[TowerTalk] trap antenna performance

[Jim Lux]

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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