Hardware Reference
In-Depth Information
on the radio frequency in order to transmit it. The radio
frequency acts as a carrier wave, carrying the audio signal.
Digital radios superimpose digital signals on the carrier
wave, so there must be a digital device on either end to
encode or decode those signals. In other words, digital
radios are basically modems, converting digital data to
radio signals, and radio signals back into digital data.
those signals is very short compared to lower frequency
signals. To cook food, microwave ovens generate energy
in this range to excite (heat up) the water molecules in
food. Some of that energy leaks from the oven at low
power, which is why you get all kinds of radio noise in the
gigahertz range around a microwave.
Motors and generators are especially insidious sources of
radio noise. A motor also operates by induction; specifi-
cally, by spinning a pair of magnets around a shaft in the
center of a coil of wire. By putting a current in the wire,
you generate a magnetic field, which attracts or repulses
the magnets, causing them to spin. Likewise, by using
mechanical force to spin the magnets, you generate a
current in the wire. So, a motor or a generator is essen-
tially a little radio, generating noise at whatever frequency
it's rotating.
Radio Interference
Though the antennas you'll use in this chapter are omni-
directional, radio can be blocked by obstacles, particularly
metal ones. A large metal sheet, for example, will reflect a
radio signal rather than allowing it to pass through. This
principle is used not only in designing antennas, but also in
designing
radio frequency (RF) shields
. If you've ever cut
open a computer cable and encountered a thin piece of
foil wrapped around the inside wires, you've encountered
an RF shield. Shields are used to prevent random radio
signals from interfering with the data being transmitted
down a wire. A shield doesn't have to be a solid sheet
of metal, though. A mesh of conductive metal will block
a radio signal as well—if the grid of the mesh is small
enough. The effectiveness of a given mesh depends on
the frequency it's designed to block. It's possible to block
radio signals from a whole space by surrounding the space
with an appropriate shield and grounding the shield. You'll
hear this referred to as making a
Faraday cage
. The effect
is named after the physicist Michael Faraday, who first
demonstrated and documented it.
Because there are so many sources of radio noise,
there are many ways to interfere with a radio signal. It's
important to keep these possible sources of noise in mind
when you begin to work with radio devices. Knowledge of
common interference sources, and knowing how to shield
against them, is a valuable tool in radio troubleshooting.
Multiplexing and Protocols
When you're transmitting via radio, anyone with a com-
patible receiver can receive your signal. There's no wire
to contain the signal, so if two transmitters are sending
at the same time, they will interfere with each other. This
is the biggest weakness of radio: a given receiver has no
way to know who sent the signal it's receiving. In contrast,
consider a wired serial connection: you can be reasonably
sure when you receive an electrical pulse on a serial cable
that it came from the device on the other end of the wire.
You have no such guarantee with radio. It's as if you were
blindfolded at a cocktail party and everyone else there had
the same voice. The only way you'd know who was talking
to you was if each person clearly identified himself at the
beginning and end of his conversation, and no one inter-
rupted him during this time. In other words, it's all about
protocols.
Sometimes radio transmission is blocked by unintentional
shields. If you're having trouble getting radio signals
through, look for metal that might be shielding the signal.
Transmitting from inside a car can sometimes be tricky
because the car body acts as a Faraday cage. Putting the
antenna on the outside of the car improves reception.
Bodies of water block RF effectively as well. This is true for
just about every radio housing.
All kinds of electrical devices emit radio waves as side
effects of their operation. Any alternating current can
generate a radio signal, even the AC that powers your
home or office. This is why you hear a hum when you
lay speaker wires in parallel with a power cord. The AC
signal is inducing a current in the speaker wires, and
the speakers are reproducing the changes in current as
sound. Likewise, it's why you may have trouble operating
a wireless data network near a microwave oven. WiFi
operates at frequencies in the gigahertz range, commonly
called the
microwave range
, because the wavelength of
The first thing everyone at that cocktail party would have
to do is agree on who speaks when. That way they could
each have your attention for awhile. Sharing in radio com-
munication is called
multiplexing
, and this form of sharing
is called
time-division multiplexing
. Each transmitter gets
a given time slot in which to transmit.
