Well, if you look inside one of these antennas, most of them are made out 
of pretty light coax.  I had one come apart, the top piece came unscrewed 
and fell to the ground.  The top half just hung down as it was just rg 58 
or smaller coax.  The commercial ones, not dual band though, are very 
heavy indeed and of course, cost 7 or 8 hundred bucks.  The design of 
these is as I recall quarter wave pieces of coax, center to shield, shield 
to center and so on.  In the commercial ones, they use half inch hard 
line.  Years ago there were articles in q s t or  somewhere that described 
how to take a commercial antenna cut for say 150 and move it down to two. 
You basically put foil sleves along the length of the antenna.  This added 
paralel capacatance for each element and effective lowered the frequency. 
I'd love to find these areicles.
73s
Butch Bussen
wa0vjr