BEAM FORMING NETWORKSBEAM FORMING NETWORKS((BFN’sBFN’s))

EE EE 525 Antenna Engineering525 Antenna Engineering

BFN’sBFN’s

cconstrained feedonstrained feedss

sspace pace (optical) (optical) feedfeedss------------------------------------------------------------------------------------------------ transform feedstransform feeds (constrained and/or (constrained and/or

optical)optical)

● ● Series feedSeries feed● ● Parallel feedParallel feed● ● True time delay feedTrue time delay feed● ● Multiple beam matrix feedMultiple beam matrix feed

- Butler matrixButler matrix- Blass matrixBlass matrix

Types of Constrained Feed Types of Constrained Feed SystemsSystems

Elements of Constrained Feed Elements of Constrained Feed SystemsSystems

●● Transmission linesTransmission lines●● HybridsHybrids●● Magic T’s Magic T’s ●● Directional couplersDirectional couplers●● Coaxial linesCoaxial lines●● StriplinsStriplins

Types of Constrained FeedsTypes of Constrained Feeds

● ● Series feedSeries feed● ● Parallel feedParallel feed● ● True time delay feedTrue time delay feed● ● Multiple beam matrix feedMultiple beam matrix feed

- Butler matrixButler matrix- Blass matrixBlass matrix

Series FeedSeries Feed

Parallel FeedParallel Feed

True Time Delay FeedTrue Time Delay Feed

True Time Delay FeedTrue Time Delay Feed

•ttime delay from wavefront to input feed is ime delay from wavefront to input feed is the same for every paththe same for every path● ● waves via all paths add inwaves via all paths add in phase at the feed phase at the feed point forpoint for every frequency component in the every frequency component in the pulse pulse ●● tthere is no reduction in the peak value of here is no reduction in the peak value of the received pulsethe received pulse●● ttime ime ddelay = ndsinelay = ndsinθθ/c/c●● not suitablenot suitable frequency scanning frequency scanning●● suitablesuitable for wideband applications for wideband applications

Butler Matrix Feed SystemButler Matrix Feed System

●● is a passive feeding N x N network with is a passive feeding N x N network with beam steering capabilitiesbeam steering capabilities

●● consist of hybrid junctions (or directional consist of hybrid junctions (or directional couplers) and fixed phase shifters.couplers) and fixed phase shifters.

●● (N/2) log(N/2) log22N hybrids and (N/2) logN hybrids and (N/2) log22 (N – 1) (N – 1) fixed phase shifters are required to form fixed phase shifters are required to form the network.the network.

● ● hybrids can be either 90° or 180° 3-dB hybrids can be either 90° or 180° 3-dB hybridshybrids

Butler MatrixButler Matrix

Butler MatrixButler Matrix

--ddistrubutes RF signals istrubutes RF signals to to

radiating elementsradiating elements

--pprovidees orthogonal rovidees orthogonal beamformingbeamforming

Butler MatrixButler Matrix

●●multiplemultiple beamformingbeamforming can can bebe achieved by achieved by exciting two orexciting two or more beam ports more beam ports with RFwith RF signalssignals at at thethe same time. same time.

●●two adjacent beams two adjacent beams cannot be formed cannot be formed simultaneously as simultaneously as theythey willwill add up to add up to produce a singleproduce a single

beambeam

The Butler Matrix as a Fourier The Butler Matrix as a Fourier TransformerTransformer

- - thethe field amplitude field amplitude atat thethe nnthth output element due to output element due to unitunit

excitation at the mexcitation at the mthth beam port beam port

--uusing superposition for an arbitrary sing superposition for an arbitrary inputinput distributiodistributionn f(m)f(m) will result in awill result in a superposition of discrete plane superposition of discrete plane waveswaves weighted by f(m)weighted by f(m), , resulting in theresulting in the Fourier Fourier TransformTransform : :

Butler MatrixButler Matrix

Butler MatrixButler MatrixAdvantagesAdvantages

● ● Simple network using few component types easily Simple network using few component types easily implemented in stripline or microstripimplemented in stripline or microstrip

● ● bbeams generated are of the Woodward-Lawson type eams generated are of the Woodward-Lawson type withwith

narrow beamwidth, high directivity and are orthogonalnarrow beamwidth, high directivity and are orthogonal

● ● the ideal Butler matrix is the analog equivalent of thethe ideal Butler matrix is the analog equivalent of the discrete Fourier Transformdiscrete Fourier Transform

● ● low-losslow-loss as minimum insertion loss in hybrids, as minimum insertion loss in hybrids, phase shifters and transmission linesphase shifters and transmission lines

● ● Design of large matrices is easyDesign of large matrices is easy

Butler MatrixButler Matrix

DisadvantagesDisadvantages

● ● beam-width and beam angle vary with beam-width and beam angle vary with frequency; thus the Butler matrix forms frequency; thus the Butler matrix forms phased-steered beams that squint with phased-steered beams that squint with frequencyfrequency

● ● has a complex interconnection scheme for has a complex interconnection scheme for large matrices large matrices

Blass Matrix Feed SysyemBlass 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

Blass MatrixBlass Matrix

Blass MatrixBlass MatrixAdvantagesAdvantages

● ● TThe he interconnection layout of the circuit is interconnection layout of the circuit is simple as no crossovers or multilayers are simple as no crossovers or multilayers are neededneeded

● ● Time-delayed beams produced do not squint Time-delayed beams produced do not squint with frequencywith frequency..

