On Sun, 8 Jul 2001 23:59:29 -0600 "Tod Olson" <firstname.lastname@example.org> writes:
> I am planning to install a 40 meter beam (40-2CD) on the same tower
> and mast
> as my triband beam (TH7). Should I place the 40 meter beam over the
> beam or the other way around. Assume that there will be at least 8
> feet of
> separation between the two antennas and that the lowest antenna will
> be no
> higher than 72 feet above ground.
I'd put the antenna with the smallest windload (40) on top.
Since the far-field pattern is composed of roughly equal parts
of the direct ray and the ground-reflected ray, it doesn't matter
if the near-field pattern is asymmetrical in the vertical plane.
This assumption holds for horizontally polarized beam antennas over
reasonably flat terrain.
> I have had a suggestion that I turn the booms so that they are 90
> degrees to
> each other.
User experience here on Towertalk indicates that the KT-34XA, TH-6,
and TH-7 stack nicely with the 40-2CD with the booms aligned. If
you mount the 40 at 90 degrees, I'd really worry about that 22'
40m boom, end-loaded by the 40 reflector, and the coax run to the
40 driven element being in the middle of the tribander's elements.
With the booms aligned, the only thing you have to worry about is
the length of the unused
feedline dangling off the 40, when you are using the tribander.
That feedline can make the 40 driven element resonant in one of
the tribander bands. You can test for this interaction by shorting
and unshorting the 40 meter feedline, observing swr and rear
rejection on the tribander. If there is interaction, just change
the length of the 40 meter feedline until it goes away. You don't
have to worry about the 40 reflector. It won't bother the tribander.
Dave Hachadorian, K6LL
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