2009, Frequency Reconfigurable Antenna for Multi Radio Wireless Platforms

66 February 20091527-3342/09/$25.00©2009 IEEE

Frequency-Reconfigurable

Antennas for Multiradio

Wireless Platforms

Looking to increase the functionality of currentwireless platforms and to improve theirquality of service, we have explored themerits of using frequency-reconfig-urable antennas as an alternative for

multiband antennas. Our study included ananalysis of various reconfigurable and multi-band structures such as patches, wires, andcombinations. Switches, such as radio-fre-quency microelectromechanical systems (RF-MEMS) and p-i-n diodes, were also studiedand directly incorporated onto antennastructures to successfully form frequency-reconfigurable antennas.

While it is true that a variety of reconfig-urable antennas have been successfully devel-oped, their use has always been hindered by theneed for a large number of switches, complicated dcwiring and their limited capability of serving one fre-quency band at a time. The concept of extending multi-band antennas to be reconfigurable has evolved as well. Asan example, hybrid reconfigurable multiband antennas have

Songnan Yang is with the University of Tennessee–Knoxville/Intel Corp., Santa Clara, CA 95054, USA; Chunna Zhang and Aly E. Fathy are with the University of Tennessee–Knoxville 37996, USA;

Helen K. Pan is with Intel Corp., Hillsboro, OR 97124, USA, and Vijay K. Nair is with Intel Corp., Chandler, AZ 85226, USA.

Digital Object Identifier 10.1109/MMM.2008.930677

Songnan Yang, Chunna Zhang, Helen K. Pan, Aly E. Fathy, and Vijay K. Nair

© EYEWIRE

Authorized licensed use limited to: Bambang Samajudin. Downloaded on September 30, 2009 at 08:11 from IEEE Xplore. Restrictions apply.

recently been demonstrated and should become aviable alternative to the currently popular multibandantenna. In mobile multiradio platforms particularly,implementation of such reconfigurable multibandantennas would greatly reduce the complexity—andincrease the capability—of the antenna system as awhole. In addition, the filtering requirements of thefront-end circuitry can be greatly reduced by the supe-rior out-of-band noise rejection that frequency reconfig-urable antennas can provide.

Pros and Cons of Alternative Approaches toAntennas for Multiradio Wireless PlatformsThese days, more and more radios are being integratedinto a single wireless platform to allow maximum con-nectivity. Extensive efforts are currently underway todevelop multiradio mobile platforms, such as laptops,mobile Internet devices (MIDs), and smart phones, toaddress several wireless services scattered over a widefrequency range. These efforts use multiple wirelessmodules collocated on the same device. From the anten-na perspective, this can be achieved by using many indi-vidual antennas, multiband antennas, or—as we areproposing here—frequency-reconfigurable antennas.All of these approaches have their advantages and dis-advantages, which are summarized in Table 1.

Currently, multiband antennas, which are designed toaddress more than one band/service at a time, are themost practical and affordable single wireless modulesolution, even though dedicated single-band antennas

would provide superior performance. However, multi-band antennas face serious challenges as more and morewireless services are packed into ever smaller devices.For example, as Figure 1 shows, because of the rapid pro-liferation of radios in mobile platforms, laptop comput-ers are expected to contain more than six radios by 2009.On the antenna side, each one of these wireless modulesmay support more than one frequency band of the sameservice (as shown in Table 2). Most of the modulesrequire two antennas for diversity, and some need asmany as three antennas for multiple-input, multiple-out-put (MIMO) operation. So, even with multiband anten-nas, the laptop computer will soon run out of space forantennas. For devices with smaller form factors, such assmart phones and MIDs, the scenario is even worse. Asa result, the antennas from different wireless moduleswill need to be placed next to each other, a situation thatcauses very poor isolation between radios and couldlead to a failed connection when multiple radios areoperating simultaneously. This is primarily caused bythe close spacing and the low out-of-band rejection capa-bility of multiband antennas.

To demonstrate the advantages of reconfigurableantennas in comparison to multiband antennas, weexposed a global positioning system (GPS) receiver to ajamming signal. The first setup had the receiver con-nected to a multiband antenna, while the second setuphad the receiver connected to a frequency-reconfig-urable antenna. The multiband antenna we utilizedoperates at both the GPS and the 2.4 GHz wireless local

February 2009 67

TABLE 1. Comparison of antenna solutions for wireless mobile platforms.

Multiband/Wideband Frequency-Reconfigurable Characteristic Multiple Antennas Antennas Antennas

Usage model Single-band antenna supports One antenna supports all One antenna supports many one frequency of frequency bands of wireless standardswireless service wireless service/module

Number of antennas Frequency bands × services × Wireless modules × diversity Diversity (such as main/aux diversity or MIMO)

Space requirement Multiple antennas require Reduced space, but wide Minimal space requirementmany spaces bandwidth frustrates

miniaturization efforts

Front-end complexity Loose filter specs, simple Many stringent filters required, Relaxed filter specifications, but front-end introducing high insertion complex reconfigurable

loss and cost front end required

Individual radio performance Excellent Good; lower receiver sensitivity Acceptable performance, due to insertion loss at front additional loss introduced end (switches, diplexer, etc.) by switches

Radio coexistence Little spacing between Poor out-of-band rejection, Degraded true simultaneous antennas, strong coupling transmitted signal of other operation, as antenna between radios radios may cause supports one service

noise jamming at a time

Cost Increased number of cables High-cost stringent filters Cost of low-loss, low-power-contributes most of the cost required in the front end consumption switch

(RF-MEMS) is high


68 February 2009

area network (WLAN) band, while the reconfigurableantenna supports either the GPS or the WLAN serviceupon reconfiguration. Both antennas had identical gainand their received signals were subjected to the samestringent filtering of the GPS front end, so the resultingcarrier-to-noise ratios (C/Ns) of the received GPS sig-nals should have been the same. A signal generator wasused to emit a 20-dBm signal at 2.4 GHz, emulating atransmitted signal from a WLAN radio collocated onthe same multiradio platform. The signal was thendirected toward both systems by a horn antenna placedabout 1.5 meters away from the receivers.

While no pronounced degradation was noticedwhen the reconfigurable antenna was used, a drop ofover 8 dB C/N was measured for the multibandantenna (as shown in Figure 2), and the location lockwas also lost. It was evident that the multiband anten-na had poor out-of-band rejection compared to the

frequency-reconfigurable antenna; hence, additionalfiltering would be required in order for the antenna toattain better noise rejection. For other radios such asWiFi, WiMax, and 3G, in which wider bandwidths andless stringent filtering are commonly employed, thescenario could be even worse.

In short, most multiband antenna solutions requirevery expensive stringent filters to improve their out-of-band rejection. Frequency-reconfigurable antennas, onthe other hand, have inherent bandpass characteristicsand generally have excellent out-of-band rejectionwithout of filters. While reconfigurable antennas canhave higher-order resonances, typically they are faraway from the operating band and therefore can beremoved with much less selective filters at a lower cost.It is essential that a fair cost comparison of these twoalternatives include the cost of the filters required forboth antennas.

