W4RNL has a number of articles on quads, along with some

design equations implemented as a BASIC program. The wire

diameter is a critical variable - fatter wires require a

large element diameter. Any formula that doesn't take

this into account is suspect.

You can see some of his research here:

http://www.cebik.com/quad/quadlist.htmlI've built 2-, 3- and 4-element quads from his formulas

and they have all come up right on frequency without

any tuning or adjustment.

There was an article in the first issue of the ARRL Antenna

Compendium that had design charts for quads (that also

included the effect of the wire diameter.) One of the

interesting outcomes was that quads with an odd number of

directors would have wider bandwidth and were more forgiving

to tune than those with an even number of directors.

I don't know of any software that implements his charts

and formulas, however.