I\'m trying to find a way (or find out whether it\'s possible) to create an android app that can control transmissions on a cellphone\'s aerial and get it to transmit a desired
Just to add to Commonsware and Luke's answers - in nearly all jurisdictions or countries there will be regulations covering who and what is allowed broadcast over any given frequency (in fact most have laws which even govern which frequencies you are allowed listen to, although these don't tend to be easily or strictly policed...).
There are unlicenced parts of the spectrum which are often used by low powered communications such as Bluetooth, WiFi, CB radio and similar short(ish) range transmissions. As suggested above, interfacing an Android device to a Software Defined Radio that is able to transmit in these frequencies might allow you experiment with your idea. The SDR would have to be a transceiver - one that can transmit rather than just receive which unfortunatly for you rules out some of the cheaper TV dongles which have been shown to allow you build a cheap SDR based receiver such as the ones discussed here:
http://www.rtl-sdr.com/about-rtl-sdr/
http://sdr.martinmarinov.info