A couple of thoughts on the Mag 340N interacting with tri-banders:
1) The biggest issue, by far, is the fact that full size 40 meter antennas
have a secondary resonance very near (or even in) the 15 meter band. The
design of the N elements moves that secondary resonance away from the 15
meter band, which significantly reduces interaction with tri-banders. The
reason that the resonance moves is that the reactance of the linear loading
(or capacity hats for that matter) changes with frequency - so shortened
monoband antennas can be designed to have the secondary resonance wherever
you want (more or less) by playing with the size and location in the element
of the loading device. You don't need a computer to test this, just take an
N element (or antenna) and look at where it is resonant with an antenna
analyzer of some sort. The secondary resonance of the N antennas is well
away from all other ham bands.
2) N6NF has a 340N at 73', a C-31 below it at 40' and another C-31 above it
at 80'. They all work just fine, with no interactions that I can find with
my MFJ or notice in heavy contest use.
***dan, K6IF
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