I guess the theory is that on 20 it is a bit long for resonance, which now 
looks like a 3/8 wave.  Any antenna used on multiple bands will work 
differently on each band.
I am trying to understand at what point the SWR drastically effects the 
efficiency of the vertical.  The variables seem to be reflected power and 
transmitter shut back when it sees the reflected power.  That can be 
eliminated with a tuner, but I don't know if doing that actually delivers 
more signal to the antenna or not.  Working with shielded line and a tuner 
is new to me.
I am going to try a remote automatic tuner at the base of the antenna.  That 
sounds like the best way to go for efficient multi-band operation.  I think 
that with coax, the tuned reflected power probably warms the coax rather 
than is radiated by the vertical.  It looks like stretching the efficient 
use of the antenna from 17 meters to 29.2 for A.M. Phone is asking a lot, 
which makes sense to me.