Dear Markus, your calculations are fully adequate only while ionosphere
does not work. I.e. for less then about 100 km (very approximative). At
180 km and 800 km ionosphere influence is certanly worth. Generally
speaking ionosphere should increase sigs. But it may be some variations
because of interference of different earth-ionosphere waveguide modes 
which may have as same phases as opposite phases. Fore large distanses i
expect behaviour E ~ 1/sqrt(d). This can be simple derived from energy
conservation low. Certanly there should be someone additional exponent
exp(-d/a) because of dissipation but i expect a >> 1000 km. Then exponent
is neglectable for distances described. 

Regards,
Alexander/RA9MB