Buried in a list published by the UK's Ofcom (equivalent to USA's FCC) of potentially Wi-Fi offending household devices and appliances is the innocuous Fairy Light - a product that has become very popular at Christmas in the last few years. For some reason all the news media has glommed onto this tidbit to make a big deal out of it. The aforementioned list was issued as part of a promotion for Ofcom's new 'Wi-Fi Checker' smartphone app.
A search for exactly why these fairy lights might cause Wi-Fi signal degradation did not turn up any information about how they might generate EMI. If there is a switching AC-DC converter somewhere in the string, then that could be it, but the power level would have to be fairly low compared to the Wi-Fi signal. It could be that the possibly of a large and dense tangle of wires around the Christmas tree or around windows and doors is the electromagnetic wave's menace, but if that is so, then even the old fashioned C5, C7 and miniature incandescent light strings would be a problem as well. If I had a spectrum analyzer, I'd run a test to discover what the spectrum of fairy lights looks like.
Per Ofcom's website:
"The Ofcom Wi-Fi Checker, which runs on smartphones and tablets, allows consumers and businesses to discover the quality of their wireless internet signal wherever they live or work - as well as offering practical steps to help people get the best from their connection. Wireless broadband may not be working as well as it could in nearly six million UK homes and offices, according to Ofcom research published today. This is often caused by the Wi-Fi set-up in the house slowing down broadband. It could be down to something as simple as interference from other electronic devices, such as a microwave oven, baby monitor, a lamp - or even Christmas fairy lights."
Posted December 1, 2015