Hellow, Stefan.

> If that calculation is reasonably correct, what distance could be 
> reached with 1,4mW @ 8,9 kHz 

With such a condition you'll get about 2.5 uV/m at 100 km. Seems it can be
recievid. If there is no atmospheric and industrial noise it should be
very strong signal. But all depends on noise on 9 kHz. I have no ideas
about noise on 9 kHz. I neglet ionosphere in estimations. But at D=100 km
it should be approximately right. 

Anyway if you can use 100 m high antenna then few of 100's km you should
reach. May be substantionally more. For large distances dependance of E
on D changes. On the distances more then ~100 km field should have
behaviour E ~ sqrt(1/D). This yelds only 10 dB attenuation if distance
became 10 times more. Thus if you'll have 20 dB over noise at 100 km (if
noise is 0.25 uV/m for example) then you'll have ~10 dB over noise at 1000
km.  

Do it if you have such an oportunity! It is very interesting.

Regards,
Alexander