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Newnes Radio and RF Engineering Pocket Book
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Propagation of radio waves
Frequency and wavelength
There is a fixed relationship between the frequency and the wavelength,
which is the distance between identical points on two adjacent
waves (Figure 1.1 ), of any type of wave: sound (pressure), electromagnetic
(radio) and light. The type of wave and the speed at which
the wavefront travels through the medium determines the relationship.
The speed of propagation is slower in higher density media.
Sound waves travel more slowly than radio and light waves which,
in free space, travel at the same speed, approximately 3 × 108 metres
per second, and the relationship between the frequency and wavelength .
The isotropic radiator
A starting point for considering the propagation of radio- or lightwaves
is the isotropic radiator, an imaginary point source radiating equally
in all directions in free space. Such a radiator placed at the centre of
a sphere illuminates equally the complete surface of the sphere. As
the surface area of a sphere is given by 4πr2 where r is the radius of
the sphere, the brilliance of illumination at any point on the surface
varies inversely with the distance from the radiator.
Formation of radio waves
Radio waves are electromagnetic. They contain both electric and magnetic
fields at right angles to each other and also at right angles to
the direction of propagation. An alternating current flowing in a conductor
produces an alternating magnetic field surrounding it and an
alternating voltage gradient – an electric field – along the length of
the conductor. The fields combine to radiate from the conductor as in
Figure 1.3.
Other propagation topics
Communications in the VHF through microwave regions normally
takes place on a ‘line-of-sight’ basis where the radio horizon defines
the limit of sight. In practice, however, the situation is not so neat and
simple. There is a transition region between the HF and VHF where
long distance ionospheric ‘skip’ occurs only occasionally. This effect
is seen above 25 MHz, and is quite pronounced in the 50MHz region.
Sometimes the region behaves like line-of-sight VHF, and at others
like HF shortwave.
Scatter
There are a number of scatter modes of propagation. These modes
can extend the radio horizon a considerable amount. Where the radio
horizon might be a few tens of kilometres, underscatter modes permit
very much longer propagation. For example, a local FM broadcaster
at 100MHz might have a service area of about 40 miles, and might
be heard 180 miles away during the summer months when Sporadic-
E propagation occurs. One summer, a television station in Halifax,
Nova Scotia, Canada, was routinely viewable in Washington, DC in
the United States during the early morning hours for nearly a week.
Sporadic-E is believed to occur when a small region of the atmosphere
becomes differentially ionized, and thereby becomes a species
of ‘radio mirror’. Ionospheric scatter propagation occurs when clouds
of ions exist in the atmosphere. These clouds can exist in both the
ionosphere and the troposphere, although the tropospheric model is
more reliable for communications. A signal impinging this region may
be scattered towards other terrestrial sites which may be a great distance
away. The specific distance depends on the geometry of the
scenario.
Refraction modes
Refraction is the mechanism for most tropospheric propagation phenomena.
The dielectric properties of the air, which are set mostly by
the moisture content, are a primary factor in tropospheric refraction.
Refraction occurs in both light or radio wave systems when the wave
passes between mediums of differing density. Under that situation,
the wave path will bend an amount proportional to the difference in
density of the two regions.
Super refraction
A special case of refraction called super refraction occurs in areas
of the world where warmed land air flows out over a cooler sea
(Figure 1.16 ). Examples of such areas are deserts that are adjacent to
a large body of water: the Gulf of Aden, the southern Mediterranean,
and the Pacific Ocean off the coast of Baja, California. Frequent
VHF/UHF/microwave communications up to 200 miles are reported
in such areas, and up to 600 miles have reportedly been observed.