Smart Wearables
The substrate is the antenna. Sit on a felt-backed patch and the permittivity rises, the resonance moves, and the link budget goes with it.
— mm
Resonant dimension including the fringing correction
Compare the compression shift against the bandwidth: if the shift is a large fraction of it, the antenna will not survive being worn, and no amount of matching at the bench will fix that. The transmission-line model used here is quick and approximate — it ignores the ground plane size, the feed and the human body underneath, and a body at a few millimetres both detunes the patch and absorbs a large share of its radiated power. Moisture is the other uncontrolled variable, since a damp substrate has a materially higher permittivity and loss than a dry one. Confirm on a vector network analyser with the antenna on a phantom, not in free space.
Wearable Antenna Fabric Permittivity & Patch Dimensions — free, with the formula and a worked example, at Textile School.