Spectrum analyzer is a widely used signal analysis instrument. It can be used to measure the signal frequency, level, waveform distortion, noise level, spectral characteristics, etc..
The standard antenna can also be used to measure the field strength. Level conversion and impedance matching problem. The actual on - line measurement is the measurement of the induced voltage of the standard antenna. Therefore, as long as the frequency meter readings into the voltage unit, coupled with the antenna antenna coefficient can be measured to be measured field strength.