Lower and Upper Side Band, why is it so?

Foundations of Amateur Radio Today we enjoy radio using all manner of different so-called modes. The ones that most people are familiar with are FM and AM. In fact digital radio DAB+ is another example of a mode. In Amateur Radio we have a few more to play with, Single Side Band, or SSB, countless other digital modes, CW, or Carrier Wave are all different approaches to getting information from one place to another. If you have a radio that uses SSB, you'll soon notice that there are two versions of SSB, something called LSB and something called USB, or Lower and Upper Side band. If you tune around the bands you'll soon notice that some stations are using Lower Side Band and others are using Upper Side Band and there seems to be no rhyme or reason to it. Actually, there is a method to the madness. First of all, commercial and military HF radio all use Upper Side Band all the time. Radio Amateurs are a bit more traditional and we have the following basic convention. For voice signals on frequencies lower than 10 MHz, we use Lower Side Band, and for signals above 10 MHz we use Upper Side Band. There are exceptions to this. RTTY, one of the digital modes uses Lower Side Band regardless of frequency and all the other digital modes use Upper Side Band. Confused yet? Here's another way to think of it. Everyone uses Upper Side Band, except for RTTY and Voice below 10 MHz on Amateur Radio. Why are we doing this cookey thing to ourselves in Amateur Radio? First thing to note is that it's not random, we didn't just wake up one morning with this idea and said, lets do that. Side Band was first figured out mathematically in 1914. A year later it was made into reality by John Carson who used it to carry more long distance telephone calls across the AT&T phone system. There were on-air experiments and in 1933 the ARRL board instructed the technical staff of QST magazine to investigate the feasibility of single side band carrier-less phone transmission on amateur frequencies. There is lots to read about this in the January 2003 edition of QST magazine, if you're interested, it's a fascinating read. From an engineering perspective, radios built during the birth of Side Band, or Single Side Band Suppressed Carrier, to give its full name, used different methods to create a side band signal. One method was to filter out the part of the side band you didn't want, the other was to use phasing to add or subtract two signals and create a side band signal. Creating a filter was hard, creating a phase difference was much simpler to achieve. Now, one of the effects of using this method of making a side band signal was that you had a place where the signals would add and another where they would subtract. Where was this place you ask? One guess. 10 MHz anyone? So, for these radios built during the birth of side band, Upper and Lower Side Band came as a side-effect of creating a simple and reliable system to make the signal. Today we have alternatives which make this 10 MHz magic spot pretty much obsolete, but there are still 1951 Collins 75A-1 radios on air today and we like to talk to each other, so why fix something that isn't broken? That's why we use Upper and Lower Side Band. Remember, everything uses Upper Side Band, except RTTY and Amateur Radio phone below 10 MHz. I'm Onno VK6FLAB