In order to reduce the minimum antenna measurement distance, the aperture reduction is needed. While the aperture reduction will cause the gain of the antenna reduced. The method of loading the elongated dielectric rod on the rectangular waveguide can remarkably improve the gain of the antenna without increasing the antenna aperture. A simulation study was carried out on the antenna with a dielectric rod loading covering 18–36 GHz. The dielectric rod is loaded on waveguide WR42 fed by the parallel coaxial. The CST simulation results show that the reflection coefficient of the antenna is lower than −10 dB in the frequency range of 18–36 GHz, the gain is stability in 10–16 dBi, and high efficiency is reached at high frequency, which is 95%.