QotW: Why does phone signal vary so much?

Why doesn't my phone have constant signal when it doesn't move?...
14 January 2020

USING A PHONE

Hands holding a phone

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Question

Derek asks: "I can sit at my work desk, with my phone by my side, and the signal strength display goes from saying "no service" to 4 bars out of 5, within a couple of minutes, for no obvious reason. It varies like this all day, every day. I can understand why some places have good coverage, and others have bad coverage, but why should it vary so much in one place?"

Answer

Katie Haylor asked signal expert Ramsey Faragher from Focal Point Positioning and Cambridge University to dial up an answer to Derek’s question about wandering mobile phone bars...

Katie - When you dial up your buddy, a signal is being sent from the transmitter in your phone to a cell phone mast and from here it travels to the receiver on your friend's phone, facilitating communication. We asked signal expert Ramsey Faragher from Focal Point Positioning and Cambridge University to dial up an answer about those wandering mobile phone signal bars.

Ramsey - Signal fluctuations can typically be caused by two main effects. The first one is objects moving in and out of the line of sight between the phone and the radio tower, things like vehicles on the road outside. But if the signal is fluctuating quite rapidly, then the second effect is likely to be dominating here and that's called multipath interference. Because radio signals don't just pass completely through non-metallic objects, some of the energy is actually reflected as well, in a typical room that can be lots of copies of the same signal bouncing around. Often there's just one dominant loud signal still, but in Derek's case it's possible that in his room there is no main dominant signal and all of these different copies of the same signal are interfering with each other.

Katie - Just like water waves, radio waves have peaks and troughs. And if the signals come together at Derek's phone all nicely lined up, peak to peak, then the signal will actually boost a little. But if they align peak to trough, they can cancel each other out and the signal power can drop to next to nothing.

Ramsey - Signals can sometimes take quite complicated paths to get deep inside a building and it could be the movements of objects that are dozens of metres away from Derek that are actually causing these fluctuations he's seeing. The wavelength of radio signals used by mobile phones is about 20 centimetres. And so people and objects and vehicles that are moving around even just a few centimetres in the area around Derek, they can be what are causing these fluctuations he's seeing, as all of the radio signals that bounce off these objects vary in their path lengths by a few centimeters and sometimes boost and sometimes cancel each other out. So my top tip for Derek, if he's struggling to get his messages out, with his fluctuating signal strength, is to gently shake his phone. Just a few centimeters is enough and it will allow him to average through the multipath interference and it will increase his chances of getting that critical tweet out of the door.

Katie - Thank you Ramsey for beaming us that answer. Next time we're getting physical with this fertility question from Jure.

Jure - Hello Naked Scientists, I would like to know why a woman's body doesn't reject sperm as a foreign object.

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