Potential Applications of PBG Engineered Structures in Microwave Engineering: Part II
The second part of this article reviews the potential applications of electromagnetic bandgap (EBG) structures in enhancing the performance of waveguides and antennas. A conductor-backed coplanar waveguide (CB-CPW) free of spurious transmission and a t...
In the new millennium, the explosion of info-communication technologies has brought many new broadband design challenges. To meet these challenges, designers are required to fulfill more functionality per unit volume. In the first part of this article, the basic PBG configurations and application of PBG structures in enhancing the performance of microwave devices and components were reviewed.1 In this second part, the potential applications of EBG structures in enhancing the performance of waveguides and antennas are presented. For example, in conductor-backed coplanar waveguides (CB-CPW), the leakage transmission must be forbidden. In the case of TEM waveguides, the field distribution should be uniform. Integration of PBGs in CB-CPW and waveguides can solve these problems.
Nowadays, microstrip antennas are widely used in high performance aircraft, spacecraft, satellite and missile applications, where size, weight, cost, performance, ease of installation and aerodynamic profile are of prime importance. Microstrip patch antennas also find applications in a broad range of military and commercial applications, due to their advantageous features, including lightweight, low profile, low cost, conformable to planar and nonplanar surfaces, and their compatibility with HMIC, MMIC and micromachined technologies. They are very simple and mechanically robust when mounted on rigid surfaces, and fabrication can easily be accomplished using modern printed-circuit technology. Despite their dimensional advantages and versatilities, microstrip patch antennas suffer from low gain, low power, low efficiency, high Q, poor scan performance, spurious feed radiation and poor polarization purity; their crucial limitation, however, is the inherent narrow bandwidth that is typically only a fraction of a percent or at most a few percent. Only in limited applications is a narrow bandwidth desirable, such as in security systems. Thick substrates can be used to increase the bandwidth (up to about 35 percent) and efficiency (up to 90 percent).2 However, thick substrates stimulate propagation of surface waves that are not desirable as they do not couple with the space waves. They extract power from the total power available for direct radiation. Surface waves propagate within the dielectric slab and are scattered at bends and surface discontinuities, such as the edges of the dielectric and finite ground plane. Consequently, surface waves degrade the antenna efficiency, pattern and polarization characteristics. Surface wave propagation drastically degrades performance, not only in microstrip patch antennas, but also in microwave devices. Conventional methods for surface wave suppression are cavity enclosed patch antennas, although cavity enclosed devices are difficult to fabricate and may be incompatible with MMIC and MEMS batch processing. PBG engineered patch antennas and microwave components easily alleviate these problems.
A compact design is now a requirement. Substrates with high dielectric permittivity are used to minimize the size of the elements and are suitable for establishing tightly bound fields to minimize undesired radiation and coupling. Radiation efficiency is poor, however, and surface waves propagate within the dielectric slabs. Thus, a design with a high dielectric constant substrate is undesirable for compact design if performance enhancements in terms of bandwidth, gain and smoother radiation patterns are not managed adequately. PBGs can provide solutions to such problems. Photonic bandgap (PBG) structures ensure better performance in terms of bandwidth, gain and smoother radiation patterns. Being a slow wave structure, PBG also helps in reducing the size of microwave devices.
In this article, EBG-assisted waveguides and antennas are reviewed to show their improved performance over conventional waveguides and antennas. The suppression of transmission leakage by inclusion of a PBG structure in a CB-CPW waveguide will be demonstrated. It will be shown that a PBG-assisted TEM waveguide provides a more uniform field distribution than a conventional rectangular waveguide. PBG antennas show improved performance in terms of gain, bandwidth, directivity and phase steering. PBG-assisted VSAT antennas also provide more bandwidth and sufficient isolation between ports.
