The implantable antennas designed for MICS band have gain in the range of -10 dB to -45 dB. Due to the restrictions given by FCC, the amount of power delivered to antenna is also less.
I do not know much about [implantable] antennas, but I can guess that they are probably very electrically small. Gain is a combination of antenna efficiency and antenna directivity. Small antennas have both low directivity and poor efficiency. We need wide beams from large antennas to get high directivity. The efficiency of small antennas tends to be low because they are a poor impedance match.
I agree with Raymond. If an antenna is less than 1/6 wavelength across it will be difficult to match, and either mismatch losses or resistive losses in the matching circuit will make it inefficient. Also because it is small it will have a low-directivity pattern with a maximum directivity of about 1.5, so any efficiency less than 60% will result in negative gain. In addition to this the body may resistively load the reactive near-field of the antenna, further reducing its efficiency.
Antennas much less than 1/6 wavelength across can have very bad efficiency, like 1% or less, even with good design, and without being surrounded by lossy flesh!
the substrate is very lossy . the substrate has conductivity of 10 S/m and this is the main reason for -ve gain and low efficiency
I agree with Nessim, the poor gain is mainly due to the huge absorption of the biological material arround the antenna.
Since the implantable antennas are miniaturised in size gain should be minimum to give sustainability
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