# S meter
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{{Short description|Radio signal strength indicator}}
{{distinguish|Field strength meter}}
thumb|right|238px|S-Meter of a Ten-Tec Orion amateur radio transceiver.
An '''S meter''' (signal strength meter) is an indicator often provided on [communications receiver](/source/communications_receiver)s, such as [amateur radio](/source/amateur_radio) or [shortwave](/source/shortwave) broadcast receivers. The scale markings are derived from a system of reporting signal strength from S1 to S9 as part of the [R-S-T system](/source/R-S-T_system). The term '''S unit''' refers to the amount of signal strength required to move an S meter indication from one marking to the next.
==Technical description==
Analogue S meters are actually sensitive [microammeter](/source/microammeter)s, with a full scale deflection of 50 to 100 μA. In AM receivers, the S meter can be connected to the main detector or use a separate detector at the final [IF](/source/intermediate_frequency) stage. This is the preferred method for [CW](/source/continuous_wave) and [SSB](/source/single_sideband) receivers. Another approach in the days of electronic tubes (valves) was to connect the S meter to the screen grid circuit of the final IF amplifier tube. A third option is to connect the S meter to the [AGC](/source/automatic_gain_control) line through a suitable level conversion circuit.[{{cite book |title=The Radio Amateur's Handbook |publisher=[American Radio Relay League](/source/American_Radio_Relay_League) |year=1978 |editor=Tony Dorbuck |location=Newington, CT |page=256}}]
In FM receivers, the S meter circuit must be connected to the IF chain before any limiter stages. Some specialized [integrated circuit](/source/integrated_circuit)s for FM reception like CA3089 and CA3189 provide a DC signal to drive a 100 μA S meter.[{{cite book |author=G.R. Jessop |title=VHF UHF manual |edition=4th |publisher=[Radio Society of Great Britain](/source/Radio_Society_of_Great_Britain) |year=1983 |location=Potters Bar, UK |pages=4.38 |isbn=0-900612-63-0}}]
== IARU Region 1 Technical Recommendation R.1==
thumb|right|LCD emulation of an S meter on the Ten Tec ''Jupiter'' transceiver
In the 1930s, it was already agreed that S9 corresponds to 50 μV at the input terminal of the receiver,[{{cite book |title=Introduction manual to the National HRO |year=1939 |publisher=National Company, Inc |location=Malden, Mass |page=6}}][{{cite book |title=The Radio Amateur's Handbook |publisher=[American Radio Relay League](/source/American_Radio_Relay_League) |year=1978 |editor=Tony Dorbuck |location=Newington, CT |page=255}}] but this was not a measure of the power received as the input impedance of receivers was not standardized.
The [International Amateur Radio Union](/source/International_Amateur_Radio_Union) (IARU) Region 1 agreed on a technical recommendation for S Meter calibration for [HF](/source/High_frequency) and [VHF](/source/Very_high_frequency)/[UHF](/source/Ultra_high_frequency) transceivers in 1981.[International Amateur Radio Union Region I (1981). [https://web.archive.org/web/20050316191147/http://www.algonet.se/~k-jarl/ssa/IARU/smeter.html IARU Region 1 Technical Recommendation R.1]. Brighton, England, UK.]
IARU Region 1 Technical Recommendation R.1 defines S9 for the HF bands to be a receiver input power of -73 [dBm](/source/dBm). This is a level of 50 [microvolts](/source/volt) at the receiver's antenna input '''assuming''' the input [impedance](/source/Electrical_impedance) of the receiver is 50 [ohms](/source/Ohm_(unit)).
For VHF bands the recommendation defines S9 to be a receiver input power of -93 [dBm](/source/dBm). This is the equivalent of 5 microvolts in 50 ohms.[{{cite book| publisher=[International Amateur Radio Union](/source/International_Amateur_Radio_Union) Region 1 |year=2021 |title=VHF Managers Handbook, Version 9.01 |editor=Jacques Verleijen |page=115 }}]
The recommendation defines that a difference of one S-unit corresponds to a difference of 6 [decibel](/source/decibel)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", or simply "20 over 9" (or even the simpler "20 over").
==Examples==
A weak signal with signal strength of S2 corresponds to received power of -115 [dBm](/source/dBm) or 0.40 microvolts [RMS](/source/Root_mean_square) in 50 ohms on HF.
A strong signal with signal strength of S8 corresponds to received power of -79 [dBm](/source/dBm) or 25 microvolts [RMS](/source/Root_mean_square) 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
{| class="wikitable"
|-
! S-reading
! HF
!
! Signal Generator emf
|-
!
! μV (rms, Relative to 50Ω)
! dBm
!dB above 1uV
|-
| S9+10 dB
| 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 on traditional analog receivers are not calibrated and in practice can only provide a relative measure of signal strength,[{{citation|website=qsl.net|title=What Does Your S-Meter Reading Mean ? (Two Receivers Compared)|first=Mark|last=Connelly|date=June 1, 2001|url=https://www.qsl.net/wa1ion/receiver/s_meter.htm|quote=[A] calibrated receiver can be used as a "transfer standard" with measurements that have real meaning, rather than just being somewhat-blind relative indications.}}] based on the receiver's [AGC](/source/Automatic_gain_control) voltage. Some S meters on traditional analog receivers are calibrated to read S9 for an input of -73 [dBm](/source/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 radio](/source/Software-defined_radio)s (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 signal strengths at any one frequency will remain meaningful, even if they are not from one frequency to another.
==See also==
* [Magic eye tube](/source/Magic_eye_tube)
* [VU meter](/source/VU_meter)
==References==
{{reflist}}
==Further reading==
{{refbegin}}
* {{cite magazine |magazine=[QST](/source/QST) |publisher = [American Radio Relay League](/source/American_Radio_Relay_League) |date=June 2015
| volume=99 |issue=6 |author=Doug DeMaw |title=What does my S-meter tell me? |issn=0033-4812 |pages=49–51}}
{{refend}}
==External links==
{{commons category|S meters}}
{{refbegin}}
*[http://www.seed-solutions.com/gregordy/Amateur%20Radio/Experimentation/SMeterBlues.htm# Measurements of amateur radio transceiver S Meter calibrations]
{{refend}}
Category:Receiver (radio)
Category:Amateur radio
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Adapted from the Wikipedia article [S meter](https://en.wikipedia.org/wiki/S_meter) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/S_meter?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.