Methods for mode identification and accurate measurements of the unloaded Q ($Q_0$) of a dielectric-loaded $TE_{01{delta}}$ mode cavity resonator with HTS endplates are proposed. A resonator with a sapphire rod and $YBa_2Cu_3O_{7-x}$(YBCO) endplates was prepared and its microwave properties were studied at temperatures above 30 K. The $TE_{01{delta}}$ mode $Q_0$ of the resonator, designed to work as a tunable resonator with variations in the gap distance (s) between the sapphire rod and the top YBCO, was more than 1000000 at s = 0 mm and at 30 K with the resonant frequency of 19.56 GHz. The $TE_{01{delta}}$ mode $Q_0$ decreases as s increases for s < 2 mm until mode couplings between the $TE_{01{delta}}$ mode and other modes appeared at s = 2 mm. Significant dependence of the $TE_{01{delta}}$ mode $Q_0$ on the input and output coupling constants was also observed. Applications of the open-ended $TE_{01{delta}}$ mode cavity resonator for a tunable resonator with a very high Q as well as a characterization tool for the surface resistance measurements of HTS films are described.