1. A system comprising: orthogonally oriented antennas configured to generate signals in response to a magnetic field, where the signals correspond to components of magnetic field vectors in space; a circuit coupled to the antennas and configured to: determine polarities of the signals based on phase measurements between the signals; reduce a possible number of magnetic field vector interpretations based on the determined polarities of the signals; reduce a possible number of angles or angle differences between the magnetic field vectors based on the reduced possible number of magnetic field vector interpretations; compare the reduced possible number of angles or angle differences to predetermined angle or angle differences; and detect a relay attack based on results of the comparing.