Cisco makes it easy to calculate bridge distances by using the Cisco
distance calculations spreadsheet that is available from the Cisco Web site.
The user must follow several basic steps:
- Select the product line being used.
- Next select the proper antenna for both sites. For other non-Cisco
antennas, enter the gain in dBi. The concept of the units dBd was formed to
capture the gain of an antenna relative to a dipole antenna. A dipole is
considered the standard basic horizontal antenna, and comparisons to it seemed
to be more meaningful than comparisons to the isotropic radiator. If the
antenna gain is provided in dBd, simply add 2.15 to the number to convert to
dBi.
- Then select the cable used on both sites. If using something other than
standard Cisco antennas, enter in the length and cable loss per 30 meters (100
ft.) in the appropriate place. For Cisco cables this is 6.7dB /100 feet at
2.4Ghz. If a different cable is used, contact the cable vendor for this
information.
- Add any other losses due to splitters, connectors and so forth into the
miscellaneous column.
Remember these are theoretical values, but they should provide a very
good comfort level for proper operation. These values are for line of sight and
provide a 10dB fade margin that provides assurance that the calculations will
work.
To determine the bridging distance, the following items are
considered:
- Antenna gains are given in dBi, based upon a theoretical isotropic antenna,
not dBd that is based upon a dipole antenna.
- To convert from dBd to dBi add 2.15 to the dBd. As a result, 0dBd =
2.15dBi.
- Cable lengths are a loss and are subtracted.
The antenna and radio parameters include cable losses at the receiver
and transmitter sites, the antennas used at both sites, and the performance of
the receiver and transmitter. Receiver gain changes with data rate. Always use
the maximum data rate values needed by the customer.
Distances for these
formulas are calculated in miles. For any given frequency, the atmosphere
offers losses. This loss is a standard for any radio at that frequency. In this
case, use the middle frequency of 2442 Mhz.
A sample radio and antenna
parameters is calculated in Figure
. This
calculation uses a 20 dBm value for the transmitter power, 2 dBi to 13.5 dBi
yagi antennas, and two cables of 6 m (20 ft) each. The values are entered into
the formula to calculate maximum distance.