[Milsurplus] HF antennas with ART-13 etc

[Pete Williams]

G'day list...
Question.. why is is so difficult to effectively couple an aircraft type HF
tx to an indiscriminate length of wire?--- at ground level
The ART-13 for example, will, in an aircraft, couple all frequencies from
around 2 MHz to 18MHz with wire lengths from 20 thru' to 60feet.
Yet when I try the same act at ground level, there's much angst, lttle up
the wire I suspect, and a shack full of hot RF. !
Yes I know about connecting a 300pf cap to make a pi.section final but it
gives mediocre success.
I do get joy when a 300ohm feed line from a CF dipole is connected one lead
to to the antenna terminal and other to chassis.--- why is it so?
I suspect that in metal fuselage  aircraft, the aircraft does most of the
radiating with the length of antenna wire unreeled acting as the other leg
of the feedline . 
I also appreciate that the tx is designed to load into low impedance  but
don't see how this equates to operation at ground level or even at 50,000
feet.
Any input on a confusing subject welcome-- with solutions.--
Thanks - in ignorance.
Pete VK3IZ