I want to create flex PCB stickers that are cheap and you can just slap on stuff to reshape the field to couple better with implants.
I did some calculations about “on-board” capacitors that are just copper pours so we can create the stickers without needing to populate parts or add any z-height. My calculations indicate that two 1cm² copper plates on a 0.13mm thick polyimide flex PCB will create a capacitor of 22.5pF
C = ε0 * εr * (A/d)
1cm2 on 0.13mm thick polyimide:
C = 8.85⋅10−12 * 3.3 * (0.0001m2 / 0.00013m)
C = 22.5pF
It also scales linearly with the copper area, so this is definitely doable in a larger sticker form factor in parallel with some antenna traces to get a resonant repeater to reshape the field.
Unfortunately there’s less than 1cm² of area in the center of a flex implant so I’ll have to get crafty if I want to create a resonant repeater board small enough to be installed with the 5mm flex needle and work with the Coil on Module (CoM) credit cards.