Microwave Journal
www.microwavejournal.com/articles/43342-compact-uwb-patch-antenna-with-open-loop-resonator-for-dual-band-rejection
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Compact UWB Patch Antenna with Open-Loop Resonator for Dual-Band Rejection

Ibrahim Fortas, Electrical Systems Engineering LIST Laboratory, University of M’hammed Bougara Boumerdes, Mouloud Ayad, Department of Telecommunications University of Setif Setif, and Bachir Zoubiri, Division Telecom, Center for Development of Advanced Technologies, CDTA
February 10, 2025

An innovative design for a compact ultra-wideband (UWB) patch antenna with improved frequency rejection features integrates a dual-ellipse structure in the patch geometry fed by a coplanar waveguide (CPW). The antenna is constructed on a low-profile FR-4 substrate measuring 18 × 19 × 1.5 mm. Four open-loop resonators are incorporated between the patch and the ground plane to provide rejection capabilities for two specific undesired frequency bands: WLAN (5.2 to 5.8 GHz) and X-Band satellite downlink (7 to 8 GHz). The prototype exhibits promising UWB performance and dual-band rejection using metamaterials, providing valuable insights into compact UWB antenna design for applications in wireless communication.

In the field of wireless communications, the use of UWB technology seeks to achieve high data rates at limited distances. UWB technology is defined by its capacity to function across an extensive frequency spectrum, typically 3.1 to 10.6 GHz, by Federal Communications Commission regulations established in 2002.1 This expansive frequency range includes numerous narrow bands, such as WiMAX (3.3 to 3.7 GHz), 5G sub-6 GHz (3.4 to 3.8 GHz), WLAN (5.15 to 5.75 GHz) and others, causing significant interference.

A band-notch refers to a specific frequency range within the broader frequency spectrum that is intentionally suppressed or attenuated. Band notching is commonly employed in antenna design to reject or minimize interference.2 Several techniques are used, e.g., slots,3,4 defected ground structures (DGSs),5,6 electromagnetic band gaps (EBGs),7,8 resonators9,10 and metamaterials.11,12

Figure 1

Figure 1 3D metamaterial unit cell.

These techniques are employed to reject certain frequency bands, although some exhibit suboptimal rejection performance. While certain techniques are complex, i.e., they can only be realized by specialized technologies, others fall short of achieving a compact design. Researchers have used metamaterial structures, based on their unique electromagnetic properties, particularly negative permittivity and negative permeability, to achieve enhanced performance as band-reject filters.13 This technique employs precise control over the metamaterial’s response to electromagnetic waves; however, the process of designing a compact UWB antenna employing metamaterial structures that effectively reject unwanted bands is challenging.

This article describes an UWB patch design featuring a compact dual elliptical shape fed by CPW. Four open-loop resonators are used to reject radiation across two separate frequency bands: WLAN (5.2 to 5.8) GHz and the satellite downlink band (7 to 8 GHz). Rejection is significantly increased by integrating metamaterials between the patch and ground plane.

METAMATERIAL UNIT CELL DESIGN

Figure 1 illustrates the open-loop resonator metamaterial unit cell printed on a 1.5 mm thick FR-4 epoxy substrate with a relative permittivity, εr, of 4.3 and a loss tangent of tan, δ, of 0.025. A plane electromagnetic wave incident in the x-direction approaches the unit cell, with the magnetic field oriented along the z-axis and the electric field along the y-axis. Perfect electrical conductors serve as boundary walls along the y-axis (at the x/z-oriented sides). This configuration facilitates the design of rings that resonate near the desired frequency and enables metamaterial characterization through the calculation of S-parameters and the retrieval of effective electromagnetic characteristics εeff and μeff.

To further investigate the impact of metamaterials, a standard parameter retrieval technique is used to calculate the effective magnetic permeability.14,15 Real and imaginary components of the magnetic permeability are acquired with CST Studio software (see Figure 2). It is evident that the planar representation of the metamaterial structure exhibits a frequency range characterized by negative permeability in a specific band. This evaluation is an estimation. Nevertheless, simulation and permeability retrieval do provide a reasonable indication of the presence of metamaterial properties, even at the individual cell level. This not only facilitates resonator design but also offers an alternative justification for the results obtained.

Figure 2

Figure 2 Real and imaginary parts of retrieved permeability.

Figure 3

Figure 3 Permeability for different values of Lm.

Figure 3 shows the retrieval of the metamaterial’s permeability, including both its real and imaginary parts, for various unit cell lengths, Lm. The results indicate a direct influence of unit cell length on the frequency at which the band effect manifests. This reveals an inversely proportional relationship between the unit cell’s length and the frequency of observed bands in the metamaterials. This enhances the understanding of the design parameters’ impact, particularly regarding negative permeability.

