Is is possible to non-destructively sense a signal through a wire such as a coax cable or telephone cable? In this case both the send and return wires are bound together and cannot be separated. The answer would seem to be no since the radiation would be quite small and only due to deviations from the idea case.
What I would like to do, just for fun, is non-destructively("remote") sense an audio signal from a coax or telephone wire.
Electromagnet pickup and microphone amplifier. Coax is leaky, but it's more of an RF thing; I don't know how leaky at AF. Maybe you'll still detect something.
Twisted pair obviously has a near field which works. Such telephone detectors have been used for decades (including a famous submarine recorder placed over important Soviet telephone lines).
Tim
-- Deep Friar: a very philosophical monk. Website:
On a sunny day (Wed, 3 Feb 2010 11:59:17 -0600) it happened "Jon Slaughter" wrote in :
Audio playback head.
more
ter
as
to
ns
-It leaks plenty at audio frequencies too. It's easy to see the ~20kHz 'crap' from the fluorescent lamps in my room leaking into a piece of coax.
George H.
a s o
In the near-field (only a wire-radius or three from the cable), you don't need "radiation" to get pickup. The telephone case is easy, just use a capacitive pickup (if you want to get fancy, you can modulate the capacitance and pickup from several twists at once, creating deliberate 'crosstalk').
The coax case is less simple; by symmetry, no E-field or current pickup can be expected. Your probe can use induced current easily if you slit the shielding and put a pickup into the slot, or can rely on e-field leakage (it has to be a probe that's on the size scale of the braid crossovers). CATV foil-shield cable defeats the second scheme, and the first makes an evident change in the wiring.
There's plenty of other ways that could get into a measurement, it needn't be the differential-mode of the coaxial cable that's doing the pickup. Triboelectricity, for instance, makes an acoustic input channel.
Since energy has mass, weigh the cable.
Close capacitive pickup on phone wire works reasonably well. Capacitive pickup of audio and low frequency RF from coax cables would be dicer but not impossible (braids typically not 100 percent).
Have something to add? Share your thoughts — no account required.
Ask the community — no account required