What is the first bit of information we typically give or receive when we begin an on-air QSO? It’s the signal report, of course. Sure, we’ll talk about the weather, brag about our stations, and share our location. But first, we want to know what kind of signal we are getting to the other operator’s radio and vice-versa.
Most receivers and transceivers have some variety of S-meter. It can be a single-needle analog meter, a bar graph style indicator using LEDs, an LCD display, or even a graphic image on a screen. In many cases, the meter can also multi-task to measure other parameters like RF power, ALC, and SWR.
Its purpose is to indicate the relative strength of signals passing through your receiver. S-meters are not intended to be absolute value measuring instruments, according to most radio manufacturers, as there are so many factors that can affect meter readings.
Let’s say Kenny Woods was rag-chewing with his buddies on the local 6m net. They compare signal reports. Skip Longpath, who lives about two miles away, says Kenny’s signal is 30 dB over S9. Morris Kode, several blocks away, says Kenny’s signal is only S9.
Could one of these guys be wrong or have a bad receiver? Maybe, or maybe not. Even if these guys lived in the same neighborhood and were using the same transceiver model and the same type of antenna, they still might not get the same readings. Why? It’s all relative.
Technical Stuff
Without doing a deep-dive into electronic circuits, here’s where an S-meter fits into the receive chain.
Old-school analog S-meters are sensitive microammeters, generally with a full scale deflection of 50 to 100 μA. Their modern LED/LCD display counterparts have a similar range. In AM receivers, the S-meter can be connected to the main detector or use a separate detector at the final IF stage, which is the preferred method for CW and SSB receivers.
A more common approach is to connect the S-meter to the AGC line through a level conversion circuit. In FM receivers, the S-meter circuit must be connected to the IF chain before any limiter stages. Some specialized integrated circuits for FM reception provide a DC signal to drive an S-meter.
The newer SDRs (Software Defined Radios) process signals differently and determine S-readings by directly measuring RF signal amplitude. Many SDR systems with bit depths of 14-bits or more are accurate from one end of the S-scale to the other right out of the box. They produce more consistent readings than conventional superhet receivers.
Accuracy
S-meters show the relative strength of a signal on your receiver. Of course, with CW and SSB signals, the S-meter will be bouncing around a bit, so it is subject to some interpretation. More important, signal levels can vary from radio to radio and one band to another.
S-meters are fairly accurate at the S9 level because this is the calibration point used at the factory. There are two reference values used for calibration approved by ITU/IARU. For frequencies below 30 MHZ, S9 is defined as a voltage of 50 μV over 50 Ω (or -73dBm) 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.
Each S-unit reflects a +/-6 dB change in signal strength. For example, when 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, such as S9+20, meaning 20 dB above S9.
Most of the currently popular amateur radio HF rigs (other than SDRs), as well as rigs that were sold over the last decade or two, are only calibrated at the S9 point of the scale. This leaves some room for error. As the meter moves above or below S9, the accuracy diminishes. Usually the readings are reasonably acceptable between S6 and S9, but most readings below S5 are off. Above S9, the accuracy can also suffer.
Though readings may not agree from one radio to another, one thing is certain. The stronger the signal, the higher the meter reading.
RST Signal Reports and S-Meters
Back in 1934 when the RST system was implemented, most receivers did not have S-meters. Those that did had widely varying readings. This was the reason an operator’s ears were used to determine the appropriate Readability and Signal Strength numbers.
In today’s world, it’s unusual to find a transceiver without some kind of S-meter. And if the meter is there, many hams are tempted to use the meter reading as the reported strength. It seems logical–RST signal strength numbers are on a scale of 1-9 and S-meters also have readings from 1-9. Very convenient . . .
Then there’s the practice of ignoring S-meters and the proper use of RST. It’s common for DX and contest stations to give out rubber stamp signal reports. They’re trying to work as many stations as fast as possible and don’t want to be bothered with accurate signal reports, so everyone gets a 59 or 599. All you know for sure is that the DX station copied your signal.
Influencing S-Meter Readings
Here’s a short list of things that can affect S-meter readings:
AGC: Often, there’s little correlation between a radio listener’s impression of signal strength and the actual strength of the received signal on an analog receiver. The receiver’s AGC holds the audio output fairly constant despite changes in input signal strength.
Preamp: Turning on the preamp can make both the signal and noise floor level rise and the S-meter reading will go up. Adding the attenuator will do the exact opposite.
RF Gain: On many rigs if you turn down the RF gain, the S-meter reading goes up, even though the strength of the signal coming in hasn’t changed.
Propagation: The meter reading can be more an indication of the path and band conditions. A station signal is one factor but only a small part of the equation when propagation is the dominant factor.
RX Filter: The S-meter is showing you the total RF power inside of the filter. The larger the filter, the more RF power and the higher the S-meter reading.
General Relativity
It doesn’t take an Einstein to realize there are imperfections in S-meter readings. Remember, they are only a guide as to signal strength since there’s often no consistency in S-meter calibration among different receivers. Most S-meters on traditional analog receivers can only provide a relative measure of signal strength based on the receiver’s AGC voltage. Some S-meters on traditional analog receivers are calibrated to read S9 for an input of -73 dBm but may not reflect the correct 6 dB increase per S-unit.
Also, the accuracy of a signal report, including an S-meter reading, is still dependent on the operator and his/her interpretation. Is that report an average of the peaks or a momentary peak on that rapidly moving meter? If I’m not even moving the meter, could it be that the op on the other end runs with the pre-amp off, or is my signal really weak? And there are even more parameters that affect my signal strength at the receiving end–things like antennas and propagation.
Your S-meter may be inaccurate and nowhere near the IARU’s recommendations, but it is inaccurate, non-standard and non-linear for everybody. Just don’t try to compare the S-meter reading on your rig with your buddy’s TS-890. They won’t necessarily be the same.
So here is the good news. S-meters are completely accurate–as long as you’re using them to give relative signal strength reports.
