On the coupling of energy in parallel plane waveguides

Many EM phenomena may be framed in the context of the excitation and propagation of waves between parallel conducting planes at frequencies below f₀ where only the TEM or “zero” mode propagates. For a localized source, propagation is in the radial TEM mode, described by Hankel functions, with E and H decreasing much slower in the far field (r^-1/2) than free-space radiation (r ^-1). The excitation of these waves has frequently been underestimated. Source currents below the TEM cutoff frequency f₀ couple capacitively, with a coupling factor determined by a simple distance ratio. Higher frequency sources are more complex, but computer analysis shows that rapid transients launch wavelets whose frequency is somewhat below f₀ to satisfy quasistatic boundary conditions, with total energy comparable to that “radiated” at the source frequencies. The combination of strong coupling and slow decrease with distance may help explain “anomalously” large EM signals over a broad range of frequencies and geometric scales, from ELF energy between the Earth and ionosphere which may sustain for milliseconds the mesospheric optical emissions known as “red sprites” to VHF energy in portable electronic devices (PEDs) which may have sometimes interfered with avionic systems. The coupling of energy into the boundaries is thought to be due largely to excess charge momentarily released into the conductors by passing wavelets