● ● Shaped beams can be produced by Shaped beams can be produced by controlling the coupling ratios of the couplerscontrolling the coupling ratios of the couplers

Blass MatrixBlass Matrix

DisadvantagesDisadvantages

● ● Each coupler on any given feed-line must Each coupler on any given feed-line must have a different coupling value >> have a different coupling value >> complicates design !!complicates design !!

● ● array configuration requires more couplers array configuration requires more couplers than the Butler matrix >> greater cost and than the Butler matrix >> greater cost and weight !!weight !!

Optical (Space) Feeds• Transmission typeTransmission type

• Reflection typeReflection type

Space-Fed ArraysSpace-Fed Arrays

less expensive to construct less expensive to construct compared tocompared to corporate-fed arrays corporate-fed arrays

suffer suffer from from spillover spillover andand reflection reflection losslosseses and do not offer good pattern and do not offer good pattern control for sidelobescontrol for sidelobes

Optical Feeds - Principal Optical Feeds - Principal FeaturesFeatures

free space exists free space exists between the between the feed(s) and the feed(s) and the radiating apertureradiating aperture

aperture aperture distribution is distribution is determined determined mainlymainly by the pattern of by the pattern of the feed.the feed.

The larger the FOV, The larger the FOV, the greater the the greater the complexity and the complexity and the cost of the antenna cost of the antenna system. system.

Transmission TypeTransmission Type array elements and phase shifters array elements and phase shifters

are connected to an array of pickup are connected to an array of pickup elements, illuminated by a feed at a elements, illuminated by a feed at a given focal distancegiven focal distance

Phase shifters are set to provide the Phase shifters are set to provide the required phase increments.required phase increments.

Reflection TypeReflection Type The concept is the same with the The concept is the same with the

transmission model, excepttransmission model, except the the presence ofpresence of short short circuits circuits behind the behind the phase shiftphase shiftersers

amount of required phase shift at amount of required phase shift at each element is half that of the each element is half that of the transmission case transmission case

Transmission & Reflection TypesTransmission & Reflection Types

Phased Array With Phased Array With ParaboloidParaboloid

sQsQsQ

•The The reflectarray reflectarray aperture is aperture is placed in the placed in the region forward region forward of the focus.of the focus.

•Picks up the Picks up the converging converging fieldfield andp andphase-hase-shifts it to shifts it to refocus on the refocus on the primary feed(s)primary feed(s)

Optical Transform FeedsOptical Transform Feeds feed systems feed systems in which in which the input to the the input to the

feed and the resulting aperture feed and the resulting aperture distribution of the array are related by distribution of the array are related by one or more Fourier transforms. one or more Fourier transforms.

Optical Transform FeedsOptical Transform Feeds ((large lens fed by a small lenslarge lens fed by a small lens))

REFERENCESREFERENCES Lo Y.T. , Lee S.W.,’Antenna Handbook’,Van Lo Y.T. , Lee S.W.,’Antenna Handbook’,Van

Nostrand Reinhold,1988, Nostrand Reinhold,1988, http://innovexpo.itee.uq.edu.au/2001/projects/s804http://innovexpo.itee.uq.edu.au/2001/projects/s804 113/thesis.pdf113/thesis.pdf http://www.dcjenn.com/pubs/leeAPS.pdfhttp://www.dcjenn.com/pubs/leeAPS.pdf Jasic, H., Antenna Engineering HandbookJasic, H., Antenna Engineering Handbook Johnson, R.C., Jasic H., Antenna Engineering Johnson, R.C., Jasic H., Antenna Engineering

HandbookHandbook Ming H.C., Tsandoulas G.N., A dual-reflector optical Ming H.C., Tsandoulas G.N., A dual-reflector optical

feed for wide-band phased arrays, IEEE feed for wide-band phased arrays, IEEE Transactions on Antennas and PropagationTransactions on Antennas and Propagation

Mailloux, R.J., Phased Array Antanna HandbookMailloux, R.J., Phased Array Antanna Handbook Hansen, R.C., Phased Array AntennasHansen, R.C., Phased Array Antennas Mailloux, R.J., Space Fed Subarrays using a Mailloux, R.J., Space Fed Subarrays using a

Displaced Feed, InternetDisplaced Feed, Internet