A deeply subwavelength (~0.02?0 at 10 MHz) bandgap structure is proposed for suppressing the ground wave excited by HF radar antennas installed on a coastal sand land. The surface wave supported by the air/sand interface can no longer exist when the sand is substituted with the proposed structure. No guided-wave modes can be supported by the substitution bandgap structure. As a result, the radiation of the radar antenna is mostly toward the air region, and therefore its sky-wave efficiency can be enhanced significantly. For practical implementation, low-cost construction materials are used as the substrate of the proposed bandgap structure. Besides, the proposed structure is covered with a superstrate for stealth purpose. It is shown that the sky wave-to-ground wave ratio (SGR) of HF sky-wave radar can thus be enhanced, and the SGR enhancement reaches up to 4.67 times by adding the proposed bandgap structure in front of the radar antenna installed on the sand land. The design concept can also be applied to radar systems operating near the HF band, e.g., VHF radars.