Transmission between two identical antennas is unidirectional, i.e., non-reciprocal. Sketch of a whistler wave antenna which exhibits directionality by matching the helicity of antenna and wave fields. It consists of a loop on the axis of a low aspect-ratio torus such that the resultant fields are helical. The direction of currents in the loop and torus determines the sign of helicity, hence direction of preferred radiation. Snapshot of the magnetic field components of a whistler wave packet excited by a current pulse to a loop-torus antenna of positive helicity.
Bottom panel shows the axial field component in the central y-z plane on both sides of the antenna.
The amplitude asymmetry demonstrates the directionality of the helicity antenna. Left hand diagram shows the experimental setup. A long flat electrode is biased so as to draw a current pulse in a uniformly magnetized background plasma. The right hand contour plots show the axial current density in two orthogonal planes after the current sheet has been established. Space-time evolution of an EMHD current sheet.