Fixed Beam Forming Techniques
Some array applications require several fixed beams that cover an angular sector.
Several beam forming techniques exist that provide these fixed beams. Three examples
are given here.
Butler matrix The Butler matrix is a beam forming network that uses a
combination of 90° hybrids and phase shifters. An 8x8 Butler matrix is shown in Fig 3-5.
The Butler matrix performs a spatial fast Fourier transform and provides 2n orthogonal
beams. These beams are linearly independent combinations of the array element patterns
When used with a linear array the Butler matrix produces beams that overlap at
about 3.9 dB below the beam maxima. A Butler matrix-fed array can cover a sector of up
to 360° depending on element patterns and spacing. Each beam can be used by a
dedicated transmitter and/or receiver, or a single transmitter and/or receiver can be used,
and the appropriate beam can be selected using an RF switch. A Butler matrix can also
be used to steer the beam of a circular array by exciting the Butler matrix beam ports with
amplitude and phase weighted inputs followed by a variable uniform phase taper.