[TowerTalk] TH-11DX and 40/30m dipole

[N1BUG]

I am sure this subject has been beaten to death many times but I am 
not finding all of the wisdom I seek either on the bookshelf or in 
the archives of this list.

I have a TH-11DX at 100 feet. I plan to build a 40/30 meter 
rotatable dipole which I want to put above the TH-11. I assume 
(uh-oh) that with the close frequency spacing of 20-17-15-12-10m it 
will be difficult or impossible to get resonances of the 40-30m 
antenna far enough away from all of those bands to get away with 
putting the elements in the same plane. This assumption however 
leads to two questions:

1) I'm not sure this can be answered in general terms, but: How far 
removed from a band does a resonance have to be in order to not 
cause significant problems? I don't mind a little distortion of 
pattern or a dB loss of gain but significant SWR changes are not to 
be tolerated.

2) Is there a simple way to measure said resonances of the 40/30 
meter dipole that will be good enough for this purpose? I have an 
MFJ antenna analyzer at my disposal.

Assuming putting the 40/30 dipole in the same plane as the TH-11 
elements is out brings me to my next question.

3) If mounted in the same plane Will the boom of the TH-11 cause 
problems for the 40-30 meter dipole? How far above the boom should I 
be trying to mount it for good performance? Four feet could be done 
very easily but I worry that may be too close. I could get a few 
feet more with extra work and expense. Anything beyond 8 feet is 
probably not possible in this instance.

I would appreciate any comments or advice I can get before I get too 
deep into this project.

Thanks,
Paul N1BUG