User Manual

TRACER 4206 System Manual Section 2, Microwave Path Engineering Basics
612804206L1-1B © 2002 ADTRAN, Inc. 15
where the variables in the equation are defined as
P
R
received power (Watts)
P
T
transmitted power (100 mW maximum for TRACER 4106/4206 - adjustable)
G
T
transmit antenna gain
G
R
receive antenna gain
λ carrier wavelength (c / ƒ) (meters)
d path distance (meters)
L other losses (RF coaxial cable, etc.)
The actual transmit and receive antenna gain values are strictly dependent upon the physical characteristics
of the antennas installed for each link. Typical gains are between 20 and 40 dB. For example, a 4 foot
diameter Parabolic dish has 34.2 dB of gain at 5.8 GHz. The carrier wavelength is the physical wavelength
of the main RF carrier being used for communication, and is usually approximated at the center frequency
of the band, which is 2441.75 or 5787.5 MHz. This gives a wavelength of 12.29 cm at 2441.75 MHz and
5.18 cm at 5787.5 MHz.
The FCC specifies the maximum transmitter power that may be used for antennae of a given gain. FCC
rules Part 15, Subpart 247 allow for a maximum power of 1 watt (30 dBm) into antennae of a gain less than
or equal to 6 dBi. At 2.4 GHz (TRACER 4106), the 1 watt maximum transmitter power must be reduced
by 1 dB for every 3 dB of antenna gain over 6 dBi. Since the TRACER 4106 maximum transmit power is
100 milliwatts, only antennas with gains above 36 dBi (12 foot diameter parabolic dishes) require any
reduction in transmit power. For the 5.8 GHz band (TRACER 4206), there is no reduction in transmitter
output power required for antennae gains greater than 6 dBi.
The path distance is simply the physical distance between the transmit and receive antennas. For the
TRACER 4106/4206 these distances can range up to 30 miles. The final parameter L incorporates all other
signal power losses in the microwave link, most of which are caused by antenna feed.
4. ANTENNA GAIN
Best performance will result from the use of a parabolic dish antenna. Antenna gain is determined by the
size of the dish, with typical features detailed below. Dish manufacturers will be able to supply gains for
other types of antennas.
Table 1. Antenna Gain for Given Dish Diameters
Dish Diameter
(in feet)
Gain at 2.4 GHz (in dBi)
(TRACER 4106)
Gain at 5.8 GHz (in dBi)
(TRACER 4206)
2 21 28.5
4 27 34.2
6 31 37.5
8 33 40.7
10 35 42.5
12 37 44.2