Chapter Outlines

This dissertation is organized in the sequence described below.

In Chapter 2, three versions of the coupled twin slots fed by the CBCPW are presented in order. First of all, the leakage phenomenon that occurs at the discontinuity of the CBCPW-fed slot dipole is illustrated visually. Then the CBCPW-fed slot dipole coupled with a straight slot is designed, fabricated, and experimented. Second, the CBCPW-fed slot dipole coupled with an arc-slot slot is presented. Some cautions about CBCPW-fed slot antenna designs are also discussed. The simulation and measurement results are demonstrated. Finally the miniaturized version is presented. Simulation and measurement results about the performance of the reduced-size antenna are shown. The smoother radiation patterns that prove the ground diffractions are alleviated are also presented.

In Chapter 3, two parallel-plate slot arrays are presented in order. First, the novel parallel-plate longitudinal slot array fed by the CBCPW is proposed. Design considerations about the choices of the slot orientation, the feed-line dimension, the inter-element spacing, and the slot dimensions are explained in detail. The reflected

wave from the end of the feed-line is also taken into consideration to compensate for the power tapering along the radiating apertures. A 5 x 6 and two 5 x 2 arrays with and without the feed-line termination are simulated, fabricated and tested. Next, the parallel-plate transverse slot array is presented. An array with an off-broadside main beam is designed to illustrate the basic concept and the predictable main beam position.

Then another array with a broadside main beam is designed to demonstrate the frequency-scanning property. Simulation and measurement results are presented.

Finally, conclusions are drawn in Chapter 4.

ε

_h S

W

Fig. 1. 1 Structure of the coplanar waveguide (CPW).

ε

_h S

W

Fig. 1. 2 Structure of the conductor-backed coplanar waveguide (CBCPW).

Chapter 2

Coupled Twin Slots Fed by Conductor-Backed Coplanar

Waveguide

2.1 T HE S TRAIGHT -S LOT C ASE

The coupling mechanism of the twin slots fed by the conductor-backed coplanar waveguide is investigated. The goal is to improve the antenna efficiency by using the phase cancellation technique and at the same time keep the structure as simple as possible. The leakage phenomenon is illustrated visually and the effects of the antenna parameters are examined numerically for choosing the appropriate variables. Finally the proposed antenna is fabricated and tested, showing 69.8 % of antenna efficiency.

2.1.1 I NTRODUCTION

Coplanar-waveguide-fed slot antennas are attractive due to their uniplanar structures and the ease of fabrication [2]. To render the radiation from bidirectional to

unidirectional, conducting planes are usually placed at the back of the antennas. The performance, however, suffers because the conductor-backed coplanar waveguide (CBCPW) and the conductor-backed slot are both leaky structures due to the incursion of the parallel-plate mode leakage [27]. The parallel-plate TEM mode has zero cut-off frequency, so the leakage phenomenon occurs at all frequencies. One major trend toward solving the leakage problem of conductor-backed slots is the phase cancellation technique originally proposed in the 1980s to deal with the surface wave problem of dipoles [79] as well as slots on electrically thick dielectric substrates [80] using infinitesimal elements. This technique uses twin broadside slots half a guided wavelength apart to cancel the undesired propagating power. Successful implementations of this technique for conductor-backed slots can be found in [88] and [90], where the slot lengths are 0.84 and 0.95 wavelengths long in [88] and about one wavelength long in [90]. For these excellent works, the complexities always come from the feeding structures, since both slots are fed directly and separately. Two dielectric layers and feeding circuits are used in [88], whereas two dielectric layers and two feeding ports are required in [90].

In this chapter we propose a new feeding mechanism for the twin slots configuration, of which the first slot dipole is directly fed by the CBCPW, whereas the second slot is coupled by the parallel-plate mode leakage excited at the discontinuity of

the first slot dipole. By means of this feeding method, the resulting geometry is single-layered with only one feed and without any additional feeding circuits or via holes. The simplicity and conciseness greatly enhance the usability of CBCPW-fed slot antennas for practical applications.

2.1.2 A NTENNA S TRUCTURE AND D ESIGN

The single slot dipole fed by the CBCPW acts more like a mode converter than an efficient antenna. It converts most of the power from the CBCPW mode to the parallel-plate mode, leaving the radiated power very small. The typical antenna gain is far below 0 dBi and the radiation efficiency below 10 %. Using the simulator HFSS from Ansoft, Fig. 2.1(a) illustrates this phenomenon, where the surface current density at the interface between the dielectric and the top metal layer is plotted. At the junction of the CBCPW feed-line and the slot dipole, the parallel-plate mode is strongly excited and propagates radially into the substrate region. In contrast, as Figs. 2.1(b) and (c) show, if an additional slot with suitable dimension is placed in front of the first slot dipole at a distance about half a guided wavelength of the parallel-plate mode according to the phase cancellation technique, the leakage is reduced significantly. Although the leakage suppression is not complete, the gain and the efficiency are greatly improved, as

will be shown shortly.

