This paper presents an ultra-wideband out-of-phase filtering power divider based on sector-shaped spoof surface plasmon polariton (SSPP) unit cells. By employing a microstrip-to-slotline transition, the odd-mode SSPP is effectively excited, enabling ultra-wideband operation with an approximately 180 \(^{\circ }\) phase difference between the two output ports. The proposed filtering power divider exhibits enhanced slow-wave characteristics and a compact circuit size. The dispersion and transmission characteristics of the sector-shaped SSPP unit cell are investigated and compared with those of a conventional rectangular SSPP unit cell under identical transverse dimensions, demonstrating a stronger slow-wave effect. Based on these characteristics, a theoretical model of the ultra-wideband out-of-phase filtering power divider is established, followed by fabrication and experimental validation. The measured results show that the proposed power divider achieves an approximately 180 \(^{\circ }\) phase difference over a wide frequency range from 1.34 GHz to 5.23 GHz, corresponding to a fractional bandwidth of 118.4%, while maintaining low insertion loss and good return loss. With a compact size of only 0.78 \(\lambda _g^2\) and simple implementation, the proposed design exhibits strong potential for SSPP-based power dividers and other passive microwave devices.
