PRODUCT NOTE 7106
GRE A
MERICA
, Inc. 425 Harbor Blvd. Belmont, California 94002 phone (650) 591-1400 fax (650) 591-2001
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COAXIAL CABLE
How the connection from the antenna to GINA is accomplished can be as important as
the antenna. If much of the wireless signal is lost before it reaches the antenna, or the radio, the system won't
work well. The transmission line of coaxial cable must be chosen to match the 50 ohm antenna impedance
and to limit the losses due to the cable length. A reasonable choice for up to 100 feet of cable is Belden 9913
with a loss figure of about 4.5 dB/100 ft. [NOTE: The loss in dB is linear with distance, so the drop for 50 ft
would be 2.25 dB.]
DATA QUALITY
GINA needs -95 dBm of “clean” received signal to provide a minimum Bit Error
Rate of 1 error out of 1,000,000 bits sent. It is wise to have some “insurance” or fade margin, so the design
signal strength at the radio will be -80 dBm allowing 15 dB of extra margin.
SYSTEM CALCULATIONS
By putting all of these calculations together, it is possible to determine the
viability of a wireless link before installing the equipment. The process is simply to add the gains, subtract the
losses with the goal being a received signal better than -80 dBm.
Example:
Suppose that it is necessary to send data 5 miles over a “flat” terrain. The AGL of the receive and
transmit antenna should be between 45 and 50 feet for best results. [NOTE: A lower 15 ft AGL can work but
signal losses and ground reflections do occur that will reduce the overall performance.] GINA's transmitter
RF power is +30 dBm. The receive and transmit antennas each have a gain of +6 dBi. By adding those
numbers the total system gains +42 dB. Five miles of space loss is approximately -110 dB with 2.7 dB of
coaxial cable losses (60 feet) at each of the transit and received ends, the total loss is about -116 dB (rounded
up). Then -116 dB and +42 dB combined will equal -74 dB which is the calculated received signal strength.
This -74 dBm is a stronger signal by 6 dB than the minimum design -80 dBm signal, four times better, and the
path will work very well (when dealing with minus numbers, less is higher).
If the AGL is lowered to 15 feet at each end, then an obstruction loss of at least 10 dB will occur. Now the
received signal is - 84 dBm or about 60% weaker than the design minimum. The system still works but it is
more susceptible to interference and fade.
SITE
There are some important details that must be considered before the antenna is installed. Check the
proposed antenna site for other radio transmitting antennas; the GINA antenna must be as far as possible from
them. If the antenna is a directional antenna, Yagi or panel type, it must be pointed away from any other
transmitting antennas. If an omnidirectional antenna is used, it should be mounted above or below the field of
other transmitting antennas. Stay away from locations that are sites for TV stations, RADAR stations, paging
systems, all high powered, pulsing modes of RF energy, and other spread spectrum systems. Taking these
precautions can save a lot of time and effort. The external antenna parts are vulnerable to many problems and
they must be installed for easy repair. If the antenna system fails, the wireless system fails!
INSTALLATION TIPS
If it is necessary to provide a tower to elevate the antenna, then the structure must
safely support the weight of any service personnel. When a mast is used, it is desirable that the top be
reachable with a self standing ladder for antenna orientation. Lightning protection is very important and the
National Electrical Code and/or the local building codes should be consulted for proper grounding
procedures, especially on tall buildings. Consider a coaxial cable lightning arrestor as well as an antenna that