We have designed a low‐loss dielectric sample holder for positioning very thin, metallic, semiconductor samples in a TE102cavity for ESR studies at liquid‐helium temperatures. The sample holder can be rotated ±15° from the center position, allowing accurate positioning of the high‐conductivity sample at minimum electric field. This results in maximizing theQfactor and allowing symmetric excitation on both faces of the sample. We have also devised a method of setting the microwave phase to desired value to better than 0.3° by utilizing the hyperfine lines of a dilute As‐doped Si sample.