We put up a two high stack of C31XR's at N6NF's station about 10 years
ago. They have performed beautifully every since.
The antenna's are part of a bigger array, including a three element 40
meter yagi and an 80 meter dipole, all Force12, which are mounted on a
heavy duty crank-up tower.
If I remember correctly, the top C31XR is right at the top of the
tower, mounted at the base of the mast. That would put it at 72'.
The lower C31XR is mounted on a ring rotor at the top of the second
section, which would put it at 40' - giving about 30' of separation,
or roughly 3/8 wavelength on 20 meters, 5/8 wavelength on 15 meters
and 7/8 wavelength on 10 meters. They are phased using an Array
Solutions Stackmatch.
I have read that the optimal stacking separation for "bigger" antennas
is 5/8 wavelength, so I suppose that this was pretty close to optimal
spacing - keeping in mind that the C31XR has a longer effective boom
on the higher bands (and so stacking the antennas further apart on the
higher bands works).
I highly recommend this configuration - these yagi's have played
extremely well for us.
***dan, K6IF
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