On Tue, 16 Mar 2010, Paul Nicholson wrote:
> 
> For conservation of energy, it cannot be higher than
> that, so that gives an upper bound.
> 
> Then there must be the exponential factor for attenuation...

Certanly exponential factor exp(-r/a) should be present. I mentioned this 
before. But i have no ideas how to estimate roughly (at least) parameter
(length) a. Intuitively i expect  a = few of 1000s km. Then exp is
neglectable (about 1) at 800 km. But this should be calculated. At the
earth bondary one can use Leontovich bondary condition for such
calculations, it is simple and parameters are known roughly. But what to
do with ionosphere boundary... To neglect dissipation in ionosphere looks
not reasonable. Certanly it is solvable problem but one need to work much,
particulary study literature to find any estimations of plasma parameters
of D-layer, particulary free path length of electron and electron
concentration for diferent height. Then Maxwell's equations to be solved. 
Anyway this require not couple of hours, much more. But i have very few
time for this now. May be much later... To solve the problem (rather
volumable problem) is nice task for physicist. I expect it was solved many
years ago in book by  V.L. Ginzburg. But some training is usefull anyway.
If one have time...:-)

Regards,
Alexander/RA9MB