Beamformers - General Information

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Perhaps the most common application of quadrapole networks is to provide proper phasing for circularly polarized antenna feeds. By switching the input used, the "sense" of the antenna can be switched from right hand circular (RHC) to left hand circular (LHC) polarization.

A slightly more complex beamforming network is the four element beamforming matrix as illustrated in Figure 4. This configuration is created by taking the outputs from a pair of two beam matrices (i.e. two quadrature couplers) and feeding the outputs to a second level of hybrid junctions. In this case it is necessary to insert fixed 45º phase shifters in two paths to achieve the phasing necessary for proper combining. This routes a signal originating from a single point in space to a particular output port of the matrix. In this way the precise three dimensional coordinates of the signal source can be identified. This information is particularly important in radar warning and similar systems where the precise location in three dimensional space is essential.

Figure 4: Four Element Beamforming Matrix

An eight element beamforming matrix, as shown in Figure 5, requires a total of 20 passive networks including fixed 22.5º and 67.5º phase shifters. At this level and beyond, however, important economies can be realized in reducing the number of passive networks required.

Figure 5: Eight Element Beamforming Matrix

More complexity arises as the number of antenna elements increases and the number of matrix levels increases as might be required of a focused, multi-element system. The actual number rises in a binary fashion, i.e., 2, 4, 8, 16, 32, etc., as does the complexity and precision required of each passive network in the matrix.

Designing beamformers such as these requires a detailed knowledge of achievable performance characteristics of the four basic building blocks: 0º power dividers, quadrature couplers, hybrid junctions and fixed wideband phase shifters. The design challenge is a bit easier if the performance requirement is for a narrowband unit since phase errors can be corrected by adding line length. But as the required bandwidth is expanded, the need for highly stable and accurate components becomes progressively more critical.

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