Factors affecting the value of uncertainty in measurements of electromagnetic radiation of radio-electronic equipment in real-world conditions are summarized. The actual uncertainty in this case is never exhausted the instrumental and methodological components. The fundamental difficulty of expanded uncertainty determining is highlighted. The main factors affecting the accuracy of measurements and recommendations for reducing of measurement uncertainty are listed. It is proposed to apply the quality scale reliability in conjunction with the quantitative scale of uncertainty for electromagnetic radiation measured parameters in real-world conditions.