1 00:00:00,160 --> 00:00:03,440 Speaker 1: Audible dot com is the leading provider of downloadable digital 2 00:00:03,480 --> 00:00:07,160 Speaker 1: audio books and spoken word entertainment. Audible has over one 3 00:00:07,200 --> 00:00:10,080 Speaker 1: hundred thousand titles to choose from to be downloaded to 4 00:00:10,119 --> 00:00:13,640 Speaker 1: your iPod or MP three player. Go to audible podcast 5 00:00:13,760 --> 00:00:16,120 Speaker 1: dot com slash brain stuff to get a free audio 6 00:00:16,160 --> 00:00:18,560 Speaker 1: book download of your choice when you sign up today. 7 00:00:19,480 --> 00:00:23,640 Speaker 1: Welcome to Brainstuff from hastaff works dot com where smart happens. 8 00:00:31,360 --> 00:00:34,120 Speaker 1: Hi Am Marshall Brain with today's question, how does a 9 00:00:34,320 --> 00:00:39,400 Speaker 1: CB radio antenna work? Truck drivers still use CB radios 10 00:00:39,479 --> 00:00:42,720 Speaker 1: all the time and their antennas are actually pretty interesting. 11 00:00:43,159 --> 00:00:47,040 Speaker 1: The idea behind an antenna is to do two things. First, 12 00:00:47,159 --> 00:00:51,239 Speaker 1: it captures radio frequency signals that are then converted to 13 00:00:51,280 --> 00:00:56,200 Speaker 1: electrical signals by the receiver. And it takes electrical signals 14 00:00:56,200 --> 00:01:00,000 Speaker 1: from the transmitter and converts them into radio frequency signals 15 00:01:00,440 --> 00:01:03,960 Speaker 1: to broadcast them through the air. This second function is 16 00:01:04,000 --> 00:01:07,679 Speaker 1: where tuning comes into play, because an antenna has to 17 00:01:07,800 --> 00:01:12,000 Speaker 1: radiate radio frequency signals, something that's done best when the 18 00:01:12,120 --> 00:01:16,160 Speaker 1: length of the antenna precisely matches the wavelength of the 19 00:01:16,200 --> 00:01:20,440 Speaker 1: transmitted radio frequency. You can determine the proper length of 20 00:01:20,480 --> 00:01:24,080 Speaker 1: an antenna by using this formula, the length of the 21 00:01:24,120 --> 00:01:28,480 Speaker 1: antenna in feet is equal to nine four over the 22 00:01:28,680 --> 00:01:32,800 Speaker 1: frequency being transmitted in mega hurts. The CB portion of 23 00:01:32,840 --> 00:01:37,360 Speaker 1: the spectrum begins at one mega hurts, so a full 24 00:01:37,400 --> 00:01:40,679 Speaker 1: wavelength antenna would be a bit more than thirty nine 25 00:01:40,680 --> 00:01:44,440 Speaker 1: point three four ft long. That's obviously a little long 26 00:01:44,520 --> 00:01:47,400 Speaker 1: to attach to your bumper, so people tend to use 27 00:01:47,440 --> 00:01:50,480 Speaker 1: antennas that are a fraction of the wavelength, like a 28 00:01:50,520 --> 00:01:52,920 Speaker 1: half or a quarter or an eighth. Those are all 29 00:01:53,040 --> 00:01:56,760 Speaker 1: common lengths for antenna's In the case of CB, the 30 00:01:56,840 --> 00:02:00,360 Speaker 1: one quarter antenna, at just under ten feet long, is 31 00:02:00,400 --> 00:02:03,760 Speaker 1: the common whip antenna that you see on many cars 32 00:02:03,800 --> 00:02:07,440 Speaker 1: and trucks. The trouble is that there are many channels 33 00:02:07,440 --> 00:02:11,760 Speaker 1: on a modern CBE transceiver, and each of those corresponds 34 00:02:11,840 --> 00:02:15,120 Speaker 1: to a different frequency. It's not practical to have a 35 00:02:15,200 --> 00:02:20,000 Speaker 1: separate antenna for each frequency, so antenna designers have to compromise. 