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Progress In Electromagnetics Research Symposium Proceedings, Suzhou, China, Sept. 1216, 2011 1285

Gain and Bandwidth Enhancement of a Microstrip Antenna Using

Partial Substrate Removal in Multiple-layer Dielectric Substrate

Neeraj Rao and Dinesh Kumar V.

PDPM Indian Institute of Information Technology, Design & Manufacturing Jabalpur, India

Abstract In this paper, a novel antenna design for simultaneous improvement of patch an-tenna gain and bandwidth has been proposed using partial substrate removal in a multiple-layerdielectric by suppression of surface wave losses beneath the substrate surface by embedding alow-dielectric (air) void. In this way, improvement in gain is achieved by fractional removalof substrate. Bandwidth of the patch antenna is increased by using multiple layer dielectricsubstrates. It is known that bandwidth of microstrip antenna is small and varies directly withthe size of the patch, Increasing size of patch to improve bandwidth makes it large and bulky.Thus to overcome the bandwidth limitation without increasing size unacceptably, a patch onmultiple-layer scheme has been designed to reduce the effective dielectric constant. This ensuresthat bandwidth increases greatly while size is nearly unaffected. Performance parameters likebandwidth and reflection loss have been evaluated and compared with that obtained from singlelayer dielectric substrates. Results suggest considerable improvement in bandwidth using the

proposed patch antenna design on multi-layered dielectric substrate. The designed patch has asize of 12 8 mm2 and gives a bandwidth and gain of 314 MHz and gain 4.035 dB at 6.479 GHz.

1. INTRODUCTION

A microstrip patch antenna is widely used in compact and portable communication devices dueto its small size, thin profile configurations, conformity and low cost. In spite of these remark-able advantages, the patch suffers some serious drawbacks like low bandwidth (due to small size).Bandwidth can be increased but at the cost of size of the patch, making it large and bulky. Toovercome this problem, a multiple-layer dielectric substrate has been used to improve bandwidth,in this paper [1]. There are different types of losses in antenna, one of which is surface wave lossdue to of the permittivity of the material and thickness of the substrate [25]. Due to excitation ofsurface waves, patch antenna also suffers from reduced gain and efficiency as well as unacceptablyhigh levels of cross-polarization and mutual coupling within an array environment at high frequen-

cies. In this paper, parts of the substrate surrounding the patch have been strategically removedto suppress surface wave losses, and thereby increase gain [610]. It is known that for a particu-lar resonant frequency, the bandwidth increases with increase in size of patch antenna for a highdielectric [11]. The patch antenna of low dielectric has a moderate bandwidth but large size [12].Bandwidth decreases with increase in the dielectric value of the substrate. The two substrates havebeen combined so as to include the quality of both high and low dielectric, i.e., high bandwidthand low patch size respectively [1]. A microstrip patch antenna implemented on silicon (r = 11.9)has moderate bandwidth with a small sized patch and that implemented on glass (r = 4.6) has ahigh bandwidth. In this paper, a multiple-layer substrate is made by sandwiching one layer of glass(r = 4.6) in between two layers of silicon (r = 11.9) so as to reduce the antenna size as much aspossible. Here, silicon (r = 11.9) has been used directly below the patch so as to maximize the sizereduction. As a result, the size is increased slightly, but the bandwidth is increased greatly. Themethod to improve the gain is to reduce the loss of the microstrip antenna. The gain of the antenna

can be increased by reducing the loss due to surface wave propagation [1317]. One method todo so is by replacing the substrate of patch with low dielectric values or with air (r = 1). Peri-odic structures of electromagnetic band-gap (EBG) can be used to block the surface waves frompropagating in a certain band of frequency. A common method to generate EBG structures is todrill holes in substrate to synthesize a lower dielectric constant substrate [18]. However, in thispaper, a rather simpler version of EBG structure approach has been used by partially removingthe substrate surrounding the patch instead of drilling periodic holes. Removing the substratepartially stops the propagation of surface wave in the substrate, which reduces the power coupledin backward direction and enhances the forward coupled power [6]. As a result the gain increases.

