How does a waterfall display work?

Foundations of Amateur Radio With computers becoming more and more ensconced within the confines of our radio shack the variety of information available is increasing regularly. The introduction of a waterfall display has dramatically simplified the process of detecting what the activity level is on a particular band. If you've never seen a waterfall display, it's often a real-time, or nearly real-time display of radio activity. Leaving aside the mechanics of how this comes about, or how much you see, generally it's presented as a picture that changes over time. In reality it's a very compact way of showing a lot of information. You can think of it as a chart, showing the horizontal axis as frequency, the vertical axis as time and the colour as signal strength. So as you look from left to right you'll look at higher and higher frequencies. For example, the left side might be 7 MHz and the right side might be 7.3 MHz. Halfway along is 7.150 MHz. Similarly, now, as in zero seconds ago is at the top of the chart and 1 minute ago is lower. Depending on how fast you've set it to update the whole screen might represent 10 seconds, 10 minutes or 10 hours of information, entirely flexible, entirely configurable, entirely arbitrary. If you think of the colour black as having no signal strength and the colour red being maximum signal strength, then the brighter the colours, the more signal there is. A morse code signal might turn up as a series of dits and dahs running down the screen, with the oldest one being at the bottom and the newest one at the top. An AM signal might show up as a thick line with a bright colour, that's a high signal strength in the middle and lighter colours or low signal strength towards the edges. Every mode has its own visual characteristic and there are even modes that allow you to read information within a waterfall display. One of the other things you'll see in a waterfall display is strange artefacts, things like a diagonal line for example. If you think of what a diagonal line represents as a radio signal, it's something that has a strong signal at a particular time and frequency. A moment later it's changed frequency and a moment later it's done it again. The steepness of the line is dependent on two things, the speed that the frequency changes and the speed that the waterfall is updating. Before waterfall displays, the way you'd experience such a signal would be something that flashes up as a low to high swoop, or a high to low swoop, depending on your listening mode and the direction of the frequency change. So what is that signal? Well, it's likely to be something called an Ionospheric Sounder. It does what you think it does. Ping the ionosphere across multiple frequencies. The station doing this is listening for a return echo to see if the ionosphere is reflective for that particular frequency at that particular moment. The information can be used to create a map of what the ionosphere is doing right now, which in turn is used to figure out what frequency to use to make a contact. I should also mention that there is a signal identification wiki which shows and plays various identified and unidentified radio signals, hours of fun for the family. I'm Onno VK6FLAB