An air-filled X-band rectangular waveguide has dimensions a = 2.286 cm and b = 1.016 cm. If the waveguide has copper walls (ε = ε_o, μ = μ_o,σ = 5.8 x 10⁷ S/m), find the attenuation in dB/m due to the wall loss when the dominant mode is propagating at 8.4 GHz.