I just wanted to add that modeling or field testing the lower 1/4 wave cone by itself will provide NO useful information as to how it behaves on the Sigma. Once you remove the end fed 1/2 wave from the top of this 1/4 wave coaxial cone, all you're left with is essentially a 1/4 wave of transmission line. If you think that proves it's just a 1/2 wave J-pole, you're overlooking a lot. It is the connection of this end fed 1/2 wave to the top of the cone that creates the beneficial CMC radiation on the outside of the cone. The Key difference between this and any other antenna you have seen is the 1/4 wavelength, 90 degree phase delay taking place inside the cone that feeds the upper 1/2 wave. That just happens to bring it back into a constructive phase with the CMC radiating off the cone. Some have been quick to quote experts like W8JI for pointing out the CMC can be a significant enough factor as to cause "the entire end fed design to fall apart once the mast and feedline are added". What do you think could happen if you provide a low impedance path to a resonant cone for the CMC to radiate off of once the top 1/2 wave has been phase shifted to line up with the cone? Why wouldn't we expect the same degree of change in a positive direction once the phase is aligned? This is what makes the Sigma IV the "non apparent collinear" antenna L.B. Cebik claimed it was.