[TowerTalk] trap antenna performance

[Jim Brown]

All of this makes sense to me. Also, a trapped is a shortened antenna, 
so radiation resistance can be lower relative to loss components.

And the difference need not all be dissipation. Any antenna with "gain" 
concentrates its radiation in the desired direction(s) by "stealing" it 
from other directions. As Jim notes, this is the result of phase 
relationships between the elements as a result of their dimensions and 
spacing, and if those relationships are non-ideal, the "stealing" 
process doesn't work as well, so the power goes somewhere else besides 
forward.

73, Jim K9YC

On Mon,4/13/2015 7:19 AM, Jim Lux wrote:
> 1500/2/6 watts, a lot of watts per trap.  Even it not this much just
> > loss, there would be ample power to melt the insides of the trap
>
> this is an interesting question...
>
> I wonder if it's a "sensitivity to small changes in component values" 
> issue?  Designing a trap antenna so that when its brand new it has the 
> right performance shouldn't be challenging: there's cut and try, if 
> nothing else.
>
> But traps are kind of difficult to model (at least in NEC) at a very 
> detailed level.  So maybe if the L or the C of the trap changes 
> significantly, then the trap resonance changes, which then changes the 
> apparent series L or C in the element.
>
> As Markku pointed out, the Q of the trap has to be fairly high, or 
> they'd melt from the losses.
>
> So it could even be things like manufacturing variability.  A trap 
> change in resonant frequency by 2 or 3 percent (a change in L and/or C 
> of 4-6%.. is that reasonable?) could change the apparent impedance of 
> the trap at the use frequency from inductive to capacitive.