Not sure if it's been mentioned elsewhere, but as ADS-B is line of sight (basically the same as a VHF signal), the theoretical range can be worked out by the following:
Theoretical Range = √1.5 x Aircraft Height Above the Antenna
Eg. √1.5 x 30,000ft = 212Nm's
Then as has been pointed out above, that's all dependant on site error's of your aerial as well. Plus the cableing, etc. It doesn't take much for a weak signal that far out, to then be shielded by a tree, or a distant hill.
Mick
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