On Sun, 12 Nov 2006 01:29:20 GMT, Owen Duffy wrote:

On Sat, 11 Nov 2006 16:19:48 -0800, Danny Richardson
wrote:

On Sat, 11 Nov 2006 22:07:14 GMT, Owen Duffy wrote:

Programs tend to underestimate the losses of these traps.

Owen,

What is your estimation of the loses? Will it be more than a dB?

Danny,

I recall modelling dipoles incorporating such traps with NEC some time
ago. My recollection is that the loss is of the order you suggest,
which might seem insignificant in the context of a path budget, if it
wasn't for the fact that it might about to a significant amount of
power to be dissipated in the trap, depending on the power level /
mode.

Operation at resonance exacerbates the situation.

My sentiment is not so much that the traps are a bad idea, but the
design tools that are around seem to take shortcuts and are
inconsistent. The design tools would make one think that the trap
designs are better than they really are.

Unfortunately, a lot of ham lore also suggests that traps are much
worse than they actually are. The typical proponent of feeding a
"G5RV" with ladderline-balun-tuner rather than dreaming of using
traps probably believes that he has the more efficient system.

IMHO, for the case in question; 80 and 40 M operation with BW not an
issue, I would use coax traps and be done with it.

The major objection to traps in my estimation is the reduced BW that
results from the wire shortening and inductive loading on the lower
bands, not the additional loss. Any of the single feedline multiband
antenna *systems* that I can think of have higher loss than a
single-band resonant dipole. You can have your loss in traps or stubs
or you can have it in the transmission line/balun/tuner. Your choice.

I have placed a couple of files here that might be useful to the OP:

http://k6mhe.com/n7ws/QST_Correspondence_3_84.pdf

http://k6mhe.com/n7ws/QST_Correspondence_8_85.pdf

Also, Owen, I think you said earlier that the coax capacitance isn't a
linear function of line length because of transmission line effects.

The referenced authors have demonstrated that because of mutual
coupling between the coax conductors and the way the trap is
configured the line is actually just a capacitor.