Yes you can design a cylindrical antenna that would couple better with our x-series implantable transponders. However, doing so is not as straightforward as you might think. Unlike an electric field emitting antenna like an FRS radio or cell phone or wifi hot spot, a passive RFID antenna is simply a magnetic field inductor, or a loop antenna similar to AM radio. It involves both a wire coil as well as an accompanying parallel and series capacitive elements that help create and maintain resonant tuning at the desired operating frequency. Simply replacing the wire coil part of the "antenna" will require obtaining the capacitive values already designed into the reader and creating a complimentary coil for those values, or somehow replacing the entire antenna including capacitive elements.