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Speaker 1: Welcome to Brainstuff from hastaff works dot com where smart happens.
Speaker 1: Hi Am Marshall Brain with today's question, how does a
Speaker 1: CB radio antenna work? Truck drivers still use CB radios
Speaker 1: all the time and their antennas are actually pretty interesting.
Speaker 1: The idea behind an antenna is to do two things. First,
Speaker 1: it captures radio frequency signals that are then converted to
Speaker 1: electrical signals by the receiver. And it takes electrical signals
Speaker 1: from the transmitter and converts them into radio frequency signals
Speaker 1: to broadcast them through the air. This second function is
Speaker 1: where tuning comes into play, because an antenna has to
Speaker 1: radiate radio frequency signals, something that's done best when the
Speaker 1: length of the antenna precisely matches the wavelength of the
Speaker 1: transmitted radio frequency. You can determine the proper length of
Speaker 1: an antenna by using this formula, the length of the
Speaker 1: antenna in feet is equal to nine four over the
Speaker 1: frequency being transmitted in mega hurts. The CB portion of
Speaker 1: the spectrum begins at one mega hurts, so a full
Speaker 1: wavelength antenna would be a bit more than thirty nine
Speaker 1: point three four ft long. That's obviously a little long
Speaker 1: to attach to your bumper, so people tend to use
Speaker 1: antennas that are a fraction of the wavelength, like a
Speaker 1: half or a quarter or an eighth. Those are all
Speaker 1: common lengths for antenna's In the case of CB, the
Speaker 1: one quarter antenna, at just under ten feet long, is
Speaker 1: the common whip antenna that you see on many cars
Speaker 1: and trucks. The trouble is that there are many channels
Speaker 1: on a modern CBE transceiver, and each of those corresponds
Speaker 1: to a different frequency. It's not practical to have a
Speaker 1: separate antenna for each frequency, so antenna designers have to compromise.
Speaker 1: Usually they pick a frequency in the middle of the
Speaker 1: spread and choose the antenna length to correspond to that.
Speaker 1: When it compromised like this is made, you have to
Speaker 1: see whether it's a good compromise. This is done by
Speaker 1: measuring the standing wave ratio or s w R of
Speaker 1: the antenna and the cable between the antenna and the radio,
Speaker 1: and then tuning the antenna until the SR is acceptable.
Speaker 1: Every antenna and every antenna feed line have a characteristic
Speaker 1: impedence or an opposition to electrical current. In an ideal situation,
Speaker 1: the impedances of the line and the antenna match perfectly,
Speaker 1: and a hundred scent of the electrical energy sent to
Speaker 1: the antenna is converted to radio energy and radiated into
Speaker 1: the atmosphere. In a less than ideal case, when the
Speaker 1: impedances aren't perfectly matched, some of the electrical energy sent
Speaker 1: to the antenna won't be converted to radio energy, but
Speaker 1: it will be reflected back down the feed line. The
Speaker 1: energy reflecting back from the antenna causes standing waves of
Speaker 1: electrical energy in the feed line. The ratio of the
Speaker 1: highest voltage on the line to the lowest one is
Speaker 1: the standing wave ratio. In a perfectly matched system, the
Speaker 1: sw R is one to one. To tune an antenna,
Speaker 1: use an SWR meter attached between the transmitter and the
Speaker 1: antenna feed line. Depending on the meter, you can either
Speaker 1: use a button on the meter to generate a signal
Speaker 1: on the various channels, or you can key the microphone
Speaker 1: on the CB transceiver to generate a signal while you
Speaker 1: look at the sum you are reading. In general, if
Speaker 1: the s w R never goes above one point five
Speaker 1: to one, you're in good shape. If the s WR
Speaker 1: does go above one point five to one, then watch
Speaker 1: the meter on different frequencies to see the trend develop.
Speaker 1: The s WR will be greater either on the higher
Speaker 1: channels or the lower. If the s w R is
Speaker 1: greater on the lower channels, then try gradually lengthening the
Speaker 1: antenna by moving it in its base. If the s
Speaker 1: w R is greater on the higher channels, try shortening
Speaker 1: the antenna. Do you have any ideas or suggestions for
Speaker 1: this podcast? If so, please send me an email at
Speaker 1: podcast at how stuff works dot com. For more on
Speaker 1: this and thousands of other topics, go to how stuff
Speaker 1: works dot com and be sure to check out the
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