D-1
APPENDIX D. FUNDAMENTALS OF RADIOTELEMETRY
D.1 RADIO WAVES
Radiotelemetry is the process of transferring
information (data) in the form of radio waves.
The data is transferred on a carrier wave which
normally has a sinusoidal form. Therefore, the
carrier wave can be described entirely by the
frequency, amplitude, and phase with respect to
a reference.
The commonly used term for radiotelemetry,
RF, refers to radio frequency, which in actuality
is the frequency of the carrier wave.
Radio waves can be divided into three
categories: 1) ground waves, 2) direct waves,
and 3) sky waves. All communication with
Campbell Scientific's RF networks are done via
direct waves. Direct waves travel "line-of-sight"
at a maximum distance of approximately 25
miles.
Low frequency radio waves (5-10 mHz) can
travel for thousands of miles using the ground
wave portion of the radio wave. The ground
wave is that portion of the radio wave which
travels just above the surface of the ground.
Conversely, the sky wave radiates to the
ionosphere where a certain percentage of the
energy is reflected back to earth. At the higher
frequencies used for data transmission the
ionosphere is penetrated by the radio wave and
too small of a percentage is reflected back to
earth. However, neither the ground wave or sky
wave is used in Campbell Scientific's RF net-
works.
Energy is lost from radio waves as they travel
away from the transmitting antenna. One
reason for this is the loss due to dispersion of
energy over a larger area; analogous to water
waves reducing in size (energy) as they get
farther from the source. Second, is that energy
is absorbed by the earth over the distance of
travel. Eddy currents cut down signal power,
and intervening terrain and buildings can
prevent a signal from being strongly received.
The higher the frequency, the stronger the
radiation field. However, at higher frequencies
more energy is absorbed by the surface. The
VHF and UHF frequencies can travel only a
short distance between radio stations. The
direct wave, where there is no obstacles
between stations, will transmit farther than any
indirect waves which have been transmitted
through or reflected from obstacles.
The carrier wave can be thought of as the radio
wave which "carries" the data from one radio to
the next. The "data" consists of an electrical
signal which rides with the carrier wave. The
process of placing the signal on the carrier
wave is called modulation. The signal is also in
the form of a wave, but usually the signal has a
much lower frequency. The carrier with the
modulating signal is called the modulated
carrier.
The signal wave isn't used as a carrier wave
because radio transmission must be of a high
frequency to keep radio components small,
antennas small, filtering efficient, and to isolate
the radio waves from the common low
frequency man-made noise.
The main forms of modulation are amplitude,
frequency, and pulse modulation. Frequency
modulation (FM) is used by Campbell Scientific.
D.2 ANTENNAS
An antenna is a device which captures and
radiates radio waves. The antenna at the
transmitting station is excited by the transmitting
radio. The antenna converts energy from the
radio to radiated energy. Electrons within the
antenna oscillate at the frequency of the radio
thereby producing radio waves. These radio
waves radiate out from the antenna at the
speed of light (299,800 km/s).
The transmitted radio wave will cause electrons
in the receiving antenna to oscillate at the
carrier frequency. The AC current thereby
produced in the antenna is transferred to the
radio for demodulation.
The antenna is constructed for a particular
frequency, operating radius, and gain. Length,
diameter, number of elements, and element
spacing are among the items that can be
changed to alter antenna performance at the
design stage.
