I don't recall the company name but I saw a company recently, as I surfed 
the net, that makes a shorten 80/40 meter trapped rotating dipole.  You can 
reduce the electrical length of an element by 20 percent without losing 
gain, they say, and there are rotary dipoles for the lower bands that use 
traps to make up the loss in length.  There are some, in fact, using 40 foot 
length tubing with traps but getting that short will reduce DB gain.  If you 
have a hundred foot tower, on the other hand, something that short would 
certainly radiate but it is a toss up when compared to a simple inverted v 
that high off the ground.

Phil.
K0NX