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This is the range of angles for which the Antenna Gain [1] remains
larger than -3dB of the peak value. A beam width of 20 degrees hence
implies that the antenna radiates gain -3dB or higher over 20 degrees
(main beam - 10 and main beam + 10 degrees). Sometimes the same for
all angles (beam is 3 dimensional) but sometimes will have an azimuth
beam width of say 60 degrees and elevation beam width of 10 degrees)
Figure 1 : Illustrating the 3 dB beamwidth of an 8dBi patch antenna.
The beamwidth is 43
Think of it like this:
* Narrow Beamwidth (Dish, Helical, Yagi) for Point-to-Point links
* Wide Beamwidh (omni, patch) for local area distribution
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MATH
For a reflector antenna it may be expressed as
HPBW = a = k l / D
where k is a factor that depends on the shape of the reflector and
the method of illumination. For a typical antenna, k = 70 (1.22 if a
is in radians). Thus the half power beamwidth decreases with
decreasing wavelength and increasing diameter.
For example, in the case of the 2 meter antenna, the half power
beamwidth at 6 GHz is approximately 1.75. At 14 GHz, the half power
beamwidth is approximately 0.75 . As an extreme example, the half
power beamwidth of the Deep Space Network 64 meter antenna in
Goldstone, California is only 0.04 at X-band (8.4 GHz).
The gain may be expressed directly in terms of the half power
beamwidth by eliminating the factor D/l . Thus,
G = h (p k / a )2
Inserting the typical values h = 0.55 and k = 70 , one obtains
G = 27,000/ (a)2
where a is expressed in degrees. This is a well known engineering
approximation for the gain (expressed as a numeric). It shows directly
how the size of the beam automatically determines the gain. Although
this relation was derived specifically for a reflector antenna with a
circular beam, similar relations can be obtained for other antenna
types and beam shapes. The value of the numerator will be somewhat
different in each case.
For example, for a satellite antenna with a circular spot beam of
diameter 1 , the gain is 27,000 or 44.3 dB. For a Ku-band downlink at
12 GHz, the required antenna diameter determined from either the gain
or the half power beamwidth is 1.75 m.
A horn antenna would be used to provide full earth coverage from
geostationary orbit, where the angular diameter of the earth is 17.4 .
Thus, the required gain is 89.2 or 19.5 dB. Assuming an efficiency of
0.70, the horn diameter for a C-band downlink frequency of 4 GHz would
be 27 cm.
Taken From ATI Space and Communications [2]
* WLAN Installation Manual
* (http://wireless.ictp.trieste.it/handbook/C4.pdf Antenna Basics
Links:
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[1] http://melbournewireless.org.au/help/?AntennaGain
[2] http://www.aticourses.com/antennas_tutorial.htm
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