Background and Motivation

In the past decade, wireless communication has grown rapidly. Many systems have been widely used, e.g. Global Position System (GPS), Wi-Fi for Wireless Local Area Network (WLAN), WiMAX, Bluetooth, and Global System for Mobile Communication (GSM) and Digital Cellular System (DCS). As these systems gain popularity, the demand for wireless products has highly increased, particularly for mobile devices. With the rapid development of the integrated circuit technology, the wireless mobile devices tend to be compact and integrated with multiple systems.

For example, a USB WLAN card today is only typically 12 mm × 50 mm that is smaller PCMCIA ( 50 mm by 100 mm ) (λ0/4 of 2.4GHz is 32.5 mm). Also, more than one wireless system is evident inside a smart phone today, e.g. GSM, Bluetooth, GPS, WiFi. Moreover, for increasing the data through put, the multiple-input multiple-output (MIMO) antenna system was developed. The system employs multiple antennas which simultaneously transmit and receive within the same system.

These examples have to integrate multiple antennas in one device. As a result of the increased demand for miniaturized wireless devices, compact antenna designs with high integrating ability are desired. Since the small devices have limited space reserved for antennas, size reduction becomes the primary concern [1]. Antennas have to be arranged closely giving serious coupling issue in application. However, the coupling will be a serious problem in applications. Therefore, antenna design in modern wireless products has to face important issues on miniaturization and decoupling [2]-[4].

This dissertation focuses on the development of miniaturized antennas and the isolation enhancement between closely-spaced antennas. There are already many design methods for miniaturized antennas – most of which utilize the structure natural resonance of the structure (which corresponds to the length of the antenna). Most miniaturization designs can be simply categorized into three groups – bending the

capacitance in single a LC resonator.

Metamaterial have recently bacomes popular in antenna design. There have been many published designs on this subject but the input matching or radiation mechanisms are usually not discussed. A survey of these designs will be discussed in the next section. This dissertation tries to develop a new design method of compact antennas and to provide the equivalent circuit analysis for demonstration.

The design methodology stems from equivalent circuit analysis to synthesize the resonance and process input matching. This design concept aims to diminish the consideration of wavelengths. As such, the compact antenna size is possibly smaller if resonance is achieved by discrete reactive elements in stead of the physical length. This dissertation will also study the conventional designs of bending the resonant path and dielectric loading.

The coupling between antennas will degrade the communication system performance, in terms of efficiency due to the cause of power dissipated because of coupling. The solution to this problem is to prevent the other antennas from absorbing the power. In this case, the antenna will be a reactive element with current on it but without power dissipation. This can be achieved by increasing the isolation between antenna terminals, i.e. increasing port isolation. Another solution is to decouple antennas for no current distribution on inactive antennas. Both conditions have already been discussed widely. A survey for above methodology will be presented in the next section. However, many of today’s published designs are bulky and most of them are not flexible enough for practical applications.

Today’s antennas tend to be compact and so does the decoupling structure for smaller devices. Therefore, this dissertation proposes two compact designs for decoupling and port isolation enhancement. The port isolation enhancement design can process two very closely-spaced antennas. The miniaturization methods of decoupling that respond to the miniaturized antennas also use the circuit approach design for size reduction.

1.2 Literature Survey

In this section, the conventional miniaturization designs and decoupling methods will be presented briefly. The miniaturization methods introduced here are divided into three parts: 1. resonant path alignment, inductive loading or capacitive loading, 2.

dielectric loading, and 3. using metamaterial. In the first section, some compact printed dual-band design will also be mentioned due to their popularity.

1.2.1 Miniaturized Antennas

Most compact antennas are resonant antennas. In resonant type wire antennas, length is a main factor in determining the antennas’ frequency of operation. For a conventional monopole, it requires a quarter wavelength to resonate and for a loop antenna, it requires one wavelength. To have a compact design, one can simply arrange the resonant path inside a small area or reduce the physical length by inductive or capacitive loading while keeping same electrical length. The conventional design of the resonant path alignment is to bend, fold or turn the path into a smaller size – even a spiral or helical shape is acceptable. Figure 1.1 shows a loop antenna that is folded to be compact for handset application [1].

For mass production, cost is always the first priority. The following examples are given with low cost printed antennas. The printed slot antenna consisting of a slot line, can be miniaturized using similar methods. For example, the most used method is to spiral the slot while keeping its resonant length as shown in Figure 1.2 [5]-[7].

