I came across this in the ARRL Antenna Book (p. 7-21) 20th Edition.

For such a simple set-up, why haven't I heard of anyone using it?

I understand some commercial verticals use this design in some way.

From what I have read, the open-sleeve design has a simple lambda/4
vertical with two side vertical elements of given d diameter and
distance D from the center vertical (monopole).

Given the improvement of SWR bandwidth shown, why isn't this a more
popular approach? (or is it, and I just don't know it?)

John
AB8WH