From G3SEK site:

IMPORTANT WARNING!
If you are copying an existing Yagi design, you must copy:

all the element lengths
all the element diameters
the element mounting method - why?
That means: copy EXACTLY! If you change any of these important details, you
are building a new design!
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Gosh, just what m I getting myself into here. All I have is an NEC file for
a 156-162 Mhz yagi.That's a long way from having a design is it not.