Ceramic-polytetrafluoroethylene composite material-based miniaturized split-ring patch antenna

1 Introduction

Antenna miniaturization has been an interesting research topic for the past decades [1]. The basic approach to miniaturization of antenna element was addressed by Chu in 1948 [2] and Harrington in 1960 [3]. So far, antenna miniaturization and its limitations were addressed by Chu in 1948 [2]. Several techniques have been studied for antenna miniaturization, such as using an artificial magnetic conductor [4], electromagnetic band gap structure [5], reactive impedance surface [6], metamaterial [7], multilayer substrate [8], stacked patch [9], and fractal shape [10]. Conventionally, miniaturized antennas suffer from narrow bandwidth, low gain, and low efficiency [3]. As a result, antenna miniaturization with multiband capabilities, wider bandwidth, and higher gain and efficiency has become necessary to be integrated with modern microwave circuitry [11–13].

Polytetrafluoroethylene (PTFE) was a lucky accidental invention by Dr. Roy Plunkett in 1938 [14]. Polymerized materials for microwave antennas were extensively investigated in 2005 [15]. The dielectric characteristics of ceramic-PTFE (nF₂C=CF₂→{F₂C—CF₂}) allows for high-frequency modulation with the influence of an electric field bias operating perpendicular to the signal propagation direction [16]. Consequently, this property of the ceramic-PTFE provides a significant advantage to develop high-frequency microwave and millimeter-range devices [17–19]. Recently developed ceramic-polymerized composite materials have low loss and high dielectric properties, allowing their use for antenna miniaturization. The high dielectric constant can be obtained and characterized from the ceramic mixture, thus reducing the size of the radiating patch significantly. It can be interpreted by using the following equation [Eq. (1)] of patch antenna miniaturization.

The effective wavelength λ is determined by the permittivity of the material ε_r, and λ₀ is the wavelength in a vacuum. However, there is a trade-off between the size of the radiating patch and the gain of the antenna that can be understood from Eq. (2).

The length is denoted by l_e and resistance is denoted by R_r in Eq. (2). The gain will be reduced by decreasing the length of the patch and increasing the wavelength and resistance. However, to improve the gain and antenna compactness, the design parameters need to be optimized. Split-ring patch antenna on high-permittivity dielectric material substrate is a promising candidate for compactness with better performance. Many researchers have studied and investigated the split-ring resonator (SRR) antennas [20–22]. However, the reported antennas are either large in size or have a narrow bandwidth and low gain and efficiency.

We present a planar compact SRR antenna on high-dielectric ceramic-filled PTFE composite material substrate for C band WLAN and X band satellite applications in this paper. The measurement results of the proposed antenna exhibit an impedance bandwidth of 1.5 GHz, 500 MHz, and 300 MHz and range from 5 to 6.5, 9.1 to 9.6, and 10.7 to 11 GHz, respectively. It also achieved 2.13, 3.59, and 3.54 dBi average gains at first, second, and third band, respectively. The simulated radiation efficiencies of 87.3%, 88.5%, and 93.1% were observed at three resonant frequencies, 5.6, 9.5, and 10.9 GHz, respectively. The radiation patterns at three resonant frequencies are also measured. The surface current distribution and the effect of the dielectric properties of substrate materials are analyzed.

2 Antenna design and configuration

The proposed antenna was designed and analyzed using a finite element-based full-wave high-frequency electromagnetic simulator HFSS v.13. The proposed antenna was designed with compact dimensions of 12 mm×16 mm on 1.9-mm-thick ceramic-PTFE composite material substrate. PTFE is an artificial fluoropolymer from tetrafluoroethylene; the compound and its molecular structure are shown in Figure 1 [16]. The relative permittivity of PTFE alone is 2.1. By combining ceramic with PTFE, the relative permittivity increased to 10.2 [23]. The permittivity of the material can be modified by adjusting the composition ratio. Due to the low water absorption and thermal sensitivity, ceramic-filled PTFE is suitable for external use.

