Accuracy
Most S meters are not calibrated and in practice can only provide a relative measure of signal strength based on the receiver's AGC voltage. Some S meters are calibrated to read S9 for an input of -73 dBm but do not provide the correct 6 dB per S unit correspondence.
Often the correlation between a radio listener's qualitative impression of signal strength and the actual strength of the received signal is poor, because the receiver's AGC holds the audio output fairly constant despite changes in input signal strength.
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