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Now that you have completed this chapter, let's review some of the new terms, concepts, and ideas that you have learned. You should have a thorough understanding of these principles before moving on to chapter 3.

The INDUCTION FIELD contains an E field and an H field and is localized near the antenna. The E and H fields of the induction field are 90 degrees out of phase with each other.

The RADIATION FIELD contains E and H fields that are propagated from the antenna into space in the form of electromagnetic waves. The E and H fields of the radiation field are in phase with each other.

A HARMONIC FREQUENCY is any frequency that is a whole number multiple of a smaller basic frequency. For example, a radio wave transmitted at a fundamental frequency of 3000 hertz can have a second harmonic of 6000 hertz, a third harmonic frequency of 9000 hertz, etc., transmitted at the same time.

A VERTICALLY POLARIZED antenna transmits an electromagnetic wave with the E field perpendicular to the Earth's surface. A HORIZONTALLY POLARIZED antenna transmits a radio wave with the E field parallel to the Earth's surface.

A WAVEFRONT is a small section of an expanding sphere of radiated energy and is perpendicular to the direction of travel from the antenna.

RADIO WAVES are electromagnetic waves that can be reflected, refracted, and diffracted in the atmosphere like light and heat waves.

REFLECTED RADIO WAVES are waves that have been reflected from a surface and are 180 degrees out of phase with the initial wave.

The Earth's atmosphere is divided into three separate layers: The TROPOSPHERE, STRATOSPHERE, and IONOSPHERE.

The TROPOSPHERE is the region of the atmosphere where virtually all weather phenomena take place. In this region, rf energy is greatly affected.

The STRATOSPHERE has a constant temperature and has little effect on radio waves.

The IONOSPHERE contains four cloud-like layers of electrically charged ions which aid in long distance communications.

GROUND WAVES and SKY WAVES are the two basic types of radio waves that transmit energy from the transmitting antenna to the receiving antenna.

GROUND WAVES are composed of two separate component waves: the SURFACE WAVE and the SPACE WAVE.

SURFACE WAVES travel along the contour of the Earth by diffraction.

SPACE WAVES can travel through the air directly to the receiving antenna or can be reflected from the surface of the Earth.

SKY WAVES, often called ionospheric waves, are radiated in an upward direction and returned to Earth at some distant location because of refraction.

NATURAL HORIZON is the line-of-sight horizon.

RADIO HORIZON is ONE-THIRD farther than the natural horizon.

The IONOSPHERE consists of several layers of ions, formed by the process called ionization.

IONIZATION is the process of knocking electrons free from their parent atom, thus upsetting electrical neutrality.

RECOMBINATION is the opposite of ionization; that is, the free ions combine with positive ions, causing the positive ions to return to their original neutral atom state.

The D LAYER is the lowest region of the ionosphere and refracts signals of low frequencies back to Earth.

The E LAYER is present during the daylight hours; refracts signals as high as 20 megahertz back to Earth; and is used for communications up to 1500 miles.

The F LAYER is divided into the F1 and F2 layers during the day but combine at night to form one layer. This layer is responsible for high-frequency, long-range transmission.

The CRITICAL FREQUENCY is the maximum frequency that a radio wave can be transmitted vertically and still be refracted back to Earth.

The CRITICAL ANGLE is the maximum and/or minimum angle that a radio wave can be transmitted and still be refracted back to Earth.

SKIP DISTANCE is the distance between the transmitter and the point where the sky wave first returns to Earth.

SKIP ZONE is the zone of silence between the point where the ground wave becomes too weak for reception and the point where the sky wave is first returned to Earth.

FADING is caused by variations in signal strength, such as absorption of the rf energy by the ionosphere.

MULTIPATH FADING occurs when a transmitted signal divides and takes more than one path to a receiver and some of the signals arrive out of phase, resulting in a weak or fading signal.

Some TRANSMISSION LOSSES that affect radio-wave propagation are ionospheric absorption, ground reflection, and free-space losses.

ELECTROMAGNETIC INTERFERENCE (emi), both natural and man-made, interfere with radio communications.

The MAXIMUM USABLE FREQUENCY is the highest frequency that can be used for communications between two locations at a given angle of incidence and time of day.

The LOWEST USABLE FREQUENCY (luf) is the lowest frequency that can be used for communications between two locations.

OPTIMUM WORKING FREQUENCY (fot) is the most practical operating frequency and the one that can be relied on to have the fewest problems.

PRECIPITATION ATTENUATION can be caused by rain, fog, snow, and hail; and can affect overall communications considerably.

TEMPERATURE INVERSION causes channels, or ducts, of cool air to form between layers of warm air, which can cause radio waves to travel far beyond the normal line-of-sight distances.

TROPOSPHERIC PROPAGATION uses the scattering principle to achieve beyond the line-of-sight radio communications within the troposphere.

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