Roy Lewallen wrote:
The simplified idea of a trap as a short circuit on other bands also
leads to bad decisions. On any frequency band below its resonant
frequency, it acts like a loading inductor; on bands above resonance,
like a capacitor.

Until it gets to the 1/2WL self-resonant frequency where
it yields the 180 degree phase shift described by Kraus.

Quoting Kraus: "A coil can also act as a 180 degree
phase shifter ... The coil may also be thought of as
a coiled-up 1/2WL element." This coil is placed at a
current minimum point in a phased array.

This does NOT mean that the length of the wire used in
the coil is 1/2 wavelength. It just means the coil is
series resonant with a 180 degree phase shift.
--
73, Cecil http://www.w5dxp.com