The brain–machine interface (BMI) is a new area of research and it is still in the development stage. The key component for the brain–machine interface is internal and external antennas. In this paper, we propose an implantable antenna for wireless brain signal sensing and monitoring using an inhomogeneous multi-layer model of the human head. A model with seven layers composed of skin, bone, fat, dura, cerebro spinal fluid (CSF), grey matter, and white matter was adopted for our multi-layer model. The antenna was embedded below the bone and above the dura of the head phantom. Artificial tissue emulating (ATE) materials were fabricated in semi-solid form and measurement was carried out to check the permittivity and loss tangent of each semi-solid ATE. Implanted antennas for wireless brain signal monitoring data must be compact, lightweight, and biocompatible. The proposed antenna was designed with Taconic RF-35 as the substrate with an overall size of $10\,\,\text{mm}\,\,\times\,\,10\,\,\text{mm}\,\,\times\,\,0.5\,\,\text{mm}^{3}$ . The proposed antenna has a −10 dB reflection bandwidth of 2.42-2.50 GHz and a gain of −25 dBi at the broadside direction. The top and bottom insulators with a thickness of 0.5 mm each were also designed with a Taconic RF-35 substrate. A good agreement between simulated and measured results was achieved for the proposed antenna for both in free space and inside of head phantom.