# Planar C-Band Antenna with Electronically Controllable Switched Beams

Authors: Mariano Barba; Juan E. Page; Jose A. Encinar
Journal: International Journal of Antennas and Propagation
 (2009)

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## Abstract

The design, manufacturing, and measurements of a switchable-beam antenna at 3.5 GHz for WLL or Wimax base station antennas in planar technology are presented. This antenna performs a discrete beam scan of a60∘ sector in azimuth and can be easily upgraded to 5 or more steps. The switching capabilities have been implemented by the inclusion of phase shifters based on PIN diodes in the feed network following a strategy that allows the reduction of the number of switches compared to a classic design. The measurements show that the design objectives have been achieved and encourage the application of the acquired experience in antennas for space applications, such as X-band SAR and Ku-band DBS.


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## Body

## 1. Introduction

The  well-known features of planar
antennas are low profile, low mass, potential low cost, and ease of mounting; they make this antenna technology very attractive for many communications
applications of both terrestrial and space systems. Moreover, for space
applications, their planar geometry is very interesting because the
simplifications that its geometry introduces in the packaging for the launching
and deployment systems.On the other hand, the well-established
printed circuit technology and the consolidated market of planar microwave
devices allow the introduction of nonlinear components, such
as PIN diodes, varactor
diodes, and MEMS, which can be used to reconfigure or reshape the beam by
controlling or changing the RF signals. In this way, the added flexibility to
the antenna can give an agile response to the continuous changing needs in
communication systems. A good review of the advantages of reconfigurable
antenna for mobile communication systems can be found in [1]; these advantages
are very similar to fixed wireless services such us wireless local loop (WLL)
or Worldwide Interoperability for Microwave Access (Wimax). The ideal situation
would be that one where a complete pattern reconfiguration is attainable in the
antenna according to the system needs, in fact, this is achieved by using MIMO
systems, however, they require many antennas, many antenna locations, and
signal postprocessing. There is a very interesting way of developing agile
antennas including partial and limited reconfigurability in just one single
antenna by, as mentioned, the addition of controllable phase shifters in the
antenna circuits at the RF level. Since base station antennas usually generate
a pattern that covers a sector in azimuth with the appropriate beam shaping in
elevation implemented by a linear array [2], the antenna can be designed to
scan a sector typically covered by one passive antenna [3]. Alternatively, the
antenna can be designed to switch the beam pointing direction to the area with
a greater service demand. These solutions lead to electronically reconfigurable
antennas at a moderate cost and complexity and very low power consumption. This
concept of antenna reconfiguration is very suitable for space applications
where switching, scanning, or reconfiguration of the beam is required to
improve the performances of the system.In this paper, a low-cost switchable
antenna is proposed for base stations of WLL and Wimax systems in the 3.5 GHz
band. The objective of this development was to obtain a low-cost low-complexity
antenna covering a 60° sector with several switchable beams. A new topology is
proposed for the beam switching, based on the implementation of 1 bit phase shifters
at each level of a conventional corporate feeding network. The distributed
location of phase shifters in the feeding network yields a simplification of
the antenna architecture. The concept of the proposed
switchable antenna is demonstrated by the design,
manufacture, and measurements of a prototype with three beams.The development presented in this
paper was focused on the scanning technology, circuits, and concepts to be
applied to Wimax-WLL systems, but bearing in mind the use of the knowledge to
develop switchable and reconfigurable antennas for other applications. In
particular, the use of the concepts developed in this antenna for space systems
is under consideration and most of the experience acquired with this
development will be directly applied to other designs in progress for X-band
SAR and Ku-band DBS applications.


