Demonstration that power line communication
turns electrical lines into antennas
Any type of signal that
runs along an electrical wire will generate an electromagnetic field that
radiates outwards. The wire is
turned into a de facto antenna.
This can cause problems
of various kinds, such as interfering with radio reception and sometimes even
making electronic equipment malfunction.
It can also be a problem for people who are particularly sensitive to
these kinds of radiation.
The invention of systems for
sending communication signals along electrical wires has been met with
complaints from radio amateurs and broadcasters. Their concerns were that these power line communication or
power line carrier (PLC) systems would turn out to cause a lot of interference
as the electrical wires radiate the signals unintentionally.
The industry building the
PLC systems responded that “there is no problem”.
One of the objectors was
the British Broadcasting Corporation (BBC), which saw the PLC systems as a
threat to the reception of the BBC World Service programs. The BBC World Service is broadcast
worldwide, especially to countries which have censored media. These transmitters use the same
frequencies as some of the PLC operators, frequencies that are particularly useful
for long-distance radio broadcasts.
BBC research engineer
Jonathan Stott stated in his White Paper:
there
is the difficulty for radio-system users that the signals PLT [PLC] injects do
not simply travel from point to point along the wiring, they also escape as radiated emissions [emphasis in
original]
Met with refusal by the
PLC-industry to take the problem seriously, the BBC engineer Jonathan Stott and
his German colleague Dr. Markus Wehr came up with a simple demonstration of the
problem.
They used an
off-the-shelf home-networking PLC system called HomePlug. This system can be used to let
computers, printers, etc. be networked in a home, using the household wiring to
transfer the signals.
A HomePlug modem is connected to a
computer’s USB port and also plugged into a wall outlet. Each computer or printer has its own
HomePlug modem.
The HomePlug modems used
in this demonstration are powered by the USB port. They only use the wall outlet to communicate through, not to
power the modem. They do not check
whether or not there is any power available in the outlet, they just send out
their signals and listen for incoming signals. In other words, these HomePlug modems can work even if there
is no electricity in the outlets they are plugged into.
Instead of plugging the
HomePlug modem into a wall socket, Mr. Stott plugged it into a power
strip. He also inserted the plug
for a string of Christmas tree lights into the power strip. The power strip was not plugged into
the wall, so it had no power and the lights on the Christmas tree were not
on. There was no connection
between the HomePlug modem and the household wiring.
And yet, the HomePlug
network could freely communicate with another computer using another HomePlug
modem, which was plugged into a wall outlet.
The electrical wires in
the walls worked as a transmitting and receiving antenna for the one
computer. The wires for the
Christmas tree worked as transmitting and receiving antenna for the other
computer. There was no physical
connection. The communication was
wireless, even though it was not intended to be. This is an elegant way to visually demonstrate that the
wires are turned into unintentional antennas.
This demonstration is
described more briefly in: British
Broadcasting Corporation R&D White Paper WHP 099: PLT and broadcasting
— can they co-exist? by J. H. Stott, November 2004. It is available on http://www.bbc.co.uk/rd/pubs/whp/whp099.shtml.
The BBC produced a brief
video with this demonstration, which is available on: http://downloads.bbc.co.uk/rd/pubs/whp/whp-pdf-files/whp099-downloads/whp099-videofile/1.rm
Note that PLC is called
PLT (Power Line Transmission) in British terminology.
January 2012 (updated March
2013)