Bad boy bulbs
Techno Talk
Mark Nelson
Unwelcome interference is again our topic this month, but on a different kick. This time, the investigation
team performed splendidly, tracking down a hapless householder who was jeopardising the safety of the
8.85 million passengers who use Glasgow airport each year.
T
his month’s EMR nightmare
starts with the oldest, and you’d
think humblest of electrical items
– the lightbulb. Someone unwittingly
bought some light bulbs that should
never have been put on sale in the first
place, and which had defects that were
recognised 70 years ago.
Mission impossible?
Pilots flying aircraft in and out of Glasgow
airport complained they were suffering
radio blackouts when they were between
6,000 and 10,000 feet in the air.
Voice communication between aircrew and the controllers on the ground
was wiped out by mystery interference,
meaning crews were unable to hear vital
air traffic control messages. Locating and
identifying the source of the jamming
signal aloft was well-nigh impossible on
account of the height of the aircraft and
their flying speed. Finding the source
was a veritable needle-in-a-haystack
puzzle, but OFCOM’s investigators were
not easily deterred.
An ‘area of probability’ was narrowed
down by using flight-tracking software to
correlate the location of planes reporting
the issue with the corresponding location
on the ground. With this done, the quest
turned into a ground-level investigation
centred on a single town. This task involved using receivers aboard vehicles
and driving through the suspected area
until the interference was heard.
Old-fashioned footwork
Having pinned down the area where the
signal was strongest, the investigators
then used handheld equipment to cover the remainder of the search area on
foot. The team visited a number of likely
properties and they eventually located
the source. But what was jamming the
sensitive aircraft radios so effectively?
If you’re a regular reader of this column
(and if not, why not?), you may well be
screaming: ‘Oh no, it’s those blessed LED
light bulbs that hoot from DC to daylight’,
but you’d be wrong. In fact, the culprits
were squirrels – or rather their cages. Let
me explain, albeit in a roundabout way.
10
The problem with squirrels
First, let’s recall a very apposite statement made in 1905 by the philosopher
George Santayana in his book, The Life
of Reason. It read, ‘Those who cannot
remember the past are condemned to repeat it.’ The radio jamming at Glasgow
airport proves his point perfectly.
Back in 1905, the incandescent lamp
bulb was not particularly efficient and to
counter this, manufacturers were doing
their damnedest to squeeze the maximum
amount of light from a glowing filament.
One technique involved threading the
wire filament up and down the length of
the bulb multiple times in a circular fashion using a system of hooks and frames
until it resembled a squirrel cage. In those
days, keeping pet squirrels in cages was
not uncommon, although nowadays it’s
considered pretty cruel by most people.
After the sale of most incandescent
bulbs was phased out by European legislation, lamp manufacturers discovered
a loophole that allowed them to sell
‘decorative’ incandescent lamps, and the
squirrel cage bulb has returned. These
are usually made of tinted glass and have
the word ‘rustic’ in their description;
for example: https://amzn.to/32C1BTL
Get to the point!
OK, so what’s the connection between
radio jamming and Santayana? Precisely
this – OFCOM’s photo of the errant
lamp bulb that was wiping out air traffic control frequencies (118 to 121MHz,
remember this!) showed it was a squirrel cage ‘deco’ bulb. Electromagnetic
radiation does not occur with modern
incandescent bulbs that use short, coiled
filaments, but the multiple straight wires
used in squirrel cage lamps radiate like
mad and can affect FM radio transmissions (88 to 108MHz) and the nearby air
band (118 to 121MHz). The mechanism
is so deeply technical that I cannot paraphrase it in a sentence, but you can read
about it here: https://bit.ly/pe-jan21-fm
Back in the early 1950s this potential for
harmful interference was already recognised in America, where power companies
were changing out straight-filament bulbs
for consumers (read Popular Science
magazine, April 1953, https://bit.ly/pejan21-ps). Nobody cares today, though,
because those who do not remember the
past are condemned to repeat it.
Happy ending?
OFCOM confirms that the interference
at Glasgow was caused by four ‘vintage’
lightbulbs that the homeowner had recently bought online (not necessarily the
ones mentioned above). The house was
directly underneath the flightpath of the
aircraft and so every time a plane passed
and the lamps were in use, the air crew
suffered the interference. The bulbs were
replaced with non-interfering types and
checks with National Air Traffic Services
and aircraft operators confirm that the
area is now free of interference. OFCOM’s
spectrum enforcement team is following
up the case with the lightbulb suppliers,
to make sure no more dangerous bulbs
are sold to unwitting customers.
Tracking down the disturbance that
was not merely annoying but positively
life-threatening was a highly impressive
effort. This work, by telecoms regulator
OFCOM, confers great credit on their
spectrum assurance team, who found
the unknowing culprit.
So that’s all right then? Hardly. Squirrel
cage lamp bulbs are still sold widely;
as I write this article, eBay has 1,284
offers of them for sale, while Amazon
has more than 2,000. All manner of other online sellers and out-of-town DIY
sheds sell them as well. How many of
those bulbs have been tested for EMC
compatibility? Manufacturers – and
UK distributors – are responsible for
exercising due diligence regarding the
CE marking they place on their products, but how many of them are aware
of their legal responsibilities for public
safety? And what if they are made on
the other side of the world, where CE
merely signifies ‘Chinese export’? Local
trading standards authorities are responsible for enforcing regulations, but they
may not be aware of non-conformant
merchandise. So, as always – caveat
emptor – let the buyer beware!
Practical Electronics | January | 2021