## 2. Antenna Description

The antenna concept for the base
station antenna with switchable beams addressed in this design is detailed in Figure1. It is made up of a horizontal array of vertical subarrays. The vertical
subarray produces the desired elevation pattern. For base station antennas, the
requirements for the elevation patterns typically include shaping for upper
lobe reduction, and often squared cosecant and null filling. According to these
requirements and the gain specifications, these arrays have at least 8 elements, but sometimes up to 12 or 14
elements. In the antenna presented in this paper, the horizontal array has been
implemented to produce the desirable beam scanning in azimuth by the inclusion
of the proper electronically controllable feed network, which gives the
different feed distributions needed. In this work, the main objective was to
obtain the demonstrator of the switchable-beams antenna. Therefore the vertical
array was obtained with just two radiating elements with equal amplitude and a
phase shift to obtain a 2° degree tilt in the elevation pattern. The option for
several elements in the vertical array is indicated by the saw lines in Figure 1.
The reader is referred to [4, 5] for more discussions on the synthesis and the
implementation of elevation beam-shaped patterns.Figure 1
Antenna architecture.


### 2.1. Radiating Elements

The radiating element is a dual-stacked
patch coupled to a microstrip line through a slot.  It has been designed to obtain the sufficient
bandwidth and matching level for the required frequency band in the final
antenna. Its design objectives, band from 3.3 GHz to 3.7 GHz, matching level
better than −22 dB, have been established considering that this element has to
be included in the array as its radiating element as well as considering the
performance degradation that will be found in the prototypes as a result of the
tolerance effects. This element is very similar to the one used in [3] for a
lower frequency band. For reader convenience, their details and geometry are
included in this paper; however, to focus this paper on the most relevant
issues, the discussions about its design are avoided.  Figure 2 shows the geometry including the
conductors of the radiating element and their description. In Table 1, the
stack of the antenna and the parameters of the elements of the radiating
element are listed.Table 1
Definition of the
lay-up for the radiating element.
DescriptionTypeDielectric layersConductorsH mmDKSIZE mm × mmUpper patchConductor19.83 × 19.83Upper substrateDielectric121.6Lower patchConductor24.64 × 24.64Lower substrateDielectric61.6Ground planeConductorSlot: 20.42 × 1.55RF substrateDielectric0.53.2Microstrip, stubConductorw  =  1.9; stub  =  4.73Air15Back scattererConductor4 × 50FR4Dielectric1.54.1Figure 2
Radiating element. Top view and definition of conductors.The chosen separation distance for
all of the radiating elements of both arrays is 60 mm (0.7λ). This value has been taken as the maximum
distance between the elements, where the disturbance introduced by the grating
lobes level is not significant for the system requirements.As has already been mentioned, the
vertical array for this prototype consists of two elements fed with equal
amplitude and with a phase shift that produces an elevation tilt of 2°. Figure 3 shows the arrangement of the vertical array for this prototype.Figure 3
Vertical subarray topology.


