S-Units and S-Meters
The “S” in S-unit or S-meter stands for “(signal) Strength”. Originally used in the early 20th century as part of the RST reporting code, this was a subjective assessment of the received signal, based on the following 9 point scale.
- Faint signal, barely perceptible
- Very weak
- Fairly good
- Moderately strong
- Very strong signals
The term “S-unit” refers to the amount of signal change between two adjacent points on the scale.
As technology improved, radios were fitted with signal strength meters, allowing quick and easy estimation of signal strength. People began to rely more and more on the S-meter, rather than the original reporting code. But this created a problem, because each receiver was different and manufacturers couldn’t agree on the calibration.
Over the years there have been many unsuccessful attempts to create a standard for S-meters, but any attempt to assign standard signal levels to the above table were doomed to failure because it is a subjective scale, affected by many variables.
For example, the background noise level varies from one band to the next, and on the HF bands may vary with time of day, season, and sunspot activity. On VHF bands, where background noise is not usually a significant factor, it is variations in receiver sensitivity that affect the way a signal’s strength is perceived. Differences in aerials, feeder losses and local interference also affect the subjective signal strength.
Thus a signal voltage which is assessed as “barely perceptible” on a noisy HF band, with an inefficient aerial and severe local interference, may be perceived higher on the scale if a directional aerial is used, due to the relative reduction in noise and interference, even if the absolute signal level is lower! The same signal voltage may be perceived as “good” on a quiet VHF band.
Nevertheless, an imperfect standard is better than no standard, so in 1981 the International Amateur Radio Union (IARU) Region 1 agreed on a technical recommendation for S Meter calibration for HF and VHF/UHF transceivers. The calibrations are different because HF signals are usually limited by “atmospherics”, whereas VHF signals are only limted by the much lower thermal noise.
IARU Region 1 Technical Recommendation R.1 defines S9 for the HF bands to be a receiver input power of -73 dBm. This is a level of 50 microvolts at the receiver’s antenna input assuming the input impedance of the receiver is 50 ohms.
For VHF bands it defines S9 to be a receiver input power of -93 dBm. This is the equivalent of 5 microvolts in 50 ohms.
The recommendation defines a difference of one S-unit corresponds to a signal difference of 6 decibels (dB), equivalent to a voltage ratio of two, or power ratio of four.
Signals stronger than S9 are expressed in decibels above the S9 level, e.g. “S9 + 20dB”, or “20 over 9”.
The recommendation is quantified in the following table:
Modern FM receivers are unlikely to resolve intelligible voice at signal levels below -129 dBm, and many fall short of that figure. So S3 is roughly the minimum intelligible signal threshold for NBFM. This doesn’t agree with the old subjective scale.
SSB receivers can resolve much lower signal strengths, hence the positioning of S1 at -141 dBm.
The above table defines a standard to which very few, if any, receivers comply. Usually the S unit steps deviate significantly from the ideal 6dB, and may vary across the range, typically between 1dB and 5dB per unit.
Even in the range above S9, the steps rarely correspond with the meter markings. Unless the precise calibration is known, it is difficult to use such a device to accurately compare the gains of two aerials for example.
There are several reasons why manufacturers don’t stick to the IARU recommendation. In many cases, the S-meter is more or less an afterthought, the signal being sampled from a convenient point in the circuitry, which does not give the desired range or logarithmic response. And manufacturers are keen to make their receivers look more sensitive than their competitors, by displaying higher S-meter readings for a given signal.
Some radios have switchable pre-amplifiers, but these are of dubious merit on VHF. Visually they make a few S points difference, but rarely make any difference to the intelligibility of weak signals, as they boost the signal and noise equally.
In practice, the S-meters of all but “professional grade” receivers are next to useless, unless the precise calibration is known. Reports of “S7” or “two S-points improvement” are fairly meaningless as measurements, without additional information. The same signal will more than likely display a wildly different S-reading on different receivers. It would be far better to calibrate the S-meter and give reports in dBm.
There are many IC’s capable of producing a logarithmic RSSI (Received Signal Strength Indication) with a 120 dB range. Some rigs now have an RSSI display, calibrated in dBm.