The S-meter is an instrument present on the majority of radio receivers that measures the strength of the signal that is being received, and uses a special unit: the S-point. S-points are often used for RST reports.
S-points go from S1 to S9 and each S-point is defined as a 6 dB change in signal strength. This means that each time the voltage is halved (–6 dB) the signal strength decreases by one point. S9 is already a very strong signal, but to describe larger signals, steps of 10 dB are used instead of 6 dB, noted "S9+20" meaning 20 dB above S9.
Today two reference values exist: for frequencies below 30 MHz, S9 is defined as a voltage of 50 μV over 50 Ω at the receiver antenna connector; for frequencies above 30 MHz, S9 is defined as a voltage of 5 μV over 50 Ω at the receiver antenna connector. This refers to an unmodulated carrier signal (N0N) that uses almost no bandwidth; in case of real signals using a given bandwidth, this definition may not be enough since a smaller receiver bandwidth allows a weaker minimum detectable signal, but S-points are still a good tool for comparing received signals.
(Zc = 50 Ω)
|S1||–48 dB||0.20 μV||–14 dBμV||790 aW||–121 dBm|
|S2||–42 dB||0.40 μV||–8 dBμV||3.2 fW||–115 dBm|
|S3||–36 dB||0.79 μV||–2 dBμV||13 fW||–109 dBm|
|S4||–30 dB||1.6 μV||4 dBμV||50 fW||–103 dBm|
|S5||–24 dB||3.2 μV||10 dBμV||200 fW||–97 dBm|
|S6||–18 dB||6.3 μV||16 dBμV||790 fW||–91 dBm|
|S7||–12 dB||13 μV||22 dBμV||3.2 pW||–85 dBm|
|S8||–6 dB||25 μV||28 dBμV||13 pW||–79 dBm|
|S9||0 dB||50 μV||34 dBμV||50 pW||–73 dBm|
|S9+10||10 dB||160 μV||44 dBμV||500 pW||–63 dBm|
|S9+20||20 dB||500 μV||54 dBμV||5.0 nW||–53 dBm|
|S9+30||30 dB||1.6 mV||64 dBμV||50 nW||–43 dBm|
|S9+40||40 dB||5.0 mV||74 dBμV||500 nW||–33 dBm|
|S9+50||50 dB||16 mV||84 dBμV||5.0 μW||–23 dBm|
|S9+60||60 dB||50 mV||94 dBμV||50 μW||–13 dBm|
(Zc = 50 Ω)
|S1||–48 dB||20 nV||–34 dBμV||7.9 aW||–141 dBm|
|S2||–42 dB||40 nV||–28 dBμV||32 aW||–135 dBm|
|S3||–36 dB||79 nV||–22 dBμV||130 aW||–129 dBm|
|S4||–30 dB||160 nV||–16 dBμV||500 aW||–123 dBm|
|S5||–24 dB||320 nV||–10 dBμV||2.0 fW||–117 dBm|
|S6||–18 dB||630 nV||–4 dBμV||7.9 fW||–111 dBm|
|S7||–12 dB||1.3 μV||2 dBμV||32 fW||–105 dBm|
|S8||–6 dB||2.5 μV||8 dBμV||130 fW||–99 dBm|
|S9||0 dB||5.0 μV||14 dBμV||500 fW||–93 dBm|
|S9+10||10 dB||16 μV||24 dBμV||5.0 pW||–83 dBm|
|S9+20||20 dB||50 μV||34 dBμV||50 pW||–73 dBm|
|S9+30||30 dB||160 μV||44 dBμV||500 pW||–63 dBm|
|S9+40||40 dB||500 μV||54 dBμV||5.0 nW||–53 dBm|
|S9+50||50 dB||1.6 mV||64 dBμV||50 nW||–43 dBm|
|S9+60||60 dB||5.0 mV||74 dBμV||500 nW||–33 dBm|
Older receivers were calibrated using the old standard that defined S9 as a voltage of 100 μV instead of 50 μV over 50 Ω at the receiver antenna connector.
Usually S-meters in amateur radio equipment are not calibrated and are not very precise. S-meter readings may also vary from one band to another and it's always interesting to check an S-meter with a precise generator and a step by step attenuator.
|||Wolfgang Link, DL8FI. Metodi di misura per radioamatori. Franco Muzzio & C. editore, 1978, sezione 3.9.|
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