How far from the tower should I space a side-mounted 2M/440 gain vertical
such as the Diamond X-510MA?

I am considering mounting one vertical on top and two more on the sides,
with the tops of the side-mounted verticals approximately even with or a
bit below the base of the top-mounted one, all on a 50-foot Rohn 25G tower
guyed at 30 feet.

You might want to take a look at the page at

http://www.telewave.com/pdf/TWDS-7057.pdf

which is part of Telewave's antenna catalog. It shows the effect on
the pattern of Telewave's exposed-dipole antenna patterns, of
adjusting the side-mounting distance from 1/4 wavelength to 1/2
wavelength.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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