In this article, we focuson frequency-reconfigurableantenna structures that aresuitable for implementationon consumer-type mobilemultiradio platforms. Sucha n t e n n a s a re u s u a l l yequipped with switches thatare controlled by dc bias sig-nals. Upon toggling theswitch between on and offstates, the antenna can bereconfigured to support adiscrete set of operating fre-quencies. The following sec-tion serves as a generaloverview of the concepts ofconstructing single- andmultiband reconfigurableantennas. Three major anten-na types commonly used inmobile platforms, includingpatch antennas, wire anten-nas, and planar inverted Fantennas (PIFAs), are illus-trated as candidates for fre-quency reconfigurabilitywith examples from the liter-ature. In the section “SwitchIntegration and DC BiasingConsiderations,” the model-ing, placement, and dc bias-ing and control of switches inan antenna environment arediscussed, followed by“Examples of Frequency-Reconfigurable AntennaDesigns,” which discussesantennas implemented with

Figure 1. Radio system diagram of multiradio laptop computer platforms. (Image takenfrom [1], ©Intel Corp. 2007. Reproduced with permission.)

CPU

MCH

WiFiA,B,G,N WiMAX 3G UWB TV-DVB GPS 60 GHz

Bluetooth

TABLE 2. Frequency bands for wireless services and current number of antennas required.

Wireless Services Frequency Bands Number of Antennas

WiFi IEEE 802.11b/g/n: 2.4–2.48 GHz 3 × 3 MIMOIEEE 802.11a/n: 5.15–5.85 GHz

WiMax IEEE 802.16: 2.3–2.4 GHz, 2.5–2.7 GHz, Diversity: main and aux 3.3–3.8 GHz, 5.15–5.85 GHz (1 × Tx, 2 × Rx)

3G GSM 850: 0.824–0.894 GHz Diversity: main and aux GSM 900: 0.88–0.96 GHz (1 × Tx, 2 × Rx)DCS 1800: 1.71–1.88 GHzPCS 1900: 1.85–1.99 GHzUMTS: 1.92–2.17 GHz

Bluetooth IEEE 802.15.1: 2.4–2.48 GHz Single

GPS 1.575 GHz Single

UWB 3–10 GHz Single


RF-MEMS and p-i-n diode switches. In the same sec-tion, the concept of hybrid reconfigurable multibandantennas is introduced, where the advantages of bothreconfigurable and multiband antennas can be simulta-neously achieved while supporting more services. Tworeconfigurable multiband antenna designs for mobilehandset/MID and laptop computer platforms aredemonstrated. In the section “Implementation ofReconfigurable Multiband Antennas in MultiradioPlatforms,” the system level integration is discussed.Conclusions are presented in the final section.

Frequency-Reconfigurable ConceptsThere are three types of frequency-reconfigurableantenna structures that are major candidates for mul-tiradio wireless platforms: patch antennas, wire anten-nas, and PIFAs. Each employs a distinct mechanism inorder to achieve the required frequency reconfigura-bility. First, for the patches, a variety of slots are usu-ally introduced to detour the current path on the patchantennas to control their resonances. The length ofthese slots can be controlled by switches to reconfig-ure the patch antenna’s operating frequency. Second,for the wire antennas, the resonant frequency is pri-marily defined by its length or perimeter, which alsocan be controlled for reconfigurability. For example, amonopole antenna has its first resonance when itslength is about a quarter-wavelength, while a loopantenna resonates at a frequency where its perimeteris approximately one wavelength. Subsequently, vari-ous switches can be implemented to alter the length orperimeter of these wire antennas to allow operationover different frequency bands. Third, a PIFA struc-ture can be considered as a half-sized slot, where itsfeed and ground location determines the input imped-ance of the antenna. By changing the PIFA’s feed orground location, its mode of operation can be recon-figured, allowing its resonant frequency to be con-trolled. There are many other alternatives forfrequency-reconfigurable antennas, but here weaddress only these three approaches.

Patch AntennasPatch antennas are very attractive structures for varioustypes of wireless platforms, such as smart phones andMIDs. Patch antennas have a wide beam angle ofalmost uniform radiation pattern at broadside, and pre-sent inherent ground shields, which is quite advanta-geous for improving mobile handsets’ compliance withtheir specific absorption rate (SAR) constraints. Hence,a great deal of effort has been put into extending theiroperation from a single frequency to cover multiple fre-quency bands by reconfiguring the antenna structure.For example, using RF-MEMS switches, Weedon et al.developed a frequency-reconfigurable patch antennathat could dynamically support both L band (1–2GHz) and X band (8 to 12.5 GHz) for communicationand synthetic aperture radar applications, respectively[2]. As shown in Figure 3, when all switches betweenthe patches are off, the central patch operates in X band.Meanwhile, a patch structure working at L band can beconstructed by connecting all nine patch antennastogether with MEMS switches.

Such a technique of constructing a reconfigurableantenna with patches has been proposed for wireless

February 2009 69

Figure 2. C/N of GPS module using reconfigurable andmultiband antenna with a 2.4-GHz jamming signal inject-ed for a limited time (10–40 s).

42

40

38

36

C/N

Rat

io

34

32

30

2810 20

Time (s)

30 40 500

Reconfigurable AntennaMultiband Antenna

Figure 3. Frequency-reconfigurable patch antenna operating at (a) L band (switches ON configuration) and (b) X band(switches OFF configuration). (c) Perspective view. (Modeled after [2].)

SwitchLocation Coaxial

Feed

Patches

(a) (b) (c)


70 February 2009

MIMO system applications [3]. As shown in Figure 4,multiple MEMS switches are used to connect the smallconductive pads to form patch antennas of differentsizes, having different resonant frequencies. Althoughthe pixel patch approach provides great flexibility inreconfiguring the patch antenna’s operating frequencyand polarization, the number of switches required caneasily become so prohibitively high that the cost andloss of the switches and the corresponding complexbiasing circuitry make the implementation of such astructure very challenging.

Yang and Rahmat-Samii presented a more practicalway to construct a frequency-reconfigurable patchantenna by introducing a switchable slot [4]. As shownin Figure 5, a vertical slot is cut in the patch antennawith a diode switch placed across the slot in the middle.When the switch is on, the horizontal main current ofthe patch’s first resonance is only slightly disturbed ascompared to the case with no slots. Hence, this intro-duced slot has limited effect on the patch antenna’s res-onant frequency. But when the switch is turned off, thehorizontal current is forced to detour around the slotand travels a longer path; as a result, the patch antennaresonates at a lower frequency. More recently, switch-able slots have been implemented on the ground planeas well to enable frequency reconfigurability of thepatch antenna design [5]. As shown in Figure 6, threeslots are cut on the ground plane with one p-i-n diodeswitch placed across each slot. As with the previouslydescribed patch antenna with switchable slot (PASS),when the switches are on, the patch works at the higherwireless broadband Internet service (WiBro) frequency(2.3–2.4 GHz); when the switches are off, a longercurrent path results in a lower resonant frequency thatcovers the K-PCS (1.75–1.87 GHz) band. The dc biasingcircuitry for the switches in the patch antennas is rela-tively simple, and the parasitic effects of the dc biaslines, commonly seen with reconfigurable wire antennastructures, are significantly reduced because of theextended ground plane. However, in order to developan antenna suitable for mobile platforms, the size of thepatch antenna must be significantly reduced. At thesame time, it is very desirable to have more than twofrequencies supported by one reconfigurable patchantenna. Along these lines and addressing some of these

Figure 5. Prototype of frequency-reconfigurable PASS,supporting two frequency bands: (1) p-i-n diode and (2) dcbias circuitry. (Figure taken from [4].)