In a conventional coplanar waveguide (CPW), an additional ground plane is normally used on its backside to increase its mechanical strength, to enable mixed CPW and microstrip circuits and to provide a heat sink.3 This type of CB-CPW excites parallel-plate modes, resulting in the deterioration of CPW performance. Using posts to short the unwanted plate mode or using a multi-layered substrate to shift the dispersion curve of the parallel-plate mode are two different approaches to solve the problem.4 The uniplanar compact photonic bandgap (UC-PBG) is a promising candidate to stop this leakage due to its distinct stopband characteristics. It can easily be etched in the top ground planes of a CB-CPW circuit without using any extra masks or via holes. A CB-CPW is shown in Figure 1, where a ground plane is placed on the backside of a conventional CPW. A parallel-plate waveguide will be formed between top and bottom planes. Once the wave is launched, the energy will leak along a definite angle allowing a severe effect such as cross talk with neighboring circuits. The top view of a UC-PBG engineered CPW5 is also shown. The UC-PBG lattice is placed on the top ground plane. A comparison of the measured transmission coefficients between a conventional CPW, a CB-CPW and a non-leaky CB-CPW is shown. From the result, it is clear that for the conventional CB-CPW, a significant leakage is observed at all frequencies. In addition, the insertion loss of a conventional CPW is found to be below 13 GHz and starts rippling at higher frequencies from reflections caused by the SMA connectors. Though power leakage is present in the passband (DC to 9 GHz) in the proposed CB-CPW, the insertion loss in the stopband (9 to 14 GHz) has been found to improve significantly and is comparable to a conventional CPW providing the information that the leakage has almost been suppressed. This type of novel CB-CPW may find its potential application in CPW-fed slot antennas.
A Novel TEM Waveguide
A novel TEM waveguide is a very promising candidate for the feeding structure of quasi-optical power combining amplifiers. The use of waveguide in power combining is very popular as the diffraction loss can be avoided.6 Dielectric-loaded or oversized waveguides are often used in combining amplifier arrays for achieving uniform aperture field distributions.7–8 Small size dielectric-loaded waveguides are not suitable since they possess no advantages over a conventional empty waveguide.9 A high dielectric constant material can improve the performance, although it causes the bandwidth to be smaller. On the other hand, a UC-PBG structure can be used to build a TEM waveguide with a uniform field distribution. The replacement of the two sidewalls of a rectangular waveguide by UC-PBG structures forms a TEM waveguide, providing a permanent magnetic conducting (PMC) surface at the wider stopband. This structure can be fabricated on a thin substrate using a standard etching technique. The proposed PBG waveguide10 produces a fairly uniform field distribution from 9.4 to 10.4 GHz, and the phase velocity is found to be close to the speed of light ensuring the establishment of TEM propagation. The waveguide is shown in Figure 2. The field profiles for conventional and proposed UC-PBG engineered waveguides are shown in Figure 3. This type of waveguide also finds an application in TEM cells for electromagnetic compatibility (EMC) measurements.
PBG Applied to Antennas
Now the focus will be on potential applications of PBG structures for improved antenna performance.
Phase Control of Plane Waves
When an antenna is situated very close to a conducting surface, its efficiency degrades because of out-of-phase image currents. It has been shown11 that for a horizontal wire on a PBG substrate, the image currents are in phase within the bandgap. The measured reflection phase for a two-layer high impedance surface is shown in Figure 4. Due to the inclusion of the high impedance surface, the matching and gain improve significantly.
Suppression of Surface Waves
The bandwidth of a microstrip patch antenna can be improved by increasing the substrate thickness. However, this greatly degrades the antenna efficiency due to surface wave losses. Moreover, on a finite substrate, multiple reflections of surface waves from the substrate edges result in ripples in the radiation pattern. When multiple antennas share the same ground plane, surface currents can cause unwanted mutual coupling. In a PBG structure, the surface wave propagation is inhibited within the stopband.11 Figure 5 demonstrates the suppression of surface waves by a high impedance PBG structured surface.
The tangential transmission performance of a high impedance surface is shown in Figure 6. When a probe-fed patch antenna is embedded in such a PBG structure, a smooth symmetric pattern with little backward radiation is observed. The efficiency of the patch antenna is also enhanced.
R. Gonzalo, P.D. Maagt and M. Sorolla12 describe a microstrip feed rectangular patch antenna with 52 mm width and 26 mm length. The substrate has a dielectric constant of 10 and a size of 420 × 420 × 10 mm. The antenna is placed in the middle of the EBG-engineered substrate, which is shown in Figure 7. For the conventional design, the gain was found to be –2 dB; the gain was 7.5 dB when the antenna is etched on a PBG material. The back radiation was also reduced significantly, indicating increased efficiency.
