A coupling device for impedance matching a probe of a guided wave radar (GWR) system. A feed-through is for connecting to a coaxial cable or other transmission line connector that includes an inner conductor which connects to an output of a transceiver and an outer conductor that connects to an outer metal sleeve. A subwavelength coaxial transmission line (CTL) having a length from λ/5 to λ/2 is coupled to the feed-through including an inner conductor connected to the inner conductor of the feed-through and an outer conductor connected to the outer metal sleeve. A mode converter (MC) having a plurality of metal fingers (7) of length 2λ± twenty percent is connected to the outer conductor of the subwavelength CTL, where the MC includes a dielectric coating on its inner conductor connected to the inner conductor of the subwavelength CTL.