A triple-band low frequency ultra-compact metamaterial absorber (MA) is proposed, which is composed of a periodic array of 3D structures constructed in lossy FR4-epoxy dielectric slab, and another ultra-thin grounded dielectric substrate is attached to its bottom. The unit cell can be counted as a combiner of three 3D magnetic resonators, which have similar resonant mechanisms and can all be ultra-compact in size. By adjusting structural parameters of the unit cell, the proposed MA can perform well in absorptions at each low resonant frequency. For example, one MA with unit cell width 10.92 mm exhibits three absorption peaks at 2.151 GHz, 2.283 GHz, and 2.378 GHz with the corresponding absorptivities of 96.5%, 96.8%, and 99.6%, respectively; for the three low frequencies, the width of the unit cell is only 7.8%, 8.3%, or 8.6% of the free space wavelength, respectively. Furthermore, the three absorption peaks can be lowered to 1.220 GHz, 1.281 GHz, and 1.343 GHz when the relative permittivity of the ultra-thin grounded substrate is increased to 9.8. Finally, one waveguide experiment was conducted to verify the proposed MA effectively.

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