Transponders are disclosed that are configurable to operate in an HDX mode and/or an FDX-B mode and that use a switchable load impedance across the transponders' resonant antenna circuit to induce amplitude and/or frequency modulation. One embodiment of the invention includes an inductive antenna including two terminals connected to tag circuitry, where the inductive antenna forms a resonant antenna circuit tuned to resonate at the frequency of the activation signal. The tag circuitry and the resonant antenna circuitry are configurable as an oscillator, the tag circuitry is configured to be powered by electric current induced in the resonant antenna circuit, includes at least one switchable load impedance connected across the antenna resonant circuit, configured to amplitude modulate the activation signal appearing at the resonant antenna circuit terminals using a switchable load impedance, and to frequency shift key modulate the oscillator signal appearing at the resonant antenna circuit terminals.