A radiometer operating between 50 MHz and 26.5 GHz and consisting of microwave channels all with a low-noise amplifier (LNA) was established. Microwave switches were used to select an appropriate channel according to measurement frequency. The effect of variations both in input and output reflection coefficients on LNA gain was investigated. To keep the change in the gain of the LNA at minimum, the LNA output was terminated with a load whose impedance was equal to that of matched load. This technique gives the opportunity of full-band noise measurements of noise sources with one total power radiometer. The performance of the overall radiometer was tested. The difference in excess noise ratio (ENR) between the experimental results and actual values was found between -0.32 dB and 0.30 dB with an expanded uncertainty between 0.13 dB and 0.34 dB (k = 2). The results illustrate the validity of the full-band radiometer for determining ENR.