A Review on the Development of Rotman Lens Antenna

Table 1

Different types of beamforming networks.

S. number	BFN	Features	Advantages	Disadvantages
1	Digital beamforming networks	They make use of a computer or chip processor to control electronic components forming exact amplitude and phase for different channels of array elements.	They have zero phase error, flexible amplitude tapering, and infinite number of scanning steps.	They are limited to low-microwave frequencies due to low bit-bandwidth of current A/D devices [8]. It requires fast processors in order to form a coherence receiver. In case where large processing of information is involved, the computation speed becomes difficult.
2	Network beamforming network	One of the earliest BFNs which originated from the Butler matrix. It consists of alternate rows of fixed phase shifters and hybrid junctions [8, 9].	They are easy to construct and can be implemented using printed circuit boards. The produced beams are dependent on frequency.	It requires crossovers on the lines. The beam shift occurs as the frequency varies, which is not desirable in most communications links, as certain bandwidth information is expected to be sent to the same location.
3	Microwave lens beamforming network	It applies path delay mechanism to form the desired phase front at the array input. Each of the input connects to a beam port that radiates semicircular phase front within the lens cavity [10]. Array of receiving elements functioning as receivers then guides the energy into the output array.	The path-length design mechanism in the microwave lens is independent of frequency [10]; thus it is typically considered as true-time delay device [11]. The properly designed beam, receiving port positions, transmission line length, correct phase, and acceptable amplitude distributions can be achieved across the aperture.	Finite insertion losses and inherent losses occur due to the hybrid couplers, fixed phase shifters, and transmission lines that make up the matrix. The Butler matrix passive beam forming antenna also requires that the individual beam patterns be orthogonal in space.