Thanks for the replies. I guess it's better not to take a chance and risk getting a fine (and maybe criminal record, if I understood the posts in that link correctly). Anyway FR24 is a pretty good substitute in the absence of ATC, if not better in some ways.
Originally Posted by
oliver2002
Ah, it could be that it was a ads-b transponder listening station. That antenna is only 30 cm (or you can use just a DVB-T antenna that plugs into a USB dongle):
http://www.flightradar24.com/dvbt-stick
That's what you call dedication
I must say, the terrace at FRA seems to be quite popular, last time I was there a few weeks ago, it was a beautiful bank-holiday morning and there were lots of people. Although I didn't see any with scanners or antennae...