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

[Robert Lay]

My guess is that a trap should be self-resonant at a frequency above the
frequency for which you are designing.

My rationale' is based on the concept that the amount of wire beyond the trap is
going to be seen as a lumped capacitance in parallel with the trap coil, thus
lowering its resonant frequency (any parallel resonant tank circuit resonating
frequency goes down as you add parallel capacitance).

Quantitatively, one would have to know some things for which  we don't have a
good estimate, because there is much flexibility in where the trap is placed and
how much antenna extends beyond it (to the open end).

Bob Lay (W9DMK), Dahlgren, VA
w9dmk at crosslink.net
http://www.qsl.net/w9dmk

----- Original Message -----
From: "john" <johnmb at nc.rr.com>
To: <antennas at mailman.qth.net>
Sent: Sunday, April 03, 2005 6:14 PM
Subject: [Antennas] Trap design, how far from operating freq, should I hit?


> 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