WEBVTT - How does a CB radio antenna work?

0:00:00.160 --> 0:00:03.440
<v Speaker 1>Audible dot com is the leading provider of downloadable digital

0:00:03.480 --> 0:00:07.160
<v Speaker 1>audio books and spoken word entertainment. Audible has over one

0:00:07.200 --> 0:00:10.080
<v Speaker 1>hundred thousand titles to choose from to be downloaded to

0:00:10.119 --> 0:00:13.640
<v Speaker 1>your iPod or MP three player. Go to audible podcast

0:00:13.760 --> 0:00:16.120
<v Speaker 1>dot com slash brain stuff to get a free audio

0:00:16.160 --> 0:00:18.560
<v Speaker 1>book download of your choice when you sign up today.

0:00:19.480 --> 0:00:23.640
<v Speaker 1>Welcome to Brainstuff from hastaff works dot com where smart happens.

0:00:31.360 --> 0:00:34.120
<v Speaker 1>Hi Am Marshall Brain with today's question, how does a

0:00:34.320 --> 0:00:39.400
<v Speaker 1>CB radio antenna work? Truck drivers still use CB radios

0:00:39.479 --> 0:00:42.720
<v Speaker 1>all the time and their antennas are actually pretty interesting.

0:00:43.159 --> 0:00:47.040
<v Speaker 1>The idea behind an antenna is to do two things. First,

0:00:47.159 --> 0:00:51.239
<v Speaker 1>it captures radio frequency signals that are then converted to

0:00:51.280 --> 0:00:56.200
<v Speaker 1>electrical signals by the receiver. And it takes electrical signals

0:00:56.200 --> 0:01:00.000
<v Speaker 1>from the transmitter and converts them into radio frequency signals

0:01:00.440 --> 0:01:03.960
<v Speaker 1>to broadcast them through the air. This second function is

0:01:04.000 --> 0:01:07.679
<v Speaker 1>where tuning comes into play, because an antenna has to

0:01:07.800 --> 0:01:12.000
<v Speaker 1>radiate radio frequency signals, something that's done best when the

0:01:12.120 --> 0:01:16.160
<v Speaker 1>length of the antenna precisely matches the wavelength of the

0:01:16.200 --> 0:01:20.440
<v Speaker 1>transmitted radio frequency. You can determine the proper length of

0:01:20.480 --> 0:01:24.080
<v Speaker 1>an antenna by using this formula, the length of the

0:01:24.120 --> 0:01:28.480
<v Speaker 1>antenna in feet is equal to nine four over the

0:01:28.680 --> 0:01:32.800
<v Speaker 1>frequency being transmitted in mega hurts. The CB portion of

0:01:32.840 --> 0:01:37.360
<v Speaker 1>the spectrum begins at one mega hurts, so a full

0:01:37.400 --> 0:01:40.679
<v Speaker 1>wavelength antenna would be a bit more than thirty nine

0:01:40.680 --> 0:01:44.440
<v Speaker 1>point three four ft long. That's obviously a little long

0:01:44.520 --> 0:01:47.400
<v Speaker 1>to attach to your bumper, so people tend to use

0:01:47.440 --> 0:01:50.480
<v Speaker 1>antennas that are a fraction of the wavelength, like a

0:01:50.520 --> 0:01:52.920
<v Speaker 1>half or a quarter or an eighth. Those are all

0:01:53.040 --> 0:01:56.760
<v Speaker 1>common lengths for antenna's In the case of CB, the

0:01:56.840 --> 0:02:00.360
<v Speaker 1>one quarter antenna, at just under ten feet long, is

0:02:00.400 --> 0:02:03.760
<v Speaker 1>the common whip antenna that you see on many cars

0:02:03.800 --> 0:02:07.440
<v Speaker 1>and trucks. The trouble is that there are many channels

0:02:07.440 --> 0:02:11.760
<v Speaker 1>on a modern CBE transceiver, and each of those corresponds

0:02:11.840 --> 0:02:15.120
<v Speaker 1>to a different frequency. It's not practical to have a

0:02:15.200 --> 0:02:20.000
<v Speaker 1>separate antenna for each frequency, so antenna designers have to compromise.

0:02:20.560 --> 0:02:22.760
<v Speaker 1>Usually they pick a frequency in the middle of the

0:02:22.880 --> 0:02:26.160
<v Speaker 1>spread and choose the antenna length to correspond to that.

0:02:26.800 --> 0:02:29.640
<v Speaker 1>When it compromised like this is made, you have to

0:02:29.680 --> 0:02:33.000
<v Speaker 1>see whether it's a good compromise. This is done by

0:02:33.080 --> 0:02:36.600
<v Speaker 1>measuring the standing wave ratio or s w R of

0:02:36.680 --> 0:02:40.400
<v Speaker 1>the antenna and the cable between the antenna and the radio,

0:02:40.760 --> 0:02:44.640
<v Speaker 1>and then tuning the antenna until the SR is acceptable.

