I'm an electrosensitive (someone who is sensitive to electromagnetic fields such as radio frequencies). I'm trying to figure out a way in which I can use a cellphone without it's radiation affecting me. What I want to know is if the radiation comes from the antennae because I'm planning on purchasing an external antennae that I will connect to a Samsung Galaxy S3 that has a FME connector.During my research they stated that once an external antennae is connected the internal one will be disabled so I would just place the antennae at a far distance and use an amplifier to boost the signal to the cellphone. Can this idea work?
The majority comes from the antenna. However, no system is 100% perfect, and there will always be some leakage from other portions of the circuit.
Also, the rest of the phone will be generating high frequency (RF) EMI fields. The CPU in the phone typically runs at 1GHz or so, and the various peripherals all use clocked (serial or parallel) high frequency (MHz) signals, all of which radiate RF EM fields. Yes, the phone has to be designed to limit the amount radiated to pass its certifications, but that doesn't cut out 100% of all EMI.
Any electrical signal through a conductor generates an EM field, depending on the physical parameters of the conductor and the frequency. An antenna can be considered as a conductor with a very high gain in terms of radiations for a given frequency range. A transmitter is basically in its simplest form a (modulator and a) driver for the antenna, which means it is also dealing with the same signals at the same levels of (electrical) power, but the transmitter is not designed to radiate so its effective emitted power will be neglectable compared to the antenna's. The FME connector seems shielded too, so that should work, though that depends on your sensitivity (radiated power decreases in distance²).
I have discussed this with mobile phone antenna designers and seen the measurement results from a whole mobile phone. The whole phone radiates the signal, although there are hot spots and those are not necessarily at the antenna. The placing of hot spots depends on impedance at the signal frequency, geometrics, etc. I do not remember having seen measurements with an external antenna, definitely the radiation from the phone will be lower since external antenna has the lowest impedance path for RF power. A modern mobile phone might have seven antennas inside, of course the gsm having the most powerful transmitter and for example GPS antenna is receiving only.