Figure 1

The molecular compound and ball-stick model of PTFE.

The design procedure starts with the determination of the size of the radiating ring and the split part of the ring. The basic concept of the patch size was taken from the widely used mathematical formulation for patch antenna design [24]. However, the available mathematical equations are based on the rectangular patch antenna. For split-ring antenna, the size of the ring and split part is optimized using HFSS Optimetrics (HFSS v.13, Ansys Corp., Canonsburg, PA, USA). The design of the microstrip-line-fed split-ring radiating patch element of the proposed antenna is composed of the configuration of its dimensions. The basic idea of the patch dimension was taken from an established mathematical formulation [25]. Although the available mathematical modeling is based on the conventional rectangular patch for the proposed split-ring radiating patch, the dimension is determined using a test and modify method.

In Eqs. (3) and (4), W is the width and L is the length of the patch, f_o is the center resonant frequency of the fundamental band, and c is the speed of light in vacuum. The effective dielectric constant can be calculated by using Eq. (5) [9]:

where ε_r is the relative dielectric constant and h is the thickness of the substrate. Due to the fringing field around the periphery of the patch, the antenna electrically looks larger than its physical dimensions. The increment to the length Δl due to the fringing field can be expressed as [26]:

The patch width (W) has a minor effect on the resonance and has been determined by using mathematical modeling [24]. The length of the radiating patch (L), location of the feeding point, and the slots have a dominant effect on the resonance. The length of the patch (L) is 0.29λ and the width (W) is 0.22λ, where λ is the corresponding wavelength at the resonance of 5.6 GHz. The antenna design geometry and configuration are shown in Figure 2. The ring width is optimized for 4.5 mm with 2.5-mm-long split part.

Figure 2

Design layout of the proposed antenna.

A 5-mm-long, 2-mm-wide microstrip line was used to connect the radiating patch and the ground plane at the bottom that is fed by a standard 50 Ω coaxial probe at the center of the X axis along the Y axis. Generally, a high-permittivity material-based patch antenna suffers from narrow bandwidth [27]. To overcome this limitation, the bandwidth of the proposed antenna is improved by reducing the ground plane length. Reduction of ground plane length increases the cavity between the radiating patch and the ground plane, resulting in improvement of the impedance bandwidth. Figure 3 shows the effect of ground plane length on the reflection coefficient of the proposed antenna. It can be clearly seen that a 5-mm-long ground plane provides the widest bandwidth compared to 10 and 16 mm (substrate size). The proposed antenna with 10 and 16 mm exhibits a lower reflection coefficient value and narrower bandwidth. The effect of the dielectric property of the three substrate materials on the reflection coefficient was examined for the proposed antenna and is shown in Figure 4. It has been observed that an antenna with low-permittivity material substrate provides wider bandwidth, but the resonance is shifted to higher frequency that is out of the desired band and higher reflection coefficient value. In addition, the ceramic-filled bioplastic material-based antenna has a lower reflection coefficient value but provides a narrower bandwidth compared to the proposed ceramic-PTFE-based antenna. The overall size of the proposed antenna on 1.9-mm-thick ceramic-PTFE material (ε_r=10.2) substrate is reduced by 60% compared to the conventional FR4-based antenna.

Figure 3

Effect of ground plane length on reflection coefficient of the proposed antenna.

Figure 4

Effect of dielectric property of substrate materials on reflection coefficient.

3 Performance results

The proposed antenna was fabricated on double-sided copper-imprinted ceramic-PTFE board using an LPKF printed circuit board prototyping machine. The performance of the fabricated prototype of the proposed antenna was measured in a standard 5.5 m×4.5 m×3.5 m high far-field anechoic measurement chamber. A double-ridge guide horn antenna from AH Systems Inc. (model no. SAS-571) was used as a reference antenna. Pyramid-shaped electrically thick foam absorbers with <-60 dB reflectivity at normal incidence was used on the walls, ceiling, and floor. A turntable of 1.2-m diameter was used to rotate the measuring antenna with specifications of 1 rpm rotation speed, 360° rotation angle, and connected with a 10-m cable between controllers. An Agilent E8362C vector network analyzer with a range of up to 20 GHz was used for the measurements.

