Design and Fabrication of a Novel Quadruple-Band Monopole Antenna Using a U-DGS and Open-Loop-Ring Resonators

In this Article, a novel quadruple-band microstrip patch antenna is proposed for the systems operating at quad-band applications. The antenna structure is composed of modified rectangular patch antenna with a U-shaped defected ground structure (DGS) unit and two parasitic elements (open-loop-ring resonators) to serve as a coupling-bridge. The proposed antenna with a total size of 31 × 33 mm 2 is fabricated and tested. The measured result indicates that the designed antenna has impedance bandwidths for 10 dB return loss reach about 180 MHz (4.4–4.58 GHz), 200 MHz (5.4–5.6 GHz), 1100 MHz (7.2–8.3 GHz), and 700 MHz (9.6–10.3 GHz), which meet the requirements of the wireless local area network (WLAN), worldwide interoperability for microwave access (WiMAX), C and X bands applications. Good agreement is obtained between measurement and simulation results.


Introduction
In modern wireless communication systems, the requirement of multi-band antenna has played an important role for wireless services. Microstrip patch antenna is an attractive candidate to be used as multi-band antennas due to its low physical profile, easy fabrication and low cost. Many patch antennas have been recently reported in the literature to cover such applications, but most of them are single-band, dualband and tri-bands [1][2][3] and cover only the wireless local area network (WLAN) and the worldwide interoperability for microwave access (WiMAX) applications. In [4,5], the antenna produced four bands at 4.64, 5.04, 5.62 and 6.22 GHz. In addition, the antenna in [6][7][8][9][10][11][12][13][14][15][16][17][18] can generate four bands at 2.8, 4, 5.4 and 6.2 GHz to cover WLAN/WiMAX. However, none of these antennas can satisfy the C and X bands for mobile and satellite applications. In this article, a microstrip antenna based on modified rectangular patch antenna with a U-shaped defected ground structure (DGS) unit and two parasitic elements (open-loopring resonators) to control surface current distribution on the patch antenna to achieve quad-band operation is proposed.
The antenna prototype is fabricated and measured using Rohde & Schwarz R&S®ZNB vector network analyzer (VNA) operating in the 100 KHz -20 GHz frequency band. The proposed antenna produces four operating frequencies at 4.5, 5.5, 7.54 and 10 GHz which covers systems operating at WLAN, WiMAX, C and X bands applications.

The influence of open loop resonators on radiation characteristics
An investigation of the PE1 and PE2 positions is conducted to find out and to be able to control the effect of placing these parasitic elements on the antenna return loss. Fig. 1 and Fig.  2 display, respectively, the PE1 and PE2 positions with the patch and the simulated return losses for various PE1 and PE2 positions. As the Fig. 2 depicts, different positions, in the closeness of the patch, are considered to be as follows: 0.2 mm, 0.4 mm and 0.6 mm for x-coordinates and, 0 mm, 5 mm and 7 mm for y-coordinates. As Fig. 5 shows, a significant decrease in return loss, especially at 5.5 GHz, compared to the ant4 results, is observed. Moreover, the 4th resonant frequency is shifted down, from 10 GHz to 9.3 GHz. Presence of an additional resonant frequency at around 12 GHz is also noticed. Consequently, the parasitic elements positions affect, considerably, the resonant frequencies.

The alternative antenna design
The configuration of the proposed microstrip quadruple-band antenna, fed by 50 Ω microstrip feed line, is shown in Fig. 3. The design consists of modified structure of a rectangular patch antenna with a U-shaped DGS unit and two loaded parasitic elements (denoted as PE1 and PE2). The antenna is printed on a 31 × 33 mm 2 flame resistant (FR-4) substrate with a height of 1.6 mm, a relative dielectric constant of 4.3, and a loss tangent of 0.0017. The design steps to attain the proposed antenna are shown in Fig. 4. The proposed design started with a conventional rectangular patch antenna as shown Fig. 4(a). The modified structure is designed by etching three different parts (Ushaped) from rectangular patch antenna and a U-shaped unit from ground plane as shown Fig. 4(b). Then introducing resonator to act as a parasitic element as shown Fig. 4(c-e). This modified structure generates four resonant modes of operating frequencies. The parameters of the proposed design are illustrated in Table 1.

Simulation results and discussion
The simulation results of antenna with only U-shaped DGS unit (denoted as ant 1), antenna with U-shaped DGS unit and parasitic element 1 (denoted as ant 2), antenna with U-shaped DGS unit and parasitic element 2 (denoted as ant 3) and antenna with both U-shaped DGS unit and the two parasitic elements (denoted as ant 4) are depicted in Fig. 5. The simulated results of ant 1 indicate that the designed antenna achieves quadruple-band operation with resonant frequencies at around 4.5, 5.5, 7.54 and 10 GHz. By adding PE1, ant 2, the second, the third and the fourth resonant frequencies are slightly shifted towards lower frequencies where the second one is more matched. By placing PE2, ant 3, a notched band at the second resonant frequency, 5.5 GHz, is produced and a tri-band operation at about 4.5, 7.50 and 9.5 GHz is achieved. Since the antenna aimed to operate at four distinct resonant frequencies to covers the WLAN, WiMAX, C and X operating bands, a combination of a DGS unit with both PE1 and PE2, ant 4, generates a quadrupleband resonant characteristic at about 4.5, 5.5, 7.54 and 10 GHz with good impedance matching.    The simulated antenna gain is depicted in Fig. 8. One can observe that the proposed antenna exhibits acceptable gain uniformity with 5.24 dB, 3.98 dB, 5.2 dB and 5.3 dB at 4.5, 5.5, 7.54 and 10 GHz, respectively, which verifies the proposed idea. The radiation efficiency is expected to be small regarding the antenna dimensions and this also can be reflected by a low realized gain within the band. The gain in [19], given by 2.6 dB, 3.1 dB, 3.8 dB and 6.3 dB at 2.63 GHz, 4.64 GHz, 5.94 GHz and 7.78 GHz, respectively, seems to be smaller than those of our design, except at 7.78 GHz, and this could be realized due to the huge extension of the bandwidth achieved in [19]. Moreover, the pick gain in [20] is approximately same throughout the band except at elevation Y-Z-plane at θ = 90°.

Fabrication and measurement results
Based on the parameters illustrated in Fig. 3, the proposed antenna was fabricated and measured using R&S®ZNB VNA. The photograph of the fabricated antenna is shown in Fig. 9. The obtained measurement results are given in Fig.  10 and superimposed to the simulated ones. It can be observed that the measured results are in good agreement with simulated ones. The measured impedance bandwidths for 10 dB return loss reach about 180 MHz    Table 2. The measured return loss is almost 16, 23, 42, and 16 dB, respectively, for the final design shown in Fig. 4(e), (where (f 1 , f 2 , f 3 , f 4 ) and (RL 1 , RL 2 , RL 3 , RL 4 ) are respectively resonant frequencies in GHz and corresponding return loss values in dB). In what follow, the effect of the DGS unit along with the two parasitic elements PE1/PE2 is investigated.

Conclusions
A novel quadruple-band microstrip patch antenna for WLAN, WiMAX, C and X bands applications has been fabricated and the measurement results have been reported. The measured result have indicated that the proposed antenna has impedance bandwidths for 10 dB return loss attain roughly 180 MHz, 200 MHz, 1100 MHz and 700 MHz which meet the requirements of the WLAN, WiMAX, C and X bands applications. Satisfying experimental results validate the proposed antenna. Good agreement between the simulated and the measured results have been obtained.