Q2Electromagnetics Waves
Question
Q.2 Define attenuation in a waveguide.
Answer
Attenuation in a waveguide is the reduction in signal amplitude as a wave travels along the guide, arising below cutoff from evanescent (non-propagating) exponential decay, and above cutoff from finite conductor and dielectric losses.
Attenuation in a waveguide refers to the progressive reduction in the amplitude of a propagating (or attempted) wave as it travels along the guide's length, expressed as an attenuation constant α (Np/m or dB/m). Two physically distinct mechanisms produce attenuation: below cutoff (f < fc for a given mode), the propagation constant β becomes purely imaginary, so the 'wave' is actually an evanescent field that decays exponentially with distance, e^{−αz}, without truly propagating or carrying real time-average power beyond a short distance from the source — this is the basis of intentionally used 'below-cutoff' waveguide attenuators. Above cutoff (f > fc, the normal operating regime), the wave does genuinely propagate but still loses some energy per unit length to two loss mechanisms: conductor (ohmic) loss, from the finite conductivity of the guide walls (current flowing in the walls dissipates power via the skin-effect surface resistance), and dielectric loss, from any imperfect (lossy) dielectric filling the guide interior (usually negligible for the common air-filled case). Waveguide attenuation is normally very low compared with coaxial cable at the same frequency, which is one of its principal practical advantages.