30 meters is not a harmonic of 20....

WARC - added some bands to the American Band Plan that were odd harmonic's. It was easier to sell a antenna that only did WARC then to make one that did everything.

10 meters difference between 20 and 30 isn't all that much in the grand scheme of things.

The offcenter fed dipole I use for Field Days will not operate on 15 meters, nor will it work on 30 - although I never really tried to use it on 30.
It is only marginal on 12 and 17.

My Solorcon A99 seems to work well on everything from 10 to 17 meters - although if the band is not open, it doesn't perform as well as other antenna designs.

The comment that no antenna is designed for use on both 20 and 30 is a misnomer.. The Barker & Williamson BWD 90 will work on everything from 10 - 80 meters. Regardless of what anyone's opinion about it is!

The bottom line is - traps are very inefficient and unreliable - not the best antenna in the world to use on a long term basis.