Re: Signals on a single wire
- From: "Roger Johansson" <no-email@xxxxxxxxxx>
- Date: 03 Apr 2005 14:19:17 GMT
Andrew Howard wrote:
> Could someone please explain how a signal can be transmitted on a
> single wire? I have always been taught that a signal, or current,
> needs a circuit to work.
> Is the second "wire" a path between you and the negative via your
> legs, the ground, the table, then the casing or something?
Yes. And there is a capacity in big bodies, a buffer of charges, both
positive and negative which usually are in balance, and it works as a
kind of fixed point voltage-wise.
If we think of a short piece of water pipe, not connected to anything
at the ends, just air outside. A small amonut of water inside the pipe,
and a pump in the middle of the pipe.
Then if we start the pump it will pump the water in one direction,
but after a very short time there is no more water to pump. If the
amount of water from beginning in the pipe is only a nanoliter and the
pump pumps 1 liter per second the water will be gone in one nanosecond.
If the water from the beginning is a million liters, if the pipe is
connected to a swimmingpool, the pump can pump water for a million
seconds in one direction before the water stops coming.
The capacity is very different in these cases, but there is a certain
capacity
for delivering current in both cases.
The capacity determines how strong AC currents we can create if there
is no DC connection which completes the circuit.
Even if you body does not touch the ground, and you touch the probe to
an oscilloscope you can see a strong signal on the scope. That is
because your body has a certain capacity, measured in nanoFarad, and
that is enough to deliver AC current, as the AC current goes forth and
back. And because magnetic noise induces a voltage in your body.
If you touch the probe with a much smaller object, like a 1mm piece of
copper wire, you will not see much difference on the scope screen.
Because that little object cannot deliver much as much AC current as
your body, or the earth.
> Another example, which probably has a different explanation, is
> antennas on radio transmitters. How do they transmit anything, when
> there is only a single wire?
The higher the frequency the easier it is to use these small
capacitances of the neutral parts of the antenna as virtual ground
planes.
At lower radio frequencies the earth itself forms the neutral part of
the antenna.
The antenna converts electric signals into electromagnetic waves which
can travel through space.
Well, the signal is actually an electromagnetic wave already when it
travels through the antenna wire. The antenna is just a bridge between
electromagnetic waves in conductors and electromagnetic waves in space.
--
Roger J.
.
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