S-meter and signal strength


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, like "S9+20" meaning 20 dB above S9.

S-meter

Today two references values exist: for frequencies below 30 MHz, S9 is defined as a voltage of 50 uV over 50 Ohms at the receiver antenna connector; for frequencies above 30 MHz, S9 is defined as a voltage of 5 uV over 50 Ohms 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.


S-points for frequencies below 30 MHz:

Signal
strength
Relative
intensity
Received
voltage
Received power
(Zc = 50 Ohm)
S1 -48 dB 0.20 uV -14 dBuV 790 aW -121 dBm
S2 -42 dB 0.40 uV -8 dBuV 3.2 fW -115 dBm
S3 -36 dB 0.79 uV -2 dBuV 13 fW -109 dBm
S4 -30 dB 1.6 uV 4 dBuV 50 fW -103 dBm
S5 -24 dB 3.2 uV 10 dBuV 200 fW -97 dBm
S6 -18 dB 6.3 uV 16 dBuV 790 fW -91 dBm
S7 -12 dB 13 uV 22 dBuV 3.2 pW -85 dBm
S8 -6 dB 25 uV 28 dBuV 13 pW -79 dBm
S9 0 dB 50 uV 34 dBuV 50 pW -73 dBm
S9+10 10 dB 160 uV 44 dBuV 500 pW -63 dBm
S9+20 20 dB 500 uV 54 dBuV 5.0 nW -53 dBm
S9+30 30 dB 1.6 mV 64 dBuV 50 nW -43 dBm
S9+40 40 dB 5.0 mV 74 dBuV 500 nW -33 dBm
S9+50 50 dB 16 mV 84 dBuV 5.0 uW -23 dBm
S9+60 60 dB 50 mV 94 dBuV 50 uW -13 dBm

S-points for frequencies above 30 MHz:

Signal
strength
Relative
intensity
Received
voltage
Received power
(Zc = 50 Ohm)
S1 -48 dB 20 nV -34 dBuV 7.9 aW -141 dBm
S2 -42 dB 40 nV -28 dBuV 32 aW -135 dBm
S3 -36 dB 79 nV -22 dBuV 130 aW -129 dBm
S4 -30 dB 160 nV -16 dBuV 500 aW -123 dBm
S5 -24 dB 320 nV -10 dBuV 2.0 fW -117 dBm
S6 -18 dB 630 nV -4 dBuV 7.9 fW -111 dBm
S7 -12 dB 1.3 uV 2 dBuV 32 fW -105 dBm
S8 -6 dB 2.5 uV 8 dBuV 130 fW -99 dBm
S9 0 dB 5.0 uV 14 dBuV 500 fW -93 dBm
S9+10 10 dB 16 uV 24 dBuV 5.0 pW -83 dBm
S9+20 20 dB 50 uV 34 dBuV 50 pW -73 dBm
S9+30 30 dB 160 uV 44 dBuV 500 pW -63 dBm
S9+40 40 dB 500 uV 54 dBuV 5.0 nW -53 dBm
S9+50 50 dB 1.6 mV 64 dBuV 50 nW -43 dBm
S9+60 60 dB 5.0 mV 74 dBuV 500 nW -33 dBm

Older receivers were calibrated using the old standard that defined S9 as a voltage of 100 uV instead of 50 uV over 50 Ohms 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.


Home Electronics Index