Bill Buchanan - That Pesky dBm!

I love wireless (wi-fi) communications. In fact, I did my PhD around the propagation of radio waves using Maxwell’s equations. The beauty and perfection of radio waves will never leave me. The first thing you often learn about wifi is how the frequency of the wave relates to its wavelength (lambda=speed of light divided by the frequency) and how dipole antennas have to be around half a wavelength long. For AM, there are long antennas (such as, with the ones that wrap copper around a core) or can be short ones (like the dipole antenna on your wireless router). Much of the magic happens around 2.4GHz.. and which gives a wavelength of 12.5cm, and where if you measure the dipole antenna, it will be around 6cm high. Once you learn about this, you are often hooked on the wonderment of radio waves. It has solid mathematics, but is also a black art (ask any RF engineer, and they will tell you this)! Overall, too, wi-fi has freed us from those pesky twisted pair of cables and those troublesome RJ45 and RJ11 connectors. And, at the core of wifi, is signal strength, and where the stronger the signal, the more chance we have of creating a good network connection. For this, with most IEEE 802.11x standards, the bandwidth that you can use often relates to the signal strength that you have — so the further away you are from the transmitter, the more likely it is that you will have a lower bandwidth capacity. I, too, love all the different antenna shapes and designs and try to imagine how they spread their signals. But those pesky metal things get in the way and can bounce signals in other directions (which is sometimes a good thing, of course), and the other materials, such as concrete, will reduce the signal strength. For all the maths of Maxwell’s equations, a lot comes down to measurements and simulations. At home, you might have a MIMO (Multiple In, Multiple Out) transmitter, and which bounces signals of objects and transmits on multiple channels. This might give up to 540Mbps. But, the further you go away from this, the bandwidth reduces until it will drop to nearer 11 Mbps. And, so RSSI (Receiver Signal Strength Indicator) is an important measurement as it defines how good your signal strength is — at a point in time. This will obviously vary as you move and as other things move around you. But, at the other end, if you have too much signal strength, you can breach health and safety regulations. Currently, this is around 100mW, and you need to have a good reason if you need higher power levels than this, as too much radio power — especially around 2.4GHz — might affect someone’s health. So, let's talk about that troublesome (and powerful) unit called dBm, and where it is all about adding and subtracting, and not those difficult maths operations of multiplying and dividing. Believe in John Napier's logs to help our wifi systems: