S meter
Encyclopedia
Communications receiver
A communications receiver is a type of radio receiver used as a component of a radio communication link.-Features:Commercial communications receivers are characterised by high stability and reliability of performance, and are generally adapted for remote control and monitoring...
s, such as amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
receivers or shortwave
Shortwave
Shortwave radio refers to the upper MF and all of the HF portion of the radio spectrum, between 1,800–30,000 kHz. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m which marked the original upper limit of the medium frequency band first used...
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
RST code
The RST code is used by amateur radio operators, shortwave listeners, and other radio hobbyists to exchange information about the quality of a radio signal being received. The code is a three digit number, with one digit each for conveying an assessment of the signal's readability, strength, and...
. 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.
IARU Region 1 Technical Recommendation R.1
The International Amateur Radio UnionInternational Amateur Radio Union
The International Amateur Radio Union is an international confederation of national amateur radio organisations that allows a forum for common matters of concern and collectively represents matters to the International Telecommunication Union...
(IARU) Region 1 agreed on a technical recommendation for S Meter calibration for HF
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
and VHF
Very high frequency
Very high frequency is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency , and the next higher frequencies are known as Ultra high frequency...
/UHF
Ultra high frequency
Ultra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
transceivers in 1981.
IARU Region 1 Technical Recommendation R.1 defines S9 for the HF bands to be a receiver input power of -73 dBm
DBm
dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...
. This is a level of 50 microvolts
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
at the receiver's antenna input assuming the input impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
of the receiver is 50 ohms.
For VHF bands the recommendation defines S9 to be a receiver input power of -93 dBm
DBm
dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...
. This is the equivalent of 5 microvolts in 50 ohms.
The recommendation defines a difference of one S-unit corresponds to a difference of 6 decibel
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
s (dB), equivalent to a voltage ratio of two, or power ratio of four.
Signals stronger than S9 are given with an additional dB rating, thus "S9 + 20dB", or, verbally, "20 decibels over S9".
Examples
A weak signal with signal strength of S2 corresponds to received power of -115 dBmDBm
dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...
or 0.40 microvolts in 50 ohms on HF.
A strong signal with signal strength of S8 corresponds to received power of -79 dBm
DBm
dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...
or 25 microvolts 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
| S-reading | HF | Signal Generator emf | |
|---|---|---|---|
| μV (50Ω) | dBm | dB above 1uV | |
| S9+10dB | 160.0 | -63 | 44 |
| S9 | 50.2 | -73 | 34 |
| S8 | 25.1 | -79 | 28 |
| S7 | 12.6 | -85 | 22 |
| S6 | 6.3 | -91 | 16 |
| S5 | 3.2 | -97 | 10 |
| S4 | 1.6 | -103 | 4 |
| S3 | 0.8 | -109 | -2 |
| S2 | 0.4 | -115 | -8 |
| S1 | 0.2 | -121 | -14 |
Accuracy
Most S meters are not calibrated and in practice can only provide a relative measure of signal strength based on the receiver's AGCAutomatic gain control
Automatic gain control is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels...
voltage. Some S meters are calibrated to read S9 for an input of -73 dBm
DBm
dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...
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.