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Radio Signal Propagation
Radio propagation is a general term used
to clarify how radio waves behave when they
are transmitted, or are propagated from one
point on the Earth to another. Propagation
on Earth is affected by a number of factors
determined by its path from point to point.
This path can be a direct line of sight path
or an over-the-horizon (also known as radio
horizon) path helped by refraction in the
ionosphere of the Earth’s atmosphere.
Radio-wave propagation in the terrestrial
environment is an enigmatic phenomenon whose
properties are difficult to predict. This is
particularly true for radio applications
where terrain features (hills, trees,
buildings, etc.) and the ever-changing
atmosphere provide scattering, reflection,
refraction, and diffraction obstacles with
dimensions of the same order of magnitude as
the wavelengths.
Lower frequencies (between 30 and 3000 kHz)
have the property of following the curvature
of the earth via ground wave propagation in
the majority of occurrences. The interaction
of radio waves with the ionized regions of
the atmosphere makes radio propagation more
complex to predict and analyze than in free
space. Ionospheric radio propagation has a
strong connection to space weather (which is
variable and therefore unreliable during
periods of solar activity such as solar
flares).
Since radio propagation is somewhat
unpredictable, radio propagation mapping is
not always 100% accurate and is only helpful
to get a general estimate of areas that the
radio signals may reach in a near perfect
condition. Also the land cover vegetation,
atmospheric conditions, weather and other
elements are always effecting the radio
signal propagation; hence, all the radio
mapping predictions do have some
limitations, however, the approach taken in
the BAS represents the most practical means
of defining an accurate picture of broadcast
reach without undertaking a very expensive
(and limited in different ways) physical
signal strength survey.
The antenna
The beginning and end of a communication
circuit is the antenna. The antenna can
provide gain and directivity on both
transmits and receives. Antennas have
practical uses for the transmission and
reception of radio frequency signals (radio,
TV, etc.). In air, those signals travel
close to the speed of light in vacuum and
with a very low transmission loss, giving
radio broadcasts their long range. The
signals are absorbed when propagating
through more conducting materials, such as
concrete walls, rock, etc. When encountering
an interface, the waves are partially
reflected and partially transmitted through.
There are two fundamental types of antennas,
which, with reference to a specific three
dimensional (horizontal, vertical or
circular) plane are either:
1. Omni-directional
(radiates equally in all directions), or
2. Directional (radiates more in one
direction than in the other.
Antenna parameters
There are several critical parameters that
affect an antenna's performance and can be
adjusted during the design process. These
are:
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Resonant frequency
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Impedance
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Gain
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Radiation pattern
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Polarization
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Efficiency
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Bandwidth
Transmitting antennas may
also have a maximum power rating, and
receive antennas differ in their noise
rejection properties. All of these
parameters can be measured through various
means.
To reduce the potential for problems, the
radio station’s antenna should be located:
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as far away from
neighboring houses as possible, and
preferably higher than them;
-
away from AC power lines
which could affect its operation;
-
away from cables for the
telephone, cable television or
television antennas; and
-
as far away from any
electronic equipment as possible.
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