A citizens band (CB) radio is often used by truck drivers to communicate while on the road, but how does it work? Marshall Brain gives a detailed explanation of how a CB radio's antenna works in this episode.
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Episode Transcript
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Welcome to Brainstuff from hastaff works dot com where smart happens.
(00:31):
Hi Am Marshall Brain with today's question, how does a
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
(00:56):
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
(01:20):
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
(01:40):
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
(02:03):
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.
(02:26):
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
(02:49):
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
(03:09):
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
(03:31):
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
(03:54):
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
(04:15):
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
(04:38):
this and thousands of other topics, go to how stuff
works dot com and be sure to check out the
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