BEAM FORMING NETWORKS BFNs EE 525 Antenna Engineering

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BEAM FORMING NETWORKS(BFNs)EE 525 Antenna
Engineering
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BFNs

• constrained feeds
• space (optical) feeds
• ------------------------------------------------
• transform feeds (constrained and/or optical)

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Types of Constrained Feed Systems

• ? Series feed
• ? Parallel feed
• ? True time delay feed
• ? Multiple beam matrix feed
• Butler matrix
• Blass matrix

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Elements of Constrained Feed Systems

• ? Transmission lines
• ? Hybrids
• ? Magic Ts
• ? Directional couplers
• ? Coaxial lines
• ? Striplins

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Types of Constrained Feeds

• ? Series feed
• ? Parallel feed
• ? True time delay feed
• ? Multiple beam matrix feed
• Butler matrix
• Blass matrix

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Series Feed
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Parallel Feed
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True Time Delay Feed
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• True Time Delay Feed
• time delay from wavefront to input feed is the
  same for every path
• ? waves via all paths add in phase at the feed
  point for every frequency component in the pulse
• ? there is no reduction in the peak value of the
  received pulse
• ? time delay ndsin?/c
• ? not suitable frequency scanning
• ? suitable for wideband applications

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Butler Matrix Feed System

• ? is a passive feeding N x N network with beam
  steering capabilities
• ? consist of hybrid junctions (or directional
  couplers) and fixed phase shifters.
• ? (N/2) log2N hybrids and (N/2) log2 (N 1)
  fixed phase shifters are required to form the
  network.
• ? hybrids can be either 90 or 180 3-dB hybrids

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Butler Matrix
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Butler Matrix

• -distrubutes RF signals to
• radiating elements
• -providees orthogonal beamforming

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Butler Matrix

• ?multiple beamforming can be achieved by
  exciting two or more beam ports with RF signals
  at the same time.
• ?two adjacent beams cannot be formed
  simultaneously as they will add up to produce a
  single
• beam

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The Butler Matrix as a Fourier Transformer

• - the field amplitude at the nth output element
  due to unit
• excitation at the mth beam port
• -using superposition for an arbitrary input
  distribution f(m) will result in a superposition
  of discrete plane waves weighted by f(m),
  resulting in the Fourier Transform

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Butler Matrix
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Butler Matrix

• Advantages
• ? Simple network using few component types easily
  implemented in stripline or microstrip
• ? beams generated are of the Woodward-Lawson type
  with
• narrow beamwidth, high directivity and are
  orthogonal
• ? the ideal Butler matrix is the analog
  equivalent of the
• discrete Fourier Transform
• ? low-loss as minimum insertion loss in hybrids,
  
• phase shifters and transmission lines
• ? Design of large matrices is easy

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Butler Matrix
Disadvantages ? beam-width and beam angle vary
with frequency thus the Butler matrix forms
phased-steered beams that squint with
frequency ? has a complex interconnection scheme
for large matrices
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Blass Matrix Feed Sysyem

• employs a set of N antenna array element
  transmission lines that intersect a set of M beam
  port lines (with directional couplers at each
  intersection)
• matrix is terminated with matched loads
• upper feedline radiates a broadside beam
• Feed line tilt angle and propagation constants
  determines beam position

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Blass Matrix
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Blass Matrix

• Advantages
• ? The interconnection layout of the circuit is
  simple as no crossovers or multilayers are needed
• ? Time-delayed beams produced do not squint with
  frequency.
• ? Shaped beams can be produced by controlling the
  coupling ratios of the couplers

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Blass Matrix

• Disadvantages
• ? Each coupler on any given feed-line must have a
  different coupling value gtgt complicates design
  !!
• ? array configuration requires more couplers than
  the Butler matrix gtgt greater cost and weight !!

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• Optical (Space) Feeds
• Transmission type
• Reflection type

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Space-Fed Arrays

• less expensive to construct compared to
  corporate-fed arrays
• suffer from spillover and reflection losses and
  do not offer good pattern control for sidelobes

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Optical Feeds - Principal Features

• free space exists between the feed(s) and the
  radiating aperture
• aperture distribution is determined mainly by the
  pattern of the feed.
• The larger the FOV, the greater the complexity
  and the cost of the antenna system.

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Transmission Type

• array elements and phase shifters are connected
  to an array of pickup elements, illuminated by a
  feed at a given focal distance
• Phase shifters are set to provide the required
  phase increments.

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Reflection Type

• The concept is the same with the transmission
  model, except the presence of short circuits
  behind the phase shifters
• amount of required phase shift at each element is
  half that of the transmission case

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Transmission Reflection Types
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Phased Array With Paraboloid

• The reflectarray aperture is placed in the region
  forward of the focus.
• Picks up the converging field andphase-shifts it
  to refocus on the primary feed(s)

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Optical Transform Feeds

• feed systems in which the input to the feed and
  the resulting aperture distribution of the array
  are related by one or more Fourier transforms.

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Optical Transform Feeds (large lens fed by a
small lens)
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REFERENCES

• Lo Y.T. , Lee S.W.,Antenna Handbook,Van
  Nostrand Reinhold,1988,
• http//innovexpo.itee.uq.edu.au/2001/projects/s804
  
• 113/thesis.pdf
• http//www.dcjenn.com/pubs/leeAPS.pdf
• Jasic, H., Antenna Engineering Handbook
• Johnson, R.C., Jasic H., Antenna Engineering
  Handbook
• Ming H.C., Tsandoulas G.N., A dual-reflector
  optical feed for wide-band phased arrays, IEEE
  Transactions on Antennas and Propagation
• Mailloux, R.J., Phased Array Antanna Handbook
• Hansen, R.C., Phased Array Antennas
• Mailloux, R.J., Space Fed Subarrays using a
  Displaced Feed, Internet