cell towers tend to operate in the 900 to 1.2 ghz range just like any radiation range time and shielding are factors there are electromagnetic meters that can show you your exposier, sometimes a cheap AM radio can on static can give interesting results. the higher the frequency the smaller the fish and the better "net" you need. the shielding doesnt need to be thick or even block light very fine metal screen or metalic mylar may give shielding, a multimeter that shows conduction and the film is grounded properly can be effective. also flat pannels attached at 90 degree angle will reflect the radiation away, if the cell company complains you can use the argument that the freq energy is too low to be harmful to people should also not be harmful to equipment especially at double the distance. im not an expert so take with grain of salt,, might take a look at amateur radio info or find a ham operator,, even if it doesnt directly effect you it may effect other items like electronics, also walls are no garentee of protection.