Propagation: Antennas and radio waves

What is it exactly that these sometimes oddly shaped devices do in the overall big scheme of radio transmission and reception?

Once upon a time aircraft were only operated when the pilot had a clear view of the ground and visual reference was the only means of navigation. Adaptation of the magnetic compass and development of the gyroscope were pivotal components in promotion of maneuvering when outside vision was impaired. Commercial aviation only became a reality after the introduction of radio navigation. The information age is truly upon us and even our aircraft have a need to know what's going on. To facilitate air to ground and ground to air or even air to air voice and data transmission an array of antennas are strategically placed on the airframe. So what is it exactly that these sometimes oddly shaped devices do in the overall big scheme of radio transmission and reception?

All things on the earth, in the water, or even in the air are showered continually with waves of energy. Some of these waves stimulate our senses and can be seen, felt, or heard. For instance, we can see light, hear sound, and feel heat. Radio waves are propagated, which means "moved through a medium." This is most easily observed by light rays. When a light is turned on in a darkened room, light rays travel from the light bulb throughout the room. When a flashlight is turned on, light rays also radiate from its bulb, but are focused into a narrow beam. You can use these examples to picture how radio waves propagate. Like the light in the room, radio waves may spread out in all directions. They can also be focused (concentrated) like the flashlight, depending upon the need. Radio waves are a form of radiant energy, similar to light and heat. Although they can neither be seen nor felt, their presence can be detected through the use of sensitive measuring devices. The speed at which both forms of waves travel is the same; they both travel at the speed of light.

Historical innovators

The first antenna was devised by the German physicist Heinrich Hertz. During the late 1880s he carried out a landmark experiment to test the theory of the British mathematician-physicist James Clerk Maxwell. This would prove that visible light is only one example of a larger class of electromagnetic effects that could pass through air (or empty space) as a succession of waves. Hertz built a transmitter for such waves consisting of two flat, square metallic plates, each attached to a rod, with the rods in turn connected to metal spheres spaced close together. An induction coil connected to the spheres caused a spark to jump across the gap, producing oscillating currents in the rods. The reception of waves at a distant point was indicated by a spark jumping across a gap in a loop of wire.

The Italian physicist Guglielmo Marconi, considered the principal inventor of wireless telegraphy, constructed various antennas for both sending and receiving, and he also discovered the importance of tall antenna structures in transmitting low-frequency signals. With the early antennas built by Marconi and others, operating frequencies were generally determined by antenna size and shape. In later antennas frequency was regulated by an oscillator, which generated the transmitted signal.

Electromagnetic fields

An electromagnetic wave consists of two primary components: an electric field and a magnetic field. The electric field results from the force of voltage, and the magnetic field results from the flow of current. Electromagnetic fields that are radiated are commonly considered to be waves and electromagnetic radiation in space can be interpreted as horizontal and vertical lines of force oriented at right angles to each other. These lines of force are made up of an electric field (E) and a magnetic field (H), which when combined make up the electromagnetic field. The electric and magnetic fields radiated from an antenna form just such an electromagnetic field which is responsible for the transmission and reception of electromagnetic energy through free space.

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