The geometry and the photograph of the proposed twin slots fed by the CBCPW are shown in Fig. 2.2. The gray area represents the metal portion and the white ones are the etched slots and the feed-line. The relative phases of the fields on the first and the second slots depend on the distance d as well as the lengths and widths of the slots, L1, L2, S1, and S2. When the distance d is half a guided wavelength of the parallel-plate mode according to the phase cancellation technique and the slots are of the same length, it is found that usually the phases are not exactly equal, resulting in an off-broadside main beam in the E-plane (y-z plane). The phases can be made equal if the slot lengths are allowed to be discrepant. Thus for each L1 value, we can find a corresponding L2

that results in a broadside main beam, assuming that the transverse dimensions (x-y plane) of the substrate and the ground are infinite so the influence of the edge diffractions on radiation patterns is temporarily ignored. The following simulations are carried out using the package software IE3D from Zeland. Fig. 2.3 illustrates the simulated results at 5 GHz, where the normalized L2 (L2 / L1) is plotted versus L1. The effect of different slot widths is also considered. From Fig. 2.3 we see that L2 might be greater than, equal to, or smaller than L1, depending on the choices of L1 and the widths of the slots. Note that since the CBCPW is a leaky line, the length of the feed-line (Lf) should not be too long. In our simulations the feed-line length is chosen to be 15 mm,

with the corresponding loss estimated to be 0.45 dB.

For each combination of L1 and L2 that results in a broadside main beam, the corresponding efficiencies and gains are calculated and shown in Fig. 2.4. Here the radiation efficiency is defined as the ratio between the radiated power and the input power, whereas the antenna efficiency as the ratio between the radiated power and the incident power. The difference between the incident power and the input power is the return loss at the input. Fig. 2.4(a) reveals that the radiation efficiency has local maximums when L1 is near integer multiples of λ, where λ is approximated by

2

λ , which is 37.2 mm at 5 GHz for the present case. However, the serious

input mismatch makes the overall antenna efficiency below 50 % when L1 ≒ λ. On the other hand, the highest antenna gain is achieved when S2 = 2.7 mm and L1 = 78 mm ≒ 2λ, as can be seen in Fig. 2.4(b). The corresponding antenna efficiency is 64.7 % but the input return loss is just about 10 dB. If a better input matching condition is required, smaller L1 values should be chosen such that the curves of the radiation and antenna efficiencies are closer to each other. In our experiment we choose L1 = 60 mm for a better input return loss and a slightly higher antenna efficiency, although the gain is smaller due to the smaller antenna size. Note that although it seems like increasing S2

results in higher efficiencies and gains, these performances soon begin to saturate and finally fall off. Therefore in our simulations higher S2 values are not pursued further.

2.1.3 E XPERIMENTAL R ESULTS

A twin slot fed by the CBCPW is fabricated and tested with the following parameters: L1 = 60 mm, L2 = 59 mm, S1 = 1.5 mm, S2 = 2.7 mm, d = 13.1 mm, Lf = 15 mm, W = 2.5 mm, and G = 0.8 mm. The FR4 substrate with h = 1.6 mm, εr = 4.2, and tanδ = 0.02 is used in the fabrication. Note that for antenna applications, the substrate

material with lower dielectric constant and loss tangent would be more preferable.

However, in this work, the FR4 substrate was used merely because it is more accessible to us and much cheaper than other substrates. The dimensions of the substrate and the ground plane are 150 mm in the x-direction and 85 mm in the y-direction.

Fig. 2.5 plots the frequency response of the simulated and measured input return losses and gains. The measured input return loss is seen to have about 0.1-GHz frequency shift from the simulated one, whereas the simulated and measured gains are very close to each other. Considering that the simulation tool IE3D does not take the finite ground and the finite substrate into account and the mechanical tolerance of the fabrication process would unavoidably result in some discrepancy between the physical dimensions of the simulation model and the test piece, the results are quite satisfactory.

At 5 GHz the measured return loss is 24.1 dB and the measured gain is 5.64 dBi. The

measured 10-dB return loss bandwidth is 5 %, extending from 4.88 GHz to 5.13 GHz.

Fig. 2.6 shows both the simulated and measured E- and H-plane radiation patterns at 5 GHz. Mild ripples are observed in the measured E-plane co-polarization pattern. This is caused by the edge diffractions of the finite ground plane, which is a common phenomenon in slot antenna E-planes. The measured cross-polarization levels are fairly low in the E-plane and a bit higher in the H-plane, but are still below -15 dB in all directions. Note in Fig. 2.6(a) that the simulated cross-polarized component in the E-plane is invisible. This is because the component remains lower than -40 dB, which is the lower bound of the radial axis in the figure. The measured front-to-back ratio is higher than 17 dB.