(1)

(2)

Figure 4. Frequency-reconfigurable pixel patch antenna schematics at (a) 4.1 GHz and (b) 6.4 GHz. (Figure taken from [3].)

Connected ComponentDisconnected Component

Actuator Down

Actuator Up

(a) (b)


design challenges, a design example is given in the“Reconfigurable Mini Nested Patch Antennas,” where aminiaturized reconfigurable nested patch structure inte-grated with MEMS switches is discussed in detail.

Wire AntennasWire antennas are constructed using a given arrange-ment of conductive wires located away from theground plane. The basic wire antenna structuresinclude monopole antennas, dipole antennas, and loopantennas. They usually offer wider bandwidths com-pared to their microstrip line antenna counterparts,which are generally constructed above a large groundplane. Wire antennas can also be compacted to a smallvolume with generic shapes through meandering. Suchadvantages in bandwidth and design flexibility makewire antennas ideal candidates for mobile platformswith ground planes of limited size, such as smartphones and MIDs. The wire antenna’s operating fre-quency can be reconfigured by adding or removingparts of its length or perimeter by means of switches.When this is done, the current distribution at the newresonant frequencies is usually very close to that of theoriginal resonances. Hence, the new structure canexhibit almost the same radiation characteristics andinput impedance as the original wire antenna structure.These properties make wire antennas excellent choicesfor frequency-reconfigurable antenna implementation.

Kiriazi et al. presented a reconfigurable dipoleantenna fabricated on a high-resistivity silicon (HRS)substrate [6]. In this design, RF-MEMS switches arefabricated on the same HRS substrate as the dipoleantenna. They are placed in line with the dipolestructure to dynamically control the length of theantenna and consequently its operating frequency.The 4.9-GHz and 9-GHz frequency bands are sup-ported by the proposed reconfigurable dipole anten-na in two configurations. A similar idea with a dipoleantenna on HRS was reported by Panagamuwa et al.using silicon photoconducting switches instead [7].Figure 7 is a photograph of the antenna. As a replace-ment for the dc voltage carried by metallic feed lines,the photoconducting switches are addressed by lightsignals generated by laser diodes and carried to theantenna structure by fiber-optic cables. The introduc-tion of optically controlled switches completely elim-inates the effects of the dc bias lines that is usuallyseen when RF-MEMS and p-i-n diode switches areutilized in reconfigurable wire antennas. Three fre-quency configurations have been achieved uponturning both switches on, or only one switch, orkeeping both switches off.

Optically controlled switches have been success-fully implemented in frequency-reconfigurablemonopole antennas as well [8]. In this design, opto-electronic switches are placed in series with a seg-mented monopole antenna structure. The illumina-

tion of the switches is controlled to toggle the switch-es between a high- and low-resistance state, effective-ly changing the resonant frequencies of the monopoleantenna. This optical antenna is analogous to thereconfigurable monopole antenna utilizing RF-MEMSswitches that was presented by Gupta et al. in [9],where the antenna is fabricated on quartz and HRSsubstrates in order to accommodate the RF-MEMSswitches. The fabricated monopole antenna supportsboth the 4-GHz and 6-GHz frequency bands when theswitch is on and off, respectively.

Along these lines, a reconfigurable maze antenna [10]concept was introduced by the authors. In this, the

February 2009 71

Figure 6. Frequency-reconfigurable patch antenna withthree slots on the ground plane, supporting the WIBROand K-PCS frequency bands. (After [5].)

p-i-nDiode

Ground

PatchSlots

Figure 7. Optically switched frequency-reconfigurabledipole antenna supporting 2.26, 2.7, and 3.15 GHz.(Figure taken from [7].)

Silicon Switches

Balun

CPW

CPS

SMAConnector


72 February 2009

perimeter of a rectangular loop is controlled by RF-MEMS switches. As shown in Figure 8, the edges of a rec-tangular loop could be indented to form a larger perime-ter, resulting in a lower resonant frequency operation inthe same predefined area. The indentation process can berepeated at various levels, as the figure shows, to func-tion at relatively lower frequencies [10]. In addition, thesegmented loop structure can be dynamically controlledto form a certain loop perimeter by changing the varioussegments’ interconnectivity. This control would requireusing multiple switches such that the various loop seg-ment dimensions could be designed to provide multiple-

resonances at a well-prescribed set of frequencies.Meanwhile, the antenna’s radiation pattern and its inputimpedance at these resonances should be effectively verysimilar, which is quite attractive for wireless services.

Even though these examples are just a few imple-mentations of reconfigurable wire antennas, theydemonstrate the potential of using wire antennas toachieve frequency reconfigurability. However, the phys-ical size and implementation cost are the main draw-backs limiting the above designs in consumer-typemobile platforms. With the goal of size and cost reduc-tion in mind, a frequency-reconfigurable wire antennadesign example is shown in “Reconfigurable MultibandMeandered Monopole Antennas.” In this design, ameandered monopole structure is used to reduce theantenna size while providing more operating frequen-cies. P-i-n diode switches are utilized and integratedwith antennas built on regular low-cost flexible dielec-tric material to reduce the implementation cost.

Planar Inverted F AntennaThe PIFA structure is a very popular type of antenna formobile platforms. It usually consists of a radiating wirethat is short-circuited to the antenna’s ground plane

with a shorting pin located acertain distance from the feedpoint. The location of the feedpoint can be optimized toimprove the input match at res-onance, which generally hap-pens when the length of thePIFA antenna is approximatelyone-quarter wavelength. PIFAstructures also exhibit accept-able radiation impedanceswhen they are placed veryclose to a large ground plane,making them a popular choicefor laptop computers, as theycan be easily integrated intothe rim of the display. The tech-niques discussed in the patch

and wire antenna cases can also be implemented onPIFA structures to enable frequency reconfigurability.Panala et al. demonstrated a thermally controlled fre-quency-reconfigurable antenna with switchable L-shaped slots on its main plate [11]. As shown in Figure9, thermal MEMS switches are placed across the slot onthe PIFA. The operating mechanism is similar to that ofthe PASSs, where the surface current distribution on themain plate is altered by reconfiguring the slot length.

Because of the special placement of the MEMSswitch in [11], the global systems for mobile communi-cations (GSM) frequency band is always supported,while additional resonant frequencies can be achievedin ranges from approximately 1 GHz to 3 GHz with dif-ferent switch locations.

Figure 8. Reconfigurable loop (maze) antenna configura-tions. (a) 2.4 GHz, (b) 2.0 GHz, (c) 1.8 GHz, and (d) 5.2GHz. (Figure modeled after [10].)

(a) (b) (c) (d)

Figure 9. Frequency-reconfigurable slot loaded PIFA structure with thermal MEMSswitches. (Figure taken from [11].)

