Carrier Current

*Profile
 Carrier current transmitters are devices that transmit sound over
 powered lines (usually electrical mains) by modulating a carrier
 frequency (the 60 Hz carrier wave present on all electrical lines,
 if its connected to a power line) with audio. The signal is sent
 down the line to be picked up by a specially designed receiver. If
 you leaf through enough gadget catalogs, you're bound to see a
 carrier current transmitter marketed as some magic product that
 "lets you turn any power outlet in your home into a phone jack!".
 What the catalog fails to tell you is that everyone else served by[Image]
 the same pole transformer (usually everyone else on the block) can
 turn their power outlet into your phone jack too. These devices
 are known as carrier current telephones. Some carrier current
 products (usually wireless intercoms) have several variations on
 the same model operating at different frequencies in order to
 combat the 'crossed wires' effect. Carrier current surveillance
 transmitters are almost exclusively hidden in plug-in devices,
 such as lamps or surge protectors.

Some commonly found carrier current frequencies:

9 kHz - 490 kHz Carrier Current 47 CFR 15.219 Auth (250 mv max.)
490 kHz - 1.705 MHz Lossy Cable 47 CFR 15.221 Auth (1000 mv max.)
450 kHz - 30.00 MHz PLA Systems 47 CFR 15.207 Auth (30 mv@ft/30ft max.)
3 - 200 kHz 300 mw High Grade Pro Bugs (over 500$ each)
100 - 200 kHz 50-100 mw Older VLF Bugs
120 - 200 kHz 30-50 mw Pre 1990 intercoms
200 - 300 kHz 30-50 mw Post 1990 intercoms
300 - 400 kHz 250-450 mw TELCO Line transmitters (355 kHz popular)

*Detection

Carrier current devices can be detected as standard wire line devices with
a TDR or oscilloscope, or with a carrier current receiver (which can be
purchased from <http://www.martykaiser.com>). Using an oscilloscope to test
for carrier current devices calls for connecting the scope and checking for
any RF on the line other than the 60 hz sine wave thats present on all
power lines. There's been some discussion of using commercially available
carrier current receivers (from carrier current telephone extensions or
intercoms) as fill-ins for a good receiver. This isn't the best of ideas,
because carrier current devices can transmit on any frequency below 500 kHz
(150-300 kHz being VERY popular), while commercial receivers are generally
tuned to only a small fraction of that band. An electrical tester designed
to test for RF noise on power lines would work as a detector, but could
easily be triggered by RF on the line. Physical searches for carrier
current devices should include removing the covers of all power outlets,
and the disassembly of anything that runs on AC power.

*Countermeasures

The most obvious way to prevent native carrier current threats is to
prevent friendly threats. Don't use carrier current telephones or
intercoms. Audio jamming will prevent the microphones on CC surveillance
devices from picking up conversations. A good surge protector or power line
conditioner <http://www.blackbox.com> will incorporate a filter that will
block carrier current emissions. Make sure that the power line conditioner
or surge protector you select blocks RFI down to at least 1000 Hz. Its
possible to use blocking capacitors or low-pass filters (blocking above 60
Hz) on AC power lines in order to negate carrier current devices, but
certainly not recomended for anyone lacking in electrical knowlege.

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