We have presented link budget calculations for SATCOM downlink in a previous post, where we described mathematical equation to calculate input SNR of ground terminal (receiving system). For details, read here: Link budget calculations for SATCOM – Downlink.
The SNR equations involves a parameter called as G/T of the terminal where G is the gain of terminal antenna and T is the noise temperature of the receiving system including antenna itself, RF chain, and LNB with the assumption that LNB has sufficient gain to make any noise contributions after the LNB negligible.
The purpose of this post is to explore the gain calculations of Electronically Steerable Antenna (ESA) of ground terminal in terms of scan angle and cosine roll-off.
The above diagram shows the beam pointing coordinates of an ESA terminal highlighting the broadside vector and boresight vector.
Broadside vector is perpendicular to the plane of antenna and antenna gain is maximum along this vector and is called as peak gain. For fixed objects applications, antenna is always placed so as to align the satellite (beam direction) along broadside vector to get maximum gain out of the antenna and hence, best possible SNR for downlink.
Boresight vector is the direction of the beam towards satellite in moving applications, and is defined in terms of two angles: theta (scan angle) and phi.
Scan Angle is the angle (theta) between broadside and boresight vectors. It is the first angle that determines the direction of beam.
- As the scan angle increases, gain of the ESA reduces.
- Note that, for downlink SNR calculations, we only need scan angle
Phi is the other angle that determines the direction of beam (boresight vector) and is measured in the plane of the antenna from a reference point.
Peak Gain (dBi) is achieved along the broadside vector (when theta is zero) and provided by the manufacturer in the units of dBi.
Cosine roll-off is an antenna coefficient and describes the ESA gain reduction as the scan angle increases. This is also provided by the manufacturer. Note that it is also called as scan roll-off.
Gain (dBi) of the antenna is finally calculated in terms of peak gain, scan angle , and cosine roll-off as follows: