The rapid development of wireless mobile communication and the system integration with shrinking the mobile handset continuously demand compact and built-in antennas. However, conventional very high frequency (VHF, 30–300 MHz) antennas have large physical dimensions, and therefore, are not suitable for mobile applications. Reducing their physical dimensions without affecting their electrical performance has been challenging up till now Kong et al. 2008.
Magnetodielectric materials are expected to play an important role in the miniaturization of antennas. Theoretically, if loaded with magnetodielectric materials, with miniaturization factor, n = (μr′∊r′)1/2, larger than 1 (miniaturization factor of free space), an antenna's physical dimensions can be reduced by a factor of n while its electrical dimension stays unchanged Mosallaei and Sarabandi 2004, Buell et al. 2006. For practical applications, the magnetodielectric materials should have matching ∊r′ and μr′, so that they will possess a characteristic impedance close to that of free space. As a result, no reflections take place between the antenna substrate and the surrounding free space, thereby reducing the energy trapped in the substrate Kong et al. 2008, Petrov et al. 2008. Another requirement is that the materials must have sufficiently low dielectric and magnetic loss tangent to ensure good performance of antennas made of them. Recently, some studies have shown that ferrite materials are potential candidates of the magnetodielectric materials for antenna miniaturization Kong et al. 2007, Kong et al. 2008, Petrov et al. 2008, but the performance of magnetodielectric antenna is not satisfied due to large magnetic loss tangent in the range of 100–200 MHz.
In this letter, the synthesis and use of a low–loss Ni–Mn–Co ferrite for antenna miniaturization and performance improvement are reported. We also present a compact helical antenna using the ferrite material for mobile handset applications covering terrestrial digital multimedia broadcasting (T-DMB, 174–216 MHz) band. Samples with ∊r ′ ≈ μr′ ≈ 7–9, and dielectric and magnetic loss tangent less than 0.001 and 0.01 below 200 MHz, respectively, were used as substrates for the antenna. Details of these studies are provided in the following sections.
CHARACTERISTICS OF Ni–Mn–Co FERRITE
The Ni–Mn–Co ferrite (Ni0.76Mn0.24−xCoxFe2O4 with x = 0–0.04) was made by mixing the oxides, followed by ball milling, calcination, and sintering at 1050 °C–1250 °C. The sintered powders were analyzed using X-ray diffractometer (XRD) with Cu Kα radiation. XRD data showed single phase of spinel ferrite. The magnetic hysteresis loops were measured with vibrating sample magnetometer (VSM), as shown in Fig. 1. The saturation magnetization (Ms) is 54.95 emu/g, and the coercivity (Hc) is 45.86 Oe.
Fig. 1. Hysteresis loop of the Ni–Mn–Co ferrite.
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Fig. 2. Magnetodielectric properties of the Ni–Mn–Co ferrite. (a) Real permittivity and permeability. (b) Dielectric and magnetic loss tangent. (c) Normalized impedance and miniaturization factor.
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Two types of green samples, disk (diameter of ∼20 mm and thickness of ∼2 mm) and coaxial cylinder (outer diameter of ∼20 mm, inner diameter of ∼10 mm, and thickness of ∼2 mm), were prepared. Disk samples were used for measurement of permittivity, while cylinder samples were used for measurement of permeability. The samples were sintered at 1100 °C for 6 h and resintered at 1150 °C for 3 h. Complex permeability and permittivity of the ferrite were measured using an Agilent E4991A RF impedance/material analyzer with the 16453A (permeability) and 16454A (permittivity) test fixtures, respectively. Typical magnetodielectric properties of the Ni–Mn–Co ferrite are presented in Fig. 2. The real permittivity and permeability are 7–9, and the dielectric and magnetic loss tangent is lower than 0.001 and 0.01 below 200 MHz, respectively. At higher frequencies above 300 MHz, however, one notices a rapid decrease in permeability and an increase in magnetic loss tangent. These permeability and magnetic loss data show desirable characteristics for use as antenna's substrates covering T-DMB band.
Fig. 3. (a) Geometry of the miniaturized T-DMB antenna using ferrite material. (b) Structure and detailed dimension of the helical radiating element.
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Fig. 5. Measured radiation patterns at 195 MHz for the miniaturized T-DMB antenna. (a) In x–y plane. (b) In x–z plane. (c) In y–z plane.
