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Dr Ghulam Ahmad

My publications


Ahmad G, Brown TWC, Underwood CI, Loh T (2017) An efficient algorithm for electrically large reflectarray antenna design automation,Proceedings of the 2017 International Workshop on Electromagnetics: Applications and Student Innovation Competition pp. 133-134 IEEE
Reflectarrays are becoming a potentially attractive replacement of parabolic reflectors for high gain requirements. A large reflectarray consists of thousands of elements. To predict their performance a simulation model is required which is very cumbersome to build manually due to a large number of elements. It takes exhaustive efforts, keen attention to details and significant amount of time to build such a simulation model. When several iterations of modelling are required it worsens the issue even further. We have presented here an algorithm as an automated solution to this problem by interfacing Matlab® with an electromagnetic simulation software. It is very generic, time efficient and makes the modelling easy with least intervention of the designer.
Ahmad G, Loh T, Brown TWC, Underwood CI (2017) On the Phase Selection of Millimeter Wave Quantized Reflectarrays,Proceedings of the 2017 International Applied Computational Electromagnetics Society Symposium IEEE
Microstrip printed reflectarrays are becoming a potential replacement of parabolic reflector and phased array antennas due to their simple design, low cost and ease of manufacture to attain high gain and wide angle beam pointing at millimeter waves (mm-waves). Significant challenges are faced while implementing continuous phase reflectarrays at mm-waves. However, discretizing the required reflection phase provides a practically implementable solution. This contribution addresses the selection of phase states and its scattering in a phase discretized mm-wave reflectarray. The performance of two 1.5 bit phase quantized reflectarrays having closely spaced geometrical features is analyzed at 60 GHz. This study provides a better understanding to achieve a wider bandwidth response in practically implementable mm-wave reflectarrays.
Ahmad G, Brown TWC, Underwood CI, Loh T (2017) How coarse is too coarse in electrically large reflectarray smart antennas?,Proceedings of the 2017 International Workshop on Electromagnetics: Applications and Student Innovation Competition pp. 135-137 IEEE
Millimeter wave (mm-wave) bands are becoming potentially attractive candidates for next generation communication systems. It is envisioned that high gain smart antennas will be one of the key enabling technologies for such systems. At mm-wave bands, where electrical size of an individual antenna becomes very small, the inclusion of a reconfigurable mechanism in the antenna becomes a great challenge due to real estate constraints. In these scenarios a designer has to decide on the number of bits in a phase shifter for antenna beam steering which will result in an optimum design. This contribution addresses the issue of phase quantization in mm-wave high gain reflectarray smart antennas to achieve an optimum performance. Implementing coarse phase quantization greatly reduces the complexity at mm-wave bands. A case study is presented to highlight the effects of coarse phase quantization using various numbers of bits.
Ahmad G, Brown T, Underwood C, Loh T (2017) Millimetre Wave Reflectarray Antenna Unit Cell Measurements,LAPC 2017 Proceedings Institution of Engineering and Technology
Reflectarray antennas are a potential candidate solution to realize high gains at millimetre waves (mm-waves). A reflectarray contains a large number of spatially illuminated unit cells. The performance of a good reflectarray design is manifested by the behaviour of its comprising unit cells. An established technique to characterise a unit cell is by placing it inside a waveguide to achieve periodic boundary conditions. This usually requires custom waveguide products; making the tests difficult and expensive. Additionally, when the unit cells are reconfigurable as in a smart reflectarray it is hard to take the DC bias lines out of the waveguide without using custom made waveguide parts. This contribution address the issue of unit cell placement inside the waveguide and proposes simple unit cell structures to avoid custom made waveguide parts. The idea was verified by measuring a series of unit cells at mm-waves in various configurations and a practically acceptable agreement was found. The proposed structures greatly simplify the reconfigurable unit cell testing.
Ahmad G, Underwood C, Brown T, Loh T (2017) Role of Surface Waves in Improving the Reflection Properties of a Millimetre Wave Reflectarray Unit Cell,LAPC 2017 Proceedings The Institution of Engineering and Technology
A steady increasing trend towards millimetre waves (mm-waves) for next generation communication has initiated an intensive research in the field of mm-wave antenna technologies. Reflectarray antennas being one of the potential candidates offer significant advantages over parabolic and phased array antennas at mm-wave bands. In a well-designed reflectarray, the overall performance is mainly determined by its comprising unit cell(s). Most of the recent reflectarray designs are based on printed microstrip technology. It is well known that surface waves get generated in printed microstrip technology and contribute to loss in the radiated signal power in the intended direction. This paper analyses the effect of surface waves in the reflection properties of a printed microstrip millimetre wave reflectarray unit cell. The analytical results are compared with measured data at 32 GHz and an excellent agreement was observed. It was observed that surface waves, though generally considered to have malign effects in antennas, play a significant positive role in the reduction of reflection loss magnitude at unit cell level.
Ahmad Ghulam, Brown Tim W.C., Underwood Craig I., Loh Tian H. (2018) An investigation of millimeter wave reflectarrays for small satellite platforms,Acta Astronautica 151 pp. 475-486 Elsevier
This article reports two contributions related to reflectarray antenna design at millimeter waves (mm-waves). First, a closed form analytical formulation is provided for the prediction of reflection properties of square/rectangular mm-waves reflectarray unit cells based on various quality factors and the theory of waveguide coupled resonators. To ensure a high accuracy at mm-waves, the effects of fringing fields, surface waves, metal conductivity, and metal surface roughness are included in the analysis. This analysis program greatly facilitates the parametric studies of a unit cell's constituting parameters to converge on an optimum design solution. Secondly, the concept of phase quantization is proposed for a cost effective realization of mm-waves reflectarrays. The developed formulation in the first contribution was used to design two 3 bit phase quantized, single layer, 19 wavelength, passive reflectarrays at 60/GHz. The test results are compared with simulations and a very good agreement was observed. These findings are potentially useful for the realization of high gain antennas for mm-wave inter-satellite links in small satellite platforms.