BEAM FORMING NETWORKS
(BFN’s)
EE 525 Antenna Engineering
BFN’s
 constrained feeds
 space (optical) feeds
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 transform feeds (constrained and/or optical)
Types of Constrained Feed Systems
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Series feed
Parallel feed
True time delay feed
Multiple beam matrix feed
- Butler matrix
- Blass matrix
Elements of Constrained Feed
Systems
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Transmission lines
Hybrids
Magic T’s
Directional couplers
Coaxial lines
Striplins
Types of Constrained Feeds
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Series feed
Parallel feed
True time delay feed
Multiple beam matrix feed
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Butler matrix
Blass matrix
Series Feed
Parallel Feed
True Time Delay Feed
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 = ndsinθ/c
● not suitable frequency scanning
● suitable for wideband applications
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
Butler Matrix
Butler Matrix
-distrubutes RF signals to
radiating elements
-providees orthogonal
beamforming
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
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 :
Butler Matrix
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
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
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
Blass Matrix
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
Blass Matrix
Disadvantages
● Each coupler on any given feed-line must have a
different coupling value >> complicates design !!
● array configuration requires more couplers than the
Butler matrix >> greater cost and weight !!
Optical (Space) Feeds
• Transmission type
• Reflection type
Space-Fed Arrays
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less expensive to construct compared to
corporate-fed arrays
suffer from spillover and reflection losses
and do not offer good pattern control for
sidelobes
Optical Feeds - Principal Features
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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.
Transmission Type
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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.
Reflection Type
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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
Transmission & Reflection Types
Qs
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)
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.
Optical Transform Feeds
(large lens fed by a small lens)
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
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