This study introduces a passive approach for achieving tunable perfect absorption of microwave radiation, using non-Hermitian physics to manipulate electromagnetic waves without conductive elements. By using common dielectric slabs in irregular configurations within a rectangular waveguide, we can completely absorb electromagnetic energy, efficiently converting it into heat, while minimizing conductive losses. This method contrasts with the use of traditional metamaterials that rely on conductive components to absorb energy. Using the transfer-matrix method and sequential quadratic programming for optimization, our research identifies specific nonuniform placements of FR4 slabs that maximize absorption within targeted microwave bands. Experimental validation, executed in a standard WR90 waveguide connected to a vector network analyzer, confirms the theoretical and finite-element-simulation results, underscoring the effectiveness of our passive strategy. This work advances our understanding of non-Hermitian systems in electromagnetic wave control and offers significant potential for applications in reducing electromagnetic pollution and enhancing secure communications.

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• Light propagation, transmission & absorption
• Optimization problems
• Optoelectronics
• RF energy harvesting devices
• Metamaterials
• Non-Hermitian systems
• Waveguides
• Microwave techniques