Slot

DuroidSubstrate

MEMS SwitchShort-CircuitsDuroid SupportFeed Coaxial

dc Current

Microstrip Linesdc Bias Current

Rectangular Element 75 × 30 mm2Ground Plane100 × 85 mm2

Figure 10. Dual-frequency-band reconfigurable PIFA formobile handsets [12]. (Figure taken from [12]; arrows added.)

Branch 1

Ground

Branch 2

Switch Locations


Meanwhile, Wu et al. have presented a dual-frequency-band reconfigurable PIFA structure formobile handset applications [12]. As depicted inFigure 10, the PIFA consists of two branches, the lengthsof which provide resonances at 850 MHz and 1.9 GHz,respectively. Hence, the antenna simultaneously sup-ports all U.S. GSM standards in one state. By using thesame techniques applied to wire antennas, the length ofthese PIFA branches can also be controlled to achievedifferent operating frequencies. For example, and asshown in Figure 10, the length of the two radiatingbranches can be reconfigured to concurrently supportall European GSM standards (i.e., 900 MHz and 1.8 GHz)in a second state (by turning the switches off).

Recently, new frequency reconfiguration mecha-nisms that take advantage of the unique structure ofPIFA have been introduced to reconfigurable multi-band antennas [13], [14]. Instead of reconfiguring thePIFA structure with switches in the body of the anten-na, the grounding and/or feed points of the PIFAs canbe switched to achieve different operating frequencies.Using this concept, we have developed a multibandreconfigurable antenna having a multibranched PIFAstructure with a switchable grounding location [14]. Asshown in Figure 11, two sets of p-i-n diodes are placedacross the antenna branches and the ground plane.Different sets of PIFA branches can be excited by tog-gling the dc biases of the switches, resulting in PIFAconfigurations operating at distinct sets of frequencies.

Moreover, PIFA structures with switchable ground-ing/feeding locations also enable the utilization oflumped element tuning circuitry in order to achieve dif-ferent operating frequencies with the same PIFA struc-ture. For example, Mak et al. implemented a reconfig-urable multiband PIFA with switchable groundinglocation [15]. As depicted in Figure 12, lumped elementtuning circuits are placed between the PIFA and itsground plane, in addition to the switch that providesgrounding of the PIFA. Once the switch is turned off,the PIFA is loaded with additional reactance, and it thenresonates at a distinct set of frequencies. Similarly, asshown in Figure 13, Boyal et al. have successfullyimplemented a frequency-reconfigurable PIFA utilizingcapacitive MEMS switches as the tuning componentsbetween the feeding and grounding points of the PIFAstructure [16]. This antenna is capable of supportingfive frequency bands in a few configurations.

PIFA structures with switchable grounding locationsenable simple techniques to be applied to integrateswitches. dc bias circuitry can be placed away from theradiating element, which significantly reduces the para-sitic effects. A variety of types of switches can be usedwithout additional effort in switch modeling once theyare placed on or near the ground plane. As a result, thedc bias lines in [14] are completely eliminated, and dif-ferent switching devices such as RF-MEMS, p-i-n diodes,and MESFET switches can now be easily implemented,

as demonstrated in [15]. Switch integration in reconfig-urable antennas is a great challenge and is addressed inthe next section. A design example is discussed in detailin the section “Reconfigurable Multiband Twin PIFA forMultiradio Laptop Platforms.”

February 2009 73

Figure 11. Reconfigurable multiband twin PIFA struc-tures with switchable grounding points using p-i-n diodeswitches. (a) Front side and (b) back side. (Figure modeledafter [14].)

L1

L2L4L3

L5

W1 W2

Switch 1 Switch 2

Ground

L6

L10

L8

L9L11

Ground

Switch 3

(a)

(b)

Figure 12. Schematic of PIFA with switchable ground sup-porting GSM, DCS/PCS, universal mobile telecommunica-tions system (UMTS), and WLAN frequency bands in twomultiband configurations. (Figure taken from [15].)

15.5

10.5

85

42

0

C3

23

21L2

42

3 7

3

2.5

11.5

SW

Port 0

y

z

x

L1 = 6.8nHC3 = 1pF


74 February 2009

Switch Integration and DC Biasing ConsiderationsAs previously mentioned, one of the main challenges ofreconfigurable antenna development is the integration ofswitches. Commonly, two major types of switches, p-i-ndiodes and RF-MEMS, are used in developingfrequency-reconfigurable antennas for wireless applica-tions. The advantages of p-i-n diodes include a very lowdriving voltage, a relatively high power handling capa-bility, and extremely low cost. Since there are no movingparts in p-i-n diode switches, they also exhibit very highreliability. However, such diodes require a dc bias cur-rent in their on state, which consumes a significantamount of dc power. RF-MEMS switches, on the con-trary, are biased by high dc voltages and actuated bybuilt-up static charges, and hence no current is drawnand they consume almost no power. Since RF-MEMSswitches rely on physical metallic connections, they offerexceptionally low insertion loss, relatively high isolation,extremely high linearity, and very wide bandwidth [17].However, the mechanical movement within RF-MEMSswitches still requires prohibitively high actuating volt-age (greater than 60 V in most cases) and yields compar-atively low speeds. Currently, the high cost of RF-MEMSswitches is another factor that hinders their proliferationin commercial products. In the following sections, theplacement, biasing scheme, and modeling of both typesof switches is discussed in detail.

Switch ModelingAs shown in “Frequency-Reconfigurable Concepts,” inmost reconfigurable wire antenna implementations, theswitches are directly applied in series with the antennastructures. Such an arrangement offers great flexibilityin the antenna structure design, but it introduces a chal-lenge in the modeling of the RF-MEMS switches in anantenna environment. Conventionally, S parameters

are used in RF circuit designs to model MEMS switch-es. However, when MEMS switches are placed in serieswith the antenna structure and distant from the groundplane, S parameters become invalid without properground references.

Mechanical structural models, on the other hand,can always be utilized to model the MEMS switcheswith a 3-D full-wave electromagnetic (EM) simulationtool. Nevertheless, the details of the internal structureof the switch are seldom available for off-the-shelf com-ponents. A simplified MEMS switch model has beensuccessfully introduced in [18], where the internalswitch structure is simplified as a combination ofmicrostrip line, silicon substrate, and bond wires. Inthis case, the Ansoft HFSS EM simulation tool has beenused, and the simulated results of the antenna structurewith this model are close to the measured results.However, the cosimulation of objects with different sizescales is very time-consuming.

As an alternative, we have introduced an approxi-mate method for modeling RF-MEMS switches in theantenna structures using lumped elements in [10], [19],where the effects of the switches can still be evaluated byEM simulation but at a faster speed. First, the equivalentcircuit model of the RF-MEMS switch was generatedfrom its S parameters measured in a microstrip line envi-ronment using a curve fitting method. In our case, Sparameters provided by the manufacturer were usedand, as shown in Figure 14(a), two networks were gen-erated to represent the on and off states, respectively, ofa Radant SPST-RMSW100 RF-MEMS switch [20]. Thesimulated S parameters of the lumped element modelsare shown in Figure 15, where good agreement betweenthe developed equivalent model and the measured Sparameters (provided by the manufacturer) of the actualRF-MEMS switch has been achieved over a wide fre-quency range.

