The fifth generation (5G) mobile network is at the cusp of becoming the dominant form for wireless communication and internet access. Its advantages over previous generation cellular technology in speed and latency are already enabling new applications of big data and edge-cloud cooperation. However, in order to enable effective adoption of this technology across all industries, it is vital that integration of 5G communication is simple, low cost and comprehensive.
Aerospace and terrestrial multi-point high-throughput links are becoming a particularly challenging, but potentially groundbreaking new use cases for 5G communication. For instance, the internet access currently served to aircraft today is delivered via air-to-ground and satellite hybrid networks such as the European Aviation Network1. However, such networks provide limited internet speeds up to 50 Mbps to be shared between all passengers and necessary aircraft communication. By implementing 5G in place of these LTE networks, data rates and latency can be improved by an order of magnitude2. Not only does this grant a better experience for passengers for streaming and communication but allows for live data transfer between the aircraft systems and ground services. Such data will enable live remote analysis on the cloud to improve the efficiency, safety, and cost of air travel3,4,5. Furthermore, this data can be sold to third parties such as climate and meteorological research. Unmanned Aerial Vehicles (UAVs) and drones can similarly benefit from wideband 5G coverage as it will enable autonomous drones that can operate cross-border in any region of the world6.
Yet, to enable these benefits effectively there are several technical requirements in relation to the antenna performance and design. The antenna array must be low-profile in order to ease integration and retrofitting while maintaining minimal drag. Secondly, wideband coverage is vitally important for complete global compatibility. While 5G has been predominantly standardized, the frequencies licensed in each region differ (e.g., 3.4–3.8 GHz in the EU and 3.7–5 GHz in the USA)7. As such, in order to cover 5G globally with a single system, a bandwidth from 3.3 GHz to 5 GHz8 at a minimum is required for mid-band 5G, even though only approximately 0.4 GHz bandwidth is necessary in any one location and its corresponding band. High gain beamforming is also necessary in order to effectively receive and transmit high speed communication over long distances. This further complicates the array design.
Planar wideband antenna arrays are the obvious choice to meet these requirements. However, previous such proposals have encountered a number of challengers that have impeded the development of such a solution. Indeed, existing single element designs such as slotted patches9,10,11,12 can provide reasonable bandwidth (< 35%), and more complex topologies such as spiral antennas13,14,15, using a traveling wave model, can offer even greater bandwidth coverage (> 50%). However, among these designs the radiation pattern direction can vary drastically across the operational bandwidth. This may be perfectly acceptable for many applications and, indeed, has been successfully exploited for many switch beam steering topologies16,17,18. Alas, for applications such as 5G and LEO satellite communication where array beam steering is required, such variation in unit element beam pattern will heavily restrict the beam steering capabilities of the array. Additionally, wideband beamforming performance will always be impacted by the physical geometry of the array structure as the optimal element distance will change with frequency. Careful optimization of the array geometry is therefore required to ensure consistent gain and steering performance across the bandwidth. To the best of our knowledge, it is for these reasons that there are currently no documented planar spiral antenna arrays capable of providing the wideband beam steering performance presented in this work.
For instance, some topologies of spiral antenna arrays have been developed that are capable of providing a wide operating bandwidth19,20. However, they lack good beam steering due to the large element size and variation of unit element radiation pattern over frequency. A wideband spiral antenna phased array designed by Fang, H. R.19 demonstrates the challenges mentioned previously and emphasizes that beam steering performance is significantly affected by the variation in unit beam pattern over the operational bandwidth. As mentioned earlier, the array pattern is also impacted by the distance between array elements, and indeed the unit element in the Fang’s work19 is larger than λ/2 for most of the operational band. Further, in both Fang’s design19 and a wideband array designed by Hovsepian, A.20 4-arm spiral is used that requires a complex feeding structure/network that increases the cost of the system. Another wideband antenna array presented by Hinostroza Sáenz, I. D.21 uses a simpler 2-arm spiral design that is more compact than that of other existing designs19,20 and is therefore able to reduce the element spacing to < λ/2. This allows for steering across its bandwidth up to ± 30°. Nevertheless, modern aerospace applications now need beam steering across a range greater than ± 30° not met by Hinostroza Sáenz’s work21. Further, to maintain the small antenna diameter and large bandwidth a thick cavity structure (0.35λmax) has been required. Hence, the high drag area of the antenna structure means that even with the reasonable performance demonstrated in Hinostroza Sáenz’s work21, the design would be unsuitable for applications such as aerospace and UAVs. Such restrictions, similarly, pose system integration challenges in other new terrestrial high throughput applications. For example, high throughput Low Probability of Interception (LPI), which is presented as an application at the end of the paper.
To solve these challenges, in this paper, a low-profile (< 0.13λ5GHz), compressed spiral antenna array is presented with wideband functionality covering 3.3–5 GHz. This work expands on an antenna design previously developed by the authors of this paper22. Here, the optimisation process of this antenna is fully detailed. Further, this element is placed in a 4x4 array prototype that is optimised with a triangular topology that provides exceptional scanning capability across the operational bandwidth. This optimised topology is greatly enhanced over the initial antenna array design22 and is a key achievement of this work. Here, measurements of this prototype are presented, analyzed, and compared against existing works. With is further optimisation, this work is the first fully documented low profile wideband single-arm spiral antenna array with this level of beam steering performance. Crucial to this work, the spiral antenna element is compressed (spiral diameter < 0.35λmax) such that it provides an axial beam across all operational frequency bands. In summary, this work enables global mid-band 5G with a single, low profile antenna array. The antenna element design and optimization are detailed in Section II. In Section III this proposed spiral antenna element is placed in a 4x4 triangular lattice array, enabling beam steering functionality. The array provides elevation steering of -40⁰ ≤ θ ≤ +40⁰ at all azimuth angles. The array has then been fabricated and its performance has been measured and analyzed with a bespoke beamforming network. Additional applications and implementation of this work is also provided.