|
A Guide to FM Reception
(cont)
by Dave
Downing
What
is FM Radio | FM's Strengths |
FM's Weaknesses | Why
Reception Quality Varies | Multipath Distortion
| Interference | Receiver
Overloading | Docket 80-90 | Antennas
for FM Reception
Most FM listeners receive many different
reflections of their favorite station at the same time. The truck idling at
the stoplight on the next block, the concrete and steel buildings in the
industrial park at the other end of town, the water tank on top of a nearby
hill, and the hill itself all produce
reflections, and even your own body (mostly salt water, an excellent
electrical conductor) can reradiate FM signals that happen to strike it.
Because radio signals that reflect off different objects travel different
distances to
reach your radio's antenna, yet they move at
the same speed (the speed of light), they will take greater or lesser
amounts of time to reach the radio depending on how far they've come.
Radio waves are like waves in the ocean; both
have crests and troughs. When two reflections arrive at your antenna, the
crest of one may arrive at the same time as the crest of another. When that
happens, the two reflections are said to arrive in phase, and
your radio will see them as a single wave
having the strength of both. This is good, as it means you will get better
reception than either reflection would give you by itself.
By contrast, when the crest of one reflection
arrives at the same time as the trough of another, the reflections are said
to arrive out of phase, and your radio will only see the difference in
strength between the two, which is always weaker than the stronger
reflection. When reflections exactly equal in
strength arrive out of phase, the radio will see nothing at all, and you
won't be able to hear any of the signal.
When the weather changes, when the truck idling
at that stoplight starts moving, and when you move from one side of the room
to another, the phase relationship between various reflections reaching your
radio will change, and so will the quality of your
reception. Changes in temperature or humidity
can also affect reception, as can the presence of leaves on trees or snow on
the ground. You may find yourself constantly adjusting the antenna on your
radio. Worst of all, the strength of many reflections is
frequency-dependent; as an FM signal varies in
frequency depending on the music or sound that it carries, reflections will
grow stronger or weaker, and when two or more such combine at our antenna
they can produce distortion by selectively canceling
parts of the signal. Since this form of
distortion is caused by a signal following multiple paths between a
transmitter and receiver, it is called multipath distortion. Multipath
distortion is the bane of the FM listener's existence.
If you receive something else on your favorite
station's frequency, it's probably due to one of these causes:
malfunctioning equipment, receiver overload, unusual atmospheric conditions,
or a recent FCC rulemaking. Once in awhile someone puts an
illegal "pirate" radio station on the
air, but that's not a common occurrence. Moreover, most "pirates"
run low power and operate only a few hours a week.
When you hear a strange signal where your
favorite station should be, the first thing you should do is try to identify
it. Is it a broadcast station? A CB or ham radio transmission? A taxicab
dispatcher or air traffic controller? Can you hear any call letters? Does
the station's programming match that of any other station on the dial? Is it
just one station, or is there more than one?
If the intruder is a local station, you can
suspect receiver overload or a failure in a piece of equipment at a local
radio station. If there is a transmitter site nearby, your problem is almost
certainly caused by receiver overload. For example, near the intersection of
127 and I-96 in the south end of Lansing there is one radio tower that is
the location for several radio stations. If you travel near this site, or in
close proximity of any radio transmitter tower, your radio may be getting
more energy than it can handle. You will find that your radio will not
receive any single station anywhere on the dial in a situation like this. If
you are driving in your car, all you can do is wait until you have driven by
a transmitter that may be the cause of the interference, then you will be
able to tune into a station again.
What
is FM Radio | FM's Strengths |
FM's Weaknesses | Why
Reception Quality Varies | Multipath Distortion
| Interference | Receiver
Overloading | Docket 80-90 | Antennas
for FM Reception
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