This study models and analyses the fabrication errors of an ultra-wideband (UWB) Schottky diode power detector using miniature hybrid-microwave integrated-circuit (MHMIC) technology. The fabricated balanced power detector is composed of two zero-bias GaAs Schottky diodes, a 90°-hybrid coupler, and two pairs of broadband butterfly open stub reflectors. The circuit is designed on a thin film ceramic substrate having a thickness of $127~\mu \text{m}$ with a $1~\mu \text{m}$ gold conductive layer, and a 20 nm Titanium Oxide ( $TiO_{2}$ ) resistive layer. The simulations use a computer model of the broadband coupler from on-wafer measurements to obtain an authentic fabrication error analysis. Moreover, the trade-off between the maximum efficiency and the fabrication error tolerance of the balanced power detectors is discussed. It is shown that the performance of the balanced power detector is dependent on different fabrication errors. Based on the measurement results, one of the fabricated detectors with minimum fabrication errors demonstrates a return loss of better than 10 dB over the entire frequency band of 60 to 90 GHz.