Abstract: Wideband equiangular spiral antennas are typically applied to detection and tracing of passive radar seeker. In order to reduce the height of the equiangular spiral antenna, a low-profile slot equiangular spiral antenna fed by a microstrip to slotline and backed by a cavity is proposed. This microstrip to slotline Balun (unbalance to balance) transforms unbalanced electrical distribution produced by coaxial line into balanced electrical distribution to feed the slot equiangular spiral antenna. Measure results indicate that a wide band (1 : 9) of Voltage Standing Wave Ratio (VSWR), good radiation pattern and circular polarization are realized. The height of back cavity for a unidirectional radiation is 0.05L(L is the wavelength of lowest operation frequency). Lower working frequency band is expanded by filling the back cavity with a ring-shaped rectangular absorber. Measured results show that a frequency band of 1: 6.4 (voltage standing wave ratio VSWR below 2), better than 4 dB antenna gain, good circular polarization and radiation pattern are achieved. Planar feed structure and shallow cavity contribute to a low-profile slot equiangular antenna. Measured results verify the effectiveness of the microstrip to slotline Balun used to feed the slot equiangular slot antenna.