### I. Introduction

### II. Proposed Antenna Design and Measurement

*ɛ*

_{r}*=*2.2, tan

*δ*= 0.0009) having dimensions

*w*

_{3}×

*l*

_{3}×

*t*

_{1}(width × length × thickness). The dipole patches have a length

*l*

_{1}that can determine the resonant frequency, and its width

*w*

_{1}can enhance the impedance-matching characteristics at the resonant frequency. To achieve wideband characteristics, the radiator edges in the center of the substrate are truncated by width

*d*

_{1}and length

*d*

_{2}. Note that a half-wavelength dipole antenna length generally has a single operating frequency with a narrow bandwidth [18], while a patch dipole can achieve a wideband characteristic by changing the width and length of the radiator, allowing a different current path for each operating frequency [19]. Thus, the geometry of the radiators becomes hexagonal, which can directly connect to the transmission line. In addition, the wide impedance-matching characteristics can be further improved by adjusting the slope of the truncated part for the radiators. The parallel plate transmission line printed on the TLY-5 substrate has a width

*w*

_{2}and a length

*l*

_{2}. This transmission line is perpendicularly connected to the truncated part of the dipole radiators having a gap

*t*

_{2}. In particular,

*l*

_{2}can be operated as a quarter-wave transformer to match the impedance between the dipole radiators and the chip balun. The feeding network is designed using an RO4003C (

*ɛ*

*= 3.35, tan*

_{r}*δ*= 0.0027) PCB with a chip balun and differential microstrip lines to substitute for the bulky and complicated microstrip balun. The differential feeding lines having a length

*l*

_{4}and a width

*w*

_{4}are designed to have a line impedance of 100 Ω, and they are connected to each radiator by two via pins for a 180° phase difference excitation. An SMA connector is used to excite a signal for the chip balun through the microstrip line with width

*w*

_{5}and length

*l*

_{5}.

*S*-parameter, the microstrip feeding line, and the microstrip differential lines. Through the sequential simulations, the optimized design parameters and values are listed in Table 1.

*zx*- and

*zy*-planes at 3.5, 4.5, and 5.5 GHz. The measured cross-polarization levels at the boresight direction are lower than −17.5 dBi and −13.2 dBi in the

*zx*- and

*zy*-planes, respectively, at 3.5 GHz. The measured results agree well with the simulations. The measured half-power beam-widths (HPBWs) are 107° and 86° in the

*zx*- and

*zy*-planes at 3.5 GHz, and those at 4.5 GHz are 154° and 94°, respectively. Fig. 6 presents the total efficiency and the radiation efficiency according to frequency. The total efficiency of the proposed antenna considering the mismatch efficiency with frequency is simulated. The total efficiency is over 82% in the operating frequency range from 2.75 to 5.8 GHz, and it is 90.8% at 5.5 GHz. The radiation efficiency value is obtained as more than 98.3% across the whole operating frequency range.

### III. Analysis

*L*

_{f}to excite the 1:2 chip balun with a balanced impedance of 100 Ω. Through the chip balun, the parallel transmission line of the proposed antenna is connected to the parallel RLC element with

*R*

_{1},

*L*

_{1}, and

*C*

_{l}representing the resonance of the dipole radiator. In particular, the parallel plate transmission line is operated as a quarter-wave transformer, and its characteristic impedance can be obtained using an equation as follows [21]:

*Z*

*is the input impedance of the feeding network, and*

_{f}*Z*

*is the dipole patch radiator impedance based on parallel RLC elements.*

_{a}*Z*

_{0}is the characteristic impedance of the parallel plate transmission line obtained based on a full EM simulation. Fig. 7(b) shows the characteristic impedance responses of the parallel plate transmission line according to the frequency. The simulated and circuit-based characteristic impedance results are 96.45 Ω and 96.41 Ω, respectively, at 2.72 GHz (

*l*

_{2}≈ λ/4). The results confirm that the parallel transmission line of the proposed antenna works as a quarter-wave transformer for wideband characteristics. Fig. 7(c) illustrates the input impedance responses of the simulation and the equivalent circuit model according to the frequency, where blue and red lines indicate resistance and reactance values. The results of the equivalent circuit are in good agreement with the simulated results. This shows that the equivalent circuit is well modeled and represented for each part of the proposed antenna. The detailed values of the lumped elements are listed in Table 3.

*w*

_{2}and

*l*

_{2}are conducted to observe the sensitivity of the reflection coefficients, as shown in Fig. 8. Parameter

*w*

_{2}can adjust the reflection coefficient levels at the high-end frequency band, and

*l*

_{2}can affect the operating bandwidth due to the change in the capacitive couplings between the dipole radiator and ground. The optimum

*w*

_{2}of 3 mm and

*l*

_{2}of 29.7 mm are then obtained considering the stable reflection coefficient below −10 dB in the operating frequency band.

*w*

_{1}), length (

*l*

_{1}), and truncated part (

*d*

_{2}). In this analysis, we focus on the observation of the fractional bandwidth, and the maximum reflection coefficient. The maximum reflection coefficient is defined as Γ

_{max}to observe the stability of the wideband operation for the proposed antenna in the operating frequency band from 2.75 to 5 GHz. Fig. 9(a)–9(c) show the simulated parametric study results of the fractional bandwidth, and the maximum reflection coefficient. Results are obtained while varying the parameters in the ranges 4 mm ≤

*l*

_{1}≤ 24 mm, 0 mm ≤

*d*

_{2}≤ 10 mm, and 30 mm ≤

*w*

_{1}≤ 50 mm. Parametric studies indicate that the proposed antenna can achieve an over 80% fractional bandwidth with a Γ

_{max}of below −11.4 dB when using optimized parameters (dashed lines).

### IV. Conclusion

*zx*- and

*zy*-planes, respectively, at 3.5 GHz, and those at 4.5 GHz were 154° and 94°, respectively. To analyze the proposed antenna, an equivalent circuit was modeled, taking into account important antenna parts. The equivalent circuit results confirmed that the parallel transmission line of the proposed antenna worked as a quarter-wave transformer to achieve wideband characteristics. In addition, through parametric studies, the proposed antenna achieved over 80% stable fractional bandwidth with a Γ

_{max}of below −11.4 dB. For future work, additional or different designs will further be considered, such as a tapered transmission line and a cavity-backed ground to overcome the pattern distortion in the high-end frequency band.