2. SIGNIFICANCE OF A MULTIPLE-LAYER SUBSTRATE

The wavelength of an EM wave at 5 GHz is about 6 cm in air [1]. The size of the antenna at 5 GHzis typically in the order of centimeters, which take a lot of area in wireless devices. To address

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1286 PIERS Proceedings, Suzhou, China, September 1216, 2011

the issue of narrow bandwidth, many methods can be found in literature, typical methods beingE-patch and U slot, aperture coupling [19]. These approaches enable broadband characteristicsusing air or low dielectric constant substrates, but at the cost of large volume and need of a largeground plane. For small-size compact antenna, the requirement is not only the smaller size butalso a smaller ground plane. A compact rectangular patch microstrip antenna with small volumehas been proposed in [20]. It has broadband characteristics due to multiple layer substrates. Thedielectric constant of the substrate is closely related to the size and the bandwidth of the microstrip

antenna. A trade-off relationship exists between antenna size and bandwidth [21]. But if we usemultiple-layer substrates of different materials, the effect of trade-off relationship can be reduced.The use of high and low dielectric constant materials for multiple layer substrates puts effectivedielectric constant to a value at the centre of both high and low dielectric constant materials [20].HRS (High Resistivity Silicon) was chosen as a substrate. An antenna fabricated on HRS substratehad small patch size but it also had narrow bandwidth. However, a glass substrate in the middleof silicon substrates lowered the effective dielectric constant. By using a Si/Glass/Si multi layersubstrate, the bandwidth is increased greatly while the size is increased slightly. In this paper wehave designed the antenna which has a broad bandwidth and a higher gain [22].

3. PROPOSED ANTENNA DESIGN

It is known that for a particular resonant frequency, the bandwidth increases with increase in size

of patch antenna for a high dielectric [6]. The patch antenna of low dielectric has a moderate band-width but large size. Bandwidth decreases with increase in the dielectric value of the substrate [6].The two substrates have been combined so as to include the quality of both high and low dielectric,i.e., high bandwidth and low patch size respectively. Figure 1 shows the proposed antenna designfor a circular patch antenna on multiple-layer dielectric. A microstrip patch antenna implementedon silicon (r = 11.9) has moderate bandwidth with a small sized patch and that implementedon glass (r = 4.6) has a high bandwidth. In this paper, a multiple-layer substrate is made bysandwiching one layer of glass (r = 4.6) in between two layers of silicon (r = 11.9) so as toreduce the antenna size as much as possible. Here, silicon (r = 11.9) has been strategically useddirectly below the patch so as to maximize the size reduction. As a result, the size is increasedslightly, but the bandwidth is increased greatly. The proposed fractional substrate removal designconsists of replacing certain fractions of the patch substrate by air in certain configurations. Ithas been tested for fractional substrate removal in various configurations like-along radiating andnon-radiating edges, making trenches of different widths all-around the patch, below the patchand on all its sides. Figure 1 shows the design using removal of a fraction of substrate from thesubstrate surrounding the patch. The performance metrics gain (dB), half power Beam Width(Eplane) and half power Beam Width (Hplane) in degrees has been computed and analyzed forthe proposed structure.

(a)

(b)

(c)

Figure 1: Proposed antenna design Patch on multiple-layer dielectric. (a) Isometric view. (b) Top view.(c) Side view.

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4. SIMULATION RESULTS

The advantages of the proposed multi-layer substrate for a rectangular patch were verified usingthe Computer Simulation Technology Studio SuiteTM 2011 simulation tool. Simulation results andperformance parameter bandwidth has been calculated using the proposed multilayer dielectric andwere compared with that obtained from single-layer silicon and single-layer glass substrate. Table 1shows the dimensions and the simulation results of the tested microstrip antenna designs.

