[TowerTalk] trap antenna performance

[Jim Lux]

On 4/13/15 12:40 AM, Markku Oksanen wrote:
 > On "Low Performance Trap Antennas" :  Is it really known what is the
 > cause for the apparently very large difference between a correctly
 > designed trap beam, say a TH6DXX or similar and one of the not so
 > good performers?  Trap loss can't be a big part because even
 > relatively low power loss at a given trap, say 50 W, will burn the
 > thing very quickly.  Assume 3 elements, 6 traps that get exposed on a
 > given band and -3dB gain compared to better beam, this would be some
 > 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.