Diversity - a forgotten radio concept

Diversity - a forgotten radio concept. So much of Radio Frequency (RF) performance was a dark art when there were still lots of people that understood it. It is not then surprising that 20 years into a world of radio modules offering IEEE standards with an easy ‘AT Command’ set of instructions that the level of understanding on RF may have reached a new low. Many have forgotten that Bluetooth, WiFi and even cellular itself are even radio systems, especially since they took antenna off mobile phones.

It is easy to overlook that the mobile signal that your phone is receiving may actually be taking a complex path, bouncing off passing buses, the foil insulation in the wall, the glass panel in your meeting room before reaching your phone. Usually all of the above at the same time. As all of these tangle of signals phase in and out it creates dead spots where only a step away reception is loud and clear. Subconsciously we know this and often wander around a room while on the phone hunting for that magic spot where you really do get good reception. For a few moments at least.

But in Machine to Machine (M2M) we don’t have the luxury of moving the unit around in real time to optimise its position. In M2M that’s why antenna performance becomes critical in the overall performance of the unit- the M2M kit is very rarely placed with RF in mind. It is where it needs to be because of all the other factors- power, security or just ‘the only space left’. The antenna needs to make the most out whatever radio reception is available.

It’s why Mobius won’t sell airtime to somebody using USB Dongles. The antenna is such low gain and usually fairly limited directional performance (as well as the consumer nature of the electronics inside) that there are simply too many issues to overcome to make the system reliable.

So having an external antenna is a really important start. When Mobius is then asked why this is a better terminal rather than something cheaper, part of the answer is often that it supports a Diversity Antenna. So not only does it have a good external antenna but it gives you the option of adding another second antenna a critical distance apart, catching the radio wave at a different point in the cycle. It allows the terminal to unscramble all of those reflections and build one ‘good’ wave from the different signals at the two points. In short a Diversity antenna may work when a single one cant.

So up until the point M2M can get up and walk around the room looking for a better signal, it is worth looking at the hardware and asking how can I make the most of what is there at any given moment. Using Diversity Antennas, like using Mobius airtime, is another piece of the puzzle in building a more reliable M2M system. 

Written by Douglas Gilmour

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