Beamforming networks are commonly employed in both commercial and military systems, and are used in ground, airborne, and space-based applications. They are ideal for controlling multiple antennas in a system, such as a phased-array radar antenna. They make use of the ease and reliability of electronic steering in numerous systems—including fourth-generation (4G) cellular, direction-finding (DF), signal-intelligence (SIGINT), and electronic-intelligence (ELINT) systems—as an alternative to mechanical steering. Although beamforming networks can sometimes suffer drops in gain under certain steering conditions, they generally offer much longer and reliable operating lifetimes than mechanically steerable antenna arrays. Providing reliable and repeatable beamforming networks still relies on proven and consistent manufacturing and testing capabilities.
A beamforming network makes it possible to electronically steer a system’s antenna beams without mechanically shifting the antenna. A passive beamforming network is typically formed of passive RF/microwave components, such as power combiners/dividers and phase shifters, to provide the required phase and amplitude characteristics for the signal energy between the system antenna and the system’s transceivers. Passive beamforming networks, which are fabricated with traditional circuit technologies [such as microstrip, stripline, and coplanar-waveguide (CPW) transmission lines], can be placed close to an antenna or even integrated within the antenna. Newer design approaches are also including advanced circuit techniques, such as substrate-integrated-waveguide (SiW) circuits, in attempts to miniaturize passive beamforming networks for a given frequency range.
Beamforming networks perform vector manipulation on two or more input signals to generate the same number of processed output signals. The output signals are fed to an antenna array to produce the three-dimensional antenna beams that might otherwise result from physically moving or steering the antennas. In essence, the beams are being steered rather than the antennas. Beamforming networks make use of the reciprocal nature of antennas, and support both transmit and receive functions. In some systems, received signals may be frequency downconverted to intermediate-frequency (IF) signals before being applied to a beamforming network.
Antenna beamforming is accomplished by adjusting the amplitudes and phases of the signals for the different elements found in an antenna array. Rather than moving the antenna to move the beam, the amplitude and phases of the elements are varied to move the antenna beam.
These element feeds are shifted electronically so that the antenna’s main beam and its sidelobe levels are effectively controlled, and its beams are steered.
Beamforming networks have the advantage over a mechanically steered antenna of producing multiple beams simultaneously. This gives the system engineer more flexible signal processing options. A fixed passive beamforming network can produce several beams simultaneously, all performed for a fraction of the cost of a traditional fully electronically scanned beamformer. Passive beamforming offers an effective bridge between price and performance.
The passive components used in the beamforming network, such as power dividers/combiners and phase shifters, must be manufactured to tight and repeatable mechanical tolerances. Such tolerances, and their electrical performance, can be checked with the aid of proper high-frequency test equipment—including RF/microwave vector network analyzers (VNAs)—and by analysis of their S-parameters. Companies such as TRM Microwave design and manufacture beamforming networks in compact housings, achieved by combining many of the required passive components into a single integrated microwave assembly (IMA).
To understand the operation of a beamforming network, Fig. 1 shows the schematic diagram of a simple 1 x 4 beamformer that can be formed with a pair of power dividers and a 90-deg. hybrid. As the relationship of the input port to the output ports shows, the network generates pairs of in-phase (I) and quadrature (Q) outputs from a single input signal. The circuit is simple enough to manufacture to excellent mechanical tolerances and can be designed for surface-mount housings or for a package with coaxial connectors.
A Butler matrix is a more complex form of beamforming network, typically with 4, 8, or 16 inputs and the same number of outputs with high isolation among them. It is a passive reciprocal network which works in a similar fashion whether transmitting or receiving. A simple 4 x 4 Butler matrix can be assembled with quadrature couplers or a combination of 180- and 90-deg. hybrids; in each case, the amount of phase shift achieved at the output ports depends on which input port is sampled (in the present example, four separate antenna patterns are generated simultaneously).
The progressive phase shifts at the output ports can be used to create an antenna beam or radiation pattern, depending on the phase shifts and which input is used. Quadrature couplers and hybrid junctions are passive components that can be designed and manufactured with fairly broad bandwidths, high isolation between ports, and relatively low loss in the signals, above the normal reduction in power levels suffered as a result of signal divisions.
For example, a 4 x 4 Butler matrix built from four quadrature couplers (along with their 90-deg. phase offsets) and appropriate phase shifters can generate four output signals at 45-deg. phase offset increments with one port used as the input, 135-deg. phase offset increments with another port as the input, 270-deg. phase offset increments with the third possible input port used as the active input, and 315-deg. phase offset increments with the fourth input. The relationship of the amplitudes from input to output can also be adjusted, with the design of a Butler matrix targeting the use of such passive components as couplers, hybrids, and phase shifters for a specific center frequency and bandwidth.
Model BM88701 is an 8 x 8 beamforming network constructed from 180-deg. hybrids and coaxial lines (Fig. 2). It achieves fixed phase delays and operates within two specific frequency bands from 1025 to 1095 MHz with excellent electrical performance. It can handle 8 W average power and as much as 400 W peak power and exhibits input and output VSWR of 1.30:1. Using proven passive components, the 8 x 8 beamforming network suffers insertion loss of only 1 dB or less (above its nominal power division) and provides at least 18-dB isolation between ports.
The performance of a beamforming network is usually dictated by its application in particular, the antenna array or system with which it will be used. A number of different passive RF/microwave design topologies are available when considering a beamforming network for a particular application. Manufacturers such as TRM Microwave will rely on appropriate circuit materials and transmission-line approaches—including suspended stripline, stripline, airstrip, and microstrip technologies—to meet a customer’s requirements for mechanical layout, size, weight, and electrical performance.
Editor’s Note: This article is based on a more detailed white paper from TRM Microwave. To receive a free copy, complete the request form at www.trmmicrowave.com/request-white-paper.
Shaun Moore / Applications Engineer
280 S. River Rd.
Bedford, NH 03110
FAX: (603) 627-6025