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# Antenna beamwidth calculator

Enter the diameter of your parabolic dish in m (metres) and the operating frequency in GHz (Gigahertz)

A typical VSAT is 1.2m diameter and transmits at 14 - 14.5 GHz

The calculation results are:

. The total side to side beamwidth of the antenna main beam.
. Half the side to side beamwidth, if you are concerned to determine the offset angle from the beam centre.
. Antenna gain. dBi means relative to an isotropic omni-directional antenna.
. The antenna gain shown in top line of the results is the on-axis gain.

The results are increasingly approximate beyond the -3 dB contour. The first null may appear at an angle off the boresight similar to the -3 dB full beamwidth.

Default values:
Maximum main beam gain calculation assumes an efficiency of 0.65
The shape of the main beam assumes parabolic illumination with level at dish edge = -10 dB down.

 Diameter of your satellite dish Frequency Efficiency 0.6-0.8 metres GHz

 dB contour down Full beam width deg. Half beam width degr. Gain dBi 0 0 0 0.25 0.5 1 2 3 4 4.5 5 6 7 8 9

7 July 2016: Comment about why -10 dB beamwidths and beyond are not shown?

The problem is that from -10 dB down onwards the beam width indicated might be significantly misleading. If I include the lower levels I will get complaints that it does not match people's measured results.

The beamwidth of the beam coming off the main parabolic reflector (or from a flat array of radiating elements) depends much on the shape of the power distribution across the main large aperture.

For example, if the feed is abnormally large diameter it will illuminate the centre of the dish very brightly and the edges hardly at all. This leads to a broad, lower gain beam but with very low sidelobes. There is negligible noise pick up from the ground and the antenna noise temperature is low. Because the dish edges are hardly illuminated the dish diameter appears smaller. It would fail to meet an expected gain specification.

For example, if the feed is abnormally small it will illuminate the dish evenly. This would also apply to a phased array if all the elements had the same power level. This leads to a very narrow main beam, with high directivity but with lots of high level sidelobes. The central beam is main beam and is very narrow but it does not give high gain, as you might expect, because a lot of the total gain has been lost in the sidelobes which point in unwanted directions. The noise temperature is high due to excessive ground pick up and interference to and from adjacent satellite is high. It would fail sidelobe envelope testing.

Antenna design is a compromise. The best large antennas do not have exact hyperbolic and parabolic subreflector and main reflector. They have slightly distorted shapes. The idea is to get a main dish illumination so that it is broadly flat but with a low in the centre, opposite to the subreflector obstruction, and low near and at the edges to minimise sidelobes. The pattern of a good dish may have highish first sidelobes (e.g. -13 to -15 dB) but these are at close angles off boresight that are less than the satellite to satellite spacing and then low sidelobes further out to meet the tight specified sidelobe mask, to minimise interference to and from other satellites.

Simple receive only antennas often have about -10 dB edge illumination, but do suffer from adjacent satellite interference, thus some TVRO antennas that are distinctly elliptical. Their sidelobes are lower in the sideways direction of the broader dish width. VSAT antennas, which can cause serious transmit interference, need to have -16 dB edge illumination to keep sidelobes down.

-10 dB edge illumination means that if you put a power meter in the middle of the dish it would give a reading 10 times higher than if held at the dish edge or just sideways of the dish edge. ( 16 dB = 40 times) Note that for human safety you should never get into the region between the feed and the dish or the cylinder of the beam. There is extreme safety hazard that you eye lens will quickly cook if in a microwave beam.

The results of my calculator are only an approximate simulation. The professional method, which should give more exact results, uses complex antenna design software, but often still needs, as an input the measured pattern, power and phase angles of the feed itself.

Best regards, Eric

 Any problems or comments, please e-mail me Eric Johnston This calculator is copyright (c) 2007 Satellite Signals LtdOriginal : 10 Oct 2007.   Amended 9 April 2012 to include the possibility of altering the efficiency, in the range 0.6 to 0.8 Page last updated 5 Nov 2016, 20 Jan 2019 HTML5