Simulated and measured reflection coefficients of the proposed antenna are shown in Figure 5. There is good agreement between simulation and measurement results. The measured reflection coefficient vs. frequency graph shows that 1.5 GHz range from 5.0 to 6.5 GHz, 500 MHz range from 9.1 to 9.6 GHz, and 300 MHz range from 10.7 to 11 GHz, respectively, were achieved. A slight inconsistency occurred between the simulated and measured reflection coefficient results, which can be due to the SMA soldering effect or losses from the cable and connector between the antenna and the controller, even though the complete setup was calibrated before measurement using the Agilent autocalibration kit. The achieved gain of the proposed antenna is shown in Figure 6. It can be clearly observed that maximum gains of 2.13, 3.59, and 3.54 dBi were achieved at the first, second, and third band, respectively. One of the most important parameters for antenna design is radiation efficiency. Figure 7 shows the simulated radiation efficiencies of the proposed antenna. The radiation efficiencies of 87.3%, 88.5% and 93.1% have been observed from the proposed antenna at the three bands, respectively. Radiation efficiencies of the proposed antenna at the three resonant frequencies are more than 80%, which means that more than 80% of the signal can be transmitted. Therefore, minimum power is required for data transmission. The measured E and H plane radiation patterns of the proposed antenna at three resonant frequencies are shown in Figure 8. From the radiation pattern it can be easily seen that the cross polarization effect of the proposed antenna is lower than the co-polarization effect at all resonant frequencies. The simulated surface current distribution of the proposed antenna at three resonant frequencies is shown in Figure 9. It can be observed that the intensity of the distributed current is stronger in the upper band compared to the lower band. At the lower band, the strongest current flows in the right side near the feed line and lower part of the ring, whereas at the upper band, the highest density of current flows in the feed line and upper side and near the split part of the ring. A comparison between the proposed and some existing similar types of antennas are tabulated in Table 1. From the comparison it can be observed that the proposed antenna performs better with smaller dimension than other reported antennas in terms of bandwidth and gain. Although some of the antennas offer higher gain or wider bandwidth, overall size is compromised.

Figure 5

Simulated and measured reflection coefficient of the proposed antenna.

Figure 6

Achieved gain of the proposed antenna.

Figure 7

Simulated radiation efficiency of the proposed antenna

Figure 8

Measured radiation pattern at (A) 5.6 GHz, (B) 9.5 GHz, and (C) 10.9 GHz.

Figure 9

Simulated surface current distribution of the proposed antenna at (A) 5.6 GHz, (B) 9.5 GHz, and (C) 10.9 GHz.

4 Conclusion

A high-permittivity ceramic-filled PTFE composite material substrate-based miniature split-ring antenna with a compact dimension of 0.2λ×0.3λ×0.03λ is proposed. Measurement results of the proposed antenna exhibit the operating bandwidths of 26.76% (5.0–6.3 GHz), 5.3% (9.1–9.6 GHz), and 3.6% (10.7–11 GHz) as observed from the antenna prototype. Average gains of 0.69, 3.52, and 3.54 dBi were achieved at the first, second, and third band, respectively. The simulated radiation efficiencies of 87.3%, 88.5%, and 93.13% were observed at three resonant frequencies of 5.6, 9.5, and 10.9 GHz, respectively. The effects of the dielectric properties and ground plane lengths on the reflection coefficients of the proposed antenna were analyzed. Moreover, E and H plane radiation patterns of the antenna prototype were also measured. Finally, surface current distribution at three resonant frequencies was also studied.