Systems and methods for making a marker. The methods comprise: obtaining a marker housing having first and second cavities formed therein; disposing a first resonator in the first cavity and a second resonator in a second cavity; and placing a bias element at a location on or in the marker so that the first and second resonators are (a) equally spaced apart from the same bias element and (b) biased by the same bias element when the marker is in use to oscillate at a frequency of a received transmit burst.