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[TowerTalk] trap antenna performance

To: towertalk <>
Subject: [TowerTalk] trap antenna performance
From: Jim Lux <>
Date: Mon, 13 Apr 2015 07:19:55 -0700
List-post: <">>
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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