Second, the developed model can beeasily integrated into the EM simulationtools, in which the components in theequivalent circuit model are treated aslumped element boundary conditions[19]. A section of the simulation model ofan RF-MEMS switch in a wire antenna isshown in Figure 14(b) as an example.Since the switches are connected inseries with the antennas’ wires without aground plane beneath them, theirground terminals are floating. Sub-sequently, shunt elements to ground inthe equivalent circuit model have lessand less influence on the antenna as theswitches are placed farther away fromthe antenna’s ground plane. In ourimplementation of the RF-MEMS switch-es onto the bent monopole reconfig-urable antenna [19], all shunt elements

Figure 13. Reconfigurable PIFA supporting five frequency bands, usingcapacitive MEMS switches as tuning elements. (Figure taken from [16].)

D. Feed

PCB Feed

C. Switched Connection(For Impedance Matching)

B. Short to Ground

dc Bias Lines

A. Switched Connection(For Frequency Tuning)


to ground (connected between source or drain toground) are eliminated as an approximation. Such asimplified lumped element model provides almostidentical simulation results as compared to the simpli-fied mechanical structure model [18], with a greatreduction in simulation time.

Switch Placement and DC Biasing/ControlSwitch placement and biasing/control strategiesdepend on the type of the switch and the topology ofthe antenna. In most cases, dc control lines need to be

added in the antenna structure whenever switches areplaced away from the ground plane. These dc lines, ifplaced in the vicinity of the radiating elements, wouldaffect the antenna’s resonance characteristics. In addi-tion, the parasitic radiation introduced by improperlyisolated dc lines will generate significant distortion inthe radiation pattern. Typically, one-quarter wave-length dc lines are generally not appropriate at fre-quencies below 5 GHz because of their long physicallength. In addition, they are only feasible for antennastructures that are backed with a large ground plane,

February 2009 75

Figure 14. (a) Equivalent circuit model and (b) simulation model (bent monopole antenna) for off and on states. (Image taken from [19].)

R2

C1 C1R1

L1R1

C2C2 R2

GroundGround

Output(Drain)

C1 = 0.17 pF, C2 = 0.048pF, C3 = 0.17pFR1 = 6.5kΩ, R2 = 8.6kΩ, R3 = 5.6kΩ

Switch Off

C1 = 0.15 pF, C2 = 0.15pF, R1 = 0.12Ω,R2 = 1.7kΩ, L1 = 0.87 nH

Switch On

C3

R3Input

(Source)

Output(Drain)

Input(Source)

(a)

(b)

SeriesElements

ShuntElementsto Ground

Figure 15. Performance of the equivalent circuit model. (a) Return loss of ON state, (b) insertion loss of ON state, and (c)isolation of OFF state. (Image taken from [19].)

−20

−15

Ret

urn

Loss

(dB

)

−25

−30

−35

−40

−45

−15

−10

Isol

atio

n (d

B)

−20

−25

−30

−35

−0.15

−0.10

Inse

rtio

n Lo

ss (

dB)

−0.20

−0.25

−0.30

S-Parameter Provided by Manufacturer Equivalent Circuit Model

(a) (b) (c)

1 2 3 4Frequency (GHz) Frequency (GHz) Frequency (GHz)

5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8


76 February 2009

such as the microstrip line and slotted patch antennas.These biasing structures have been successfully imple-mented in reconfiguring either the radiation pattern[18] or the polarization [4] of various antenna designs.However, the inherent narrowband performance oftheir antennas’ structures, as well as the quarter-wavelength-long dc lines, have limited their implemen-tation in frequency-reconfigurable antennas. Inductorswith ferrite cores also could be used as perfect RFchokes over a wide frequency range. However, they aretoo bulky to be integrated into these antenna structures.

Alternatively, and to simplify the structure, largeresistors have been used (for example, in our bentmonopole antenna design [19]) to reject the parasiticradiation from the dc lines by offering high RF isola-tion. However, this approach carries the anticipatedcost of slower switching speed and slightly lowerantenna efficiency. A common problem is that the dclines also can radiate and can degrade the radiating pat-terns of frequency-reconfigurable antennas. Therefore,the routing of the dc lines within the antenna’s struc-ture can be designed to shield or suppress their para-sitic radiation. As shown in the design example of thereconfigurable mini nested patch antenna (in the sec-tion “Examples of Frequency-Reconfigurable Antenna

Designs” and [21]), the developed design was imple-mented on an FR4 substrate, where the nested patcheswere printed on the top layer and the dc control lineswere printed on the bottom layer. The patch was thensuspended above the ground plane, essentially sand-wiching the dc lines between the patch and the ground,which resulted in a better suppression of the parasiticradiation.

One of the most effective ways of biasing and con-trolling the switches in reconfigurable antennas is touse the radiating structure itself to carry the dc bias andcontrol signals. Strategic placement of two terminalswitches (such as p-i-n diodes) across two dc separatedsections of the antenna is required, so that the dc volt-age difference between the two sections can be con-trolled to alter the switch’s connectivity. The RF feedcable also can be used to carry the dc biasing and con-trol signals, as demonstrated in [22], where the diodeswere placed across the patch and the ground of a patchantenna, and controlled by a dc signal carried by thefeed cable. In our implementation, we have extendedthis idea to the development of reconfigurable multi-band PIFA structures, where the grounding point isswitched by the p-i-n diodes (described in the followingsection and [13]).

Examples of Frequency-Reconfigurable Antenna DesignsA few antenna prototypes with various switchingdevices, including RF-MEMS and p-i-n diodeswitches, is shown. All of these prototypes arevariations of the basic frequency-reconfigurableantenna types discussed in the section“Frequency-Reconfigurable Concepts,” and theirimplementation undertakes special design con-siderations toward mobile platform integration,such as antenna size and platform form factor.The concepts behind our design of reconfig-urable single- and multiband antennas, as well asour development of these reconfigurable anten-nas, is presented. Our initial efforts have focusedon modifying multiband structures and turningtheir resonances on and off by introducingswitches. The goal was to dynamically controlthe antenna topology, allowing it to be solelydedicated to one frequency band at a time inorder to achieve better performance. Recently, wehave directed our studies toward a new hybridapproach to provide reconfiguration capabilitiesto multiband structures and to construct recon-figurable multiband antennas, in which theadvantages of both the single-band reconfig-urable antenna and the multiband antenna canbe combined and enhanced. We also discuss size-reduction techniques for various antenna types,permitting the developed reconfigurable anten-nas to fit into mobile platforms.

Figure 16. Concept of (a) multiband patch antenna utilizing multi-ple U-shaped slots and (b) half-size slotted patch antenna using agrounding wall.

