First, let's talk about how radio waves are created — Page 139, Lesson 120

I want to walk you through the opening of Chapter 7, which is all about VHF propagation. We're starting from the very beginning, so I'll define every new term as we meet it. First, let's talk about how radio waves are created. If you feed an alternating current of a suitably high frequency into a transmitting aerial — that's the antenna — the energy doesn't stay trapped in the metal of the aerial. Instead, it radiates out into space as electromagnetic waves, which we call radio waves. This radiation of energy through space consists of two components: an alternating electrical field and an alternating magnetic field, and these two fields are at right angles to each other. The amplitude of each field varies — it oscillates — between zero and a maximum value, and it does so at the same frequency as the alternating current flowing in the aerial. Now, let's look at polarization. The term polarization describes the direction, or the plane, of oscillation of the electrical field of an electromagnetic wave. So if you have a vertical transmitting aerial, it produces — mainly — a vertically polarized radio wave. In that case, the electrical field oscillations, which we label E, occur in the vertical plane, and the magnetic field oscillations, labelled H, occur in the horizontal plane. For efficient reception, the receiving aerial should also be vertical. Conversely, if the transmitting aerial is horizontal, the receiving aerial should also be horizontal. The key relationship here is that the electric and magnetic fields oscillate at right angles to each other, and both are at right angles to the direction of propagation — that is, the direction the radio wave is travelling. There's a figure in the book, Figure 7.1, that shows the peak values E and H of the electric and magnetic fields of a vertically polarized wave. Next, the speed of propagation. Radio waves travel at the speed of light. That speed is virtually constant, and it is 300,000,000 metres per second, or 162,000 nautical miles per second. Both values are important for your calculations as a professional pilot. Finally, wavelength. The wavelength of a radio wave is defined as the distance travelled by the radio signal during the transmission of one cycle. Wavelength is normally expressed in metres. However, if the wavelength is less than one metre, we use centimetres or millimetres instead. That's the standard unit convention you'll need to remember. So to summarise what we've covered: radio waves are electromagnetic radiation from an aerial carrying alternating current; polarization is the plane of the electric field, and it dictates aerial orientation for efficient reception; the speed is constant at 300 million metres per second or 162,000 nautical miles per second; and wavelength is the distance travelled in one cycle, measured in metres or smaller units as appropriate. That sets the foundation for the rest of the chapter on VHF propagation.