### 2.2. Feeding Network

The azimuth feed network has to provide
switchable beams covering a sector of ±30°. This can be done with an array of
four elements, fed with equal amplitude and a progressive phase shift to tilt
the pointing direction as desired. The desired coverage assuming a ripple of 2 dB can be obtained by 5 different beams pointing alternatively to −21°, −10.5,
0°, 10.5°, and 21°, as shown in Figure 4.  To obtain these beam inclinations and
considering the separation between elements, the feed network should provide
five different feed distributions with a progressive phase shift between
adjacent elements of 90°, 45°, 0°, −45°, and −90°, respectively, and equal
amplitude for all of them.Figure 4
Five beams covering a 60° sector.The feeding network has been
designed and manufactured in microstrip technology.  Although for this demonstrator, it is
implemented in an independent  board to allow all of the antenna components to
be measured separately, however, in an industrial design, it can be integrated
into the same board where the input microstrip lines of the radiating element
are located, thus reducing the size, cost, losses, and complexity. The
amplitude distribution can be obtained through a corporate feed network with equal amplitude dividers.The phase shifter can be implemented
by microwave switches single-pole double-through (SPDT) and delay lines, having
each phase shifter two switches and two delay lines with the proper difference
of length to produce the desired phase shift at the centre frequency. In this
design, the SPDT switches have been implemented using two PIN diodes, resulting
in the phase shifter module shown in Figure5. The selected diode for this
application that, according to manufacturer specifications, can be used at
higher frequencies is MACOM-MA4PBL027. It presents an equivalent series
resistance in direct biasing of 3.5 Ω and an equivalent reverse capacitance
lower than 40 fF.Figure 5
PIN diodes and microstrip delay lines phase shifter module.The simplest way to implement the
feeding network using amplitude dividers and two-state phase shifters consists
of, first, obtaining the four equal amplitude signals, and after that division
and before the inputs of each subarray, including a set of phase shifters
connected serially, in order to implement the required additional phase shift
states. However, this structure can be simplified by taking advantage of the
corporate structure by including the phase shifter for more significant
shifting in the first branches of the feed network. In Figure6, this concept
to implement the phase shifting needed for this design is presented. This
figure also shows the setting of each phase shifter to obtain the 5 beams.Figure 6
Phase shifter configuration and locations to obtain the desired
beams.In order to develop the prototype and
to reduce the manufacturing costs, the number of beams for the demonstrator was
reduced to three, considering the extreme phase shiftings (−90°, 0, and +90°).
This option reduces the number of phase shifters and the manufacturing process,
proves the concept keeping the most exigent beam tilts. Therefore, the
configuration taken for the 3-beam demonstrator is the one set out in Figure 7.Figure 7
Phase shifter
configuration to obtain 3 beams, the two extreme and the central.For the 0° beam pointing direction,
only the situation with 0° phase shift has been indicated. However, it is clear
that other configurations of the phase shifter positions lead to the equal
phase distribution. Any setting of the phase shifters with equal phase shifting
at each level leads to an equal phase distribution and therefore could be
valid.The drawings of the design of the
3-beam feed network are shown in Figure8, where it can be seen how the 180°
phase shifters are at one level of the corporate network and the 90° phase
shifters at the element (subarray input) level. All of the elements of the
network such as bends, dividers, and biasing stubs have been optimized to reach
the best performances in the band of interest 3.3–3.7 GHz. Even with this
careful design, there are some parasitic effects introduced by connections of
the diodes to the lines and through the connections of the blocking capacitors.
In some conditions, as discussed before, these effects will be added each to others and can mainly degenerate the signal at the outputs. To reduce this degradation it is essential
to implement the amplitude division with Wilkinson dividers, whose topology can
be easily seen in Figure 8. High-quality microwave with low-loss blocking capacitors are needed; in this
design, ATC series 600 were used.Figure 8
Drawing of the
switchable feeding network.


## 2.1. Radiating Elements

The radiating element is a dual-stacked
patch coupled to a microstrip line through a slot.  It has been designed to obtain the sufficient
bandwidth and matching level for the required frequency band in the final
antenna. Its design objectives, band from 3.3 GHz to 3.7 GHz, matching level
better than −22 dB, have been established considering that this element has to
be included in the array as its radiating element as well as considering the
performance degradation that will be found in the prototypes as a result of the
tolerance effects. This element is very similar to the one used in [3] for a
lower frequency band. For reader convenience, their details and geometry are
included in this paper; however, to focus this paper on the most relevant
issues, the discussions about its design are avoided.  Figure 2 shows the geometry including the
conductors of the radiating element and their description. In Table 1, the
stack of the antenna and the parameters of the elements of the radiating
element are listed.Table 1
Definition of the
lay-up for the radiating element.
DescriptionTypeDielectric layersConductorsH mmDKSIZE mm × mmUpper patchConductor19.83 × 19.83Upper substrateDielectric121.6Lower patchConductor24.64 × 24.64Lower substrateDielectric61.6Ground planeConductorSlot: 20.42 × 1.55RF substrateDielectric0.53.2Microstrip, stubConductorw  =  1.9; stub  =  4.73Air15Back scattererConductor4 × 50FR4Dielectric1.54.1Figure 2
Radiating element. Top view and definition of conductors.The chosen separation distance for
all of the radiating elements of both arrays is 60 mm (0.7λ). This value has been taken as the maximum
distance between the elements, where the disturbance introduced by the grating
lobes level is not significant for the system requirements.As has already been mentioned, the
vertical array for this prototype consists of two elements fed with equal
amplitude and with a phase shift that produces an elevation tilt of 2°. Figure 3 shows the arrangement of the vertical array for this prototype.Figure 3
Vertical subarray topology.