Slots

Slots

Patch

Substrate

GroundingWall

Patch

Ground

CoaxialFeed

Ground

(a)

(b)


February 2009 77

Reconfigurable Mini Nested Patch AntennasAs discussed in “Frequency-Reconfigurable Con-cepts,” the relatively large antenna size and limited fre-quency band supported by patch antennas are twomajor limitations that hinder the implementation offrequency-reconfigurable patch antennas on multira-dio mobile platforms. As a result, in our patch antennadesign, we focused on modifying the topolo-gy of a PASSs to cover more frequency bandswith a significantly reduced size. As an exten-sion of the previous efforts to expand theoperation of patches to cover multiple fre-quency bands, we investigated the use of U-shaped slotted patches [23]. The U-shapedslots are defined to encompass many nestedpatches [24], as shown in Figure 16(a). Theoperation of these nested patches can general-ly be visualized, as the full patch operates at afundamental frequency, and the introductionof a symmetric U-shaped slot forces the cur-rent to detour around the slot, causing it tohave another resonance at lower frequency aswell. This concept can be easily extended, asaddition of more U slots would lead to evenmore resonances. Additionally, the patch andslot dimensions can be selected in such a wayso that the distinct multiple resonances of thepatches or slots specifically cover a desireddistinct group of wireless bands.

However, this conventional patch topologyis relatively large and thus is not appropriatefor a compact wireless receiver such as a smartphone or MID. Therefore, two teams suggest-ed concepts [24], [25] that reduced size by 50%by using a vertical grounding wall at the cen-ter [along the horizontal axis inFigure 16(a)] to eliminate half ofthe patch, as shown in Figure16(b). To further reduce the overallsize of the half-patch, we haveadditionally utilized the previousstructure’s symmetry on the otheraxis (vertical axis of Figure 16),thereby reducing the overall patcharea by another 50%. This antennais called the mini nested patchantenna and is shown in Figure17(a). Additionally, this simplifiedstructure can now be viewed as aseries of L-shaped stripes connect-ed at a common feed point. Thisdesign still maintains the mul-tiresonance feature—that is,multiband operation—but with agreatly reduced size.

This developed multibandstructure can also be easily modi-

fied to be reconfigurable by placing a multitude ofswitches between the L-shaped stripes, as shown inFigure 17(b). A grounding pin is also used instead of agrounding plane to separate the L-shaped stripes fromeach other. These switches can be utilized to dynamicallyreshape the structure’s interconnection/topology andaddress one frequency band at a time.

Figure 17. Concepts of (a) further reduction of slotted patch antenna byusing symmetry and (b) introduction of reconfigurability by utilizingswitches.

GroundingWall

SymmetryPlane

Patch

Ground

Groundingpin

Switches

Patch

Ground

(a)

(b)

Figure 18. Dimensions of developed mini nested patch antenna.

dc Control Lineon Bottom Layer

WiFi/WiMax

DCS/PCS/UMTS

X

YGPS

20.2 mm

MEMSSwitch 1

MEMSSwitches

2-4

GSM

19.2 mm

Foam

Shorting Point

Feeding Point

Z

Y


78 February 2009

In order to ensure a single band operation for eachstripe, the slots/gaps between the L-shaped stripes arefurther optimized, thereby ensuring that only thestripe’s lowest resonance is matched at the commoninput side. All higher-order resonances created by thevarious L stripes are either suppressed or displacedfarther away from our operating frequency range to pre-vent unwanted passbands. Figure 18 shows a sketch ofthe reconfigurable structure design, where each L-shaped branch generates a resonance at a fixed frequen-cy for a specific service. RF-MEMS switches are placedin series in the antenna structure with their dc controllines routed through the back side of the patch structure.As shown in Table 3, the antenna can be utilized for upto four groups of bands in four configurations with acombination of three to four switches (Switch 1 isalways on for the selected group of services).

A prototype of this mini nest-ed patch antenna design was fab-ricated using an FR-4 material.Foam was used to elevate thepatch from the ground plane, asshown in Figure 19(a). RadantMEMS SPST-RMSW100 MEMSswitches [20] were utilized in thisdesign to dynamically reconfig-

ure the structure with the need for only minimal dcpower consumption. However, it is obvious that withthis design, increasing the number of switches is diffi-cult because of the increasing complexity of providingboth the dc biasing networks and their associated RFrejection. During the measurement, a limited-sizeground plane (6.5 × 4.2 cm) is placed 0.8 cm beneaththe patch antenna to emulate the smart phone form fac-tor. The measured return loss at each state is shown inFigure 19(b), indicating very good performance at thespecified four bands.

Reconfigurable Multiband Meandered Monopole AntennasIn the reconfigurable mini nested patch antennadescribed above, only one frequency band is addressedat a time. As the number of services increases, however,the limitations of these antenna structures become morepronounced as the insertion losses and dc power con-sumption become prohibitively high when multipleswitching devices are used. Most important, the devel-opment of these antennas becomes more and morechallenging because of the complexity of the antennaand dc control line structures.

We have recently proposed a novel hybrid reconfig-urable multiband antenna structure that is based onboth reconfigurable and multiband antenna concepts[26]. In other words, we reconfigure the multibandantenna such that multiple switches are used to controlthe antenna structure to hop between several multi-band configurations. Through the judicious groupingof frequency bands for each configuration, the advan-tages of both the multiband and the reconfigurableantenna structures can be simultaneously achievedwhile more services are supported. Basic guidelines forthis frequency grouping are 1) different standards of thesame service can be grouped together and simultane-ously addressed and 2) conversely, different serviceswith a close frequency spectrum have to be separatedinto different groups. For example, GSM850 andGSM900 can be grouped into one bundle: when onefrequency band is used in one country, the other serviceare idle at this time. Meanwhile, other services withclose operating frequencies, such as the WLAN at 2.4GHz and cellular services around 2 GHz, need to beseparated to relax the requirements on the followingfilter sections. In the following design, we utilized boththe monopole’s fundamental and higher-order

Figure 19. Fabricated reconfigurable mini nested patchantenna with MEMS switches. (a) Photograph of fabricatedprototype. (b) Measured return loss in four single-bandstates. (Image taken from [21].)

0

−5

−10

−15

−200.5 1.0 1.5

Frequency (GHz)(b)

2.0 2.5 3.0

Ret

urn

Loss

(dB

)

GSMGPSDCS/PCS/UMTSWiFi-WiMax

Shorting MEMS

SwitchesPin

Feed

(a)

TABLE 3. Truth table of reconfigurable mini nested patch antenna design.

Switch Matrix 1 2 3 4

State 1: GSM850, GSM900 (820–960 MHz) On On On On

State 2: GPS (1.575 GHz) On On On Off

State 3: DCS, PCS, UMTS (1.71–2.17 GHz) On On Off Off

State 4: WiFi, WiMax, (2.3–2.7 GHz) On Off Off Off


resonances to accommodate more services and improvetheir out-of-band rejection.

In order to minimize the number of switchable con-figurations while still covering all services, we havestrategically grouped the frequency bands in Table 4into two sets. All cellular services are grouped togeth-er and categorized as the wireless wide area network(WWAN) configuration. This includes GSM850,GSM900, digital cellular service (DCS), personal com-munication system (PCS), and international mobiletelecommunications (IMT)-2000 [wideband code divi-sion multiple access (WCDMA)/high-speed downlinkpacket access (HSDPA)] frequency bands. Conversely,all Internet services, including 802.11.a, b, g, and n, areconsolidated in another configuration utilized forWLAN services. For each configuration, a multibandantenna is required. It is understood here that any twoservices within these specially grouped configurationswill not be required to operate simultaneously. Thepercentage bandwidth required for each band is notedin Table 4 as well, where in some cases (such as the23.7% in WWAN high band for handsets), the band-width requirement is very challenging for antennadesigns, and special bandwidth expansion techniquesneed to be employed [26].

