Dispersive delay lines are disclosed. A dispersive delay line can include a piezoelectric substrate having a first interdigital transducer electrode on a first region of the piezoelectric substrate and a second interdigital transducer electrode on a second region of the piezoelectric substrate. The dispersive delay line is arranged such that an acoustic wave is configured to propagate from the first interdigital transducer electrode to the second interdigital transducer electrode though a third region of the piezoelectric substrate. The third region of the piezoelectric substrate is configured as a waveguide and can have a thickness of less than half the wavelength of the acoustic wave. Related radio frequency modules, wireless communications devices, and methods are disclosed.