Antenna-coupled field-effect-transistors (FETs) offer high sensitivity for terahertz detection.Both the magnitude and the polarity of the response signal are sensitive to the localized terahertz field under the gate.The ability of accurate sensing the intensity patte is required for terahertz imaging systems.Here,we report artefacts in the intensity patte of a focused terahertz beam around 1 THz by scanning a silicon-lens and antenna coupled AlGaN/GaN high-electron-mobility-transistor (HEMT) detector.The origin of the image distortion is found to be connected with one of the antenna blocks by probing the localized photocurrents as a function of the beam location and the frequency.Although the exact distortion is found with our specific antenna design,we believe similar artefacts could be commonplace in antenna-coupled FET terahertz detectors when the beam spot becomes comparable with the antenna size.To eliminate such artefacts,new antenna designs are welcomed to achieve strong asymmetry in the terahertz field distribution under the gate while maintaining a more symmetric radiation patte for the whole antenna.