Design of high gain and wide band EBG resonator antenna with dual layers of same dielectric superstrate at X-bands

Abstract

A wide band EBG resonator antenna with two layer of dielectric superstrates of the same material is design which is operating in X band. The EBG superstrates act as Partially Reflective Surfaces (PRS) having positive reflection phase curve which are suspended above a double sided dipole patch antenna fed by a coaxial cable pass through ground plane. Thus a resonator cavity is constructed which increases the bandwidth and gain of the feed antenna. The measured results validate the simulation results obtain from Computer Simulation Technology (CST) microwave studio. These results show that the antenna posses a 3 dB gain bandwidth from 8.5 GHz to 11.2 GHz, with a peak gain of 15.3 dBi. Furthermore, the scattering parameter S11 is well within range from 8.6 GHz to 11 GHz.

Antenna; CST; EBG resonator antenna; superstrate

I. INTRODUCTION

Electromagnetic Band Gap (EBG) structure is extensively studied in the last decade due to its potential in controlling the electromagnetic wave for various applications. Among these applications wideband and high gain resonator antenna design were studied by different authors [¹-⁵]. Most of the conventional EBG resonator antenna the EBG structure is suspended at a proper height above the feed antenna backed by the ground plane. This type of design configuration drastically reduces the feed losses and also avoids the complex feed structure particularly at higher frequency bands. Different types of feed antennas were used by many authors but we proposed double sided microstrip dipole antenna. With the daily increase in the demands of high gain, low profile and small size antennas for modern radar and communication systems particularly in microwave and millimeter wave bands. Antennas such as reflector antennas, dielectric lens antennas, slotted wave guide antennas, horn antennas, and microstrip patch array antennas are conventionally used to fulfill these requirements. However, these types of antennas exhibits a few disadvantages such as complex structure, high cost, and high aperture size which makes EBG resonator antennas an interesting candidate to overcome these drawbacks [⁶-¹²]. Therefore, EBG resonator antennas are extensively under study for the last decade to exploits its simplicity, low aperture size and high gain. The EBG antenna is constructed by placing EBG superstrates (act like Partially Reflecting Surfaces (PRS) at the design frequency) above the ground and feed by a feed antenna at the center to form an air-filled cavity between the EBG superstrate and ground plane [¹³-²¹]. In addition to the feed antenna the major characteristics of the EBG resonator antenna, such as its design frequency, directivity, gain bandwidth and radiation patterns, are determined by the property of the superstrate. The operating frequency depends on the following equation [²²],

Where, φ_H and φ_L are the reflection phases of the superstrate used and the ground plane, respectively, while h is the distance between the superstrate and the ground plane. The boresight gain (G) relative to the feed antenna is given by

In the above relation S is the power pattern, F(θ) is the element pattern and ρ is the magnitude of the reflection coefficient of the superstrate used. To keep a low profile, normally the first resonance is taken at value N=0. With a perfectly electric conducting (PEC) ground plane φ_L = π , (1) can be rearranged as

The expression given in (3) illustrates that a wideband EBG resonator antenna could be obtained by using a superstrate with a reflection phase of positive slope at the desired frequency. Superstrate with these characteristics can be constructed by 2-D printed metal strips [¹⁸] and 1-D dielectric substrates [¹⁹], respectively, and used in several antenna designs [²⁰-²⁴].

Different types of feed antenna can be used to excite the cavity. In this manuscript we use a patch based dipole fabricated on a low loss substrate. The feed antenna is also placed at an optimized distance above the ground plane which minimizes the effects of the surface waves. The resonator cavity forms between the PRS and the ground plane have in phase fields in boresight direction which contributes to the enhancement of gain and directivity. Due to the absence of transverse modes at the operating frequency there is no need to fabricate a side wall. Moreover, in addition to the feed antenna gain and bandwidth the design of the EBG superstrates also contribute in the overall gain and bandwidth of the EBG resonator antenna. The PRS placed at a distance h from the ground plane and generate multiple reflections in the cavity. Different types of EBG structures are used as PRSs to form EBG resonator antennas have been design, such as 1-D dielectric slabs [¹], 2-D dielectric grids and rods [³], 2-D printed frequency selective surfaces (FSSs) [⁴], 2-D metallic apertures in a conducting plate [⁵], and 3-D woodpile structures [⁶]. Structures based on 1-D dielectric slabs and printed FSSs are preferable because they are simple and easy for fabrication. The EBG antenna exhibits advantages such as high directivity and high radiation efficiency. A mushroom type high impedance surface (HIS) with size-tapered patches and a single layer printed FSS were applied as the ground plane and the PRS, respectively, in the design of a wideband FP resonator antenna [¹⁰]. In this work, a wideband EBG resonator antenna having two superstrate whose reflection phase gradient is positive at the required central frequency. The antenna was simulated in Computer Simulation Technology (CST) Microwave Studio 2012. The simulated results obtain from CST are validated by the measured results.

