[../../../_private/magazine_tmp.htm]Capturing the Elusive Radio Wave
Every radio - whether AM or FM - has an antenna.
The antenna in your little AM pocket transistor radio is buried inside the unit. If you take off the back of the radio, you'll find a gray-black rod an inch or two long with several coils of small wire. Because the coil is wound on a straight rod, it receives better along one axis. Engineers call this "directionality." You call it a pain when you have to turn your radio sideways just to hear the Padres if the signal is weak.
Your portable boom box has two antennas: the AM antenna (inside) and an exterior whip antenna for FM only. To improve reception, you probably telescope the whip antenna out a bit and twist it around. Sometimes you have to hold the end of the antenna. Sometimes not. This is called "chance."
There's a lot of chance involved in trapping the signal before it gets to your ears. The process is similar to making water ripple in a pond. Let's say a stone tossed into a pond represents a radio station's transmitting antenna. The ripples are the radio waves, and the distance from the top of one radio wave to the next is the wavelength. The number of ripples per second is the frequency.
Most of the chance involved in trapping the radio signal affects FM. The FM signal is so small that it's quite vulnerable. The difference in vulnerability between AM and FM is like the difference in vulnerability between ripples produced by large stones and ripples from tiny pebbles.
If you toss a big rock into a still pond, you get big, strong, widely spaced ripples. If you toss a tiny pebble, you also get ripples; but they are smaller, weaker and closer together.
The big rock yields a big wavelength, but a low frequency similar to the AM broadcast band. The FM band is more like what the little stone produces - a small wavelength with high frequency.
In the middle of the AM band, the wavelength is more than three football fields long. The signals are so large that even a freeway overpass doesn't deter them. But FM, around 100 megahertz, has a wavelength only about 10 feet long. Suddenly, the position of a nearby truck becomes critical. An overpass can cast a "shadow."
To obtain the best reception, you must put your antenna in the best possible position - up high and in the clear. FM stations install their transmitting antennas on top of the tallest towers, buildings and mountains.
Your antenna also must be big enough to capture a good part of the wave, and if the wavelength is small enough to physically allow it, the antenna should be pointed directly at the transmitter. Hams and shortwave listeners know they have to "tune" their antennas to the exact wavelength to receive weak signals.
Just as ripples in water gradually stop, radio signals die down when their energy gets used up over distance. A radio station that wants to reach you at a distance must increase the power. Every doubling of range requires a fourfold increase of power. But sometimes that's not economical. In sparsely populated regions such as the desert, a lot of signal is wasted reaching isolated listeners. To resolve this problem, some radio stations use a "translator" - a second transmitter placed near the fringe of its listening range that retransmits to folks in the hinterlands.
Despite using the best antenna, positioned properly and aimed well, you still can have problems. For example, "multi-path distortion" occurs on the FM band with those short wavelengths. Your radio receiver hears a signal coming in straight and direct, but it also hears the signal that bounced off the building next to you and arrived a bit late. The reflected radio waves get mixed up with the direct ones and give you the hearing version of a ghosting TV picture. The only cure is to put blinders on your receiver. Get a highly directional antenna pointed in the right direction, or get close enough to the transmitter so that the direct wave overpowers the reflected one.
Another type of interference - "capture effect" - only applies to FM radios. FM receivers latch onto the strongest nearby signal, which could be another radio station on an adjacent channel. If you drive by that station's transmitter, your antenna may hear its signal more strongly than the station of your choice. It's possible that your receiver will broadcast the strong, nearby station even if you don't want to hear it.
Other types of radio waves can also interfere with your station of choice. Folks in Normal Heights can tell you what happened to their radios, TVs and even telephones when a Citizens Band radio operator illegally used 100 times too much power and saturated the neighborhood in July. Radio reception improved after "San Diego's Finest" arrested the man.
CB is just another form of radio. Consider all the radio waves generated by police cars, cellular phones, remote phones, babysitter monitors, TV stations, telephone relays and remote controls. We live in a very choppy pond!
John Bry is the news director at WCUZ in Grand Rapids, Mich. He has been in radio for 20 years and is a licensed ham radio operator.
Do you have a technical question about radio? Send your question to Sci-Tech Man, Radio San Diego Magazine, 7720 El Camino Real
Box 179 La Costa, CA 92009.
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