## 2.2. Feeding Network

The azimuth feed network has to provide
switchable beams covering a sector of ±30°. This can be done with an array of
four elements, fed with equal amplitude and a progressive phase shift to tilt
the pointing direction as desired. The desired coverage assuming a ripple of 2 dB can be obtained by 5 different beams pointing alternatively to −21°, −10.5,
0°, 10.5°, and 21°, as shown in Figure 4.  To obtain these beam inclinations and
considering the separation between elements, the feed network should provide
five different feed distributions with a progressive phase shift between
adjacent elements of 90°, 45°, 0°, −45°, and −90°, respectively, and equal
amplitude for all of them.Figure 4
Five beams covering a 60° sector.The feeding network has been
designed and manufactured in microstrip technology.  Although for this demonstrator, it is
implemented in an independent  board to allow all of the antenna components to
be measured separately, however, in an industrial design, it can be integrated
into the same board where the input microstrip lines of the radiating element
are located, thus reducing the size, cost, losses, and complexity. The
amplitude distribution can be obtained through a corporate feed network with equal amplitude dividers.The phase shifter can be implemented
by microwave switches single-pole double-through (SPDT) and delay lines, having
each phase shifter two switches and two delay lines with the proper difference
of length to produce the desired phase shift at the centre frequency. In this
design, the SPDT switches have been implemented using two PIN diodes, resulting
in the phase shifter module shown in Figure5. The selected diode for this
application that, according to manufacturer specifications, can be used at
higher frequencies is MACOM-MA4PBL027. It presents an equivalent series
resistance in direct biasing of 3.5 Ω and an equivalent reverse capacitance
lower than 40 fF.Figure 5
PIN diodes and microstrip delay lines phase shifter module.The simplest way to implement the
feeding network using amplitude dividers and two-state phase shifters consists
of, first, obtaining the four equal amplitude signals, and after that division
and before the inputs of each subarray, including a set of phase shifters
connected serially, in order to implement the required additional phase shift
states. However, this structure can be simplified by taking advantage of the
corporate structure by including the phase shifter for more significant
shifting in the first branches of the feed network. In Figure6, this concept
to implement the phase shifting needed for this design is presented. This
figure also shows the setting of each phase shifter to obtain the 5 beams.Figure 6
Phase shifter configuration and locations to obtain the desired
beams.In order to develop the prototype and
to reduce the manufacturing costs, the number of beams for the demonstrator was
reduced to three, considering the extreme phase shiftings (−90°, 0, and +90°).
This option reduces the number of phase shifters and the manufacturing process,
proves the concept keeping the most exigent beam tilts. Therefore, the
configuration taken for the 3-beam demonstrator is the one set out in Figure 7.Figure 7
Phase shifter
configuration to obtain 3 beams, the two extreme and the central.For the 0° beam pointing direction,
only the situation with 0° phase shift has been indicated. However, it is clear
that other configurations of the phase shifter positions lead to the equal
phase distribution. Any setting of the phase shifters with equal phase shifting
at each level leads to an equal phase distribution and therefore could be
valid.The drawings of the design of the
3-beam feed network are shown in Figure8, where it can be seen how the 180°
phase shifters are at one level of the corporate network and the 90° phase
shifters at the element (subarray input) level. All of the elements of the
network such as bends, dividers, and biasing stubs have been optimized to reach
the best performances in the band of interest 3.3–3.7 GHz. Even with this
careful design, there are some parasitic effects introduced by connections of
the diodes to the lines and through the connections of the blocking capacitors.
In some conditions, as discussed before, these effects will be added each to others and can mainly degenerate the signal at the outputs. To reduce this degradation it is essential
to implement the amplitude division with Wilkinson dividers, whose topology can
be easily seen in Figure 8. High-quality microwave with low-loss blocking capacitors are needed; in this
design, ATC series 600 were used.Figure 8
Drawing of the
switchable feeding network.


