There would be two configurations for a 9 element Bobtail curtain:
One would be to just keep extending the Bobtail array, one wire on the top that
is 4 wavelengths long instead of 1, and instead of 3, have 9 vertical elements.
There's two practical difficulties with this approach.
A - the element currents should have a binomial ratio for a decent broadside
pattern. In a 3-element array, the current in the center element is twice the
current in either end element, so the current ratio is 1:2:1, which is
binomial. For a 9-element Bobtail array, the actual current ratio would be
1:2:4:8:16:8:4:2:1 which is not a binomial ratio. A binomial ratio would be:
1:8:29:56:70:56:29:8:1 for a 9-element antenna giving good sidelobe supression.
B - its practically impossible to get the currents to split properly anyway due
to all kinds of interactions and losses along the way.
The other approach is to have 3 separate Bobtail curtains each fed separately.
Then the issue becomes one of properly controlling the amplitude and phase of
the feed currents for each array. Differing mutual and self impedances of the 3
arrays will make the feed network hard to realize. It's probably worse than a
4-square because each of the 3 loads in this case is a multi-element array, and
so the number of complicating interactions is a very large number.
While the antenna could be made to work, probably over a limited frequency
range, or with careful feedpoint tuning, there's probably easier ways to
achieve better performance.
-- Tom, N5EG
TowerTalk mailing list