Antennas in Material Media Near Boundaries: The Terminated Insulated Antenna
When the ambient medium in which a bare metal antenna is embedded is conducting, radial currents from the antenna into the medium decrease the axial current. This means that often only electrically relatively short antennas are useful with correspondingly small effective lengths. Radial currents can obviously be eliminated by enclosing the metal cylinder (radius a, conductivity σ c , internal impedance per unit length z i ) in a dielectric sheath (outer radius b, wave number k d = ω[μ0(ε d + iσ d /ω)]1/2, σ d ~ 0, as shown in Fig. 17.1.1. This greatly changes the properties of the antenna. In particular, the wave number of the current is no longer the wave number kI = k1[1 + (i4πz i /ωμ0Ψ)]1/2 as given by (16.2.4)—which reduces simply to the wave number k1 = ω[μ0(ε1 + iσ1/ω)]1/2 of the ambient medium except at very low frequencies.
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