Table 2.

Selected published articles that detected and characterised various solid materials. MUT, Material Under Test; IDC, Interdigital Capacitor; CSRR, Complementary Split Ring Resonator.

Ref.	Technique	Substrate Material *	Freq (GHz)	Size (mm)	Tested Material (MUT)	Remark
[39]	Splitter/combiner microstrip with IDCs and 2 CSRR structures	Fabricated on Rogers5880 ${ϵ}_r$ = 2.20 h = 1 mm t = 0.018 mm	5.6	N/A	Viz, PVC, glass epoxy, FR4, glass	The measured complex permittivity of the test specimens is found to be in close agreement with their values available in the literature with a maximum error of ≤5%. Disadvantages: Low electric field distribution (2.667 × ${10}^3$) v/m
[33]	Metamaterial SRR	Fabricated on FR4glass epoxy board ${ϵ}_r$ = 3.583 h = 1.5 mm	3.57	8 × 8	Calibration: plastic, Perspex, glass epoxy, glass Extracted: Rice flour, wheat flour, all-purpose flour, corn flour, green gram flour, gram flour, soya flour	Advantages: -Potential application in the field of food preservation and quality checking -Detecting real permittivity Disadvantage: -Low permittivity detection -Low accuracy -No weighing of flour samples -Loss tangent not determined
[34]	SRR	Fabricated on RT/duroid 6006 h = 1.27-mm	2.5	80 × 40	Viz, Teflon, polyvinyl chloride, Plexiglas, polyethylene, carbonyl iron, Ni${}_{0.6}$Co${}_{0.4}$Fe${}_2$O${}_4$, cobalt, 30% polystyrene composite	The measured relative permeability and the relative permittivity of the test specimens are found to be in close agreement with their values available in literature with a maximum error of less than 8%.
[46]	Two spiral inductors and IDC	Fabricated on FR4 h = 1.6 mm	2.2–2.8	44 × 24	Magnetic, soft cobalt steel (SAE 1018), ferrite core rubber, plastic, wood, white marble	Experimentally, it is found that complex permeability and permittivity measurement is possible with an average error of 2%.
[178]	A modified ring resonator	Fabricated on RT/duroid 5880 ${ϵ}_r$ = 2.2 h = 1.58 mm $tan\delta$ = 0.001	3.2	28 × 4	Peat soil, sand soil	The model has within a 5% error in dielectric estimation with the commercial dielectric probe for peat soil (≥28% m.c.) and sand soil (≥10% m.c.).
[38]	A metamaterial planar sensor	Fabricated on FR4 h = 0.8 mm	5.3–8.2	30 × 30	Benzene, ethyl acetate, hexane, N-pentane, polyethylene, PVC, Teflon, THF	For solid and hazardous liquid samples. Disadvantages: -Limited permittivity detection from 1 to 10 real permittivity.
[25]	Microstrip ring resonator	Fabricated on Roger5880 ${ϵ}_r$ = 2.2 h = 787 um $tan\delta$ = 0.0009 t = 17.5 um	1	141 × 87.5	Air, meat	The maximum discrepancy is about 6% only. Disadvantages: -Simulated and measured results have a discrepancy, which is observed for the loss magnitude (around 15%).

* h = thickness, t = copper thickness, ${ϵ}_r$ = dielectric constant, and $tan\delta$ = loss tangent. N/A = Not Available.