[Antennas] Trap design, how far from operating freq, should I hit?

Sun Apr 3 18:14:27 EDT 2005

I'm modifying a 15m Cushcraft trap into a 12m trap by removing
turns on the coil. I have read that it's better to design the trap to
be resonant at a different frequency than the target (operating 
frequency) to minimize losses. I'd like to understand why this is so.
I'm planning on making the trap resonant above the operating 
target, and adding a bit more length to the element "inside" the 
traps,to resonate the system at my operating freq.
My question is, how far off the operating freq should I build
the traps for? 
My goal is a 12 and 17m shortened dipole (or 2 el yagi, should
this work). An understanding of why a trap resonant at the 
operating frequency is more lossy than otherwise, would be a plus. 
Thanks!
John
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