Eddie,
as far i i am aware there are only two possible modes that current is flowing in the astroplane IA & IT
the two modes i have talked about,
all regular antennas i know of use antenna mode with the usual distribution curves
apart from antennas with transmission-lines within their construction such as j-poles & sleeve monopoles which use both modes,
you don't see that in NEC but you do see the resultant radiation due to unbalance in the stub that is caused by the mode you can't see,
it may not look like a sleeve monopole but it has to have the same currents when it has parallel conductors in close proximity that lower feedpoint impedance,
i think many folk don't understand a simple j-pole,
look at this vid explaining how a j-pole works,
its incorrect & theres nothing complicated about j-poles once you read Cebik's articles like "some j-poles that i have known" or any info on what makes transmission-lines radiate,
that mast does effect the antenna, it will carry common mode,
any conductor with a voltage differential between any two points on the conductor will radiate,
the balanced line is not balanced as he claims even before he attaches the unbalanced coax,
the top of the stub is unbalanced because one side sees the near infinite impedance of air while the opposite conductor sees the lower end impedance of a 1/2wave monopole,
which unbalances the line & causes the short leg to radiate giving the lobsided pattern with more a bit more gain in the direction of the short leg,
the wider the space at the top of the stub the more unbalanced it is which increases radiation from the short leg,
imho taper allows the same characteristic impedance for the transmission-line with increased unbalance at the top of the stub, ( i could be wrong )
i looked back in time to Heavisides work on electromagnetic wave propagation along the external region of transmission lines, tapered transmission-lines, charactersistic / surge impedance,
too complex for my old head but theres some usefull info for anybody interested enough to look at it.
look at just a couple of things that unbalance a balanced line causing it to radiate then look at the astroplane,
a balanced line will be unbalanced if you terminate the far end with an unbalanced load such as seen in the j-pole & ssfm,
3/4wave monopoles give best balance, most current on the monopole, least radiation from the sleeve, it does radiate but not a lot,
1/2wave monopoles give the worst balance, least current above the sleeve elements & high radiation from the sleeve ,
the wider the spacing at the top of the stub the more unbalanced it will be,
a balanced line will radiate if you flare or taper the conductor so they are not perfectly parallel as seen in henry's terminated sigma4 cone model,
a balanced line will radiate if you feed it asymetrically,
ie not directly central between the parallel conductors, if you offset your source you will see some radiation,,
now look at masts not connected to ground,
1/2wave masts have high end impedance low current,
1/4wave masts give low end impedance high current just like a 1/4wave whip or the legs of your 1/2wave dipole,
so how would you maximise radiation from the sleeve and the upper monopole of the astroplane while minimising current in the mast below the sleeve elements?
I think you would
use a 1/4wave upper monopole,
use isolated 1/2wave mast,
choke the coax at the end of the mast 1/4wave below the hoop,
make the space at the hoop as wide as practical for maximum unbalance while still maintaing the correct transmissionline impedance for low vswr by flaring the lines.
what would i avoid,
i would avoid odd 1/4wave masts that screw with vswr & cause the mast to carry significant antenna mode current .
IF thats not what is going on or at least most of whats going on with an astroplane i will eat my hat.
I better find the salt, hats taste like shit.
