User Manual

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Understanding Industrial WLAN – IEEE 802.11
2.3 Antenna Theory and Selection
Choosing the right antenna after a site survey is a small but important factor when planning a wireless project. The
purpose of this section is to explain what an antenna is and how to choose the right antenna to help build a reliable
wireless network.
Functions of Antennas
An antenna is a transducer that is designed to transmit or receive electromagnetic waves. It is like a converter
that converts electromagnetic waves and electrical currents back and forth. Different wireless devices use
different antennas to operate in different frequencies and to achieve, for example, a desired range. The most
important parameter of an antenna is its working frequency. For example, a 2.4GHz antenna is too weak to use
in IEEE 802.11a communication and the data rate will fall back to a very low level or even drop to ground zero.
Types of Antenna
There are two basic types of antennas, omni-directional and directional. The two types are categorized by the
direction in which they beam radio signals. Omni-directional antennas are designed to radiate signals equally in
all directions. Use this type of antenna if you need to transmit from a central node, such as an access point, to
users scattered all around the area.
Directional antennas provide a more focused signal than omni-directional antennas. Signals are typically
transmitted in an oval-shaped pattern with a beam width of only a few degrees. With higher gain, directional
antennas can also be used outdoors to extend point-to-point links over a longer transmission distance, or to
form a point-to-multipoint network.
Key Antenna Specifications
Connector types
Before you purchase an antenna for your wireless device, you should check the type of antenna connector
that your device uses. You will need to buy an antenna with a matching connector. There are several types
of antenna connectors, including MCX, TNC, N-type, SMA, and RP-SMA (RP stands for “reverse polarity” or
“reverse ping”). On WLAN devices, the most commonly used antenna connector is PRSMA and N-type for
IEEE 802.11 wireless applications.
Half-Power Beam Width (HPBW)
This parameter is measured from the antenna’s radiation pattern, and refers to the beam width at which the
antenna’s radiation drops to half of its peak value. It also refers to the antenna’s effective coverage area.
Once you get outside the half-power beam width, the signal typically drops off very quickly. A very high-gain
antenna has a very narrow angled half-power beam width, which makes the directionality high as well.
Antenna Polarity
Polarization refers to the direction in which the electromagnetic field lines point as energy radiates away
from the antenna. The simplest and most common type is linear polarization. When power is sent from
transmitter to receiver, only that portion of the beam with the same polarization can be received. An
improper antenna installation may decrease performance.
N-type(male) N-type(female) RP-SMA(female) RP-SMA(male) SMA(female) SMA(male)