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.
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