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Fig. 6. Measured maximum gain for the miniaturized T-DMB antenna.
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Another essential magnetodielectric characteristics on normalized impedance by intrinsic impedance of free space and miniaturization factor are also shown in Fig. 2(c). Normalized impedance (μr′/∊r′)1/2 is almost 1, and miniaturization factor (μr′∊r′)1/2 is about 8 at 200 MHz. Therefore, the ferrite material can miniaturize antenna by a factor of about 8. In this case, the problem of strong field confinement is minimized, and the ferrite substrate is far less capacitive when compared to the dielectric-only high permittivity materials. More importantly, since the characteristic impedance of the ferrite substrate is close to the impedance of free space, it allows for better impedance matching Kong et al. 2008.
ANTENNA DESIGN AND CHARACTERIZATION
The configuration of the miniaturized T-DMB antenna employing the Ni–Mn–Co ferrite is shown in Fig. 3. The antenna was fabricated on three-layered ferrite substrate where each layer has the dimension of 30 mm × 5 mm × 1.5 mm. The helical radiating element is constructed on middle layer of the ferrite substrate, and enclosed by upper and lower ferrite substrates. The helical radiator wound on the ferrite substrate is copper strip with the width and gap of 1 mm and the total length of about 195 mm. The physical size of the antenna is 30 mm × 5 mm × 4.5 mm and the printed circuit board (PCB) for mounting the antenna is FR4 substrate with the size of 50 mm × 110 mm × 0.8 mm. The antenna is placed 5 mm away from the PCB ground plane having a size of 50 mm × 100 mm. Fig. 4 shows the measured return loss of the fabricated antenna. The measured results were obtained using Agilent E5071B vector network analyzer. The measured resonance frequency, impedance bandwidth (3 dB return loss), and peak absorption are about 192 MHz, 25 MHz, and 30 dB, respectively. The measured voltage standing wave ratio (VSWR) at resonance is close to 1.3, and is indicative of good impedance matching with free space and the absence of reflection at the boundary between the substrate of the antenna and the surrounding medium, thereby reducing the energy absorbed in the substrate Petrov et al. 2008.
The measured radiation patterns at 195 MHz of the fabricated antenna are plotted and compared according to the cutting plane in Fig. 5. The radiation patterns at other frequencies over the T-DMB operating band are also measured, and similar measured patterns are seen over the operating band. The radiation patterns in x–y plane are nearly omnidirectional characteristics. The maximum gain and radiation efficiency were measured in an anechoic chamber. The measured maximum gain and radiation efficiency at 195 MHz are −0.4 dBi and 34.1%, respectively. The measured maximum gain according to the T-DMB operating band is varied from about −10.3 to −0.4 dBi, as shown in Fig. 6. Thus, it means that the overall shape of the radiation patterns and the features of antenna electrical performance are suitable for mobile handset applications.
The antennas based on the helical radiating element with multiple- and single-layered ferrite substrates are also studied. Table 1 shows comparison on antenna size, number of layer, and electrical performance for multiple- and single-layered antennas employing the Ni–Mn–Co ferrite. Since the helical radiator enclosed by ferrite substrates has better impedance matching, the proposed three-layered ferrite antenna shows superior performance to single-layered antenna.
Table 1 Comparison on size, number of layer, and electrical performance for multiple- and single-layered antennas using the Ni–Mn–Co Ferrite.
Antenna miniaturization and application based on a low–loss Ni–Mn–Co ferrite (Ni0.76Mn0.24−xCoxFe2O4 with x = 0–0.04) have been presented. The developed ferrite material has promising magnetodielectric properties with permittivity and permeability of 7–9, and sufficiently low dielectric and magnetic loss tangent less than 0.001 and 0.01 below 200 MHz, respectively. A miniaturized T-DMB antenna was fabricated on three-layered ferrite substrate having a total volume of 30 × 5 × 4.5 mm3. The antenna has compact size, low loss, wide bandwidth, and good radiation characteristics. We expect that this developed ferrite material will be useful for miniaturization and performance improvement of mobile handset antennas.
This work was supported by the ATC program of MKE/KEIT. (10031490, Design of a compact antenna using magnetic and dielectric composite materials for mobile handset applications).