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 [mailto:towertalk-bounces@contesting.com] On Behalf Of Jim
Lux
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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