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Gain and Beam Pattern of Antennas

 Wireless Antennas or WiFi Antennas, commonly, are described by their gain (usually in units of dBi) but there is more to choosing an antenna than just going for the maximum gain. Often it is better to choose a lower gain to distribute the transmitted power over a wider angle.


Q: Can you give me a simple explanation of antenna gain?

A: Suppose an antenna, connected to a cable, radiated all of the power, that was available to it from the cable, equally in any direction. The beam pattern would be perfectly spherical. By definition the gain of this antenna would be 0dBi. This would be a truly 3 dimensional omni antenna.

Its impossible to produce an antenna that has a perfectly spherical beam shape but in some applications this would be the ideal antenna to choose. For instance, if you were setting up a wireless LAN for a company who had offices in a three storey building, with client point computers on all three floors, the best location  for the access point would be in the centre of the middle floor and the best antenna to choose would have a gain of 0dBi.

Q: OK so what happens if we use a higher gain omni antenna instead?

A: The word "omnidirectional" in antenna terminology only refers to 2 dimensions and so some of the clients would get more power from the access point and some would get less. Say we connected up an omnidirectional antenna with a gain of 11.3dBi. The beam pattern would look something like:

omni.PNG

The beam shape is a disk. This would give additional signal to users (clients) on the same floor but users above and below would get less. You should use an omni directional antenna only when all the client users are situated in the same plane.

In the above example the 3dB beamwidth (half power) of the antenna is 9.6 degrees as the following plot  shows:

omni_plot.PNG

Q: What if I choose an antenna with an even higher gain?

A: The 3dB beamwidth would be even narrower. You need to ask if this is a good thing and if all the client users would be situated within the main beam.

Q: What happens if I use a sector antenna instead of an omnidirectional antenna?

A: A sector antenna would have a beam pattern that looked something like:

sector.PNG

sector_plot.PNG

 

Note that the angular width of the sector antenna is very similar to the angular width of the omni-directional antenna but the gain is higher. This illustrates the general principle that the higher the gain the less angular coverage is provided by the antenna.

Q: What about directional antennas?

A: Apart from the ideal 0dBi antenna, all antennas are directional to some extent. The term 'directional' is usually applied to antennas that have a beam shape which is approximately circular.

directional.PNG

This is a plot of a 15dBi directional antenna. The 3db power angles are  33.7 deg and 34.7 deg. If we choose a higher gain directional antenna the 3dB angles will be less. If we choose a lower gain the 3db angles will be more. 

Antenna farfield patterns from Microwave Studio. Courtesy of Computer Simulation Technology. See www.cst.com

 

Peter Martin


www.rfshop.co.uk