Abstract In this study, a novel frequency beam‐scanning (FBS) antenna based on composite substrate integrated waveguide/spoof surface plasmon polariton (SIW/SSPP) waveguide is proposed. The composite SIW/SSPP slow‐wave waveguide is designed by embedding a series of periodic subwavelength H‐shaped grooves on the top surface of the SIW. Its transmission characteristics own both the features from SIW and SSPP, and can be flexibly mansplained by tuning the key structural parameters. To convert the propagating composit…