There are some great answers here. I'd like to add a little. You used the term "earthed", which means you are searching for a reference to something. An antenn

There are some great answers here. I'd like to add a little. You used the term "earthed", which means you are searching for a reference to something. An antenna has its own reference: one side is at a different potential than the other due to induction from the receiving RF wave. It's easier to think about with a dipole antenna, where each half of the antenna receives a different phase of the wave: the relative difference between the poles of the antenna is the voltage induced by the changing wave. It is its own reference. Now in any circuit that is not literally grounded, there is always a reference voltage that is called ground. Everything is referenced and built to this. Whether or not it is "zero volts" is immaterial, since it is the reference: we can make it zero volts by analysis. The potential induced on the antenna is a quantifiable charge can be compared to this reference, and amplified. Hmmm. That sounded clearer in my mind, but I just wanted to point out the relative nature of a reference ground in a portable circuit.