We consider the quantum limits of the radiometer sensitivity in power and spectral density measurements of the radio emission. It is shown that taking into account zero-point field fluctuations makes it possible to resolve some paradoxes in the theory of an ideal radiometer. We estimate the limiting temperature resolution achieved when the entire spectrum of thermal radio emission is integrated. The sensitivity of a detector radiometer is analyzed with allowance for the thermal noise of the nonlinear element. The results of calculations are compared with the experimental data.