[Antennaware] Trapped dipoles

[David Gould]

I thought that a conventional 80m/40m trap dipole had the trap 
resonant at 40m, I have now read a number of different fragments of 
information that say that this is not the best approach and that the 
trap should be resonant somewhere between the two desired frequencies.

I want to build a trap dipole for 40m and 30m.  There are a couple of 
things I don't understand...
Does it matter what the trap resonant frequency is?
Does the trap resonant freq affect the "inner" length of the dipole 
from feed point to the trap to give resonance on the higher 
frequency  ( Trimming the outer length to the lower frequency would 
be straightforward)

Any help or links to some detailed description would be appreciated.

73,
Dave, G3UEG