Abstract: In order to reduce the size and gain of the high power microwave measurement receiving antenna, a small-sized expanded coaxial line antenna operating at L-band has been designed. The structure optimization was achieved by theoretical analysis and numerical calculation. The simulation shows that, with the frequency varying from 1.3 to 1.6 GHz, the gain increases from -2.0 to 0.8 dBi linearly. The maximal radiation direction is in the axial direction of the antennas physical structure. The gain and the radiation pattern of the antenna were calibrated with the Agilent E8362B network analyzer, and the calibrated results agree with the simulation ones. The gain increases from -2.3 to 1.2 dBi in the same frequency range. The width of the radiation beam is more than 60, and the axial ratio is more than 35 dB.The results show that the design can satisfy the requirements of low gain and small size for the L-band high power microwave measurement.