Supplement: Aerospace, Military and Defence
Figure 3. A true time delay on common leg and phase shifters behind the V and H feeds of antenna elements to optimize beam squint while having wideband cross polarisation capability.
whole array at the lowest operation frequency. By Equation 5, this is around 2.45ns for the example array in Figure 4.
beam squint while designing a system with cross polarisation capability. Cross polarisation is generated by setting a 90° phase shift in between the V and H feeds of antenna elements. Ensuring as close to a 90° difference as possible between feeds over the desired cross polarisation bandwidth is essential to have good cross polarisation isolation for a healthy operation. Due to the fact that having
a constant phase over frequency, PS-based ESAs have a wideband cross polarisation capability (Figure 1), unlike TTD-based ESAs that can have 90° between feeds only at a single frequency (Figure 2). The architecture in Figure 3 can be used for applying cross polarisation while mitigating beam squint. TTD coverage is set by the maximum delay ∆tMAX between most distant elements of the
There are a couple of things to consider when one thinks of using TTDs behind every antenna element instead of PSs when cross polarisation is not required. This coverage means a significantly high loss and could be challenging to implement to fit into antenna spacing. Having a 6-bit phase PS’s resolution with the given coverage would bring some design challenges along with many delay stages to be placed into TTD. If the resolution is preserved and the coverage is reduced to mitigate these drawbacks, then one would have to wrap back through zero when the coverage is exceeded (by calculating phase equivalent by Equation 4) but then the beam squint feature would ironically be lost.
Figure 4. 1024 (32 × 32) element array partitioned into 16 subarrays consisting of 8 × 8 elements.
This quick analysis shows that PSs at every antenna element followed by TTDs at the common legs of the subarrays can be useful even when cross polarisation is not required. TTDs in Figure 4 would again need to have the same coverage, but this time the resolution requirement is relaxed compared to that of TTD at every antenna element case as now they are used to align relatively larger time delays between subarrays.
Breaking down a phased array into subarray partitions reduces the cost and complexity of a system at the expense of a higher scan loss and lower beam steering resolution. By having wider beamwidth, subarrays are more tolerant to beam squint effects as they have wider beamwidth. It is apparent that beam squint and beamwidth targets are important metrics with consideration to the subarray size.
Conclusion
True time delays behind every antenna element are required for a broadband squint free operation and phase shifters behind every V and H feed of each antenna element are required for a broadband cross polarisation operation.
If cross polarisation is not required and fully squint free operation is targeted, then TTD-based design should be followed. As the frequency increases, adding PSs could help to meet the QSLL target in return for a compromised squint free operation. If cross polarisation is required, then each polarisation feed of the antenna should be followed by separate but identical PSs with a tight 90° difference above the operational bandwidth. Adding TTDs on the common leg of PSs could help to mitigate the beam squint. Whether cross polarisation is required or not, a subarray architecture with PSs behind antenna elements followed by TTDs at the common legs of subarrays can be a cost- effective solution. Note that TTD functionality can be implemented in a digital domain and all digital design can eliminate both TTDs and PSs at the expense of a higher system cost. Before diving into the countless challenges of ESA design, understanding the differences in using either TTDs or PSs vs. using them in tandem is an essential part of planning a system level beamforming architecture that meets the system requirements with a better SWaP-C.
Analog Devices has a huge portfolio of solutions, platforms, and products for all analogue, digital, and hybrid beamforming ESAs in a wide range of applications along with the capability to provide tailored power solutions for the complete signal chain.
https://www.analog.com/en
www.cieonline.co.uk. Components in Electronics October 2023 21
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