Controller Based WLANs

What is an antenna and antenna gain?

by on ‎07-02-2014 01:55 PM

Question: What is an antenna?


An antenna is a passive device that converts electrical energy into RF waves in the transmitting direction, and converts RF waves to electrical energy in the receiving direction.


Question: Why do I need an antenna and why there are many types of antennas?


Antennas are needed for many purposes:

- Increase the range of a wireless LAN.

- Control the coverage area by covering one direction more than another.

- Enhance security by reducing signal leaking with proper antenna selection.


Question: Will the antenna add any active gain to my wireless signal?


No, an antenna is a passive device. It will direct your RF signal in a certain direction as per your coverage requirement. If you talk about active gain, this is an amplifier.


Question: What are the common type of antennas?


Antennas can be divided into different types based on many factors like:

- Frequency range.

- Coverage pattern, which includes:

o  Gain (in dbi or dbd)

o  Beamwidth

o  Azimuth and Elevation charts (E and H planes)

- Indoor or outdoor


Frequency range:


As the antenna length is related to the wave length of the RF signal, some antennas work better in certain frequency ranges.  Common frequency ranges for wifi antenna are:


- ISM band (b/g band): 2.412 - 2.472 Giga Hertz


- UNII band (a band): 5.18 – 5.805 GHz


- Some antennas work better at some frequencies than others: Most .b/.g antennas work best in the middle (Chl 6)


- Many a antennas are designed for only a part of the band:  5.75 - 5.805 or 5.26 - 5.32


Coverage pattern


When discussing the coverage pattern, you should understand the following three concepts:


1-      Antenna gain:


Antenna gain measures how much the antenna can direct the signal in a certain direction more than others. The two common ways to measure gain are:


-dbi:  This unit compares the degree to which an antenna focuses its energy in certain direction relative to ideal isotropic radiator. To calculate the gain of an antenna in dbi, an RF engineer first determines the antenna’s preferred direction (the direction in which the antenna is designed to focus its energy). Then the engineer measures the power level at a certain distance from the antenna, in dbm, and calculates what power level would be at that same distance from isotropic antenna.


-dbd:  Using the same concept as dbi, but this measurement compares the antenna to theoretical half-wave antenna. The theoretical half-wave antenna has the gain of 2.14 dbi compared to isotropic antenna, as a result, 0dbd = 2.14 dbi. So if you have the gain in dbd, simply add 2.14 to get the gain in dbi.


2-      Beamwidth:


Beamwidth is the number of degrees off-axis where the antenna gain drops to half (or less by 3 db) its gain at the zero degree position (the direction where the antenna gain is at maximum).


Two vectors must be considered when discussing antenna beamwidth: vertical and horizontal beamwidths. The vertical beamwidth is measured in degrees and is perpendicular to Earth’s surface. The horizontal beamwidth is measured in degrees and is parallel to Earth’s surface.


3-      Azimuth and Elevation Chart (H and E planes):


An Azimuth (horizontal) and Elevation (vertical) chart is a standard way of representing an antenna’s coverage pattern. The Azimuth chart (H-Plane) shows a top-down view of the antenna’s coverage, and the Elevation chart (E-Plane) shows a side-view of antenna coverage.


The outer ring of the Azimuth and Elevation chart represents the strongest signal strength that was measured, and is typically marked “0 db”. Inner rings represent a signal strength of some lower db number.  The Azimuth and Elevation chart does not take distance from antenna into account; the chart shows the signal strength in each direction relative to each other direction.


The chart below shows a sample Azimuth and Elevation chart for antenna 1.




Based on the above coverage patterns, the common antenna types are:


- Omni-directional antenna:

o Sends the signal in every direction.

o Puts out an equal signal in all directions, but only in one plane.


- Patch antenna: puts out a lot more signal in one direction.


- Dipole antenna: A standard stick antenna that is basically Omni-directional. Standard dipole gain is 2.2 dBi.


- Parabolic dish

o These are for point-to-point applications.

o Very narrow signal = very high gain.

o Can be 24dBi+.




• Antennas put out signals that vary in all three dimensions. 

• Every antenna gain pattern looks like some form of distorted balloon. A perfectly equal gain antenna would have a pattern that looked like perfect sphere. 

• High-gain Omni directional antenna looks like a pancake. 

• Low-gain Omni directional antenna looks like a donut. 

• Beamwidth: The width of the signal in degrees where the signal is within 3dB of its max 


• Most Omnis have very wide horizontal beamwidth (360), but can have small vertical beamwidth (20), like a pancake.



Question: What is EIRP?


EIRP (Effective Isotropic Radiated Power) is the amount of power that would have to be emitted by anisotropic antenna to produce the peak power density observed in the direction of maximum antennagain. EIRP can take into account the losses in transmission line and connectors and includes the gain of the antenna.  

Simply, EIRP is calculated using the following formula:


EIRP = AP output power (in dbm) + antenna gain (in dbi) - cable loss or any other loss (in dbm)

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