S meter
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An S meter (signal strength meter) is an indicator often provided on communications receivers, such as amateur radio receivers or shortwave broadcast receivers. The scale markings are derived from a subjective system of reporting signal strength from S1 to S9 as part of the RST code. The term S unit can be used to refer to the amount of signal strength required to move an S meter indication from one marking to the next.
[edit] Accuracy
The International Amateur Radio Union (IARU) Region 1 agreed on a standard for S Meter calibration for HF and VHF/UHF transceivers in 1981.[1] This defines S9 to be equivalent to an input level of 50 microvolts for the HF bands at the receiver's antenna input, typically at 50 ohms impedance and 5 microvolts for the VHF/UHF bands. According to this guideline, each S-unit corresponds to a difference of 6 decibels (dB), equivalent to a voltage ratio of two, or power ratio of four. Thus, a signal strength of S8 corresponds to approximately 25 microvolts on HF. Signals stronger than S9 are given with an additional dB rating, thus "S9 + 20dB".
S meters frequently appear to be calibrated in absolute terms, but in practice can only provide a relative measure of signal strength based on the receiver's AGC voltage. Furthermore, the correlation between a radio listener's qualitative impression of signal strength and the actual strength of the received signal is often poor, because the receiver's AGC holds the audio output fairly constant despite changes in input signal strength.
[edit] External links
[edit] References
- ^ International Amateur Radio Union Region I (1981). IARU Region 1 Technical Recommendation R.1. Brighton, England, UK.
