So what is a Quad antenna?
It can be as simple as two loops and one of the loops at the resonant frequency and the other slightly larger or smaller. If the second loop is larger then you have a Reflector loop and a driven loop. If you have a smaller loop and a driven loop then the smaller loop is a director loop. At VHF and UHF frequencies, this antenna becomes small enough to be managable for significant gain. Yes it is possible to build a three or four element quad for 40 or 80 meters but the expense, material manipulation and size of real estate make this difficult for the avreage ham.
However at 6 meters and higher these antennas become more feasible for the average ham.
Personally, I have found 2m, 220MHZ and 440 Mhz a good place to design and build three to four element quads. The structural design for supports is manageable and affordable. If you require or desire more gain I would move to a quagi to add more elements at less cost.. Okay so here are the basic formulas for quads I assembled.
Three element 2m quad with a Reflector, Driven element and one director on a 3 foot beam.
Driven element is: a full wave length.plus some. so instead of 936 use :
1005(1004.5)/frequency for the length of the wire for the driven element.
The reflector is 10% longer so
1030/frequency= length of wire in feet
The director is typically 5% to 10% smaller than the driven element.
For more info on Quads http://www.iw5edi.com/ham-radio/2122/cubical-quad-design
For for more accurate calculations these sites have the calculators.