By using the method described in [108] to estimate the directivity from the half-power beamwidths of both the E- and H-plane radiation patterns, together with the measured gain data, the antenna efficiency is found to be 69.8 % at 5 GHz. As will be shown in the next section, the antenna efficiency of the CBCPW-fed slot dipole coupled with an arc-slot is calculated using the same method, and is found to be 50.6 %.

Although the structure in the next section has the advantage of occupying a smaller area, the performance of the present structure is superior in terms of the antenna efficiency, the main concern of antennas fed by the leaky CBCPW.

2.1.4 D ESIGN P ROCEDURE

The design procedure can be summarized as follows.

1. Choose L1 ≒ 1.5 ~ 1.6 λ.

2. For d being about half the guided wavelength of the parallel-plate TEM mode, find a L2 value that is close to L1 such that the coupled twin slots have a broadside main beam in the E-plane.

3. Make S2 large enough before the antenna efficiency and gain saturate.

2.1.5 S UMMARY

The CBCPW-fed coupled twin slots have been proposed and the properties demonstrated. The coupling mechanism and the effects of the antenna parameters have been studied and utilized to design a unidirectional antenna of 69.8 % antenna efficiency with a simple structure without any complicated feeding circuits. This type of antennas would be very attractive when the CBCPW feed is unavoidable and the whole structure must be kept simple.

2.2 T HE A RC -S LOT C ASE

A novel compact slot antenna fed by the conductor-backed coplanar waveguide (CBCPW) is proposed. The antenna is composed of a CBCPW-fed slot dipole and an additional arc-slot in front of the dipole. Compared to the CBCPW-fed slot dipole without the arc-slot, the antenna gain is improved significantly. The antenna occupies a small area and uses only one layer of dielectric substrate and a single feed without any via holes. The impedance bandwidth is 7.2 % and the highest in-band antenna gain is 3.4 dBi.

2.2.1 I NTRODUCTION

In this section, we propose a new CBCPW-fed slot dipole antenna that incorporates an additional arc-slot to achieve the phase cancellation with only one layer of substrate and a single feed. The excitation of the arc-slot is through the coupling of the power leaked from the discontinuity of the feed-line, as described in the previous section. The feeding mechanism alleviates the burden of designing complicated power dividing circuits. This results in a simple geometry and a compact antenna size.

2.2.2 A NTENNA S TRUCTURE

Figs. 2.7(a), (b), and (c) show the geometry of the antenna, the equivalent magnetic currents flowing on the radiating slots, and the photograph, respectively. The gray area represents the metal, whereas the white ones are the etched slots and the feed-line. As the input power travels down the feed-line and excites the slot dipole, part of the power propagates, in the parallel-plate region, toward and excites the arc-slot. When the distance d is about half the guided wavelength of the parallel-plate TEM mode, the field on the central region of the arc-slot and that on the slot dipole are in phase, which reinforces the effect of the phase cancellation. The length of the slot dipole (2 x Ld) is about λ, which is approximated by λ₀ / (εr +1)/2. On the other hand, because the length of the arc-slot is fixed by its radius, two additional sections (Lm x Sm) are attached to extend the total arc-slot length to about 1.5 * λ. As opposed to the conventional transmission lines, such as the coplanar waveguide, where the feed-line length basically merely alters the phase of the input reflection coefficient, it is not true for the CBCPW-fed antennas. Since the CBCPW is a leaky line, the input power is fed directly into the antenna as well as through the coupling of the leakage power. Therefore as the feed-line length is varied, so are the amount of the coupling and hence the characteristics of the antenna. This phenomenon is best illustrated by Fig. 2.8, where the

radiation efficiency and the antenna gain simulated by IE3D from Zeland are plotted.

When the feed-line length is changed from 5 mm to 20 mm, the variation of the radiation efficiency is about 11 % at the design frequency of 5 GHz, whereas that of the gain is about 0.9 dB. Also note in these figures that the variations with frequency are not monotonic, which would be the case if there is only material loss. These two observations reveal the distinctive feature of the CBCPW and the caution that should be kept in mind when designing CBCPW-fed antennas.

2.2.3 S IMULATION AND M EASUREMENT R ESULTS

The design is based on the FR4 substrate with dielectric constant εr = 4.2, thickness h = 1.6 mm, and loss tangent tanδ = 0.02. The central distance between the arc-slot and

the dipole slot is d = 15.65 mm, which is slightly larger than one half the guided wavelength of the parallel-plate mode at 5 GHz. The remaining parameters are as follows: Sd = 1 mm, Sa = 1.5 mm, Sm = 1 mm, Ld = 18.5 mm, Lm = 4 mm, and Lf = 14 mm. The strip and slot widths of the CPW feed-line are 2 mm and 0.5 mm, respectively, which correspond to a 50-Ω characteristic impedance. Throughout the design process, simulations are carried out using the package software IE3D from Zeland.