Basing our design on the meandered monopoleantenna structure [27], we have developed an antennathat comprises two meandered branches for multiradiomobile handsets, as shown in Figure 20(a). Only twoswitches are required in this design: one switch in eachbranch to control the monopoles’ physical lengths.According to the previous grouping, when both switch-es are on, the antenna operates in the WWAN configu-ration, supporting the lower-frequency portion of theservice. When both switches are turned off, the mono-pole elements are shorter and the antenna operates inthe WLAN configuration, as shown in Figure 20(b). Inboth cases, the higher-order resonances of the mean-dered monopole structures are utilized as well, to coverthe relatively wide bandwidth requirements, especiallyfor the high band in WWAN configuration, asdescribed in [26].

To validate our concepts, we fabricated the antennaon DuPont Kapton flexible circuit material, as indicatedin Figure 20(c). The hard-wired version (with direct

metal connection instead of electronic switches) of themeandered monopole was used for measurements, anda limited-size ground plane (4.2 × 12 cm) was fabricatedto emulate the MID form factor. The simulated and

February 2009 79

TABLE 4. Frequency grouping of reconfigurable multiband antenna configuration.

Unit: GHz WWAN Configuration WLAN Configuration

Grouped HandsetHandset 0.820.824–0.96 (14–0.96 (15%)5%) 1.71.711–2.1–2.17 (27 (23.7%)3.7%) (3.5%)(3.5%) 4.9–5.85 (14.9–5.85 (16.8%)6.8%)bands Laptop (8.3%)(8.3%) N/A (7(7.3%).3%) N/A (3.5%)(3.5%) 4.9–5.34.9–5.35 (8.8%)5 (8.8%) N/A

Service GSM850 GSM900 DCS PCS IMT2000 802.11 802.11 a/nb/g/n

Frequency 0.824– 0.88–0.96 1.71– 1.85– 1.92–2.17 2.4– 4.9– 5.15– 5.725–(GHz) 0.894 1.88 1.99 2.484 5.25 5.35 5.85

Country U.S. Europe/Asia U.S. Universal Japan U.S. Indoor Outdoor

Figure 20. Reconfigurable multiband meandered monopoleantenna topology. (a) WWAN configuration (ON), (b)WLAN configuration (OFF), and (c) fabricated prototype.

Switch 1

Ground

Ground

5.2 GHz2.4 GHz

Switch 2

SwitchLocations

50 cm CoaxialCable

42 mm

(a)

(b)

(c)

11 mm


80 February 2009

measured return losses of the two configurations,shown in Figure 21, are in good agreement.

Reconfigurable Multiband Twin PIFA for Multiradio Laptop PlatformsAs discussed in the section “Frequency-ReconfigurableConcepts,” PIFA structures with switchable groundinglocations have great potential as frequency-reconfig-urable antennas. The same concept has been extendedto provide reconfigurability of dual-band reconfig-urable structures. A PIFA structure consisting of a mainradiating branch coupled to a parasitic branch to widenthe bandwidth and provide dual-band performancewas reported in [28], [29]. These basic dual-band struc-tures of different sizes can be clustered together to sup-port more frequency bands, while the location of thefeeding point and the shorting pins can always bealtered to achieve reconfigurable multiband operations.

In [13], we implemented these novel concepts todesign a reconfigurable multiband antenna. A design

example for multiradio laptop computers platformswas presented, where only the four essential servicesfor laptop applications were supported, as shown inTable 4. In our design, two PIFA structures were placedback to back, sharing the same feed point [Figure 22(a)].Only two switches were used to switch the shorting pinlocation in order to toggle between the WWAN andWLAN configurations. As shown in Figure 22(b), whenSwitch 1 is on and Switch 2 is off, a dual-band PIFAstructure operating at the WWAN mode is formed. Thelength of the top inverted-F structure is about a quarter-wavelength at 850 MHz, and it covers the GSM850band. Beneath the F structure, a grounded L-shapedstructure is also placed close to the feed. This parasiticelement can be tuned to resonate at a higher frequency,in our case the PCS band. Similarly, when Switch 1 is offand Switch 2 is on, the F and L shaped structures of theWWAN configuration are shorted to form a shortingpin for another pair of F and L antennas designed tocover the WLAN frequencies [as shown in Figure

Figure 21. Simulated and measured return loss of the meandered monopole antenna in (a) WWAN configuration and (b)WLAN configuration.

0

−5

−10

−15

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−25

Ret

urn

Loss

(dB

)

0

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Ret

urn

Loss

(dB

)

2.5 2 3 4 5 6

Meas. w/CableCST MWS Simu.Meas. Cable Loss

0.5 1.0

Frequency (GHz)(a)

1.5 2.0

1st BandGSM850/900

2nd BandDCS/PCS/IMT2000

1st Band802.11b/g/n

2nd Band802.11a

CST MWS Simu.Meas. w/CableMeas. Cable Loss

Frequency (GHz)(b)

Figure 22. (a) Reconfigurable multiband twin PIFA topology, (b) WWAN configuration, and (c) WLAN configuration.(Image taken from [13].)

Switch 1

WWANConfiguration

GSM850 “F” Antenna WLANPCS “L” Antenna

802.11b/g/n “F” Antenna802.11a “L” AntennaConfiguration

Switch 2 GroundCoaxialFeed

(a)

(b) (c)


February 2009 81

22(c)]. This design was printed on a Kapton flexible cir-cuit material, and two p-i-n diode switches were uti-lized as well.

As shown in Figure 23, the design was very compactin size, and two Microsemi MPP4203 p-i-n diodeswitches [30] were mounted with opposite orientationacross the gaps between the main and parasitic radia-tors. These switches are controlled by applying +1 V or−1 V across the inner and outer conductors of the coax-ial cable to toggle between WWAN and WLAN modes.

P-i-n diode switches are two-terminal devices. Theycan be modeled simply using the equiva-lent circuit model provided by the manu-facturer [30]. Usually, they can be repre-sented as a series resistance in the on stateand a series capacitance in the off state.Values of these equivalent components aredetermined by the biasing conditions butindependent from the switch’s relativelocation to the ground plane. In this designexample, the p-i-n diode switches arestrategically placed in shunt between theantenna structure and the ground plane.The simulated and measured return lossesof the WWAN and WLAN configurationsof our design are shown in Figure 24. During the mea-surement, large metal plates (32 × 20 cm) were used toemulate the body of a laptop. It can be seen that thelumped element equivalent circuit model has demon-strated accurate predicted results when validated bymeasurement over a very wide frequency range.

