This is the first post in a series called “Wireless Foundations”. After working with wireless for so long, I realized that I was too close to the subject. I recently taught a wireless workshop at Hackerfarm and realized that there are concepts I’m exposed to so often, I assume they’re common knowledge. Unfortunately, a lot of it isn’t common knowledge to people that aren’t deeply involved in the wireless industry so I figured that an article series on wireless starting from the beginning might be useful. Contrary to what many people think, you don’t need to be a PhD in physics or communications to understand how wireless works. Some basic principles are really all that are required.
One of the questions I get asked most often is “What’s the range of your wireless device”. This questions seems simple but gets into complex physics questions, some of them not even fully understood by science. My usual response is “it depends” but this time, rather than just taking the easy way out, I figured I’d explain why it depends.
Our journey starts with high school math, more specifically, trigonometry and a simple sine wave. A sine wave has three basic properties: frequency, amplitude, and phase. The frequency is the speed at which is repeats itself and is usually measured in Hertz or Hz for short. Amplitude is the strength or intensity of the wave and is a measure of the power the wave possesses. Phase indicates where the wave is in its periodic (or repeating) cycle. We’ll be dealing a lot with amplitude and frequency in the coming discussion, but remember that phase plays an important part, especially when it comes to stuffing information into the wave. We’ll discuss that more in the next post.
The fact that you’re able to see these words at all is due to visible light which are a bunch of sine waves with the same properties we’ve just mentioned. Visible light falls into a category of waves called electromagnetic waves and spans everything from low frequency wireless communications signals, visible and non-visible light, and all the way into high energy radiation like x-rays and gamma rays. For visible light, we perceive amplitude as the brightness and interestingly enough, the frequency is perceived as the color. It’s a bit unromantic to think that brilliant or beautiful colors we see are just specific frequencies of electromagnetic waves that fall into the narrow band our eyes are sensitive to.
Wireless communications also relies on electromagnetic waves and is created by passing a varying voltage through a wire. The varying voltage pushes and pulls electrons inside the wire which creates bands of electrons. Areas heavily populated with electrons become very negatively charged and areas with few electrons become very positively charged. This creates an oscillating electric and magnetic field which radiates outward from the wire and becomes what we know as a radio wave.
To create a wireless radio wave, all you really need to do is create a simple voltage oscillator. In electronics, there are many ways to do this and one of the simplest is with a single inductor and a few capacitors.
An example circuit like this form the basis of a Colpitts Oscillator which is just one way to create a voltage sine wave. When the voltage sine wave is passed through a wire resonator, or antenna, it will radiate outwards. Unfortunately, a sine wave isn’t very interesting on its own, at least from a communications perspective since it doesn’t carry any information. To actually transmit information, the wave needs to change somehow. Changing a wave to carry information is called “modulation” and that brings us to the next part in the series.