The return loss and gain are plotted in Figs. 2.9 and 2.10, respectively. The

simulated resonant frequency is 5.05 GHz, whereas the measured one is 5.02 GHz. The shift in resonant frequency is less than 0.6 %. The measured bandwidth is wider than the simulated one, which may be attributed to the large loss of the dielectric substrate used in the experiment. The measured return loss at 5.02 GHz is 16.25 dB and the 10-dB return loss bandwidth is 7.2 % extending from 4.86 to 5.22 GHz. The in-band measured gain ranges from 0.55 dBi to 3.4 dBi and is 2.89 dBi at 5.02 GHz. Compared to the CBCPW-fed slot dipole without the arc-slot, which has gain far below 0 dBi, the current design increases the gain significantly, while the antenna size still kept compact.

The measured radiation patterns at 5.02 GHz are shown in Fig. 2.11. The cross-polarization level in the E-plane (yz-plane) is below -20 dB except for the region near θ = 90°, which has higher levels due to the disturbance of the connecting cable in that direction. As for the H-plane (xz-plane), the cross-polarization level is higher than that in the E-plane, but still remains under -10 dB. Especially in the direction of the main beam, the level is down to below -30 dB. For comparison, the in-band patterns at 4.9 GHz and 5.2 GHz are shown in Figs. 2.12 and 2.13, respectively. As can be seen, the pattern is quite stable with respect to the frequency variations. By using the method described in [108] to estimate the directivity from the half-power beamwidths of both the E- and H-plane radiation patterns, together with the measured gain data, the antenna efficiency is found to be 50.6 % at 5.02 GHz.

2.2.4 D ESIGN P ROCEDURE

The design procedure can be summarized as follows.

1. Choose 2 * Ld ≒ λ.

2. For d being about half the guided wavelength of the parallel-plate TEM mode, attach two sections to both ends of the arc-slot to extend the total length to about 1.5 * λ.

2.2.5 S UMMARY

A novel gain-enhanced CBCPW-fed slot antenna has been proposed. An arc-slot placed in front of the slot dipole and excited by the leakage power from the CBCPW has been shown to significantly increase the gain of the slot dipole on the leaky conductor-backed structure. The measured impedance bandwidth is 7.2 %, the highest gain is 3.4 dBi in the band, and the radiation pattern is stable within the band. This antenna is useful when the compact CBCPW-fed antenna is needed.

2.3 T HE M INIATURIZED C ASE

2.3.1 I NTRODUCTION

Slot antennas fed by the conductor-backed coplanar waveguide (CBCPW) are created when there is a conducting plane, either intentionally or accidentally, lying beneath the coplanar waveguide (CPW) and the slot antenna. The conductor-backed coplanar waveguides and the conductor-backed slots are both leaky structures due to the incursion of the parallel-plate mode leakage [27]. Therefore the CBCPW-fed slot antennas usually radiate inefficiently. On the other hand, the E-plane radiation patterns of slot antennas [109], [110], similar to microstrip patches [111], usually suffer from the space-wave and surface-wave diffractions of the finite ground plane, resulting in ripples, dips, and distortions in the radiation patterns. While the problem of pattern distortion of microstrip patch antennas had been tackled in several papers [112] – [114], little had been reported for slot antennas. In this section, we propose a novel finite ground slot dipole antenna fed by the CBCPW, derived from the prototype of the CBCPW-fed coupled twin slots discussed in the first section of this chapter. By merging the finite ground diffractions with the radiating slot, the gain of the originally leaky CBCPW-fed

Slot antennas fed by the conductor-backed coplanar waveguide (CBCPW) are created when there is a conducting plane, either intentionally or accidentally, lying beneath the coplanar waveguide (CPW) and the slot antenna. The conductor-backed coplanar waveguides and the conductor-backed slots are both leaky structures due to the incursion of the parallel-plate mode leakage [27]. Therefore the CBCPW-fed slot antennas usually radiate inefficiently. On the other hand, the E-plane radiation patterns of slot antennas [109], [110], similar to microstrip patches [111], usually suffer from the space-wave and surface-wave diffractions of the finite ground plane, resulting in ripples, dips, and distortions in the radiation patterns. While the problem of pattern distortion of microstrip patch antennas had been tackled in several papers [112] – [114], little had been reported for slot antennas. In this section, we propose a novel finite ground slot dipole antenna fed by the CBCPW, derived from the prototype of the CBCPW-fed coupled twin slots discussed in the first section of this chapter. By merging the finite ground diffractions with the radiating slot, the gain of the originally leaky CBCPW-fed