Interestingly, I am able to put out a signal on 6m using a fan
dipole designed to work from 20 through 10m. Today I got a report of
5-7 over a 1600 km path. The antenna design can be seen at
http://www.radiowymsey.org/FanDipole/FanDipole.gif .

As far as I can see none of the dipoles resonate on 6m - the 40m
dipole would if it was straight dipole with no loading coils. I
guess one of the dipoles has an odd multiple near enough.

Resonance of the antenna itself has little (some would say almost
nothing) with the antenna's ability to radiate. Resonant or not, if
you can get power into the radiator, it'll radiate!

Using a resonant antenna is convenient, in many situations, because it
can provide a convenient feedpoint inpedance (no reactance, and a
modest resistive impedance) which is easy to match to the feedline.

However, it's not necessary. You can radiate very effectively with a
non-resonant antenna, if you can match it to the transmitter's needs
in a way which doesn't introduce high losses. The matching can be
performed either at the transmitter (as you're doing it) or at the
antenna feedpoint. The latter is usually more efficient, but is often
less flexible.

Using non-resonant antennas, with a coax feedline, and matching at the
transmitter with an ATU, is an approach which tends to have a bad
reputation among modern hams, because of the high losses which can
result. Low-impedance coax feedlines carry relatively high currents,
and (in high-SWR situations) the I^2*R losses can add up fast and eat
most of your power. Add in the internal losses in an ATU when it's
tuned to match a difficult load, and you can end up with a pretty
weak-sounding signal.

The same non-resonant antenna, fed using a balanced ATU using
high-impedance balanced feedline, or matched at the feedpoint, could
be a lot more efficient.

My guess is that either your 17M or 20M segment is close enough to a
3/2-wavelength doublet on 6M (as you guess) so that its feedpoint
impedance isn't too terribly reactive. Referenced to a 50-ohm feed,
it might present a feedpoint impedance between 5:1 and 10:1... high
enough that you'll have some significant excess loss on the feedline,
but not enough to cripple your operation.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!