How accurate are those S meter reports we are given?Do S meters accurately report our signal strength, or are S meters just guess meters?

In the early days every manufacturer seemed to have their own standard.

In 1954 the Boonton Radio Corporation published an article in “THE NOTEBOOK” Fall 1954 Number 3 an article titled “Signal generator and Radio impedance” it stated that the sensitivity of a radio receiver is NOT the number of microvolts applied directly to the input terminals of a receiver to produce a standard output, even though this is frequently assumed to be the case. The Institute of Radio Engineers has defined the INPUT SENSITIVITY OF A RECEIVER as the number of microvolts required to produce standard output when applied through a dummy antenna having the characteristic impedance of the antenna with which the receiver is intended to operate, to the input terminals of the receiver.

The article goes on to talk about the source of energy from the combined system of antenna and receiver, explaining that like the open source electromotive force of a battery this voltage is  available to us only in series with the internal impedance of the power source.

It is also worth noting that a specification of the number of microvolts at the input of a receiver is not a specification of power received, due to the fact that input impedance of receivers is not standardized and could vary wildly.

Many radio manufacturers over the years set their own standard for the calibration of S Meters, National Radio Co calibrate S meters so that S9 indicates 50 microvolts input signal to the receiver. The SWAN Corporation set S9 as 100 microvolts at 50 ohms at 14MHz.

The R.L Drake Co used 50 microvolt to determine S9 but each S unit was only 5dB.

The Collins radio 75A-4 handbook sets 100 microvolts for S9.

The Collins specs for the 75S-3B however lists the input impedance as 50 ohms nominal +/- 50%. That means the input impedance can be anywhere between 25 and 75 ohms.

Popular electronics in April 1966 stated that the standard for S meters at Collins Radio is approximately 100 microvolts at 50 ohms through a 6 db pad for S9.

Looking at the handbook for the original Yaesu FT101 it outlines the S meter adjustment and states “Apply 50 microvolts and set the meter for 25dB over S9.”

It is probably worth mentioning at this point that a signal generator with an output impedance of 50 ohms when set to 50 microvolts and left unterminated the voltage at the output terminals will be 100 microvolts, connect a 50 ohm load and that voltage will indeed be 50 microvolts. By now you will realize that very few of the radio manufacturer’s specifications mention whether they are specifying unterminated EMF or terminated voltage.

Because of all this confusion in 1981 the International Amateur Radio Union the IARU decided to recommend an S meter system for signal strength reporting on the amateur bands.

Their recommendation states:

a) One S point corresponds to a level difference of 6dB

b) On the bands below 30MHz a meter deviation of S-9 corresponds to an available power of a CW signal generator connected to the receiver input terminals, of -73dBm.

c) On bands above 30 MHz this power shall be -93dBm.

I guess it was hoped that manufacturers would adopt this recommendation.

In a great majority of receivers the S meter is simply a meter measuring the AGC voltage and while S 9 may easily be set to a predetermined input level, the ability to guarantee a linear 6 db per S point is not so easily achieved.

Below is a table with values for S meter readings giving required levels in dBm and microvolts RMS relative to 50 ohms, the two right columns are measured values for my Icom IC-7300 the last column the deviation in dB from the previous S point.

S9 is correct requiring an input level of -73dBm however the S unit steps are on average 3dB per step not 6dB as per the IARU specification. This will result in actual readings of lower level S points being very much understated.

I carried out the same tests with my Apache Anan G2 using Thetis software. The Thetis software accurately conformed to the IARU specifications with all S points within 1 dB of the specification.

People using the Thetis software on Apache Anan equipment or on the Hermes Lite will have no doubt noticed that the background noise level presented by the software is always higher than the noise level shown on many of their other traditional radios. Actual noise S 5 and below will not even move the Icom IC-7300 meter off zero.

So the signal reports you receive will probably be a rough indication of your strength and will likely never be an accurate reading of the actual received signal, reports from those using SDR’s running Thetis or Power SDR software may be some of the exceptions.

So a report of five by nine or nine plus generally means you are load and clear and nothing much more than that.