Although solid, geometrically tapered microwave absorbers are preferred due to their better performance, they are bulky and must have a thickness on the order of λ or more. The goal of this study was to design lightweight absorbers that can reduce the electromagnetic reflections to less than ？10？dB. We used a very simple approach; two waste materials, that is, rice husks and tire dust in powder form, were used to fabricate two independent samples. We measured and used their dielectric properties to determine and compare the propagation constants and quarter-wave thickness. The quarter-wave thickness for the tire dust was 3？mm less than that of the rice husk material, but we preferred the rice-husk material. This preference was based on the fact that our goal was to achieve minimum backward reflections, and the rice-husk material, with its low dielectric constant, high loss factor, large attenuation per unit length, and ease of fabrication, provided a better opportunity to achieve that goal. The performance of the absorbers was found to be better (lower) than ？20？dB, and comparison of the results proved that the hollow design with 58% less weight was a good alternative to the use of solid absorbers. 1. Introduction Electromagnetic interference (EMI) is a serious threat to electronics-based civil and military infrastructures [1, 2]. Various types of natural and man-made EMI sources have been identified that can lock, upset, damage, and cause malfunctions in sensitive electronic components in extremely complex and mission-critical systems [3–5]. Electronic devices must follow the electromagnetic compatibility (EMC) standards which impose certain conditions on the electronic devices before they can be marketed [6–9]. Comprehensive EMC testing of these devices is conducted to determine their emissions and susceptibility levels within specially designed, shielded, reflectionless facilities, that is, anechoic chambers. In these chambers, geometrically tapered, synthetic, carbon-impregnated foams with thicknesses on the order of or greater are used [10]. These absorbers are made from flexible, polyurethane foam, which is a heterochain polymer, synthesized by the reaction of isocyanate (–NCO functional group) compounds and polyether polyol alcohol (–OH functional group) [11, 12]. Isocyanates are hazardous compounds and can cause significant respiratory problems, such as asthma and decreased lung function [12]. The main cause of these health hazards is the inhalation of the foam dust that contains traces of isocyanates and contaminated fibers. Agricultural waste,
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