In Figure 1.2(b), a compact open-ended meander slot antenna is shown [8]. In Figure 1.1(c) a slot ring has been miniaturized by fractal geometry [9]. The length of these spiraled or meandered slots have been effectively reduced by increasing antenna width. By bending the slot, the above methods have maintained the total electrical length which determine the resonant frequency.

According to transmission line theory, the inductive loading or capacitive loading can increase the equivalent electrical length which is useful in miniaturizing

capacitive loading, as realized by printed elements, shown in Figure 1.3(a) [10]. Chip capacitors can be loaded at the ends of the coplanar-waveguide-fed slot line for further size reduction, as shown in Figure 1.3(b) [8]. Figure 1.3(c) is the disk-loaded monopole. The antenna is considered as a series LC resonator when the disk acts as a capacitive load [11]. The simple LC model can show resonance characteristics of an antenna but does not include an impedance matching mechanism.

Aside from, a single-band compact antenna, there are many other compact dual-band antenna designs. Since 1999, the WLAN standards including the IEEE 802.11a/b/g systems were established by the IEEE 802.11 Group. In the U.S., the 802.11b/g WLAN standards are used in the frequency range of 2.4 to 2.4835 GHz while the 802.11a standards are used from 5.15 to 5.35 GHz and 5.725 to 5.825 GHz.

To enhance the communication capacity of a unit cell, a combo system with 802.11a/g or 802.11a/b/g standards have became popular. In this combo system, the antennas are like transceivers operating in both 2.45 and 5.25 GHz frequency bands; thus, a dual-band antenna with a single input port is needed for size reduction. Many dual-band antennas have been proposed in the literatures. Some of these designs use two similar resonators to achieve dual-band operations, such as a double inverted-F antenna [13] and an F shape monopole (double-L monopole), as shown in Figure 1.4(a) [14]-[17]. These are miniaturized by bending the strips into an L-shape. Also, some of these designs use a single block antenna with multiple resonant modes to achieve dual-band operations, such as a tapered bent folded monopole [18], as shown in Figure 1.4(b), meandered CPW-fed monopole [19], and, an L-shaped monopole [20].

Figure 1.1 Line alignments of a loop antenna for handset use.

(a) (b) (c)

Figure 1.2 (a)The spiraled microstrip-fed slot antenna. (b)The microstrip-fed meandered slot antenna.

(c)The fractal microstrip-fed slot ring antenna.

(a) (b) (c)

Figure 1.3 (a)The microstrip-fed slot antenna with inductive load in central portion. (b)The CPW-fed slot antenna with capacitive load at the open end. (c) The disk-load monopole antenna.

20 mm

8 mm

24 mm

19 mm 24 mm

19 mm Feed

Feed (a) (b)

Figure 1.4 (a)The compact coupled inverted-L dual-band antenna. (b)The compact CPW-fed dual-band antenna.

(a) (b)

Figure 1.5 (a)The monopole antenna printed on a dielectric slab. (b)The dielectric loaded monopole antenna.

To have enough electrical length under physical constraints, employing the use of a dielectric is an efficient solution. With a material of higher relative permittivity, one can have a larger phase constant, thereby reducing the physical resonant length [21]. Figure 1.5 shows a monopole printed on a ceramic piece [22]-[23]. In Figure 1.5(b) the antenna length is half than that in air by using the relative permittivity of 22 [23].

Unlike natural transmission line with a positive phase constant, metamaterial based transmission lines have a designable phase constant. This kind of artificial structure has been utilized by numerous guided and unguided wave applications. One notable example is Left-handed materials (LHMs) which possess negative refractive index and have drawn tremendous interests in both scientific and engineering fields.

The Left-handed transmission line (LH TL) is characterized by the phase advance whereas the Right-handed transmission line (RH TL) is characterized by phase delay along the power traveling direction. Metamaterial antennas possess different characteristics from conventional antenna designs that are based on the standard transmission line. RH TL and LH TL can be embedded into each other and named as Composite Right/Left- Handed Transmission Line (CRLH TL). [24] provides an practical application inserting LH TL into the host RH TL. Its equivalent circuit and phase constant are shown in Figure 1.6. The Zeroth-order Resonance (ZOR) makes use of the opposite phase properties of RH and LH TL and has been proved experimentally [25]. Small planar antennas utilizing the ZOR structure were published [26-27], as shown in Figure 1.7. Basically, they are just a section of synthesized CRLH TL. The physical size of such antenna can be arbitrary since its size is specified by the value of the capacitances and inductances instead of wavelength. The concept of the infinite wavelength resonant antenna was first demonstrated in [26].

(a) (b)

Figure 1.6 (a)The equivalent lumped circuit of CRLH TL. (b)The phase constant curve of CRLH TL which possesses both positive and negative values.

