Campbell Scientific RF500M Radio Modem - Manual (Page 27)

Appendix A. Fundamentals of
Radiotelemetry
A.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 is 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 that 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 nor
sky wave is used in Campbell Scientific's RF networks.
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 are no obstacles between stations, will transmit farther than
any indirect waves that 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 that 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,
A-1