Owen Duffy wrote:

Sure. I should have mentioned that after trying NEC models to get a
low VSWR solution for just two bands (one twice the frequency of the
other) I gave up figuring that it was likely that an ATU would be
needed in any case, and the traps were just an unnecessary
complication.

As you know, trapped antennas are sometimes explained by considering
the traps to be equivalent to frequency selective switches that are
o/c or s/c at certain frequencies of operation, and conveniently
disregarding the conductors on the outboard side of the o/c switches.
It is a simplistic view, it does not lead to a design that works, nor
does it explain designs that do work, but it remains the most popular
explanation to be heard on air in my experience.

Yes, I'm afraid that's true.

I try to avoid simplified explanations just because people forget that's
what they are, and begin treating them like fact. A good example is the
recent thread about there being two "theories" of ground plane
operation, one being the simplified explanation of operation as an
"image". (I still remember an April magazine article in which the author
began with the popular and common "image" idea to explain the operation
of a grounded vertical. Then he proposed drilling a hole in the ground
and putting the vertical there, fed at ground level. This, he said,
resulted in the "image" being above the ground. The perfect stealth
antenna! Anyone who knows where this appeared -- must have been around
the '60s -- would do me a favor by letting me know. I'd love to read it
again. I've searched the entire collections of QST and HR to no avail.)

But back to traps. Trap operation is indeed a lot more complicated than
most people realize. I was recently contacted by a puzzled EZNEC
customer who saw a situation where a trap had very little effect. He had
put the traps where the current was normally very low, so of course the
traps did little. Plenty of current was being induced in the wire beyond
the trap by mutual coupling from the field produced by current on the
wire on inside of the trap, whether the trap was there or not.

The simplified idea of a trap as a short circuit on other bands also
leads to bad decisions. On any frequency band below its resonant
frequency, it acts like a loading inductor; on bands above resonance,
like a capacitor. It's common to see more loss on the lower frequency
bands where the trap is a loss inductor than at the trap's resonant band.

Modeling makes it easy to find out what works well and what doesn't for
a given application. But of course, you have to know the trap's L, C, and R.

Roy Lewallen, W7EL