Feed

CRLH TL

(a) (b)

Figure 1.7 (a)The taped CPW-fed ZOR antenna. (b)The coupled microstrip-fed ZOR antenna.

1.2.2 Decoupling Methods

The use of multi-element antennas, such as MIMO antenna systems, is one of the effective ways to improve reliability and increase channel capacity. Due to the sharing of ground currents between antennas, these antennas couple strongly to each other; thereby, making it difficult to integrate multiple antennas closely in a small and compact mobile handset. For an MxN MIMO communication system, the data throughput can be pushed up to K times ( K = min(M, N) ), that of a Single-Input Single-Output (SISO) system, as long as the communication channels linked between the transmitter and the receiver are uncorrelated [2][3][29]. The correlation between the channels depends not only on the propagation environment, e.g., multi-path effect due to the reflection and diffraction of outdoor buildings or indoor partitions, but also on the coupling between the M or N antennas. High antenna coupling (or low isolation) would introduce signal leakage from one antenna to another, thus increasing the signal correlation between the channels. It will also decrease the antenna radiation efficiency due power dissipated in the coupled antenna port. The signal correlation between two receiver antennas can be reduced by increasing the antenna spacing. However, spacing is usually limited, especially for a mobile terminal which has very strict volume reserved for antennas. Another way to diminish correlation is by using multiple antennas with different radiation patterns. The patterns have to be complementary to each other in space in order to receive multi-path signals from various directions. However, the complementary patterns may not be the best solution for MIMO system. This is because for a single device, aside from the MIMO system, there are other systems operating simultaneously in the same frequency band, e.g. IEEE 802.11b and Bluetooth – both of which are popular systems that operate in the same 2.45GHz band. In this case, the coupling problem causes the same problem as MIMO system of the radiation efficiency. Thus, it causes inefficiency in the systems due to coupling.

Currently, there are numerous papers published focus on diminishing antenna coupling. Itoh and his co-workers used the defected ground structure (DGS) to increase the port isolation of dual-polarized and dual-frequency patch antennas [30]

same frequency electromagnetic band gap (EBG) structures can be used. Inserting band gap structures between antennas can help block wave coupling. Mushroom-like EBG structures are usually inserted between patch antennas to prevent the propagation of surface waves for higher isolation and better radiation patterns, as shown in Figure 1.8 [31]-[32]. These EBG structures provide conspicuous decoupling effect, but suffer from complicated structures and large structure area.

Possible loss may also be induced in the resonant EBG structures. To reduce the coupling between two planar inverted F antennas (PIFAs), Diallo [33]-[36] used a suspended metal strip linking the two antennas to cancel the reactive coupling between antennas, as shown in Figure 1.9. This neutralization technique has been also extended to patch antennas by Ranvier [37]. In [38], a decoupling circuit network was realized for two-element array by using external transmission lines. Although good isolation was achieved, only weak coupled antennas were tackled. The all-transmission-lines configuration also made the circuit bulky. The mutual coupling of two closely-packed antennas was reduced by etching slots on the ground plane [39].

The fish-bone like slots formed equivalent inductors and capacitors on the ground plane, which prevented the flowing of the coupling ground current between the antennas, as shown in Figure 1.10. A large ground plane size, which was close to the antenna size, was needed for sufficient isolation. The above methods can be divided into two cases. The first is to insert band gap structures prevent the direct propagation of electromagnetic wave between two antennas, like EBG and DGS. The driven antenna will not induce current on the other antenna. The second is to cancel the coupling current by using additional reactance network between antenna ports, such as a suspended strip, as a decoupling network. This is similar to the crosstalk elimination circuit used in telephone network. In this case, the driven antenna does induce the current on the other antenna but the decoupling network cancel the current at the antenna terminal.

In conclusion, most of the decoupling technology are bulky, especially EBG structure. The suspended reactive element between antennas is simple but not convenient for the application due to the additional empty space requirement and wire supporting mechanism. These designs are not flexible and compact enough for different PCB configurations. Therefore, this dissertation would like to propose a

more compact and flexible method for decoupling. The primary consideration for their design is to attain a compact size followed by system flexibility.

EBG wall Patch antenna

Ground

Figure 1.8 The EBG structure used to decouple two patch antennas.

Feed Shorting pin

PIFA

Suspended reactive element for decoupling

Ground

Figure 1.9 The suspended reactive element, a thin line, used to decouple two PIFAs.

Monopole 1 Monopole 2

Fish-bone like structure Ground

Figure 1.10 A fish-bone-like structure inserter on the ground plane to decouple two monopole antennas.

