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 resonators^9,10 and metamaterials.^11,12

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, ε_r, 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 ε_eff and μ_eff.

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 Real and imaginary parts of retrieved permeability.

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 Antenna design evolution.

Figure 5 Antenna geometry.

The antenna is printed on a low-profile FR-4 substrate with ε_r 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.

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.