ANTENNA DESIGN

The design process (see Figure 4) begins with the creation of an initial elliptical patch antenna in CST Studio. Then, a double-ellipse configuration fed by coplanar waveguide with an impedance of 50 Ω is used to achieve UWB performance. Finally, four metamaterial open-loop resonators are integrated into the design to effectively reject two unwanted frequency bands while further improving antenna performance.

Figure 4

Figure 4 Antenna design evolution.

Figure 5

Figure 5 Antenna geometry.

The antenna is printed on a low-profile FR-4 substrate with εr of 4.3 and δ of 0.025. Figure 5 shows the final design’s structural layout and key components. Table 1 lists the dimensions of key parameters.

Table 1
Figure 6

Figure 6 Refection coefficient for design Steps 1, 2 (a) and Steps 3 through 5 (b).

This UWB patch antenna represents an innovative departure from the conventional single ellipse design16 (see Figure 4, Step 1). Instead, it uses a dual-intersecting ellipse configuration (see Figure 4, Step 2) to improve UWB characteristics. The impedance bandwidth with the dual-ellipse structure shown in Figure 6a is greater than that of the reference single ellipse design, effectively covering the UWB spectrum from 3 to 10.5 GHz.



In this design, metamaterials are used to reject unwanted bands. Two metamaterial unit cells, M1 and M2, are positioned above the ground (see Figure 4, Step 3). Equation 1 determines the center frequency of the band-notch for a given effective dielectric constant and Equation 2 determines εeff:

Equation 1


Where:

Lm is the length of the resonator

C is the speed of light

εr is the dielectric constant of the substrate.

Figure 6b shows -4 dB |S11| at the center frequency (fn1) of the first unwanted band (5.2 to 5.8 GHz); however, the integration of metamaterials in Step 3 introduces a noticeable degradation in antenna matching, particularly above 7 GHz. To address this effectively, two rectangular stubs are added (see Figure 4, Step 4) and |S11| is reduced to under -10 dB.

Finally, in Step 5, two additional unit cells, M3 and M4, are integrated. This results in significant rejection over the second targeted band (7.2 to 7.8 GHz). |S11| reaches approximately -4 dB at the center frequency of fn2 = 7.6 GHz.

Figure 7

Figure 7 Prototype UWB antenna.

Figure 8

Figure 8 Measurement setup.


PROTOTYPE FABRICATION AND MEASUREMENT RESULTS

An antenna prototype, shown in Figure 7, is fabricated to verify the simulations experimentally. Measurements of |S11| are made using a Keysight N5224A vector network analyzer, as shown in Figure 8. Figure 9 compares simulated and measured results, showing a close correspondence.

Figure 9

Figure 9 Simulated and measured antenna reflection coefficients.

Figure 10

Figure 10 Simulated antenna current distributions at the notch center frequencies.


To provide a more comprehensive illustration of the characteristics associated with the dual band-notch features, Figure 10 shows simulated current distributions at frequencies fn1 and fn2. It reveals a concentration of current around metamaterial unit cells M1 and M2 within the WLAN band. Within the satellite data link band, current clusters around metamaterials M3 and M4.

Radiation patterns are measured in both the E-plane (YZ-plane) and H-plane (XZ-plane) at 4.3, 6.6 and 9 GHz, with the results shown in Figure 11. The results show bidirectional characteristics in the E-plane and omnidirectional characteristics in the H-plane for all frequencies of interest. Minimal changes at high frequencies are attributed to substrate power loss.

Figure 11

Figure 11 Simulated and measured antenna radiation patterns at 4.3 (a), 6.6 (b) and 9 (c) GHz.

Figure 12

Figure 12 Simulated and measured antenna peak gain.


Figure 12 shows the measured and simulated peak gain, demonstrating close agreement. Gain remains stable across the entire UWB range, reaching a maximum of 3.7 dBi at 10.3 GHz. This is accompanied by a significant decrease at fn1 (-3.8 dBi) and fn2 (-3.7 dBi). It validates the effectiveness of the metamaterials technique for rejecting radiation within the two undesired frequency bands.

A comprehensive comparison of this with other related works is shown in Table 2, highlighting distinctive aspects such as dimensions, frequency range, rejected bands, employed techniques, complexity and design technology. This design is compact and features a simple rejection technique based on metamaterials. The use of a single-faced CPW configuration not only simplifies its construction but also enhances practicality and ease of integration into diverse antenna systems.

Table 2


CONCLUSION

A novel approach to the design of a compact UWB patch antenna with improved rejection capabilities integrates a dual-ellipse structure in the patch geometry fed by CPW. It also employs four open-loop resonators to selectively target undesirable frequency bands, specifically WLAN (5.2 to 5.8 GHz) and the satellite downlink band (7 to 8 GHz). Experimental results closely align with the simulation, verifying the effectiveness of the open-loop resonators in enhancing rejection. The final design, incorporating metamaterials, demonstrates UWB performance with dual-band rejection. The use of metamaterials to reject radiation in undesirable frequency bands provides insight into the development of compact UWB antennas for applications in wireless communication systems.

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