If your tower can mechanically handle it I would recommend two separate
antennas. One for 10-20m and one for 40m.
There is a very neat design in "Low Band Dxing" for a "high performance
and light weight 40m yagi" designed by NW3Z. It will outperform the 2
element yagis on the market and has an exceptional gain f/b and swr
profile from 7.0 to 7.3 mhz.
For the multibander consider SteppIR: A nice feature of the SteppIR3 or
SteppIR4 is that you can have great performance on every frequency fro
10-20m even on the WARC bands. Also, if you were even remotely
concerned about intereaction with the 40m yagi you can detune the
SteppIR using it's control head to eliminate interaction.
Another option would be a SteppIR3 or 4 with the 30/40m adder kit. It
will give you 10-40m on one boom, but only a folded dipole on 30 and
A ten foot separation between the multibander and the 40m yagi will be
73, Scott W3TX
TowerTalk mailing list