RE:NEC-LIST: 4NEC2 Optimizer

With larger Yagis at least I find it helps to think of the ensemble as a lowpass
"filter". Leaving matching aside for a moment, consider the *possible* or
achievable gain vs frequency function.
 
On the down-frequency side (from maximum gain frequency) the achievable
gain vs frequency envelope diminishes monotonically as frequency is reduced.
In contrast on the up-frequency side (from max gain freq) once the WL gets
short enough (and this happens very quickly with increasing frequency for
multielement gain-optimized designs) one or more of the directors suddenly
flips phase and thus become reflectors. As more of the "directors" become
reflectors with increasing frequency, the sharper the cutoff, not unlike filter
behaviour with increasing numbers of circuit elements. So the gain vs frequency
envelope function is far from symmetric with frequency. The practical implication
of this is important -- working towards the low side is "easy" but that upper end
can be dangerous territory. Cutting Yagis (element length) on the "too-short" side
is always safe, although lower gain performance, but when going for max gain
one needs to be careful of that upside cutoff when pushing element lengths
towards the "too long".

It is under (within, or constrained by) this "global" gain envelope that matching is
done. That is to say, the match bandwidth of large Yagis is usually a relatively
narrow slice in frequency compared with the above global (achievable) gain-vs-
frequency envelope. Once one thinks on this a bit, it is far easier to comprehend
than it is to cogently describe in words. Well, I tried.

     At 11:34 AM 12/11/2002 +0200, Brandon Orchard wrote:
>Dear Mike
>
>I have optimised a couple of yagi's using SuperNEC's Genetic Algorithm
>optimiser(some of my MSc work) and I have listed the results below. Most of
>the results were nice and broadband with very good Gains and VSWR's .Please
>contact me if you require any of the antenna parameters for any of the
>designs listed.
>All the yagi designs below use elements with a 0.008m radius, a folded
>dipole feed, directors of all equal length, spacing between directors all
>equal, and the design frequency was set to 300Mhz (you can always scale the
>designs for the right operation frequency) .
>
>-5 element yagi(vswr for 75ohms)
>Gain BW 22.3% VSWR<2 BW 8.7% gain @ 300Mhz 10.19dBi
>Gain BW 21.7% VSWR<2 BW 5% gain @ 300MHz 10.4dBi
>
>-6 element yagi (VSWR for 75ohms)
>Gain BW 14% VSWR<2 BW 5.7% gain @300MHz 11.62dBi
>
>-12 element yagi (VSWR for 200 Ohms)
>Gain BW 29.3% VSWR<2 BW 20% gain @300MHz
>12.11dBi
>
>-15 element yagi (VSWR for 200 Ohms)
>Gain BW 11.3% VSWR<2 BW 11.3% gain @ 300MHz 15.38dBi
>Gain BW 14% VSWR<2 BW 12.7% gain @ 300MHz 14.61dBi
>
>-17 element yagi (VSWR for 200Ohms)
>Gain BW 15.7% VSWR<2 BW 11% gain @ 300MHz 14.59dBi
>Gain BW 17.3% VSWR<2 BW 12.7% gain @300 MHZ 13.97dBi
>
>where %gain BW = ((Fh-Fl)*100)/Fc
>where Fc=300MHz(design frequency) Fh=highest frequency at which gain is 3dB
>lower than gain at Fc and
>Fl=lowest frequncy at which gain is 3dB lower than gain at Fc
>
>Regards,
>Brandon
>--
>The NEC-List mailing list <nec-list_at_gweep.ca>
>http://www.gweep.ca/mailman/listinfo.cgi/nec-list

Dan Bathker

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