Flexible Electromagnetic Bandgap for Millimetre-Wave Wearable Antennas

Electromagnetic bandgap (EBG) is a class of high-impedance surfaces (HIS), which can suppress wave propagations within a certain frequency range. Since wearable antennas are intended to function close to human bodies, it is important that their performance will not be significantly altered by the effects of body proximity, and the amount of backward radiation towards the body is minimised. This project designs a flexible electromagnetic bandgap (EBG) for millimetre-wave (mm-wave) wearable antennas operating from 28 to 38 GHz. The designed EBG has the following advantages:

· In-phase reflection and surface-wave suppression to enhance the radiation, and efficiency of antennas, respectively;

· Comfortable to wear as it is made of soft conductors and fabric substrate;

· Relatively insensitive to the proximity of human bodies;

Materials and fabrication process

Polyester fabric and adhesive thin copper foil are used as the substrate, and conductive material, respectively, for both the antenna and EBG. A piece of copper foil is firstly adhered to the polyester fabric. The desired conductive pattern is then laser-cut and the excess copper removed, as shown in Figure 1.

Figure 1. Laser-cut conductive patterns from copper foil.

EBG design and prototype

The EBG can generally be modelled as a LC resonant circuit, whose L and C values are determined by the geometry and dimensions of the unit cell. Figure 2 shows the design of one unit cell of EBG and its equivalent circuit. Figure 3 shows a prototype of the proposed EBG with 3×3 unit-cells fabricated on a polyester fabric substrate, and an EBG-backed co-planar waveguide (CPW) antenna mounted on an end-launch connector.

Figure 2. Proposed EBG: (a) geometry; and (b) equivalent circuit of one unit cell.

Figure 3. (a) EBG prototype with 3×3 unit-cells; (b) EBG-backed CPW antenna

On-body performance

The performance of EBG-backed CPW antenna is evaluated on a human wearing a cotton sweatshirt and denim jeans. Three typical on-body positions are selected for this experiment: chest, forearm, and thigh. The measured reflection coefficient (S₁₁) at all three positions, and the radiation pattern when the antenna is attached to the forearm, is shown in Figure 4, and Figure 5, respectively. It can be observed that there is only a slight distortion in the radiation pattern for on-body scenario, which proves that the antenna with proposed EBG is not highly sensitive to human body proximity.

Figure 4. Measured S11 of the EBG-backed CPW antenna on human body

Figure 5. Measured radiation pattern of the EBG-back CPW antenna in free space (dashed line) and on-body (solid line) at 28 GHz

Details of our work can be found here in the IEEE Antennas and Wireless Propagation Letters. For further information, please contact: Assoc. Prof. Boon-Chong Seet (boon-chong.seet@aut.ac.nz).