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Re: [TowerTalk] trap antenna performance

To: "'towertalk'" <>
Subject: Re: [TowerTalk] trap antenna performance
From: "Paul Christensen" <>
Date: Mon, 13 Apr 2015 12:31:14 -0400
List-post: <">>
Recently, an article on p.43 in the February, 2015 issue of QST discusses
trap performance and test fixture criteria.   The article is titled:  "Test
Setup for Measuring Traps."   The author used a miniVNA device.   It's worth
having a look.  

Paul, W9AC 

-----Original Message-----
From: TowerTalk [] On Behalf Of Jim
Sent: Monday, April 13, 2015 10:20 AM
To: towertalk
Subject: [TowerTalk] trap antenna performance

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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