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A Guide to FM Reception
by Dave
Downing
What is FM
Radio | FM's Strengths | FM's
Weaknesses | Why Reception
Quality Varies | Multi-path
Distortion | Interference
| Receiver Overloading | Docket
80-90 | Antennas for FM Reception
FM (frequency modulation, also called
static-less radio) is a way to impose music, speech, video, or other data
on a radio signal by varying the signal's frequency or wavelength.
Invented by Major Edwin Howard Armstrong, FM is commonly used to broadcast audio on VHF (very high frequency,
sometimes called ultra-short wave) channels between 88 and 108 megahertz.
The sounds, but not the pictures, of both VHF and UHF (ultra-high
frequency) television programs are also transmitted by FM.
Properly designed FM radios are immune to the
atmospheric noise and static that plague AM radio reception. FM radios
also reject many types of man-made interference. Because of something
called the capture effect, if two FM signals are present on
the same channel, and one is even a little
bit stronger than the other, a radio tuned to the channel will usually
receive only the stronger signal. When a listener finds two or more FM
signals on the same channel, listeners can often take advantage of the
capture effect by varying the position or orientation of their antennas to
favor one signal over the other.
FM broadcasts can carry very high fidelity
sound; in fact, FM is used to record the sound tracks of VHS Hi-Fi video
tapes. The signal-to-noise ratio of a modern FM transmitter approaches
that of a compact disc recording, and FM's frequency response
extends from below 10 Hertz to 15,000 Hertz.
This is possible because VHF signals can occupy wider channels than the
signals used for AM broadcasting.
Here is the bad news: the same VHF channels
that allow high fidelity FM broadcasts also restrict the range of these
transmissions and make them subject to a peculiar form of distortion
called multi-path. In contrast to AM signals, which can often
be received from transmitters hundreds of
miles away, especially at night, FM signals usually do not travel very far
beyond the visible horizon. Therefore, FM transmitters must be located in
or near the communities they serve, and people who live near a transmitter site often have trouble with FM
reception because strong signals from nearby transmitters can overload
radios, preventing the reception of weaker or more distant stations.
FM radio signals travel only through free
space and cannot penetrate obstacles.
Instead, some of an FM signal striking a
building or hill will be absorbed and some reflected. Reflected signals
are very important in FM reception; people who live behind obstacles that
otherwise block their reception of an FM station can often receive a
reflection of the signal off another hill or building.
What is FM
Radio | FM's Strengths | FM's
Weaknesses | Why Reception
Quality Varies | Multi-path
Distortion | Interference
| Receiver Overloading | Docket
80-90 | Antennas for FM Reception
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