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In this thesis modes on circular antenna arrays are investigated and exploited. Circular antenna arrays are specifically chosen because of their symmetric structure which enables continuity of surface currents and symmetric mutual coupling between elements of the array. This makes circular array perfect for electronic beam steering in azimuth with no variation in gain or beam shape. Previous works have demonstrated these aspects of multiple mode generation on circular arrays but in this work we established a basis of how these modes can be put to use for various applications. The mode beams have a special property of being mutually orthogonal to each other. This enables them to be radiated in free space without interference. The aim is solely to exploit the additional degree of freedom (mode or space) to add to the diversity of variables like time and frequency. The mode diversity is the main target to be exploited. This exploitation of mode diversity was successful done by series of demonstrations using Multi-mode circular array (MMCA). The various outcomes that have resulted from mode exploitation include: 1) Envelope Correlation Coefficient (ECC) was defined as a metric to estimate the mode patterns mutual orthogonality. With ECC and other parameters we developed a selection criterion of modes that can be used for encoding parallel data for transmission on MMCA. A novel technique of extracting the encoded data by receiver data pattern capture was presented. The enhanced Bit-Error-Rate (BER) results of MMCA spatially encoded data were demonstrated. Further improvement of spectral efficiency using MMCA was evaluated using Rotman-Lens (RL) fed circular array. 2) Secondly the direction modulation circular array transmitter was demonstrated. This was used for applications in physical layer security for secure wireless data transmission. It was shown here that DM can be achieved using circular MMCA and additionally the higher order modes were used for injecting interference towards the eavesdropping/jamming signal. Further dual-beam direction modulation (DM) system was demonstrated for the first time with application to simultaneous tracking and communication with the target based on BER of the composite signal transmission. 3) Thirdly, the linearity of the modes was exploited, which allows for superposition of radiated beams using MMCA. The multiple beam formation capability of the MMCA was well exploited by using phaselocked loop (PLL) based hardware for direct digital beamforming. Here, different types of beam-forming like uni-cast for single beam, multi-cast for multiple beams, broad-cast for full azimuthal beam coverage using mode-0, and simultaneous casting of all was also presented. The beamforming techinique was experimentally demonstrated on MMCA with all hardware well described. A novel displaced phase centre circular array method was presented with results, which has useful applications for clutter reduction in Doppler radars. 4) Further the mode theory was extended to retrodirective arrays. Passive RDCA utilising RL was demonstrated with examples, where novel beam-port interconnections technique was presented. Further cascaded RL configuration was experimentally analysed and demonstrated for retrodirectivity. The surface current simulation tool was extensively used to demonstrate the retrodirective action. 5) Finally, an active retro-directive circular array (RDCA) was realised with phase-conjugate boards (PCB) used for direct pre-element phase conjugation. The initial work on linear retrodirective arrays was now extended to RDCA demonstrating full 360 ◦ coverage. Additional reconfigurability was added to control the beam by selectively switching the elements of RDCA. The decimated RDCA was demonstrated, where a selective group of active elements on RDCA are used for retrodirectivity, thereby optimising the hardware resources used for retrodirection. This was extended to large scale platforms for ESA space vehicle, with simulations demonstrating the RDCA reconfigurability. This thesis extensively exploits the benefits of MMCA for various applications, all put together in a single platform associated with modes on circular array. Future works from these results are discussed which can be beneficial to communications, radars and retrodirective arrays.