How does Single Side Band work?

Foundations of Amateur Radio

A little while ago I spent some time discussing how to test if your radio was on frequency. It generated lots of comment and email with various suggestions on other ways to do this test, but it also caused one listener to ask the question, what's this Upper Side Band and Lower Side Band thing you're talking about?

In the past I've discussed the history of these two, but I've gone back to check and it doesn't appear that I've ever actually explained what exactly Upper Side Band and Lower Side Band might be and how they work and more to the point, why they're important.

Let's start where you find these modes. In amateur radio, some bands use Upper Side Band and some use Lower. From a usage perspective it's pretty straightforward, but not obvious. Essentially everyone uses Upper Side Band all the time, except radio amateurs below 10 MHz. There is one exception in that, the 60m band - 5 MHz - uses Upper Side Band.

The mechanics aside, what is the point, how does it work and why does it matter?

If you've ever seen an AM broadcast via a waterfall display or on a spectrum analyser you'll have seen a symmetrical picture with a big spike in the middle. The spike in the middle is the carrier and the two sides are duplicate copies of each other.

If you were to do some math, you'd discover that the spike accounts for 50% of the energy that's embedded within the AM signal and you'll realise that doubling the other halves takes care of the other 50% of the energy.

If you eliminate both the spike and one half, you end up consuming 25% of the original AM signal - in terms of energy. That essentially means that you can now spend all of that available energy in your transmission and in effect get a signal that's four times stronger than the original AM signal. A better way to say that is, Single Side Band is four times as efficient as an AM signal.

Now if you took the right half of the signal, you'd end up with an Upper Side Band signal, and if you took the left half of the signal, you'd end up with a Lower Side Band signal. The signals are identical, but they're reversed.

From a technical perspective, the Upper Side Band signal represents your audio from left to right. Low, or base frequencies on the left and high or treble frequencies on the right. A Lower Side Band signal reverses that, which is why a voice sounds unintelligible if you get Upper Side Band and Lower Side Band mixed up.

The alignment of the radio to a specific frequency works because you can map the audio frequency directly to the tuning frequency. That might not be immediately obvious, but let's imagine an Upper Side Band signal at 10 MHz.

At exactly 10 MHz, the audio frequency of 0 Hz is represented, at 10.001 MHz the audio of 1 kHz is represented and at 10.002 MHz, the audio of 2 kHz is represented. If your radio is off frequency by say 50 Hz, then the sound you'll hear will be off by 50 Hz across all of those. So 10.001 MHz won't sound like 1 kHz, it will sound like 950 Hz and 2 kHz will sound like 1950 Hz.

On the other side, if you flip to Lower Side Band, 1 kHz will sound like 1050 Hz and 2 kHz will sound like 2050 Hz.

Upper and Lower Side Band, nifty solution, better signals, less bandwidth use and all in all a great way to play with radio.

Remember, everyone uses Upper Side Band all the time, except for radio amateurs below 10 MHz but not on 5 MHz.

I'm Onno VK6FLAB