The parametric description of the multiple-layer substrate is as follows:

Glass substrate (r = 4.6) of 1 mm is sandwiched between two silicon layers (r = 11.9) ofthickness 0.5 mm each.

The rectangular patch has size 12 8 mm2 and has been fed using microstrip feed.

All of the designed microstrip antenna have 15 15mm2 ground plane.

5. OBSERVATIONS

Performance metric bandwidth has been analyzed and observations regarding the radiationpattern and reflection loss obtained using the proposed patch antennas have been discussed in thissection.

5.1. Bandwidth and Patch Size

Figure 2 shows reflection loss (in dB) for different dielectric substrates.

Best bandwidth-size trade-off was observed using the proposed multiple-layer dielectric sub-strate (Si/Glass/Si) giving a bandwidth of 314 MHz (reflection loss: 9.5014 dB) at patch size8 12mm2.

Table 1: Comparison of simulation results on rectangular patch antenna using different types of substrates.

Dielectric Dielectric

constant

Thickness

(mm)

Bandwidth

(MHz)

Size of

rectangular

patch

Gain Directivity

Silicon

(single layer)

11.9 2 72.7 8 6 2.490 4.728

Glass

(single layer)4.6 2 86.4 12 13 2.326 4.528

Silicon/

Glass/Silicon

(Multiple-layer)

11.9/4.6/11/9 0.5/1/0.5 314 8 12 4.035 6.646

Figure 2: Reflection loss (in dB) for Multiple layer Dielectric with partial substrate removal. Reflection lossfor the proposed antenna reaches 9.5014 dB, it has the highest 3 dB bandwidth of 314 MHz.

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1288 PIERS Proceedings, Suzhou, China, September 1216, 2011

(a) (b)

Figure 3: Radiation pattern for patch antenna on multiple-layer dielectric with partial substrate removal in(a) Eplane, = 90. (b)H-plane, = 0. The main lobe magnitude (gain) is found to be 4.035 dB.

The smallest patch on single-layer silicon substrate was found to be 8 6 mm2 and very low

reflection loss of19.246 dB but the bandwidth was only 72.7 MHz. The rectangular patch on single-layer glass substrate was found to give a bandwidth of

86.4 MHz (reflection loss: 7.8 dB) but at a size of 12 13mm2.

When compared with bandwidth of single-layer substrate the bandwidth was found to increaseremarkably using the proposed multiple-layer substrate. Bandwidth using multiple-layer dielectricis greater than that obtained using single-layer Si substrate by while the size (patch area) wasincreased.

On comparison with bandwidth of single-layer glass substrate, multiple-layer dielectric substrategives 260% increase in bandwidth while the size is reduced by 38%.

5.2. Radiation Pattern

Figure 3 shows the simulation results of the radiation patterns for the proposed patch on

multiple-layer substrate along Eand H-planes. The microstrip antennas on glass and Si/glass-/Si substrate were found to have similar radiation patterns.

The gain of the proposed multiple-layer with partial substrate removal patch was found to be4.035 dB. This is acceptable value gain for a microstrip antenna.

Also back lobe of the radiation pattern is quite large due to small ground planes. These canbe improved by increasing the area of the ground plane.

6. CONCLUSION

The proposed antenna design using multiple-layer dielectric with partial substrate removal hasbeen found to give a bandwidth of 314 MHz with a gain of 4.035 dB at an operating frequency of6.646 GHz. This is a noteworthy improvement in bandwidth over that obtained using microstripantenna over single-layer silicon or single-layer glass substrate. These broadband characteristics

can be attributed to the multiple-layer dielectric substrate that lowers the effective dielectric valuewhile reducing the size as much as possible. The proposed antenna has a gain of 4.035 dB. Thisapproach can be extended further to improve gain and bandwidth simultaneously by incorporatingvarious electromagnetic band-gap structures in a multiple-layer configuration.

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