II. GEOMETRY OF THE PROPOSED EBG STRUCTURE

In this manuscripts different conventional electromagnetic band gap (EBG) structures are used for the improvement of gain of EBG resonator antennas [⁹-¹¹]. However, the narrow bandwidth limitation makes the final antenna having small bandwidth. This problem is address by design of PRS having positive reflection phase gradient, which makes these antenna wideband. Therefore a double layer superstrate is proposed whose reflective phase gradient is positive. The design parameters of the proposed EBG structure used for the wideband antenna are depicted in Fig. 1, which contains two dielectric substrates having the same dielectric constant. The dielectric substrates are made of Rogers TMM 10 substrate having dielectric constant value (ε_r=9.2), and loss tangent value 0.0022. The thickness of lower substrate is approximately 2.49 mm at central frequency 10 GHz. The upper dielectric substrate thickness is approximately 4.98 mm. The distance between these two dielectric substrates is approximately equal to 7.5mm.

Fig
Structure of the proposed EBG act as superstrate

Fig. 2 illustrates the reflection coefficient phase and magnitude which can be calculated by transmission line theory. A positive phase gradient around 10 GHz is observed for the proposed EBG structure due to the dual layer configuration shown in Fig.1. Most of the conventional EBG structure have negative phase gradient which exhibits relatively small bandwidth. The reflection magnitudes shown in Fig. 3 have a minimum value of 0.55 at 9.55 GHz. The detail analysis of the same type of EBG superstrate is done in [²⁵] using transmission line model. The structure is modeled using equivalent cascade transmission line network and analyzed using smith chart.

Fig. 2
Plot of the reflection coefficient phase

Fig. 3
Plot of the magnitude of the reflection coefficient

III. FEED ANTENNA

In addition to the proper design of the EBG structure the feed antenna also play an important role in the design of EBG resonator antenna. Different type of antennas are used as a feed antenna for the EBG resonator antennas such as waveguide aperture in the ground plane [¹⁹], probe-fed patch antenna [²⁰], monopole antenna [²¹], slot coupled patch antenna [²²].

In this work a double side patch dipole antenna fabricated on both side low loss substrate as shown in the Fig. 4. Fig. 4 (a) shows the top view and one pole of each dipole can be seen. The other pole of each dipole element is etched on the bottom of the dielectric substrate as shown in Fig. 4 (b). The antenna is design on a low loss dielectric substrate with a thickness of 0.8mm and having dielectric constant of 2.65. The dipole patch antenna is spaced from the ground plane by 2.5mm air gap with Teflon spacers at all corners of the substrate. These dielectric spacers are also included in the design of antenna in the CST. The air gap drastically reduces the surface wave and thus gives us good impedance matching at the desired frequency bands. The dimensions of the ground plane is 2.4λ₀ x 2.4λ₀ at 10 GHz. The dimension of the antenna and their feed are L = 6.85 mm, W = 4.6 mm, and W_feed = 1mm. This feeding antenna is designed using CST Microwave Studio 2012 and then fabricated using etching technique. The simulated S₁₁ of the antenna is validated by the measured S₁₁ as shown in the Fig. 5. There is some difference in both the results which may be associated with the fabrication tolerances and poor performance of the SMA connector at high frequencies. The polar plot for the radiation pattern of the antenna in the E-plane (yoz) and H-plane (xoz) at 10 GHz are shown in Fig. 6. The fabricated feed antenna is shown in Fig. 7. This antenna is feed from the bottom which is easy to handle in comparison to side feed antennas [²²-²⁶]. This antenna is used as a feed antenna for the design of wide band EBG antenna discussed in the next section.

Fig. 4
The fabricated model (a) Top view of fabricated dipole with dimensions (b) Bottom view of the fabricated dipole

Fig. 5
Fabricated assembled feed dipole antenna

Fig. 6
The plot of the simulated and measured scattering parameter S₁₁ for the feed dipole antenna

Fig. 7
The simulated radiation pattern of the feed dipole in E-plane and H-plane

IV. EBG RESONATOR ANTENNA DESIGN

The wideband EBG resonator antenna is design using the EBG superstrates and the wideband feed dipole antenna describe in the previous sections. The EBG structure is suspended at a height h above the aluminum ground plane with the help of pillars made of low loss dielectric materials Teflon as shown in Fig. 8.

