This paper presents a design of an OOK (On-Off Keying) envelope detector with one and a half octave bandwidth in UHF band, along with the analysis on the third-order intermodulation (IMD3) performance. A Schottky diode is employed for high-speed switching, while T and Pi networks are used for the wideband matching at UHF frequency range. To minimize ripples within the bandwidth, capacitor and resistor combination is accurately modeled and optimized at the output stage for an input resistance of 50Ω and the high output resistance. To evaluate the performance of the designed OOK receiver, mathematical analysis in the frequency domain is conducted to verify its modulation effect on the envelope signal throughout the conversion. In order to further assess the performance of the diode envelope detector, two-tone signal is applied to analyze the modulation bandwidth and IMD3 of the envelope signal through nonlinear diode and low-pass filter (LPF). As a result, the bandwidth of the detector shows 340MHz ~ 980MHz, and its IMD3 is - 44.39 dBc which is better than the typical value of -40dBc for RFID communication systems.