Hey. The standard flex antennas are flexible printed circuit boards. PCBs are made by laminating layers of material together, in this case a polyimide core, a top and bottom copper layer, and a top and bottom coverlay layer. Most of the implant antennas have white or black coverlay, but the standard is a semi transparent orange that really shows off what I’m talking about. The coverlay is important for corrosion resistance (because solder flux can be caustic and normally PCBs are out in the environment), and to reinforce the copper and prevent buckling.
The new ones are not necessarily thicker than the old copper wire antennas. They’re 0.26mm thick with a 0.2-0.4mm thick MOB package chip on top. The old ones used between 0.32-0.5mm copper wire with the mop package coplanar (not adding to the thickness). The copper of each layer on the new FPC is actually only 0.07112mm (2oz/ft^2) thick, so most of the strength is coming from the polyimide core and coverlay, aside from any durability tricks I’ve employed.
There are 2 for most flex antennas, top and bottom. If necessary for LF we could use a 4 layer FPC, but it hasn’t come up. Here this should help. Red is top and blue is bottom copper layers.
There are a lot of factors that influence how an RFID tag performs. The most important is “coupling factor” which is denoting how much of the magnetic field from the reader is making it through the center of the tag antenna and inducing power for the tag to use and modulate for communication.
If the tag is smaller then there’s less surface area in the center and less of the magnetic field will squeeze through there. The relationship is not linear, so performance increases dramatically with increased tag sizes (until they’re the same size as the reader).
The shape of the coil also affects the coupling. Most readers are large flat rectangles, so they work best with other large flat rectangles. This behavior is why there are orientation “tricks” you need to learn to get the most performance out of an implant. The x-series are the worst case scenario, because they’re not only tiny, but their antenna is a different shape than the reader so it will have an even harder time getting magnetic field through the coil.