# Flexible Phased Antenna Arrays: A Review
## 4 Major Challenges
### Mechanical Deformation and Its Impact on Performance
Moreover, frequent bending and rolling of FPAs can degrade electrical and mechanical integrity. Repetitive mechanical stress can induce small cracks in printed metallic traces leading to the disruption of the current flow, reducing the signal integrity which affects the gain and beam steering capability. Substrates such as PET or Rogers laminates experience fatigue over repeated flexing/bending, which can result in loss of elasticity. A solution to this challenge can be the use of a rigid-flex architecture which was briefly discussed in the previous sections. It combines rigid substrate tiles mounted on flexible dielectric laminates, so defining the precise region in which the device can bend and obtaining flexible and lightweight antenna arrays. For example, an RFAA maintained performance after 50 roll-unroll cycles in the application in small satellite lightweight apertures [53]. A similar study showed an array conformed to a cylindrical geometry, maintaining performance under deformation by alternating rigid-flex configuration [54]. With the aim of completely eliminating the rigid phase shifter circuitry to improve flexibility and durability under bending, a novel technique was implemented which utilized silver-coated ferrite particles to achieve tunable phase shifts by aligning the particles using a static magnetic field. The use of static magnetic fields to activate phase shifters eliminated the need for external biasing, significantly reducing complexity [59]. In another approach, a bending radius as small as 5 mm was achieved in 2021 by utilizing Taconic@fastFilm(tm) to develop a low-loss flexible PCB-based TX/RX module that maintains high RF performance under bending [64]. In general, RFAA is an efficient solution for FPA. Beamformer IC can be attached to the rigid parts of the device without the problem of bending degradation. While this is an efficient solution, maintaining electrical continuity between the flexible and rigid sections without degrading performance is still a key challenge. Furthermore, flexibility in RFAA can only be achieved as a global property with tiles that locally remain rigid but can conform globally due to the presence of the rigid tiles. Moreover, bending among the two axes (around a sphere) still remains challenging. The discussed design strategies as well as pros and cons of their phase compensation methods are summarized in Table 2. Future trends of research to face the mechanical deformation challenges lie in the development of advanced algorithms for phase compensation based on Field Programmable Gate Array (FPGA) technology. \begin{table}
\begin{tabular}{c c c c c} \hline
**Paper** & **Design** & **Phase Compensation** & **Advantages** & **Disadvantages** \\ \hline
[69] & Folded dipoles on copper ground & Spiral Match algorithm, Semi-Definite Relaxation & No additional sensors, Modular & 1D bending, inaccuracy in non-uniform bends \\ \hline
[70] & Flexible substrate with copper PCB & Mutual coupling used to infer relative phase center (differential phase measurement) & No need for external sensors, Openates under bending, High mechanical flexibility & Variations in antenna/feed network not accounted for \\ \hline
[36] & Microstrip patch array printed on a flexible substrate & Phase compensation based on the data of sensor deformation detection & Low complexity, Pattern recovery under deformation & Limited to predefined geometrical shapes \\ \hline \end{tabular}
\end{table} 
Table 2: Comparison among different designs and phase compensation strategies.