Xirrus 2x2 Omni Antenna, Справочное руководство

Страница: 2 / 12

Overview
To optimize the overall performance of a Xirrus
WLAN in an outdoor deployment it is important
to understand how to maximize coverage
with the appropriate antenna selection and
placement. This document is meant to serve as
a guideline for anyone who wishes to use Xirrus’
antennas and related accessories with Xirrus’
newest outdoor wireless products (XR-520H
and XR-2425H). The document is organized
according to the following sub-sections:
• Basic Technical Background
• Types of available Xirrus Antennas and
Accessories
• Reference Test Data
• Design Considerations and Reference
Use Cases
Technical Background
ISM bands:
The U.S. Federal Communications Commission (FCC) authorizes
commercial wireless network products to operate in the Industrial,
Scientific and Medical (ISM) bands using spread spectrum
modulation. The ISM bands are located at three different
frequencies ranges – 900MHz, 2.4GHz and 5GHz. This document
covers products that operate in the 2.4 and 5GHz bands.
ISM bands allow manufacturers and users to operate wireless
products in the U.S. without requiring specific licenses.
This requirement may vary in other countries. The products
themselves must meet certain requirements in order to be
certified for sale such as maximum Transmit Power (Tx Power)
and Effective Isotropic Radiated Power (EIRP) ratings.
Each of the ISM bands has different characteristics. The lower
frequency bands exhibit better range but with limited bandwidth
and hence lower data rates. Higher frequency bands have less
range and are subject to greater attenuation from solid objects.
Antenna Properties, Ratings
and Representation
At the most fundamental level an Antenna provides a wireless
communication system three main attributes that are inter-
related to each other and ultimately influence the overall
radiation pattern produced by the antenna:
• Gain
• Directivity
• Polarization
Gain of an Antenna is a measure of the increase in power that
the antenna provides. Antenna gain is measured in decibels
(dB) — a logarithmic unit used to express the ratio between two
values of a given physical quantity. In the general case, the gain
in dB is a factor of the ratio of output power (or radiated power)
to the input power of the antenna (that ratio is also called the
“efficiency” of the antenna). In practice, the gain of a given
antenna is commonly expressed by comparing it to the gain
of an isotropic antenna. An isotropic antenna is a “theoretical
antenna” with a perfectly uniform three-dimensional radiation
pattern. When expressed relative to an isotropic antenna, the
gain of a given antenna is represented in dBi (i for isotropic).
By that measure, a truly isotropic antenna would have a power
rating of 0 dB. The U.S. FCC uses dBi in its calculations.
Directivity is the factor that was referred to in the previous
discussion about antenna gain and its relation to efficiency.
Mathematically, the gain of an antenna is its directivity times
its efficiency. And like its gain, the directivity of a given
antenna is also expressed relative to an isotropic antenna. The
directivity measures the power density that an antenna radiates
in the direction of its strongest emission, relative to the power
density radiated by an ideal isotropic antenna (which emits
uniformly in all directions), when they are both radiating the
same total power.
Polarization of an antenna is the orientation of the electric
field of the radio wave that it produces relative to the earth’s
surface. The polarization of an antenna is determined by the
physical structure of the antenna and by its orientation. A
simple straight wire antenna will have one polarization when
mounted vertically and a different polarization when mounted
horizontally. A
linear polarized antenna radiates wholly in one
plain containing the direction of propagation of the radio wave
while, in a
circular polarized antenna, the plane of polarization
rotates in a circle making one complete revolution during one
Xirrus External Antennas Guide