Four-Element Miniaturized Implantable MIMO Antenna Design for Scalp Biotelemetry

This work presents a four-element multiple-input multiple-output (MIMO) implantable antenna for 915 MHz biotelemetry, built on a Rogers RO3010 substrate with overall dimensions of only 10×10×0.1 mm3. Getting the size this small required a semi-hexagon meandered slot geometry, which extends the electrical path considerably without touching the outer footprint. The four antenna elements are placed on the four sides of the substrate with 90° rotational symmetry over complete ground plane. Isolation was the main challenge (without any decoupling structure, port-to-port isolation was just 13.94 dB). Adding a row of through-substrate vias along the diagonal, together with the naturally low surface-wave activity of the 0.1 mm substrate, pushed this to 31.8 dB. The antenna covers 0.83–0.992 GHz (162 MHz bandwidth), which is enough to include the full 915 MHz ISM band. In simulation and in measurement, each element gives a realized gain of −26.3 dBi inside a head tissue model; the measured figure is −28.1 dBi, the small difference being consistent with fabrication and setup losses. Experimental work used minced meat as a tissue phantom. The ECC stays under 0.1 across the band, giving a diversity gain above 9.96 dB. A link budget analysis confirms the antenna can maintain a positive link margin at practical operating distances. Given its very small volume (10 mm3) size and very high isolation value (31.8 dB), this design is a practical option for compact implantable MIMO systems.