K6IF wrote:
>I think the answer basically boils down to this: boom length is a key
contributor to gain.
>The SteppIR design removes the constraint to solve for "acceptable"
bandwidth. This allows the antenna to be configured for "optimal" gain at a
given frequency with a fixed feed impedance of 22 ohms. It turns out that
if you get to optimize the elements for gain absent a requirement for
bandwidth, and you have four elements on a 32 foot boom, you get pretty
close to the optimal amount of gain possible from a 32 foot boom.
I agree with most of what you say, but the SteppIR suffers 2
major constraints on 10 meters...element placement is fixed and number of
elements is 4. I just did a quick look with YO7 and optimized a 4
element Yagi on a 32 foot boom with equal spacing, 99% weighting on gain,
0% on F/R and 1% on SWR over the limited frequency range of 28.399-28.401.
YO7 very quickly optimized this since there are so many constraints. It
came up with free space gain of about 11.0 dBi and F/R 7 dB. Adding one
director and allowing element spacing to be varied with other constraints
identical (e.g. 99% gain weighting) yielded 11.8 dBi and 14 dB F/R. Trying
6 elements was interesting. YO came up with a design which effectively
eliminated the extra director for the given optimization parameters (if F/R
were not weighted 0%, I suspect it would love to have that director). The
bottom line is you can only do so much with the restriction of 4 elements
on a 32' boom...you really need 5 elements to approach optimum gain on 10m
for that boom length.
I would guess placement of a single fixed 10m director in front of
the driven element would make major improvements in gain and F/R on 10m,
not affecting 15 and 20 much (maybe a problem on 12m however).
73, Bill W4ZV
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