II. ANTENNA DESIGN AND GEOMETRY

The configuration of proposed antenna is illustrated in Fig. 1. It mainly consists of two substrates and a cross-connected shorted strip over the circular patch. Both substrates have a relative permittivity of 2.65, a thickness of 1.0 mm and a substrate loss tangent of 0.002. The distance between them is H. The radius of the circular patch and the ground are R _p and R _g , respectively.

Fig. 1.

Geometry of the proposed antenna: (a) over view, (b) bottom view of power divider and top substrate, (c) cross-connected shorted conducting strip.

The diameter of bottom substrate is 126 mm, and the height of proposed antenna is 41 mm, as shown in Fig. 1(a). The antenna can be used for GSM1800, WCDMA, and TD-LTE systems. The proposed antenna is fed by a 50 Ω coaxial cables with connector, and can be mounted on the rooftop for the indoor scenario.

One side of top substrate consists of four arc conducting strips with the angle of α and the width of W _s , which are rotation oriented along z-axis over the circular patch. The other side of the substrate is printed with a circular patch with the radius R _p . A broadband four-way power divider is printed on one side of bottom substrate, which can provide the electromagnetic wave with uniform amplitude and 0 phase shift. The divider consists of four stepped impedance transformers, shown in Fig. 1(b). Four conducting posts are used to connect conducting strips and four-way power divider. Thus four capacitive feeds can couple the electromagnetic energy from the source to the circular radiating patch.

The detailed sizes of top shorted strip are shown in Fig. 1(c). Because the top strip is shorted to the ground with four legs, the inductive loading is introduced. The capacitive coupling is formed by arc conducting strips and circular patch, so the working frequency of proposed antenna is lowered. The simulated reflection coefficient of the proposed antenna and the antenna without top shorted strip loading are compared in Fig. 2. It can be clearly seen that the proposed antenna has a wide-band operation characteristic from 1.44 to ~2.78 GHz for S ₁₁<−10 dB, while the antenna without top shorted strip loading has a narrow operation band with the reflection coefficient of only −8.6 dB. Without top shorted strip, the resonant frequency of proposed antenna is determined by the circular patch. If the radius of the patch decreases, the resonant frequency increases. However, the input impedance for the circular patch itself is high because of strong electric filed and weak magnetic field at the center of the patch. It can be observed that there is a shift of resonant frequency when the antenna is loaded with the shorted strip, shown in Fig. 2. This is due to the coupling between the patch and the shorted strip. Because of coupling, the quality factor Q of the antenna decreases, which results in a wide bandwidth.

Fig. 2.

Simulated and measured reflection coefficients of proposed antenna at different cases.

Electromagnetic wave from the input port is split into four parts with equal power and in phase by the four-way power divider. Then electromagnetic wave is coupled to the radiating patch, and equivalent magnetic current distribution along the circular patch can be formed. Figure 3 shows the magnetic field and surface current distributions along the circular patch of proposed design at 2.0 GHz. It can be seen that the magnetic field propagates along the φ-direction, which is similar with that of a circular patch operating in TM01 mode. The surface current along the radial direction is divided into four parts, which are located at the place of capacitive feeds. The distributions of magnetic field and surface current gives rise to omnidirectional radiation at horizontal plane.

Fig. 3.

Magnetic fields and surface current distribution on the circular patch. (a) Magnetic fields, (b) surface current.

III. PARAMETRIC STUDY

To analyze the design mechanism of the proposed antenna, the key parameters study for input impedance has been carried out using commercial full-wave electromagnetic software High Frequency Structure Simulator (HFSS). There are three key parameters for designing the antenna including radius of circular patch (R _p ), the height of long conducting post (H), and the height of top shorted strip (H _s ). All other parameters remain unchanged unless specified.

Figure 4 shows the effect of different structure parameters on the simulated input impedance Z _in of the proposed antenna. It can be seen that when the radius of circular patch varies from 25 to 40 mm, the real part of Z _in changes slightly and its peak value changes from 38 to 180 Ω, while the imaginary part of Z _in gradually decreased from 15 to −2 Ω at 2.0 GHz. The proposed antenna has a better impedance matching at R _p  = 35 mm for its imaginary part of Z _in is nearby zero. Figure 4(b) shows the effect of H _s on the performance of antenna. It should be noticed that H _s has a strong effect on the first resonant frequency, and a slight effect on the second resonant frequency. When the value of H _s varies from 30 to 45 mm, the real part and imaginary part of Z _in change greatly at the first resonant frequency, and they remain the same at the second resonant frequency. The parameter of H has a great influence on the real part of Z _in and the first resonant frequency, as shown in Fig. 4(c). When the value of H varies from 11 to 14 mm, the real part of Z _in decreases and the imaginary part of Z _in changes largely at the first resonant frequency, while it keeps the same at the second resonant frequency.

Fig. 4.

Simulated input impedance Z versus frequency with different values of structural parameters. (a) R _p , (b) H _s , (c) H.

IV. EXPERIMENTAL RESULTS AND DISCUSSION

According to the above discussion and analysis, the geometric parameters of the wideband circular patch antenna are summarized in Table 1. The unit of all parameters is mm, except that the unit of α is deg. A prototype of proposed antenna is simulated, fabricated, and measured. Figure 5 shows the photograph of the fabricated antenna and the bottom of top substrate. A wideband four-way power divider is printed on the bottom substrate, and the cross-connected strip are located over the top substrate.

Fig. 5.

Photograph of fabricated antenna and top substrate.

Table 1.

Geometric parameters of proposed antenna.

The simulated and measured reflection coefficients of the proposed antenna are shown in Fig. 2. The measured impedance bandwidth for S ₁₁<−10 dB is 65.4% from 1.46 to 2.88 GHz, which covers the GSM1800, WCDMA, CDMA2000, and TD-LTE systems for indoor wireless communication system. The difference between the simulated and measured S ₁₁ is mainly due to the errors of manufacturing and assembling.

The simulated and measured radiation patterns of the proposed design are shown in Fig. 6. Radiation patterns in the azimuth plane with the unroundness less than 3.0 dB are observed at the frequencies of 1.5, 2.0, and 2.5 GHz. The omnidirectional property attributes to the in-phase feed and magnetic field distribution on the circular patch. It can be clearly seen that there is a deep null at θ = 0 direction for the elevation radiation patterns in XOZ-plane. The directions of maximum radiation are tilted several degrees from the horizon in the elevation plane, which arises due to the circular ground on the back of the bottom substrate. It also can be found that the cross-polarization level is lower than 15 dB over the wide frequency band.

Fig. 6.

Simulated and measured radiation patterns of proposed antenna at different frequencies. (a) 1.5 GHz, (b) 2.0 GHz, (c) 2.5 GHz.

Figure 7 shows the simulated and measured gains of the proposed antenna. It can be seen that the gain varies from 0.9 to 5.0 dBi in the frequency range of 1.46–2.88 GHz. It can be noticed that good agreement with a difference of less than 1.0 dB is achieved between the simulated and measured results. There are some differences between the simulated and measured models. For example, connectors and cables are not included in the simulation model. Meanwhile, some undesired effects in the measurement setup are attributed to the differences between simulated and measured results.

Fig. 7.

Comparison of simulated and measured gains of proposed antenna.