Two wide rectangle-shaped microstrip-fed 2.6-GHz slot antennas using defected ground structures (DGSs) with a low design complexity are proposed to achieve wideband harmonic suppression. To accomplish this, two rectangular DGSs (RDGSs) in the first antenna and two circular DGSs (CDGSs) in the second one with various dimensions are etched into the ground plane, which could have a wideband-stop characteristic. Simulated and measured reflection coefficients indicate that the two proposed structures effectively suppress the second and third harmonics up to 23 dB between 3.5 and 10.5 GHz with a maximum ripple of 2.4 dB. In addition, the radiation patterns and peak gains of the antennas can be suppressed at least 17 dB and 7.1 dBi, respectively, at the third harmonic frequency of 7.86 GHz.

Microstrip antennas can be conventionally integrated with active RF devices, especially amplifiers. In these cases, to maximize power added efficiency (PAE) of the power amplifiers (PAs), several methods are carried out on the PA itself. Since these approaches caused RF front-end systems to have large size, complex structures, and difficulty to design, the active integrated antennas (AIAs) were proposed. AIAs are very widespread, but they suffer from undesired harmonic radiation which must be suppressed. To this purpose, in the conventional harmonic tuning circuits, additional components such as filters were employed which increase the size and cost of RF front-ends and yield an additional insertion loss [

In the last decades, several methods have been researched in practice to decrease insertion loss and spurious radiation in slot and patch antennas with microstrip and CPW-fed method [

In this paper, two harmonic suppressed slot antennas (HSAs) with rectangular and circular DGSs (i.e., RDGS & CDGS) are parametrically investigated. The antennas having low design complexity such as less-design parameters and simpler DGS patterns can be suitable options for broadband unwanted harmonic suppression. In addition, in order to get a better understanding of the suppression performance of the antennas, the current distribution and circuit model are also discussed.

Figure ^{2}, 0.508 mm height, and dielectric constant of 3.38. The width of the microstrip-fed line is ^{2}. The proposed unequal slots can be represented as sequential shunt L-C resonators, which have totally wideband-stop operation over the third harmonic frequency.

Configuration of the two proposed antennas with their optimal parameters: (a) geometry of rectangular DGS slot antenna and (b) geometry of circular DGS slot antenna.

In the next section, it will be shown from parametric studies that the positions and dimensions of the DGS slots are important factors to suitably suppress the third harmonic frequency and tune the matching at the main resonance and the notched band.

Some of the popular DGSs are shown in Figure

Different slots for DGS: (a) rectangular DGS, (b) square head DGS (dumbbell shape), (c) triangular DGS, (d) circular DGS, (e) H shape DGS, and (f) spiral DGS.

Simulated reflection coefficients of HSA with different kinds of DGS. The position and etched square area of every DGS are identical.

In order to give a physical insight, the current distribution on the ground plane is investigated. Figure

Simulated current distribution on the ground plane at the third harmonic frequency for (a) conventional slot antenna, (b) RDGS slot antenna, and (c) CDGS slot antenna.

Circuit model for rectangular defected ground structure slot antenna.

Reflection coefficients of equivalent circuit model of the harmonic suppression antenna.

The simulation is performed using high frequency structure simulator (HFSS). By using the DGS section, both the effective permittivity and effective inductance of a microstrip line are increased [

Simulated reflection coefficients of (a) RDGS slot antenna versus ratios of

In order to validate the designs, the optimized structures of proposed HSAs were manufactured and measured carefully. Figures

(a) Reflection coefficients of the RDGS slot antenna and (b) measured input impedance of the RDGS slot antenna.

(a) Reflection coefficients of the CDGS slot antenna and (b) measured input impedance of the CDGS slot antenna.

As seen from Figures

According to the results, the proposed slot antennas having a simple structure and low design complexity are suitable choices for harmonic degradation. The photographs of the two realized RDGS and CDGS wide slot antennas are illustrated in Figure

Photographs of the fabricated RDGS and CDGS wide slot antennas: (a) back and (b) front.

Figure

Simulated radiation patterns of both RDGS slot antenna and conventional slot antenna: (a) E-plane and (b) H-plane as well as measured patterns of only RDGS antenna: (c) E-plane and (d) H-plane.

In this paper, two microstrip slot antennas using rectangular and circular defected ground structures (DGSs) with harmonic suppression characteristic over harmonic frequency band have been proposed and investigated. The reflection coefficients were less than −2.4 dB at the second and third harmonic frequencies compared to those of the conventional slot antenna. The measured and simulated radiation patterns in the HSAs indicate at least 17 dB suppression at the third harmonic frequency. According to the measured results, these antennas with the simple harmonic suppression structures are quite effective for harmonic suppression. Therefore, the proposed antennas can be suitable for active integrated antennas.

The authors thank the Iran’s Telecommunication Educational Research Center (ITERC) for its financial support.