WEBVTT NOTE What a Radio Wave Is 1 00:00:00.000 --> 00:00:03.080 Two fields, at right angles. 2 00:00:03.080 --> 00:00:08.118 Question. What are the two components of a radio wave? 3 00:00:09.018 --> 00:00:13.418 Answer. Electric and magnetic fields. 4 00:00:14.018 --> 00:00:22.538 Question. What is the relationship between the electric and magnetic fields of an electromagnetic wave? 5 00:00:23.438 --> 00:00:27.037 Answer. They are at right angles. 6 00:00:27.637 --> 00:00:41.260 Each field regenerates the other as the wave travels: a changing electric field produces a changing magnetic field, which produces a changing electric field. 7 00:00:41.260 --> 00:00:55.751 That mutual regeneration is why the wave keeps going with nothing to carry it. Radio needs no medium. Polarisation is defined by the orientation of the electric field. 8 00:00:55.751 --> 00:01:06.510 Both fields are there and both are essential; the electric one is the one we name, by convention. One speed, for everything. 9 00:01:06.510 --> 00:01:12.963 Question. What is the velocity of a radio wave traveling through free space? 10 00:01:13.863 --> 00:01:16.968 Answer. Speed of light. 11 00:01:17.568 --> 00:01:20.688 And the question that catches people: 12 00:01:20.688 --> 00:01:27.736 Question. Which of these frequencies travels at the highest velocity in free space? 13 00:01:28.636 --> 00:01:34.262 Answer. All radio frequencies travel at the same velocity. 14 00:01:34.862 --> 00:01:43.632 Frequency does not affect speed. A 3.5 MHz signal and a 1.2 GHz signal arrive together. 15 00:01:43.632 --> 00:02:05.102 What changes with frequency is wavelength, because in a fixed travel time a faster-alternating wave fits more cycles into the same distance. Wavelength and frequency. lambda metres equals (300) divided by (f MHz ) 16 00:02:05.102 --> 00:02:10.811 Question. What is the relationship between wavelength and frequency? 17 00:02:11.711 --> 00:02:16.831 Answer. Wavelength gets shorter as frequency increases. 18 00:02:17.431 --> 00:02:22.553 Inversely proportional. And the reason it comes up constantly: 19 00:02:22.553 --> 00:02:30.793 Question. In addition to frequency, which of the following is used to identify amateur radio bands? 20 00:02:31.693 --> 00:02:36.373 Answer. The approximate wavelength in meters. 21 00:02:36.973 --> 00:02:50.297 Every band has two names, and fluency between them is everyday vocabulary rather than exam trivia. The frequency ranges. 22 00:02:50.297 --> 00:03:04.953 Three ranges to memorise, each a factor of ten: Range, HF; Frequencies, 3 to 30 MHz; Amateur bands in it, 80, 40, 20, 15, 10 metres. 23 00:03:04.953 --> 00:03:24.605 Range, VHF; Frequencies, 30 to 300 MHz; Amateur bands in it, 6 metres, 2 metres, 1.25 metres. Range, UHF; Frequencies, 300 to 3000 MHz; Amateur bands in it, 70 cm, 33 cm, 23 cm. 24 00:03:24.605 --> 00:03:38.595 Note the boundaries fall in convenient places: 6 metres (50 MHz) is just inside VHF, and 10 metres (28 MHz) is just inside HF. 25 00:03:38.595 --> 00:03:55.027 That pair sits either side of the 30 MHz line and behaves very differently as a result, which is the subject of the propagation lessons that follow. 26 00:03:55.027 --> 00:04:18.677 Below HF are MF (300 kHz to 3 MHz, where AM broadcast lives) and LF; above UHF are SHF and EHF, the microwave ranges. Check yourself. Two signals, 7 MHz and 700 MHz, leave an antenna together. Which arrives first? 27 00:04:18.677 --> 00:04:27.670 Which range is 146 MHz in? 28.4 MHz? 446 MHz? What defines a wave's polarisation? 28 00:04:33.670 --> 00:04:55.466 Together. All radio frequencies travel at the same velocity in free space. 146 MHz is VHF; 28.4 MHz is HF; 446 MHz is UHF. The orientation of the electric field..