On 3/7/12 5:22 PM, Mike Baker wrote:
> I guess I need to clarify my real needs here.
> I am not putting together a 6BTV. I have one and it works fine.
> I am looking to build a trap tribander Yagi. My question was really about
> what the resonant frequency of the trap is in ANY trapped antenna. I read
> someplace it shouldn't be in the band of trapping but outside the band a bit
> because the series losses of the trap on to the next lowest trap inline
> would be higher if they were in band. That raises the question: below the
> band of desired trapping or above the band. Ad how much off band should they
> be? 1% above the top or 1% below the bottom? What? I see lots of traps made
> of coax but I was thinking more about using a coil over a solid former (like
> the Hustler traps are made) with a piece of coax as the tuning capacitor.
> (Much like what the ATB-34 Cushcraft Yagi used.)
>
> So, my real question is how far off band center should the trap be?
>
> Oh, and I know how many turns for each trap in a Hustler but what I don't
> know is the capacitor value and I can't calculate that without knowing the
> resonant frequency. A catch 22 sort of...
>
A lot of trap antennas are designed empirically.. classic cut and try..
just because they're so subject to all these weird interactions.
One approach is to make the trap like a switch that presents a high
impedance "cutting off" the rest of the antenna at the resonant
frequency of everything between the feed point and the trap...
As the frequency goes down, the trap just starts looking like an inductor.
Others use the "put the resonance a bit off" for two reasons:
1) if it's right on the band of operation, then small changes in the
trap frequency will make big changes in the feedpoint z of the antenna
2) if it's off a bit, then the trap is "part of the antenna", but
because the Z is still fairly high, there's not much current flowing
through it, so the remainder of the antenna doesn't have a huge effect,
nor do changes in the trap itself.
I suspect that one could actually build an antenna either way (resonance
high or low) as long as the resonance is close, but not right on.
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