1.2.3 Antenna Equivalent Circuit Analysis

The equivalent circuit analysis is a kind of antenna modeling, which uses a lumped element circuit to model the input impedance of an antenna. Therefore, the equivalent circuit approach used in this study uses a lumped element circuit for antenna analysis and design.

The equivalent circuit analysis of antennas has already been used for decades.

This method is usually used to explain antenna impedance and resonance mechanisms [40]-[43]. Additionally, the equivalent circuits of antennas are used in spice modeling for the analysis of antenna gain bandwidth or time-domain waveforms transmitted from antennas, etc [44]-[45].

Circuit models are used to represent antenna impedance curves. Thus, their creation usually considers antenna physics which mostly relate to resonance mechanisms. For example, a loop antenna with an anti-resonance at the operating frequency will be basically modeled by a parallel LC resonator. A parallel resonator is a basic circuit model of antennas with anti-resonance. However, a purely parallel resonator cannot depict antenna impedance curves precisely. Additional circuit parameters, such as parasitic capacitors, are required for better modeling.

Another topic relevant to equivalent circuit modeling is effective modeling bandwidth. Since lumped elements have bandwidth limitations for modeling distributed elements, the circuit model with lumped elements is also limited; thus, to describe antenna impedance over a wider bandwidth by circuit elements, distributed elements, such as a transmission line will be used.

[40], shows an antenna equivalent circuit of a strip line coupled patch antenna that considers many circuit parameters. Figure 1.11 shows the presented antenna equivalent circuit in [40]. Considering the anti-resonance of the patch antenna, RA, LA, and Cp basically form the parallel resonator of a patch. To increase modeling accuracy, LP and RP are treated as parasitic elements connected in series with Cp. Coupling through the slot of an open-ended strip line are represented by an impedance transformer – Ct and Lt. The circuit elements mentioned above form a complex antenna equivalent circuit model. The value of each circuit element can be obtained by curve fitting. Therefore, knowing the antenna input impedance is necessary for model extraction regardless of where this value is obtained – whether from measurement or from full-wave simulation. Another example on the operation mechanism of an antenna is that of a ladder CPW-fed slot antenna using multiple resonators. [41] shows a circuit model to explain and describe resonances of such an antenna. This type of circuit modeling does not have a direct relation with antenna

resonators are used to precisely fit the antenna impedance curve over a wider bandwidth since the two resonators can provide two additional poles, thereby, increasing flexibility in curve fitting.

The equivalent circuit is not only used to explain the operation mechanism and describe the impedance curve but is also for spice simulation in the time domain. In [44], a circuit model of an ultra-wide-band antenna is established and is used to calculate the waveform of receiving signal. Chapter 2 of this dissertation will show the equivalent circuit analysis for the proposed antenna structures.

(a) (b)

Figure 1.11 (a)A stripline coupled patch antenna (b)The equivalent circuit model of a stripline coupled patch antenna.

1.3 Antenna synthesis by the Equivalent Circuit Approach

In most publications, the antenna equivalent circuit is used for analysis and explanation. Design of the antenna is usually done first and then the establishment of equivalent circuit. However, the use of the antenna equivalent circuit can be expanded to antenna design. This means that the equivalent circuit must be designed first then synthesized into the antenna design. Chapter 3 will shows the synthesis of an antenna from an equivalent circuit.

This study uses the equivalent circuit approach to achieve miniaturized design.

An equivalent circuit established prior to achieve resonance at the antenna’s operation frequency. The antenna is directly synthesized from the equivalent circuit instead of the iterative full wave simulation analysis. The equivalent circuit is designed to create resonance at a specified frequency. This is used as a reference in antenna synthesis.

Next, the antenna topology is mapped from the equivalent circuit by using printed circuit elements to represent components of the circuit model. In this design procedure, resonant frequency can be easily controlled by an equivalent circuit.

However, the radiation effect of antenna is difficult to control in the equivalent circuit model. To solve this problem, there are two possible methods. The first method uses equivalent circuits to control the resonance type and resonant frequencies of antennas.

The bandwidth and radiation performance need to be fine tuned when processing the antenna layout. This means the radiation mechanism is considered during layout. This design method will be shown in Chapter 3-1.

The second method establishes circuit model of radiators by considering the radiation mechanism, and radiation resistor, during modeling. This means certain basic radiators, such as slots, should be modeled first. Thus, some preparatory work

The second method establishes circuit model of radiators by considering the radiation mechanism, and radiation resistor, during modeling. This means certain basic radiators, such as slots, should be modeled first. Thus, some preparatory work