Well calculated Neptune, that's the way to do it if you have suitable pictures. The 150 feet was what had in my Offshore Echos 157 article on offshore masts from 2009, but that article needs some updating and correction as new information has come to hand. The info I used for Radio 390 came from Bob le Roy, Tony Pine and Lawrence Bean, who are credited in the article.

Tatsfield was/is about 40 nautical miles from Red Sands and about 60 nautical miles and more from where the ships were. Was that differing distance attenuation brought into the calculation? Red Sands was the perfect site for the signal to go up the Thames. When the Mi Amigo moved south for a short trial, the signal strength in London was much higher.

I was living in Shepherd's Bush when the Mebo II came to off Clacton in 1970 and the MF signal on my Nordmende Globetrotter, with its internal aerial only, was quite weak.