What
wireless antennas should I use for Point to Point wireless data
transmission?
Directional antennas should be used for point-to-point wireless
transmission. The type of directional antenna depends upon the
power output, cable type and length, height, distance, data
rate and terrain. We recommend the use of a range table to estimate
the wireless antenna types. Whichever wireless antenna you choose,
make sure that it is FCC certified with your radio!
Is the Customer
or Client (CPE) system considered Point to Multi-Point or
Point to Point wireless?
If the CPE system (or Subscriber Unit - SU) only talks with
the POP/AP and is at a fixed location, then it is considered
to be Point to Point wireless and can use power and antenna
gain associated with Point to Point wireless systems, as shown
below. (This has been verified by FCC Certified systems using
a 26 dBm radio and a 17 dBi antenna) If a CPE system is part
of a mesh network, then it is considered Point to Multi-Point.
Is it safe to work
or stand near an active Point to Point wireless LAN antenna?
RF safety is always a concern when working around microwave
radiation. The FCC has set safety standards that are meant
to guard against potentially harmful RF fields. In a "controlled"
environment (where the people near the antenna know of the
hazards), the FCC limit is 5.0 mw/cm^2. In an "uncontrolled"
environment (where the people near the antenna are unaware),
the FCC limit is 1.0 mw/cm^2. These limits are directly related
to the EIRP of the antenna system along the main beam. Other
frequencies and powers may be calculated at this site: RF
safety Here are some other excellent links: ARRL - RF Exposure,
ARRL - RF Safety, FDA, WLANA - Health
At
2450 MHz
| EIRP
of antenna system
(dBm) |
Min.
Safe Distance in a Controlled Environment
(feet) |
Min.
Safe Distance in an Uncontrolled Environment
(feet) |
36 |
0.31 |
0.63 |
40 |
0.46 |
0.98 |
42 |
0.57 |
1.22 |
44 |
0.71 |
1.52 |
46 |
0.88 |
1.90 |
48 |
1.09 |
2.37 |
How
do I perform a Point to Point wireless site survey?
Initially, create a path profile using one of the various
mapping programs. If LOS and Fresnel zone clearance seems
good, check for trees and other unusual obstacles to LOS.
A good way to check this is to place a person at each end
of the path with a high-powered flashlight and a cell-phone.
While talking with each other, flash the light so that the
other person can see it. UHF hand-held radios (FRS or commercial
frequencies) also work well to determine LOS. Use 1 watt radios
for up to 4 miles and 5 watt radios for up to 15 miles. If
results look promising, place an AP at one end and a CPE at
the other and try connecting using 19-24 dBi grid or panel
antennas. (Do not swing both directional antennas at the same
time!) Look for interference at each end by using a spectrum
analyzer and both vertical and horizontal polarized antennas.
If you have Teletronics radios, here is a neat site-survey
tool. There are also professional consultants (e.g. Cyber-Doctors)
that can perform wireless site surveys for a fee. Wireless
site survey tools are available on the AeroNet wireless broadband
site.
What is the Maximum
Distance for a Point to Point wireless link?
The maximum distance for a standard 802.11b Point to Point
(or Point to Multi-Point) path is approximately 12 miles.
This is primarily due to timing issues in the 802.11b firmware.
Other operating systems, such as KarlNet TurboCell, Orinoco
COR or StarOS can overcome this limit and produce links up
to 70 miles, depending upon terrain.
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