The efficiency of the p-i-n switches can also be evaluat-ed with several dc bias current conditions. Unlike RF-MEMS switches, which exhibit an almost constant inser-tion loss in the on state, the insertion loss of p-i-n diodeswitches is a function of their dc biasing current. In orderto determine the relationship between the antenna’s effi-ciency and the power consumption of the p-i-n diodeswitches, the average gain value of the reconfigurablemultiband twin-PIFA structure at each configuration canbe measured under different biasing current conditionsand compared to its hard-wired counterpart. As shownin Figure 25, as the biasing current increases, the seriesresistance of the utilized p-i-n diode (MicrosemiMPP4203 [30]) decreases and the average gain increases.However, after 100mA, the gain starts to saturate, andthe associated dc power consumption becomes prohibi-tively high for mobile platforms. Obviously, even lessinsertion loss can be achieved by using higher perfor-mance diodes, as it is extremely important to reduce thedc power consumption in handheld devices.

In summary, three different frequency-reconfigurableantenna design examples have been discussed in thissection, including the nested-patch single-band recon-figurable, meandered monopole, and twin-PIFA recon-figurable multiband antennas. The immediate questionsare: Are they practical? What are the limitations of such

approaches? In the following section we address someof these valid concerns.

Implementation of ReconfigurableMultiband Antennas in Multiradio PlatformsIncorporating the reconfigurable technology into multi-radio platforms requires adjustments in system soft-ware and platform hardware architecture. This Intel-developed reconfigurable antenna technology [31]implemented in an ultrathin laptop computer is a real-life example that demonstrates reconfigurable antenna

Figure 23. Fabricated low-profile reconfigurable multiband twin PIFA forlaptops. (Image taken from [13].)

10 mm

Ground

90 mm

p-i-n Diodes

50 cm CoaxialCable

Figure 24. Simulated and measured return loss of thereconfigurable multiband twin-PIFA prototype. (a) WWANconfiguration and (b) WLAN configuration. (Image takenfrom [13].)

0

0

−5

−10

−15

−20

−25

Ret

urn

Loss

(dB

)

2.52.01.5

(a)

Frequency (GHz)

(b)

Frequency (GHz)

1.00.5

5.04.03.02.0 6.0

−5

−10

−15

−20

−25

Ret

urn

Loss

(dB

)

4.53.52.5 5.5

1st BandGSM850

2nd BandPCS

2nd BandIndoor

802.11a1st Band

802.11b/g/nCST MWS Simu.Meas. Hard WiredWith/p-i-n Diode −1V BiasMeas. Cable Loss

CST MWS Simu.Meas. Hard WiredWith/p-i-n Diode +1V BiasMeas. Cable Loss


82 February 2009

integration with a multiradio platform, maximizing theplatform’s advantages [32].

Figure 26 shows the hardware architecture for the inte-gration of a frequency-reconfigurable antenna system.This system consists of three reconfigurable multibandantenna elements that are embedded within the lid of thelaptop. They are strategically designed to support allfrequency bands required by the multiband WLAN andWWAN modules located in the laptop base. Diversitylogic circuitry dynamically routes the RF ports on thewireless module to the antennas with the matching con-figuration by connecting the antennas under WWANconfiguration to the WWAN module and vice versa. Boththe reconfigurable antennas and the diversity logic circuitare controlled by a microcontroller. The on-board systemcontrol software can access such information as currentradio band and received signal strength indicator (RSSI)to intelligently select the frequency band.

In addition, the system control software uses diversitylogic to perform antenna selection, interference mitiga-tion, and spectrum sensing. These mechanisms not onlyprovide the maximum diversity for radios that are indi-

vidually operating, but also ensures the best performanceof each radio under simultaneous operation and providesthe highest possible isolation between these radios. Thisimplementation of three reconfigurable multiband anten-nas uses a minimum number of antennas to support bothWiFi 3 × 3 MIMO and cellular radios. When more radiomodules are integrated, the space savings and gain indiversity will be even more significant.

Now, let us look toward the future of frequency-reconfigurable antennas and their practicality. There aretwo major obstacles that are currently still limiting theirwidespread implementation on multiradio mobile plat-forms. On the antenna side, the power consumption ofp-i-n diode switches is still relatively high for mobileplatforms, where the battery life is one of the mostimportant aspects. RF-MEMS switches offer almostideal power and RF performance, but their cost today isstill too high for the consumer market. On the wirelessmodule side, the stand-alone diversity logic circuitryintroduces an additional loss to the wireless link. Thisinsertion loss can be significantly alleviated when moreradios are integrated into one module. Fortunately, thetrend of merging more radios is gradually taking place,as the initial successes of merging GPS with 3G andBluetooth with UWB have recently shown. So, webelieve practical reconfigurable antenna implementa-tion is much closer to the market today.

ConclusionMultiband antennas are very desirable for current mobilewireless devices because they can cover multiple fre-quencies using a relatively simple structure, and they areusually less expensive to produce than other alternatives.However, they require complicated filters with stringentrequirements to improve their out-of-band noise-rejectionperformance. These filters, moreover, are generally bulkyand expensive. In contrast, frequency-reconfigurableantennas have great potential for reducing productioncost and offer better out-of-band noise rejection.

However, the required addedswitches could complicate thedesign and could add more dcpower consumption if RF-MEMSswitches are not used. Better per-formance can be expected fromthese reconfigurable antennas, butcurrently, the RF-MEMS produc-tion cost is a deterrent to their wide-spread use.

Choosing the optimumapproach necessitates consider-ing the cost of all components-fil-ters and switches-in cost esti-mates. In general, in designingthese reconfigurable antennas,we found it imperative to reducethe number of the required

Figure 26. Multiradio concept PC utilizing reconfigurable multiband antennas andswitching system [32]. (Figure taken from [32]).)

ReconfigurableAntenna

ReconfigurableAntenna

Antenna MEMMicrocontroller

WWAN Card

DiversityLogic

802.11nWLAN Card

Figure 25. Measured average gain drop comparing withhardwired version at various p-i-n diode biasing conditions.(Image taken from [13].)

0.0

−0.2

−0.4

−0.6

Mea

sure

d G

ain

Dro

p (d

B)

−0.8

−1.0

−1.2200150100

Diode Bias Current (mA)5020

0.86 GHz Positive Bias1.92 GHz Positive Bias2.44 GHz Negative Bias5.15 GHz Negative Bias


February 2009 83

switches and place them properly. However, locatingthe switches very close to a common ground couldeliminate many of these problems.

Reconfigurable multiband antennas are very appeal-ing as they would require fewer filters, cover many ser-vices, allow for relaxed filter specifications, and producepronounced performance improvements. But as previ-ously mentioned, this comes with highly complicateddc biasing arrangement and increased number ofswitches. Recognizing that many services cannot existat the same time, implementation of selective groupingcan help in reducing the design complexity while stillkeeping reasonable performance improvement.Ultimately, reconfigurable multiband approach shouldbe a very valuable alternative to current technology.

With the fusion of more and more radios into a singlewireless module and the commercialization of low-cost,low-driving-voltage RF-MEMS switches, we believe,reconfigurable multiband antennas will proliferate inmultiradio wireless platforms in the near future.

AcknowledgmentsThis work was supported by an IEEE MTT-S GraduateFellowship and by Intel Corporation. The authorsthank Ansoft, LCC, Computer Simulation Technology(CST), for donating the EM simulation software andRogers Corp. for providing the dielectric substrates.

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Documents

2009, Frequency Reconfigurable Antenna for Multi Radio Wireless Platforms