Campbell Scientific RF500M Radio Modem - Manual (Page 28)

Appendix A. Fundamentals of Radiotelemetry
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.
A.2 Antennas
An antenna is a device that 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.
Every antenna has a known horizontal and vertical pattern of radiation. The
horizontal radiation pattern consists of any segment of a 360 degree circle
surrounding the antenna. The horizontal pattern is important to consider when
a RF station is to communicate with more than one other RF station. The
vertical pattern is the radiating pattern in the upward and downward directions.
Any two communicating RF stations must have a minimum level of signal
power. Power is normally expressed in decibels (dB), or decibel milliwatts
(dBm). Power is lost through transmission cables (transmitting and receiving)
and over the communicating distance. Power is gained through the
transmitting radio, and the two antennas. Antenna gain is specified in decibels
in reference to a dipole, and can vary from 0 to 10 dB in common antennas. A
unity gain antenna has a 0 dB gain, therefore no additional power is added by
using these antennas.
Antenna gain is accomplished by either concentrating the radiating power in a
small sector, or using multiple radiating elements with additive patterns.
A.3 RF500M Modem
The RF500M Modem is the main communication control device in a
radiotelemetry network. The RF Modem enables a central base site to
communicate with up to 254 different RF stations.
The RF Modem is a microprocessor controlled device which codes all
transmissions for a specific communication path. Each has a hardware ID
switch for identifying different stations.
The purpose of the RF Modem is to control operation of the radio and provide
protection for data integrity. The RF Modem controls the communication
A-2