the simpler "20 over"). Examples A weak signal with signal strength of S2 corresponds to received power of -115 dBm or 0.40 microvolts RMS in 50 ohms on HF. A strong signal with signal strength of S8 corresponds to received power of -79 dBm or 25 microvolts RMS in 50 ohms on HF. Some signal generators are calibrated in dB above 1uV and have an output in emf. For example to set an HF receiver's S-reading to S9 set the signal generator output to 34 dB above 1uV Accuracy Most S meters on traditional analog receivers 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 on traditional analog receivers 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 on an analog receiver is poor, because the receiver's AGC holds the audio output fairly constant despite changes in input signal strength. Software Defined Radios (SDRs) acquire and process signals differently, and determine S-readings by direct measurement of RF signal amplitude. Consequently, many SDR systems with bit depths of 14-bits or more are accurately calibrated from one end of the S scale to the other right out of the box. In cases where this is not so, a few minutes with a signal generator to set the reference level are all that is required. Low bit depth SDRs such as an 8-bit design can be somewhat accurate, but as they distinguish much coarser differences in input levels, precision at the low end of the S scale will suffer. Even with a high quality SDR, it is worth keeping in mind that the S meter is measuring the strength of the signal at the 50 ohm input of the receiver, not at the antenna. For example, if the radio's input is 50 ohms, but the antenna's impedance is significantly higher, power transfer from the antenna into the radio will suffer, and signal levels will be lower than if they were fed to an input with a matching high impedance. Many antennas vary in impedance over various frequency ranges, particularly in the case of wideband designs. What is useful to know is that the relative