i tend to agree the astro is 1/2wave,
i think the lower 1/4wave radiates in the same manner as the sigma lower 1/4wave or the short leg of a j-pole but inverted making the astroplane a no radial endfed hat loaded 1/2wave,
heres how i imagine it works untill i learn otherwise,
the distance around the loop formed by the radials & hoop is 5/8wave so antenna mode impedance is high with little antenna mode current flow,
in parallel with the antenna mode impedance is the impedance of the transmissionline formed by the mast/radials/coax which is the high end impedance inverted through the 1/4wave transmissionline,
avanti tell us the 50ohm impedance is determined by the spacing/flare of the radials to mast so we have low 50ohm feedpoint impedance high transmissionline mode current,
looking at the open sleeve article and turning it upside down, when the mast is 1/2wave electrical ( 1/4wave below the hoop ) mast impedance is high so little current flows on the mast below the hoop, when mast length is odd multiples of 1/4wave, mast impedance is low so significant current flows on the mast,
avanti claim removing the flare in the radials causes downtilt of the pattern below the horizon and that the chosen flare gave a takeoff about 4 degrees above the horizon,
they also note that when the mast is shortened to less than 1/4wave below the hoop takeoff angle rises,
i imagine a similar scenario to other antennas, mast currents been determined by mast length and it been grounded or not,
the article eddie ordered told us avanti engineers admitted they did not understand exactly how the astroplane/astrobeam worked.
