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A compact broadband Gysel power divider with arbitrary power-dividing ratio is proposed. This divider is derived from the conventional Gysel power divider by substituting an ideal phase inverter for the 180° phase shifter, which significantly improves the bandwidth and reduces the circumference to 1 λg evaluated at the center frequency. A wide range of power-dividing ratio (1-6.25) is supported by changing the impedance ratio of the transmission lines, while the dual-band impedance matching characteristic with frequency ratio (1-2.9) is obtained only by varying the impedances of the transmission lines (20-135 Ω). Closed-form equations are derived for design parameters. The ideal phase inverter is realized by two transitions: microstrip-to-slotline and slotline-to-microstrip transitions. By utilizing this phase inverter, two microstrip Gysel dividers with power-dividing ratios of 1:1 and 2:1 and operating at 1.5/2.5 and 1/2 GHz, respectively, are designed, fabricated, and measured. The measured results are in good agreement with the simulated ones. The first measured results show a bandwidth of 80% with better than 15-dB return loss and 29-dB isolation from 1.19 to 2.76 GHz. The second measured results show a bandwidth of 95.8% with better than 12-dB return loss and 24-dB isolation from 0.8 to 2.27 GHz. To the authors' knowledge, the most wideband Gysel power divider using microstrip technology is proposed for the first time.