R. Coccioli, et al.13 describe an aperture-coupled patch antenna on a UC-PBG substrate. The reference antenna is shown in Figure 8 and the UC-PBG structured antenna is shown in Figure 9. The antenna is designed at 12 GHz on a standard 1.27 mm thick (εr = 10.2) dielectric substrate. The feed line is etched on a 0.64 mm thick substrate with εr = 10.2. To increase the coupling between the antenna and the feed line, a 0.38 mm wide and 2.29 mm long H-shaped coupling slot is used and shown clearly from the exploded top view of the reference antenna shown on the right side of the figure. The length of the radiating patch is 5.33 mm and the width is 2.54 mm. The effect of a finite substrate is taken into account. For comparison, an identical patch surrounded by three periods of UC-PBGs in each direction has also been etched on a finite substrate with the same dimensions. From the radiation measurement (not shown here) it is seen that the radiation patterns of the latter are smoother than those of the standard antenna. The peak power received by the UC-PBG patch in the broadside direction is 3 dB higher than that received by the reference patch. Smoother radiation patterns and higher gain than those for the conventional patch antenna prove the effective suppression of surface waves and an increase in radiation efficiency.
S.K. Sharma, et al.14 have proposed a simplified UC-PBG structure to enhance the antenna performance. Instead of a complex UC-PBG structure,13 they have implemented the new UC-PBG structure without the inset. The use of less metal reduces the cost. Four-, five- and six-grid patterned UC-PBG antennas show different performances. Four- and six-grid patterned UC-PBG antennas afford dual-band gain with maximum gains of 5.24 and 3.95 dBi, respectively. However, the five-grid UC-PBG shows a maximum gain of 8.32 dBi at 12.5 GHz. The reference antenna gain is found to be 2.89 dBi at 12.5 GHz. The highest gain is achieved from the five-grid UC-PBG structure. Although the gain is three times higher than that for the reference antenna, the cross polarization level in the radiation pattern increases. Its peak, however, still remains below the acceptable –20 dB level. The front-to-back ratio of the radiation pattern increases from 12 to 18.92 dB for this novel simplified five-grid UC-PBG structure.
J.Y. Park, et al. describe15 an improved low profile cavity-backed slot (CBS) antenna loaded with a 2D UC-PBG reflector. The cavity depth of the new antenna is sixteen times thinner than that of a conventional λ/4 wavelength cavity slot antenna. The performance includes a 1 dB higher gain than the reference antenna, good front-to-back ratio and no pattern distortion as compared to previous low profile CBS antennas.
C. Caloz, et al. introduced16 a novel UC-PBG structure by stacking two (or more) UC-PBG plates in the direction perpendicular to the plane of the substrate. This new structure has the merit of easy fabrication in contrast to other PBG like vias or dielectric inclusions. The periods of the lattice are a and a/2 for the bottom and intermediate UC-PBG structures, respectively. The overlapping effect of the stopband associated with each UC-PBG plate results in a dramatic increase in the stop bandwidth with excellent transmission characteristics within the passband.
M. Qiu, et al.17 developed a high directivity patch antenna with both a PBG substrate and PBG cover. The geometry of the proposed patch antenna is shown in Figure 10. The dielectric constant of the substrate is εr = 10 and substrate thickness is 10 mm. The dimension of the patch is 32 × 18 mm. The working frequency of the conventional patch antenna is 1.91 GHz. In the substrate medium, the square lattices of air holes were used with a period a = 44 mm and radius of holes R = 21 mm. The PBG substrate consists of 9 × 9 unit cells, with the middle five air holes unpunched to support the patch antenna.
The dimensions of the substrate are 396 × 396 (Lx × Ly). For the cover, the PBG structure is a rectangular lattice (Px = 60 mm, Pz = 48.5 mm), and the dielectric rods have a rectangular cross section (rx = 27 mm, rz = 23 mm). The dielectric constant is the same. The distance between the substrate and the cover is chosen to be d = 60 mm. Figure 11 shows the radiation patterns for the conventional patch antenna and EBG-assisted patch antennas at a frequency of 1.95 GHz in the E- and H-planes. The PBG substrate can increase the directivity from 6.0 dB (conventional patch antenna) to 6.7 dB, and the PBG cover can increase the directivity from 6 to 13 dB. The directivity is raised to 17.1 dB with the combination of the PBG substrate and PBG cover.