## 3. Prototypes and Measurements

### 3.1. Antenna Components

Figure9 shows the front image of
the manufactured prototype. The eight radiating elements corresponding to the
four subarrays can clearly be seen. The inputs of the four subarrays are also
seen, as well as the cables which connect the outputs of the feed network to
these inputs. In an industrial model, as it has been mentioned, the cable will
not be used, since the feed network and the radiating element input lines can
be integrated into the same board. However, at the prototype level they have
been manufactured independently to allow the measurement of all of the elements
and their validation.Figure 9
Phase antenna prototype.
View of the radiating element and vertical subarray inputs.The measured return losses at each
subarray input are shown in Figure10. It can be seen that their values are in
range of 20 dB in the frequency of operation as it was expected from the
simulations carried out during the design.Figure 10
Antenna subarrays. Return losses for each
input.The manufactured prototype of the
electronically controllable feeding network of the antenna is shown in Figure11.Figure 11
Prototype of feed network of the
switchable antenna.The set of measurements of the feed
network is quite wide. The minimum needed are the reflection parameter and the
transmission parameters (amplitude and phase) for the 3 feed distribution
configurations. Because of space considerations, some of them are not included
in this paper; the measured return losses are better of −15 dB and the
amplitude of transmission is in the range of −9 dB, giving, therefore, an
average losses of the network of 3 dB, where 1.5 dB are a result of the losses
and another 1.5 to the RF microstrip lines.In Figure12, the output phases for
the 3 configurations are shown. Figure 12(a) shows the measured output phases
for the 0° phase shift configuration, showing that the four signals are in phase. Figures
12(b) and 12(c) show the measured output phases for the +90° and −90°
progressive phase shifts. These measurements confirm that the feed network is
working properly.(a) Feed network measurement: output phases for position 1, (equal phase). (b) Feed network measurement: output phases for position 2, (−90° progressive phase shift). (c) Feed network measurement: output phases for position 3, (+90° progressive phase shift).
(a)(b)(c)


### 3.2. Complete Antenna

Figure13 shows a photograph of the
complete antenna after assembling all of the elements, including a simple but
efficient switching control matrix, which allows the phase shifters to be
biased at their proper position according to the desired phase shifting at each
configuration.Figure 13
Photograph of the complete antenna.Figure14 shows the measurement of
the return losses for the 3 pointing positions with two options for the 0°
configuration. It can be seen that its value matches the typical specification
for this type of antennas, −14 dB, which is equivalent to a VSWR of 1.5.Figure 14
Complete antenna return losses at each
pointing position.Figure15 shows the azimuth cuts in
a plane of 2° tilt for the 3 pointing directions of the far field measurements.
It can be seen how beam switching is achieved as expected. As reference, the
cross-polar measurement of the azimuth cut for the 0° beam pointing direction
is also included. It can be observed that its maximum level is lower than 25 dB
below the antenna gain.Figure 15
Antenna radiation pattern measurements.
Azimuth cut for the three pointing positions.For reference, the three-dimensional
measurements for co- and cross-polar components normalized to the maximum of
radiation for the 0° beam pointing direction are presented in Figure 16. The
maximum level of cross-polar component in the range of the main lobe (from −15°
to +15°) is lower than 25 dB below the antenna gain.Pattern measurements. Three-dimensional pattern at 0° pointing direction. (a) Copolar patterns, and (b) cross-polar patterns.
(a)(b)


