Spring 2008 Issue

FM/Repeaters—Inside Amateur Radio’s “Utility” Mode

How Many Decibels are in My S-meter?

By Bob Witte, KŘNR


Ever wonder how accurate the S-meter on your FM VHF transceiver really is? An S-unit equals 6 dB of signal change, but what is my meter really telling me? This time we’ll take a look at the S-meter characteristics of some typical FM radios.

Signal Reports

Amateur radio operators like to give and receive signal reports as part of their normal operating procedure. On the HF bands, during the first or second transmission it is normal to hear an RST (Readability, Signal Strength, and Tone) report. (“Tone” is used only for CW and RTTY, so on phone we just give the RS report.) The signal strength ranges from S1 to S9, corresponding to the reading on the receiver’s S-meter.

On FM VHF it is more common to hear a report in terms of receiver quieting. For example, a perfectly clear signal is “full quieting,” referring to how a strong FM signal pushes the noise down to zero. If there is a little noise present in the signal, you might get a report of “90% quieting.” A very noisy signal would be “50% quieting,” meaning that the received signal is about half noise.

However, we do have S-meters on most FM VHF rigs that we use to check signal strength. Through our normal operating, we get to know the weak spots in repeater coverage and how low the meter can go before we start to drop out of the repeater. The same thing is true on simplex. If another station’s signal is full scale, then you are likely to work him (or her) without any problem. If his signal barely moves the meter on your receiver, then you might have more trouble making the contact.

The Ideal S-meter

The ideal definition of the S-meter has the S9 level equal to –73 dBm or 50 µV. Each S unit corresponds to a 6-dB change in signal level. That is, a signal level of S8 should be 6 dB lower than S9, or a factor of 2 smaller in voltage (25 µV). Similarly, S7 should be another 6 dB smaller, or 12.5 µV. It is common for the S-meter to expand the scale to beyond S9, indicating “S9 + 20 dB” and “S9 + 40 dB.” The entire scale of the ideal S-meter is shown in Table 1.

No S-meter hits this scale exactly, but I have noticed that most FM transceivers are way off the mark. Through casual usage, it appeared that these meters are no where near the “6 dB per S unit” standard. This caused me to take a deeper look into the typical S-meter on a representative sample of FM VHF rigs.

The first thing you notice on today’s FM rigs is that they don’t really have “meters” in the traditional analog sense. They have a signal-strength indicator, normally a bar graph, as part of their LCD display. These bar graphs may or may not have the specific “S units” shown on the scale. This might be a clue that they don’t conform to the classic S-meter definition.

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