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Focal length of parabolic dish |
Correcting VSAT dish distortion. |
1.2m Ku band VSAT antennas (from Patriot *)
If you are learning to use the link budget calculator then you can use the technical antennas parameters below as real examples of the input variables.
When considering the uplink interference entry in the link budget you can try either the 35 or 26 dB values for the uplink cross polarisation interference. In both cases the same off-axis gain envelope applies, so you might have 7 interference entries on the uplink and 7 interference entries on the downlink as follows:
Satellite 6 deg to the left (work out the value using the off-axis gain envelope)
Satellite 3 deg to the left (work out the value using the off-axis gain envelope)
Cross polarisation interference from same satellite (use 35 or 26 dB, as above)
Beam reuse interference from same satellite (just guess, say 33 dB)
Satellite 3 deg to the right (work out the value using the off-axis gain envelope)
satellite 6 deg to the right (work out the value using the off-axis gain envelope)
In your opinion, what is the value in overall system terms of the extra cross-polarisation performance of the dual optics antenna ? What capacity loss results from some dishes being mis-pointed to -1 dB on transmit and their cross polarisation emissions degraded to 26 dB ?
1.2m Ku band VSAT antenna - single optic
Cross polarisation: on-axis 35 dB, within 1 dB beam width 26 dB.
Transmit frequency: 13.75 -14.5 GHz Receive frequency: 10.7 - 12.75 GHz
Noise temperature = 58K at 10 deg elevation, 53K at 30 deg elevation
Transmit gain=43.3 dBi Receive gain=41.8 dBi
Off-axis side lobe performance (dBi):
>29-25 log q
2.5°<q<7°
+8
7°<q<9.2°
32-25 log q
9.2°<q<48°
-10
48° < q
Focal length = 76.2 cm, f/D=0.635, Offset angle=22 deg
Explanation of "offset angle": If the front face of the dish is made vertical, the beam elevation is 22 deg.
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1.2m Ku band VSAT antenna - Gregorian dual-optics
Meets Eutelsat's rule that the cross pol performance within the -1 dB transmit beam width is better than 35 dB.
Cross polarisation on axis 35 dB, within 1 dB beam width 35 dB.
Transmit frequency: 14.0 -14.5 GHz Receive frequency: 10.7 - 12.75 GHz
Noise temperature = 58K at 10 deg elevation, 53K at 30 deg elevation
Transmit gain=43.3 dBi Receive gain=41.8 dBi
Off-axis side lobe performance (dBi):
29-25 log q
2.5°<q<7°
+8
7°<q<9.2°
32-25 log q
9.2°<q<48°
-10
48° < q
Focal length = 76.2 cm, f/D=0.635, Offset angle=22 deg
* Footnote
The company Patriot Antennas was formed in 1992 and manufactured antennas for both civil and military applications. Up to 120 people were employed. In 2007 the company was taken over by Cobham. By late 2011 the number of staff was down to 31 and the site was closed 31st Dec 2011 with the remaining work transferred to Cobham's facility in Orlando, Florida.
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► Page created 28 Sept 2004, amended 28 July 2024, 2 March 2025 |