Antenna assemblies for magnetic resonance signals comprise a non-resonant loop antenna and a high impedance differential amplifier. The amplifier can include first and second high electron mobility transistors that have gates coupled to an antenna loop that is defined on a rigid substrate. The non-resonant loop has an effective length of less than about 1/10 of a wavelength of a signal to be detected. Arrays of such loops can be defined on the rigid substrate, and HEMTs for the loops secured to the substrate.