I agree that more spacing would be better for gain, but there are other
advantages to stacking. It can fill in the radiation angle pattern and you
also have the ability to beam in multiple directions simultaneously. If
you already have a 70' tower with a beam on top and want to add another one
below, it can be a useful addition.
John KK9A
To: TOWERTALK@contesting.com
Subject: Re: [TowerTalk] C31XR Stack
From: Cqtestk4xs@aol.com
Date: Sun, 27 Nov 2011 18:56:00 -0500 (EST)
Have you modeled this? From real world experience with stacking
tribanders and monobanders, 3/8 is really too close to get any kind of pop,
especially with a long boom.
I used a stack of KT36XAs at KH7XS at 92/56/and 29 with a WX0B box. The
XAs are similar to the C31 in boom length. I found there wasn't a whole
lot
of gain on the 92/56 on 20, but with all in it was much better. The
spacing was increased from about half a wave length to almost a full wave
length. Triband stacks are always a compromise, and at my new station the
XA
stack will be at 90/60/30. (back up stack)
Bill K4XS/KH7XS
To: towertalk@contesting.com
Subject: Re: [TowerTalk] C31XR Stack
From: Dan Levin <dan@andlev.com>
Date: Sun, 27 Nov 2011 15:36:43 -0800
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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