What is an S-Meter? The Science Behind an S9 Signal

If you've ever listened to amateur radio operators talking on the air, you've likely heard them give signal reports like "You are 5 and 9!" The "9" refers to the reading on the receiver's S-Meter (Signal Strength Meter). But what does an S9 signal actually mean in terms of RF engineering?

According to the International Amateur Radio Union (IARU) Region 1 standard for HF (High Frequency) bands below 30 MHz, an S9 signal represents a receiver input power of exactly -73 dBm, which corresponds to an electromotive force of 50 microvolts (μV) across a 50-ohm antenna input. Each "S-unit" below S9 represents a 6 dB difference in signal power. To quickly make sense of these complex logarithmic relations, we highly recommend checking out the comprehensive dBm to Watt Conversion Table on our main toolkit page for a quick reference on receiver noise floors and transmitter power levels.