 470 (reflector width 49.524 in.) and also could get 18.25 @ 470 if the reflector width is extended to 61 in. if you go with the 61 in. reflector gain @ 585 drops to 18.89 and 17.48 @695.
A 6.5 foot model I had from the comparisons optimizes @ 585 to a reflector width of 46.728 in. (gain of 17.42)
It seams that optimum width depends mostly on the frequency.
Selecting a different focal point when you build the reflector could alter these results some but the general tendencies should still apply.
As you push the reflector width to the extreme, beam width increases but gain stays mostly constant at about 3 dB down from max gain..
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