36 00:02:20,560 --> 00:02:22,760 Speaker 1: Usually they pick a frequency in the middle of the 37 00:02:22,880 --> 00:02:26,160 Speaker 1: spread and choose the antenna length to correspond to that. 38 00:02:26,800 --> 00:02:29,640 Speaker 1: When it compromised like this is made, you have to 39 00:02:29,680 --> 00:02:33,000 Speaker 1: see whether it's a good compromise. This is done by 40 00:02:33,080 --> 00:02:36,600 Speaker 1: measuring the standing wave ratio or s w R of 41 00:02:36,680 --> 00:02:40,400 Speaker 1: the antenna and the cable between the antenna and the radio, 42 00:02:40,760 --> 00:02:44,640 Speaker 1: and then tuning the antenna until the SR is acceptable. 43 00:02:45,320 --> 00:02:49,520 Speaker 1: Every antenna and every antenna feed line have a characteristic 44 00:02:49,560 --> 00:02:55,079 Speaker 1: impedence or an opposition to electrical current. In an ideal situation, 45 00:02:55,200 --> 00:02:58,840 Speaker 1: the impedances of the line and the antenna match perfectly, 46 00:02:59,280 --> 00:03:02,080 Speaker 1: and a hundred scent of the electrical energy sent to 47 00:03:02,120 --> 00:03:06,080 Speaker 1: the antenna is converted to radio energy and radiated into 48 00:03:06,120 --> 00:03:09,840 Speaker 1: the atmosphere. In a less than ideal case, when the 49 00:03:09,880 --> 00:03:14,359 Speaker 1: impedances aren't perfectly matched, some of the electrical energy sent 50 00:03:14,440 --> 00:03:17,560 Speaker 1: to the antenna won't be converted to radio energy, but 51 00:03:17,639 --> 00:03:20,880 Speaker 1: it will be reflected back down the feed line. The 52 00:03:21,080 --> 00:03:25,000 Speaker 1: energy reflecting back from the antenna causes standing waves of 53 00:03:25,040 --> 00:03:28,760 Speaker 1: electrical energy in the feed line. The ratio of the 54 00:03:28,840 --> 00:03:31,640 Speaker 1: highest voltage on the line to the lowest one is 55 00:03:31,680 --> 00:03:35,440 Speaker 1: the standing wave ratio. In a perfectly matched system, the 56 00:03:35,640 --> 00:03:39,240 Speaker 1: sw R is one to one. To tune an antenna, 57 00:03:39,840 --> 00:03:43,760 Speaker 1: use an SWR meter attached between the transmitter and the 58 00:03:43,800 --> 00:03:47,560 Speaker 1: antenna feed line. Depending on the meter, you can either 59 00:03:47,720 --> 00:03:50,200 Speaker 1: use a button on the meter to generate a signal 60 00:03:50,320 --> 00:03:54,000 Speaker 1: on the various channels, or you can key the microphone 61 00:03:54,000 --> 00:03:56,960 Speaker 1: on the CB transceiver to generate a signal while you 62 00:03:57,000 --> 00:04:00,240 Speaker 1: look at the sum you are reading. In general, if 63 00:04:00,280 --> 00:04:03,320 Speaker 1: the s w R never goes above one point five 64 00:04:03,400 --> 00:04:06,280 Speaker 1: to one, you're in good shape. If the s WR 65 00:04:06,480 --> 00:04:09,200 Speaker 1: does go above one point five to one, then watch 66 00:04:09,320 --> 00:04:12,480 Speaker 1: the meter on different frequencies to see the trend develop. 67 00:04:12,840 --> 00:04:15,760 Speaker 1: The s WR will be greater either on the higher 68 00:04:15,840 --> 00:04:18,520 Speaker 1: channels or the lower. If the s w R is 69 00:04:18,560 --> 00:04:21,960 Speaker 1: greater on the lower channels, then try gradually lengthening the 70 00:04:22,040 --> 00:04:25,040 Speaker 1: antenna by moving it in its base. If the s 71 00:04:25,200 --> 00:04:28,479 Speaker 1: w R is greater on the higher channels, try shortening 72 00:04:28,480 --> 00:04:32,000 Speaker 1: the antenna. Do you have any ideas or suggestions for 73 00:04:32,040 --> 00:04:35,040 Speaker 1: this podcast? If so, please send me an email at 74 00:04:35,120 --> 00:04:38,200 Speaker 1: podcast at how stuff works dot com. For more on 75 00:04:38,279 --> 00:04:40,640 Speaker 1: this and thousands of other topics, go to how stuff 76 00:04:40,640 --> 00:04:43,400 Speaker 1: works dot com and be sure to check out the 77 00:04:43,400 --> 00:04:45,880 Speaker 1: brain stuff blog on the house stuff works dot com 78 00:04:45,920 --> 00:04:52,360 Speaker 1: home page. 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