Shielding Effectiveness Improvement of Metallic Waveguide Tube by Using Wall Losses

The metallic waveguide tubes are frequently placed in the walls of high performance shielded enclosures to provide air ventilation and strong electromagnetic interference attenuation simultaneously. However, a waveguide tube gives no attenuation for frequencies higher than its cutoff frequency. Also, there is a trade-off between the shielded frequency range and the airflow capacity, since the cutoff frequency is inversely proportional to the wave-guide radius. In this paper, the idea to shield frequencies above cutoff by coating the inner wall of waveguide tube with lossy material is investigated in detail. The attenuation constants of the TE 11 mode in a lossy circular waveguide are calculated for lossy conductive, dielectric, and magnetic material, respectively. It is shown that lossy magnetic material can provide the strongest attenuation and high-shielding effectiveness for frequencies above cutoff. This work presents a promising approach to relax the frequency-airflow tradeoff.