Whistler-Mode Radiation From a Dipole Antenna in Cold Magnetized Plasma

We treat whistler-mode radiation from a dipole antenna in a cold magnetized plasma based on the Fresnel zone construction theory. Radiated waves from the antenna with different propagation directions interfere in space. Regions of enhanced waves are formed when the interference is predominantly constructive at the observing point. The coherently enhanced wave propagates at the group velocity. If the antenna is perpendicular to the background magnetic field, the regions of strong waves form two back-to-back Fresnel zones that are parabolic-shaped cones along the background magnetic field from the antenna. The power within each cone drops with distance at a rate of $1/r$ much more slowly than the $1/r^{2}$ thinning of a spherical wave as in vacuum. The total radiation can be approximately derived by the total energy flux integrated over the Fresnel zone. The derived radiation resistance is much greater than that in vacuum. It is proportional to frequency as $1/f^{2}$ and proportional to the antenna length as $d^{2}$ . To radiate more power, a longer antenna is preferred, a result that is opposite to some previous theoretical studies.