Using a antenna computor program the main lobe at 10 degrees does
not
deviate from a circle even if the F/R is more than 30 db ( note F/R
vs F/B)
and this is comprised of vector addition mode as with a yagi design.

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Using a computer program automatically incorporates all the defects in
the programmer's reasoning and understanding of the problems
involved - plus other bugs.

Never use a computer program as the Bible.