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