APPENDIX D. FUNDAMENTALS OF RADIOTELEMETRY
D-2
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.
D.3 RF95T MODEM
The RF95T Modem is the main communication
control device in a radiotelemetry network. The
RF95T enables a central base site to
communicate with up to 254 different RF
stations.
The RF95T 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 RF95T Modem is to control
operation of the radio and provide protection for
data integrity. The modem controls the
communication sequences, sets data to be
transferred into data blocks, creates signatures
of data blocks, modulates the radio's carrier
wave, and stores information on communication
quality.
The user at the computer is responsible for
naming the desired communication path with a
setup string. This setup string contains any
repeater modem IDs and the destination
modem ID in sequence. After sending this
information out through the RF system, all of
the RF95Ts in the specified link will set
themselves in the proper mode. The RF95T
has different modes to distinguish
responsibilities at various localities within a link.
These modes are described in Section 3.1.6
"RF95T Modem Communication Protocol."
D.4 TRANSCEIVER
The purpose of a transceiver (radio) is to
transmit and receive the modulated carrier
wave.
A radio is both a transmitter and receiver. The
main component in the transmitter is the
oscillator of which the frequency of oscillation is
provided by a crystal. The crystal oscillates at a
desired frequency, which is specific for the
carrier frequency. The oscillator converts DC
power to an AC signal. This signal is then
amplified, modulated with the signal, and
transmitted to the antenna system. The
receiver consists of an amplifier, frequency
converter to slow signal, limiter to give constant
amplitude but same frequency, and
discriminator or demodulator.
The radio has a known impedance, or
resistance. Maximum power is transferred if
the impedance of the radio matches the
impedance of the antenna and cable. This
impedance is generally 50 ohms. Mismatching
of impedance will cause a lesser transmit power
and result in a higher VSWR (Voltage Standing
Wave Ratio).
When the transmission cable and antenna does
not match the impedance of the output circuit of
the radio, not all of the energy fed down the
cable will flow into the antenna. A percentage
of the energy will be reflected back forming
standing waves on the cable. The ratio of
voltage across the line at the high voltage points
to that at the low voltage points is known as the
VSWR. When the VSWR is 3.0:1 or greater,
the percentage of errors per data value is
greater than 50%. The VSWR should be kept
below 1.5:1 for error free radiotelemetry.
