the cage of the big-mac is not a sleeve like the sigma, the skinny elements are in parallel with the lower 1/4wave, electrically connected top and bottom,
enough to work as a co-linear and enough to give the maximum theoretical gain are two different things eddie, look at the physical spacing of a coaxial co-linear or the better commercial uhf co-linears vs what we have in the big-mac as a fraction of wavelength spacing,
neither the sigma nor big-mac radiate like a 3/4wave or longer monopole, to make a sigma do that you can remove the radial sleeve and retune it as a gamma fed monopole,
the tapered element will have to be longer for the same resonant frequency,
as i have stated in the past, what length i ended up with should be of no concern to anybody else, that is unless they want to use their antenna in my yard,
thats how the arrl describe it
im not suggesting theres no radiation from the sleeve, there clearly is radiation in the cst plot,
the first part of the open sleeve article in the arrl explains where and under what circumstances the currents flow in a direct fed open sleeve monopole
you have transmissionline mode and antenna mode currents flowing in the same elements in opposite directions, the magnitude of which is proportional to tm mode and antenna mode impedance respectively,
then theres common mode excitation that needs looking at, we have a transmissionline with a differential in voltage at the ends,
i have not tested the longer radials, unlike a j-pole i don't expect the extended radials will stop the antenna tuning over a wide range of lengths,
the open sleeve article deals with direct fed open sleeves where the end impedance of the monopole determines the magnitude of antenna mode currents,
adding the sleeve causes a second impedance to be seen in parallel with the antenna mode impedance,
while i have not measured the current in the upper monopole, the fact that the sigma design will tune and work ok with a very wide range of monopole lengths indicates to me that the gamma fed sigma/vector does not have to be 3/4wave to have significant current flowing in the monopole