A reconfigurable ground plane is achieved by varying the number of PBGs in the ground plane. This concept leads to the idea that PBGs can be used as beam-steerers. B. Elamaran, et al.18 suggest that the phase-lag in a PBG engineered microstrip line is approximately proportional to the number of PBG elements in the structures. The variation of phase-lag with the number of PBG elements at different frequencies is shown in Figure 12.
Based on this concept they designed a 4 × 1 array with a reconfigurable ground plane. The geometries of such antennas are shown in Figure 13. One feed line is always kept unperturbed and the others are perturbed in the following sequence: 0-1-2-3, 0-2-4-8,0-3-6-12, 0-4-8-16, 0-5-10-15 and 0-6-12-18, where the integers represent the numbers of PBG elements under consecutive feed lines. From the results shown in Figure 14, it is seen that the array steers the beam in increments of approximately 6°.
The authors recently proposed19 a PBG-assisted compact phased-array antenna where compact feed lines were designed for a 4 × 1 array. The geometry of an antenna with the compact feed line is shown in Figure 15. The return loss performances for both the conventional and compact phased-array antennas are also shown.
It can be seen that both the return loss performances agree well. On the basis of this concept a PBG-assisted compact phased-array antenna was designed. One of the geometries and the radiation patterns for different PBG distributions are shown in Figure 16. More PBGs can be placed within a small area in a compact feed line. It can be seen that due to the variation of PBGs along the feed lines, the beam-peaks are moved.
Without any PBGs under the feed line (reference array) the beam-peak is directed along the 0° axis. However, when the feed lines are perturbed with uniform circular PBGs in the ground plane the beam-peak is moved. For a PBG distribution of 0-3-6-9 the beam-peak is moved by 10° and by 15° for a 0-6-12-18 PBG distribution. The beam steering scales with the number of PBGs under the feed lines of the array.
The inclusion of PBGs also results in improved bandwidth.19 The return loss performance is shown in Figure 17. It can be seen that the 10 dB return loss bandwidth is 0.76 percent for the reference array and 1.25, 2.4 and 2.5 for 0-4-8-12, 0-5-10-15 and 0-6-12-18 PBG distributions, respectively.
A very small aperture terminal (VSAT) antenna with uniform PBG distribution was also investigated. The plan of the shared aperture dual-band, dual-polarized aperture, coupled patch antenna for a S- and C- band VSAT application is shown in Figure 18. Detailed dimensions of the antenna can be found in Ref. 20.
The S-parameters of the proposed antenna and for the reference antennas were measured. Ports 1 and 2 of the antenna system correspond to the lower and higher frequencies, respectively. The input impedance bandwidths of the reference antenna are 18.62 and 28.3 in Port 1 and Port 2, respectively. The isolation is more than 30 dB, which satisfies the requirement. For the EBG antenna, the bandwidths are 22 and 29 percent, respectively. The isolation is also more than 30 dB. The 10 dB return loss bandwidths are enhanced by the inclusion of the EBGs with sufficient isolation (more than 30 dB) between the ports.
An extensive literature survey has been carried out to find the application of PBG structures in different microwave components and devices. The planar PBG structures are more attractive as they are simple in design, easy to fabricate and compatible with MMIC design. The potential microwave applications of PBG structures have been mentioned with their beneficial effects in different passive microwave devices.
The improved performance of waveguide due to loading of PBGs has been observed. It can be seen that the parallel-plate mode in a CB-CPW with UC-PBG is significantly suppressed. The inclusion of PBGs into a conventional TEM waveguide has resulted in PMC surface having very uniform field distributions.
In the case of antennas, it can be seen that EBG structures improve the performance of the antenna in terms of phase control of plane waves, suppression of surface waves, gain, directivity and 10 dB return loss bandwidth. It has also been observed that the EBG steers the beam, which makes them a promising candidate to replace conventional solid-state phase shifters. The EBGs in a VSAT antenna also provides better performance. It can therefore be concluded that EBG structures are very promising candidates to mitigate some burning issues in RF engineering.
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- J.Y. Park, C.C. Chang, Y. Qian and T. Itoh, “An Improved Low Profile Cavity-backed Slot Antenna Loaded with 2D UC-PBG Reflector,” IEEE Antenna and Propagation Symposium Digest, Boston, MA, July 2001, pp. 194–197.
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