Let us learn a bit more on S Meter

 Radio amateurs are very fond of getting signal reports from whichever station they work. When I started off in 1985, I did not have any S meter in the Philips Prestige home radio which I used as a receiver for my ham radio activities along with a homebrew QRP transmitter. I used to give reports 'by the ear' like many other operators in those times. Those with commercial transceivers used to give S meter readings from their radios. Looking back, even that may not have been very accurate as many analog radios did not have a well calibrated S meter. Analogue S meter or signal strength meter was a sensitive microammeter with a full scale deflection of 50-100 microamperes. In 1930s, S9 corresponded to 50 microvolts at the input terminals of the receiver.


International Amateur Radio Union (IARU) Region 1 had brought out technical specifications for S meter readings in 1981. According to that, S9 on HF bands corresponded to -73 dBm. That would be 50 microvolts at the receiver's antenna input for a radio with 50 Ohms input impedance. For VHF bands, S9 was defined as -93 dBm, which was equivalent to 5 microvolts in 50 Ohms radios.

A difference of one S unit corresponds to a 6 dB difference, which was equivalent to a voltage ratio of two or power ratio of four. Signals stronger than S9 are mentioned as dB over S9 like "S9 + 20dB". Modern software defined radios have a more accurate S meter. Still, there could be differences between antennas of lower impedance and higher impedance. 

Now what is dBm? Decibels relative to a milliWatt is mentioned as dBm. One Watt would be 103 mW and hence expressed as 30 dBm. -30 dBm is 0.001 mW or 1 micro W. In that way, -73 dBm will be 0.0501 nano W and -93 dBm 0.5 pico W. Last two were mentioned in different units as the number of decimal points will be difficult to mention if mentioned in mW! Now you can understand why extreme values are better mentioned in the logarithmic unit of dB rather than with linear units.

With this background information in mind, we will have a look at the S-meter in my FT-710 radio. Signal strength in my VHF/UHF dual bander IC 2730 is shown as segments on the LCD rather than as S unit numbers.

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