The simple S-unit

What use is an F-call? You're 5 and 9, or 20 over 9, or 5 and 5. It's a phrase that you'll hear regularly in amateur radio conversations as you tune up and down the bands. If we ignore for a moment the readability signal, the first number, in this case "5", which I have to confess is pretty arbitrary. My perfect readability is not going to be the same as yours. The deafer I am, the less likely you're going to get a readability score of "5", lets look at the second number. It's a signal strength. Pretty straight forward. It goes from S0 to S9 and sometimes there are extra decibels added, 10 dB over, or 20 dB over, etc. The S meter in your radio is actually a very sensitive micro ammeter. The dial displays in S-units. What is an S-unit? Well, until 1981 there wasn't a real standard. In the 1930's they'd decided that S9 means 50 microvolts at the input of the receiver, but there wasn't a standard impedance of 50 Ohm, which we take for granted today, so the number is pretty meaningless in terms of power received. In 1981 they defined it as 50 microvolts at the receiver's antenna assuming an input impedance of 50 Ohm. It gets better. The S9 is actually defined as as -73 dBm or decibel milliwatts, or 50.12 pico Watts. Each S-unit is 6dBm, so S8 is -79 dBm, or 12.6 Pico Watt, S5 is 0.2 Pico Watt. If that wasn't enough to make your head explode, radios are rarely calibrated, so one radio's S9 isn't the same as the next one's, worse still, not every radio uses 6 dB per S-unit, so S8 for one radio might be 6 dB, for the next it might be 6.5 dB. And I should add that the Automatic Gain Control in a radio affects the S-meter as well. When you next tell someone that they're 5 and 9, or 20 over 9, just be mindful that it's useful as an indication of what's happening between your station and theirs, but it's not anything that you could use as a definite resource in the future. If you want to read more, there is much to find online. Word of warning. When you read more, your head will explode more, what I've talked about here is grossly simplified and I've not even looked at the actual electronics side of things! Amateur Radio, the more you dig, the more you find. I'm Onno VK6FLAB