## 3.1. Antenna Components

Figure9 shows the front image of
the manufactured prototype. The eight radiating elements corresponding to the
four subarrays can clearly be seen. The inputs of the four subarrays are also
seen, as well as the cables which connect the outputs of the feed network to
these inputs. In an industrial model, as it has been mentioned, the cable will
not be used, since the feed network and the radiating element input lines can
be integrated into the same board. However, at the prototype level they have
been manufactured independently to allow the measurement of all of the elements
and their validation.Figure 9
Phase antenna prototype.
View of the radiating element and vertical subarray inputs.The measured return losses at each
subarray input are shown in Figure10. It can be seen that their values are in
range of 20 dB in the frequency of operation as it was expected from the
simulations carried out during the design.Figure 10
Antenna subarrays. Return losses for each
input.The manufactured prototype of the
electronically controllable feeding network of the antenna is shown in Figure11.Figure 11
Prototype of feed network of the
switchable antenna.The set of measurements of the feed
network is quite wide. The minimum needed are the reflection parameter and the
transmission parameters (amplitude and phase) for the 3 feed distribution
configurations. Because of space considerations, some of them are not included
in this paper; the measured return losses are better of −15 dB and the
amplitude of transmission is in the range of −9 dB, giving, therefore, an
average losses of the network of 3 dB, where 1.5 dB are a result of the losses
and another 1.5 to the RF microstrip lines.In Figure12, the output phases for
the 3 configurations are shown. Figure 12(a) shows the measured output phases
for the 0° phase shift configuration, showing that the four signals are in phase. Figures
12(b) and 12(c) show the measured output phases for the +90° and −90°
progressive phase shifts. These measurements confirm that the feed network is
working properly.(a) Feed network measurement: output phases for position 1, (equal phase). (b) Feed network measurement: output phases for position 2, (−90° progressive phase shift). (c) Feed network measurement: output phases for position 3, (+90° progressive phase shift).
(a)(b)(c)


## 3.2. Complete Antenna

Figure13 shows a photograph of the
complete antenna after assembling all of the elements, including a simple but
efficient switching control matrix, which allows the phase shifters to be
biased at their proper position according to the desired phase shifting at each
configuration.Figure 13
Photograph of the complete antenna.Figure14 shows the measurement of
the return losses for the 3 pointing positions with two options for the 0°
configuration. It can be seen that its value matches the typical specification
for this type of antennas, −14 dB, which is equivalent to a VSWR of 1.5.Figure 14
Complete antenna return losses at each
pointing position.Figure15 shows the azimuth cuts in
a plane of 2° tilt for the 3 pointing directions of the far field measurements.
It can be seen how beam switching is achieved as expected. As reference, the
cross-polar measurement of the azimuth cut for the 0° beam pointing direction
is also included. It can be observed that its maximum level is lower than 25 dB
below the antenna gain.Figure 15
Antenna radiation pattern measurements.
Azimuth cut for the three pointing positions.For reference, the three-dimensional
measurements for co- and cross-polar components normalized to the maximum of
radiation for the 0° beam pointing direction are presented in Figure 16. The
maximum level of cross-polar component in the range of the main lobe (from −15°
to +15°) is lower than 25 dB below the antenna gain.Pattern measurements. Three-dimensional pattern at 0° pointing direction. (a) Copolar patterns, and (b) cross-polar patterns.
(a)(b)


## 4. Conclusions

This paper presents the design,
manufacturing, and measurements of a switched-beam antenna at 3.5 GHz in planar
technology that demonstrate the suitability of scanning beam base station
antennas for WLL and Wimax, where the beam scanning is achieved through the
inclusion of phase shifters based on PIN diodes in the
feed network, and where the strategy of phase shifter locations leads to a
reduction in the number of components. The feasibility of this type of antenna
for space applications is under consideration and the experience acquired with
this development can be used for X-band SAR applications and Ku-band DBS.



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*Source: 625327-2009-04-02.xml*