Examining the LEO to GEO Scan Loss Challenge
As the beam moves further from broadside, maintaining control of the sidelobe pattern becomes more difficult.
The challenge at the edge of scan
Alongside scan loss that comes from a reduction in gain as an antenna steers away from its central position, beam width, sidelobe behaviour and cross-polarisation also affect the antenna’s performance as the beam moves towards the edge of the scan range.As the beam moves further from broadside, maintaining control of the sidelobe pattern becomes more difficult. Increased sidelobe levels can make the antenna more susceptible to unwanted signals and can also increase the potential for interference with other spectrum users. This matters in an increasingly crowded orbital and spectrum environment, where neighbouring and overlapping allocations have to be carefully managed.
Cross-polarisation, which can become more pronounced as an array scans, is another potential issue that needs factoring in when considering how an FPA will perform. In this scenario, some of the transmitted or received energy essentially “leaks” into horizontal polarisation, reducing the quality of the wanted signal and increasing risk of interference. This consideration becomes particularly relevant when an antenna is expected to operate across different orbits and a wide range of look angles.
Need for more comprehensive testing and standardised specifications
Assessing an FPA for multi-orbit operation therefore requires a much broader view of performance than a single gain measurement. Ideally, specifications will show how the antenna performs across
the actual operating range, including gain at relevant scan angles, cross-polar discrimination and sidelobe behaviour.
Testing is obviously a crucial requirement to verify how an antenna performs, yet while measurements taken at broadside or at a single pointing angle can provide useful information, they do not
necessarily reveal how an array will behave across its full field of view. Testing across the required scan range provides a better indication of how performance changes as the beam moves
away from its central position. This is particularly relevant for sidelobes, where behaviour at the edge of the scan range can differ from what simulations or central measurements suggest.
There is also a wider issue around how FPA performance is specified. Gain, G/T, EIRP and scan range are not always presented in a consistent way, making it difficult to compare antennas on a
like-for-like basis. A more common approach to specifying performance across scan angle, temperature, polarisation and other relevant operating conditions would give operators a clearer picture
of what an antenna can deliver in the environments where it will actually be used.
Designing for the entire window of operation
Given that more elements can provide greater control over the beam and help maintain sidelobe performance across a wider scan range, the obvious response to the scan loss challenge is to increase the number of elements in the array. However, this comes with its own trade-offs.
