[TowerTalk] 3El Bobtail for 80 meters

[Tom McDermott]

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