Fig. 8
The model structure of the proposed EBG resonator antenna

These Teflon spacers with diameter 2.5mm and having a dielectric constant of 2.1 is also modeled in CST to make their electromagnetic effect also into account. The antenna is feed at the center with the help of feed antenna describe in the previous section decreases the diffractions at the edge. Furthermore, as the EBG structure design was based on the excitation of plane wave and all the design parameters of the antenna cannot be varied. However, the distance between the EBG and the ground plane h is adjustable, which is optimized to 15.5 mm to get the satisfactory results. The final optimized dimensions of the antenna are 72mm x 72mm x 36mm. The fabricated prototype antenna is shown in the Fig. 9.

Fig. 9
The fabricated prototype of the proposed EBG antenna

V. RESULTS AND DISCUSSIONS

The simulation and measured reflection coefficient (S₁₁) of the proposed EBG antenna is illustrated in Fig. 10. The simulation results are obtain from CST 2012 using full wave analysis which are validated with the help of measured results which are obtain with the help of an Agilent Vector Network Analyzer (VNA) E8363B. The simulated impedance bandwidth for S₁₁ < -10dB is equal to 2.65 GHz ranging from 8.65 GHz to 11.3 GHz. However, the measured impedance bandwidth for S₁₁ <-10dB is equal to 2.55 GHz ranging from 8.7 GHz to 11.25 GHz. These few discrepancies may be due to fabrication and assembling tolerances. It is also observed that the bandwidth of proposed EBG antenna drastically increases in comparison to the feed antenna mostly towards the lower frequencies. The gain and radiation pattern of the antenna were measured in an anechoic chamber with the help of Agilent VNA E8363B and standard gain horn antennas. The simulated and measured radiation patterns at 8.7 GHz, 10 GHz, and 11 GHz in E-plane are shown in Fig. 11, Fig. 12, Fig. 13 respectively. The simulated and measured radiation patterns are shown in Fig. 14, Fig. 15, Fig. 16 respectively.

Fig. 10
The plot of simulated and measured reflection spectra of the proposed EBG resonator antenna

Fig. 11
Polar plot of the simulated and measured radiation pattern in E-plane at 8.5 GHz

Fig. 12
Polar plot of the simulated and measured radiation pattern in E-plane at 10 GHz

Fig. 13
Polar plot of the simulated and measured radiation pattern in E-plane at 11 GHz

Fig. 14
Polar plot of the simulated and measured radiation pattern in H-plane at 8.5 GHz

Fig. 15
Polar plot of the simulated and measured radiation pattern in H-plane at 10 GHz

Fig. 16
Polar plot of the simulated and measured radiation pattern in H-plane at 11 GHz

The simulated radiation pattern obtained from the CST is validated by the measured results in general, particularly in the boresight. The measured gain is slightly lesser from simulated one due to the fabrications and measuring tolerances. The sidelobes levels are more different in simulation and measured results. This may be due to the radiation of the cable, SMA connector and fabrication tolerances. The gain versus frequency plot is plotted in the Fig. 17, which depicts a 3dB bandwidth of about 2.5 GHz ranging from 8.5 GHz to 11 GHz. As compared to the results given in [²²], the gain of the antenna is higher by about 3 dB.

Fig. 17
The gain VS frequency plot of the prototype antenna at X-band

VI. CONCLUSIONS

The EBG resonator antenna with high gain and large bandwidth feed by a double sided dipole patch antenna was proposed. An EBG superstrate composed of double layer dielectric sheets of same materials with different thickness which exhibit positive reflection phase gradient at the desired operating frequency band. This positive phase gradient contributes to the widening of bandwidth. A prototype EBG resonator antenna was design and fabricated at X-band. The simulation results obtained from CST microwave 2012 were compared with measured results obtain with the help of VNA and anechoic chamber system and were found matching at the desire frequency band. The proposed design can be extended to different frequencies by properly adjusting the dimensions of the EBG antenna.

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Publication Dates

• Publication in this collection
  June 2016

History

• Received
  16 Nov 2012
• Reviewed
  15 Nov 2015
• Accepted
  21 Mar 2016