In this paper, an eight-way substrate integrated waveguide unequal power divider with an enhanced co-phase bandwidth is presented, and demonstrated a potential application for the Ku-band radiometer antennas. The proposed divider design is based on the −35 dB Chebyshev beam-shaping algorithm to achieve at minimum −25 dB of antenna side-lobe level (SLL) and high beam efficiency (>90%). Investigations on different unequal power splitting techniques are discussed, and an efficient method for the unequal power division is proposed, exhibiting improved co-phase bandwidth and wideband matching. The proposed unequal eight-way power divider demonstrates a wider co-phase bandwidth of 10% within 10° phase error and the −15 dB return loss bandwidth of 36.8%. Furthermore, the proposed unequal divider is integrated with eight longitudinal slot radiators, designed at 18.7 GHz and exhibits low radiation loss (better than −1.3 dB). The antenna measurement results exhibit a symmetrical fan beam radiation pattern and satisfied the goal of −25 dB SLL, resulting high beam efficiency of 97% and gain of 13.45 dBi. Therefore, the proposed unequal power divider technique shows the potential to be utilised in the low SLL antenna array designs for an accurate brightness temperature measurement of radiometer systems. The application of this technique can be further extended to radar and satellite/wireless communications.