I am with a search team who use Mobile Signal to guess the whereabout of missing persons. I was told that for 900MHz GSM Signal can go as far as 30km if weather permits. I know where the station "B" is, it is about 20km away from my current location "A". At "A", I can receive somewhat weak GSM signals. Can I build a Metal Box with an opening of say 33cm (equals wavelength of 900MHz Signal), and point that opening to the station, to allow only the signal from the "B", and excluding all others nearby? Thanks.