Amateur Radio Communications Theory

Definitions
Sky-wave propagation- A type of radio-wave propagation in which
radio waves traveling upward are bent or refracted by the ionosphere back to the earth. This is one of the major means of amateur radio communication in the high-frequency spectrum. Otherwise, these radio waves would be propagated into outer space and lost. Sky-wave propagation provides a capability for long-range or DX communications using one or more “hops” in which the radio waves are reflected back to the earth.

Ground-wave propagation- Another form of propagation in which
the ground or surface wave travels along the surface of the ground or
water. This mode of propagation is important at the low and medium
frequencies. Most commercial AM broadcast stations use ground-wave
propagation during daylight hours for local urban and suburban coverage.
However, beginning at about 3 MHz, ground-wave propagation at distances greater than 100 miles (or about 160 kilometers) becomes impractical.


Refraction of radio waves- Radio waves, like light waves, are
refracted or bent when they pass from one medium to another medium
with a different density. Since radio waves traveling upward experience
less atmospheric density, they may be curved or bent. This is related to sky-wave propagation.

Sunspot cycle- The sun exhibits a periodic 11-year cycle of increasing and decreasing sunspots which affect radio communications on earth. Scientists have recorded the number of sunspots appearing on the surface of the sun for the past 300 years and have determined that the number of sunspots reaches a maximum about every 11 years. Also, the number of sunspots will vary during each maximum and minimum cycle. The maximums may range from about 60 to over 200 while minimums may drop to almost zero.

During maximum sunspot activity, excellent amateur communications
in the bands up to and including the 10-meter band are possible.
However, during minimum sunspot activity, long-range communication
is almost nil in the 15- and 10-meter bands. Also, 20-meter operation
is restricted primarily to daylight operation.

Skip distance- The skip distance is associated with sky-wave propagation
for the most part. It is defined as the distance between the transmitter
location and the point that the skywave returns to earth after striking the ionosphere. Except for the limited area subject to ground wave reception, the skip distance or zone does not allow for communications because no radio waves are reflected or bent into this zone.

Wavelength- The wavelength of a radio-frequency signal is defined
as the length in meters of that signal. More specifically, it is the length
of one complete cycle of the radio wave. Thus, a 10-meter signal or
wave is approximately 10 meters in length (or about 32.8 feet). For
example, a quarter-wave vertical antenna for 10 meters would be
approximately 2.5 meters (or about 8.2 feet) of vertical distance.

Frequency is defined as the number of complete cycles per second that a radio signal exhibits in passing from a value of maximum intensity, through zero intensity, and back to the original
value of intensity.

Frequency is measured in hertz (or simply Hz), which is directly equivalent to cycles per second; that is, 1 Hz equals 1 cycle per second.
Frequency is related to wavelength and may be used interchangeably to describe the wave motion of a radio wave. Frequency can be converted to wavelength in meters by dividing the frequency in hertz into 300,000,000. For example, 7150 kHz (which is 7,150,000 Hz) corresponds to a wavelength of 41.96 meters.

The ionosphere consists of layers of ionized air at heights above the surface of the earth ranging from about 7 to 250 miles (or about 11 to 402 kilometers). This ionization of the rarefied air particles is caused by the ultraviolet radiation from the sun. The heights of these layers vary from daylight to darkness, depending on the position of the sun with respect to the surface of the earth.