Last year I spent quite a bit of time with the VNA/SA and trusty 259 trying
to determine the precise resonant point of a Hy-gain trap. In the end it
was a failure - I could not pull the environment out of the test fixture
unless the trap end tubing was cut off which of course destroys the trap.
The measured resonant frequency changes significantly with the tubing
lengths attached to either end of the trap.
I would be very interested in a workable method to measure these traps on
the bench.
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
-----Original Message-----
From: Jim Lux
Sent: Monday, April 13, 2015 9:54 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] trap antenna performance
On 4/13/15 7:39 AM, Paul Christensen wrote:
"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.
Trap Q is more difficult to accurately model than it is to measure. A few
measurements are worth a thousand words that talk about trap loss.
I was trying to build a fixture to measure traps on a 6BTV using a VNA,
but it's a decidedly non-trivial proposition to de-embed the fixture.
(where non-trivial means I couldn't figure it out over the weekend I was
interested in doing it).
I believe Tom Rauch did some trap Q measurements and got "in the
hundreds", so the loss effects, I suspect, are negligible.. that's why
I'm thinking "detuning" of some sort.
http://www.w8ji.com/traps.htm
On a multiband dipole application, with the trap resonant frequency well
between bands, it's probably not super critical..with a parallel
resonant circuit below resonance it looks inductive and above resonance,
it's capacitive, but the Z isn't changing too fast.
In a Yagi, though, where you're looking for specific amplitude and phase
relationships, I can see it having a bigger effect.
"As Markku pointed out, the Q of the trap has to be fairly high, or
they'd
melt from the losses."
And they do melt from loss. During the 8th grade, I owned a TA-33 Jr.
When
I upgraded to the U.S. General Class license, I received a Heath SB-230 as
a
birthday gift. The moment I used the SB-230 on 20m with 500W, the driven
element traps melted. I still remember calling Mosely about the issue and
their advice was to upgrade the DE to the MP-33 model. The MP-33's DE
traps can better withstand loss. I don't know whether or not the larger
traps withstand more power due to higher Q, or just that the larger
diameter
coil and insulating material are just better at absorbing power before the
melting point occurs. Back then, I was only interested in making contacts
and not understanding antenna theory. The MP-33 worked fine, but then so
did a 15m Gotham 3L that was replaced by the TA-33 Jr. The Gotham could
very well have been a better performer on 15m than the Mosely.
Paul;, W9AC
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