0:02:45.320 --> 0:02:49.520
<v Speaker 1>Every antenna and every antenna feed line have a characteristic

0:02:49.560 --> 0:02:55.079
<v Speaker 1>impedence or an opposition to electrical current. In an ideal situation,

0:02:55.200 --> 0:02:58.840
<v Speaker 1>the impedances of the line and the antenna match perfectly,

0:02:59.280 --> 0:03:02.080
<v Speaker 1>and a hundred scent of the electrical energy sent to

0:03:02.120 --> 0:03:06.080
<v Speaker 1>the antenna is converted to radio energy and radiated into

0:03:06.120 --> 0:03:09.840
<v Speaker 1>the atmosphere. In a less than ideal case, when the

0:03:09.880 --> 0:03:14.359
<v Speaker 1>impedances aren't perfectly matched, some of the electrical energy sent

0:03:14.440 --> 0:03:17.560
<v Speaker 1>to the antenna won't be converted to radio energy, but

0:03:17.639 --> 0:03:20.880
<v Speaker 1>it will be reflected back down the feed line. The

0:03:21.080 --> 0:03:25.000
<v Speaker 1>energy reflecting back from the antenna causes standing waves of

0:03:25.040 --> 0:03:28.760
<v Speaker 1>electrical energy in the feed line. The ratio of the

0:03:28.840 --> 0:03:31.640
<v Speaker 1>highest voltage on the line to the lowest one is

0:03:31.680 --> 0:03:35.440
<v Speaker 1>the standing wave ratio. In a perfectly matched system, the

0:03:35.640 --> 0:03:39.240
<v Speaker 1>sw R is one to one. To tune an antenna,

0:03:39.840 --> 0:03:43.760
<v Speaker 1>use an SWR meter attached between the transmitter and the

0:03:43.800 --> 0:03:47.560
<v Speaker 1>antenna feed line. Depending on the meter, you can either

0:03:47.720 --> 0:03:50.200
<v Speaker 1>use a button on the meter to generate a signal

0:03:50.320 --> 0:03:54.000
<v Speaker 1>on the various channels, or you can key the microphone

0:03:54.000 --> 0:03:56.960
<v Speaker 1>on the CB transceiver to generate a signal while you

0:03:57.000 --> 0:04:00.240
<v Speaker 1>look at the sum you are reading. In general, if

0:04:00.280 --> 0:04:03.320
<v Speaker 1>the s w R never goes above one point five

0:04:03.400 --> 0:04:06.280
<v Speaker 1>to one, you're in good shape. If the s WR

0:04:06.480 --> 0:04:09.200
<v Speaker 1>does go above one point five to one, then watch

0:04:09.320 --> 0:04:12.480
<v Speaker 1>the meter on different frequencies to see the trend develop.

0:04:12.840 --> 0:04:15.760
<v Speaker 1>The s WR will be greater either on the higher

0:04:15.840 --> 0:04:18.520
<v Speaker 1>channels or the lower. If the s w R is

0:04:18.560 --> 0:04:21.960
<v Speaker 1>greater on the lower channels, then try gradually lengthening the

0:04:22.040 --> 0:04:25.040
<v Speaker 1>antenna by moving it in its base. If the s

0:04:25.200 --> 0:04:28.479
<v Speaker 1>w R is greater on the higher channels, try shortening

0:04:28.480 --> 0:04:32.000
<v Speaker 1>the antenna. Do you have any ideas or suggestions for

0:04:32.040 --> 0:04:35.040
<v Speaker 1>this podcast? If so, please send me an email at

0:04:35.120 --> 0:04:38.200
<v Speaker 1>podcast at how stuff works dot com. For more on

0:04:38.279 --> 0:04:40.640
<v Speaker 1>this and thousands of other topics, go to how stuff

0:04:40.640 --> 0:04:43.400
<v Speaker 1>works dot com and be sure to check out the

0:04:43.400 --> 0:04:45.880
<v Speaker 1>brain stuff blog on the house stuff works dot com

0:04:45.920 --> 0:04:52.360
<v Speaker 1>home page. Streaming TV shows and movies directly to your

0:04:52.400 --> 0:04:55.240
<v Speaker 1>home is a breeze with Netflix. As a Netflix member,

0:04:55.279 --> 0:04:58.200
<v Speaker 1>you can instantly watch TV and movies on your PC, mac,

0:04:58.320 --> 0:05:02.120
<v Speaker 1>mobile device, or television. Get a free thirty day trial membership.

0:05:02.279 --> 0:05:05.159
<v Speaker 1>Go to Netflix dot com slash stuff and sign up today.