Multi-Band Frequency Reconfigurable Planar Inverted-F Antenna Designs

Jing Liang, H. Y. David Yang

Department of Electrical and Computer Engineering, University of Illinois at Chicago, Chicago, IL, 60607, USA, Email: jliang6@uic.edu, hyang@uic.edu.

Abstract In this paper, novel multi-band frequency reconfigurable Planar Inverted-F Antennas (PIFA) are developed. The antennas are fabricated on a flexible polyimide tape, with conductive copper layers plated on one or both sides. This feature enables the antenna radiation traces conformably attached to a 3D polycarbonate carrier or directly on the mobile terminal chassis. The tuning varactor diodes are also assembled on the flexible-antenna radiator directly, and controlled by external DC voltages. Thus, in principle, the currents on the radiation traces can be changed by the varactor junction capacitance, and the resonant frequency of the antenna can be continuously swept over a wide frequency range. In the presentation, detailed antenna geometry, tuning varactor and DC control configuration, and antenna radiation performances will be discussed.

1. Introduction Recently, the urgent need of device miniaturization and multi-band multi-standard mobile terminals challenge the design of an efficient radiator that is small but with high performance over a wide frequency range. Reconfigurable antennas [1] that have the capability to tune their operation frequency-bands dynamically responding to external control hence attract considerable research efforts in EM community the last few years. In this paper, novel designs of frequency reconfigurable Planar Inverted-F Antenna (PIFA) [2] with varactor loading have been developed, both for single-band and double-band applications.

2. Single-Band Reconfigurable PIFA

The geometry and dimension parameters of the single-band reconfigurable PIFA is depicted in Fig. 1. As shown in the picture, the radiation body of this antenna is lifted above the main PCB. Practically, the flexible printed layout tape of an antenna is attached to a foam carrier with its electrical permittivity close to air. The radiator of the antenna is a flat planar copper trace on the flex-polyimide tape attached to the carrier, with a slot cut off in the middle. The tuning varactor is mounted over the slot providing a variable series capacitance loading to the radiator. There are three vertical copper pins in the structure, which are wrapped 90 degree and vertically extended to the bottom ground plane on the main PCB. The one at the end of the radiation trace is connected to the ground plane as a short; the other in-between is connected to a feed pad for the RF signal input; the last one adjacent to the varactor is connected to a low-pass-filter (LPF)-type bias circuit as DC control

Figure 1: (a) Prospective view, (b) top view and (c) front view of single frequency-band reconfigurable PIFA.

Tboard

Varactor

Tant

Feed Pin

Shorting PinDC Bias Pin

Wboard

Lboard

Want

Lant

Lvar

Varactor

Radiator

Varactor

DC Bias

x Y

Z

Feed Pin

Shorting Pin

(a)

(b)

(c)

Figure 2: Return losses of single frequency-band reconfigurable PIFA with LPF-type DC bias.

1500 1700 1900 2100 2300 2500−40

−35

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Cvar=0.5 pFCvar=1.5 pFCvar=2.5 pFCvar=3.5 pF

voltage to the antenna. An example of the design is for frequency-sweeping in 2 GHz range, the geometries as shown in Fig. 1(b) and 1(c) are with Lant=40mm, Want=5mm, and Lvar=10mm. The height of antenna to main PCB (Tant) is 5mm. The main PCB is a standard 62mil (1.58mm) thick FR4 board with a single copper layer at bottom as a ground plane. The dimension of the main PCB is chosen as Lboard=80mm and Wboard=50mm, similar to general cell phone size. The width of shorting vertical pin is 2mm, and the width of other two feed pins is 1mm. The distance between the shorting and RF feed pins is 2mm, and the distance between the DC bias pin and the varactor slot edge is 1mm.

The antenna performances are studied by the full-wave EM simulations with HFSS [3] firstly, and the measurement results will be given in the presentation. The RF-port return losses are provided in Fig. 2. It shows that with the varactor junction capacitance varying from 0.5pF to 3.5pF, the antenna's resonant frequency sweeps from 2.15GHz to 1.60GHz, with 29.3% dynamic tunable range. Also, the antenna instant bandwidths from the simulations are above 5% throughout the dynamic range. The detailed antenna radiation parameters within the dynamic bandwidth are given in Table 1.

In additions, the antenna gain radiation patterns are provided in Fig. 3. For simplicity, with different varactor-capacitance loadings, only the patterns at the antenna resonant frequency (listed in Table 1) are shown. Observed from the pattern shapes, the antenna maximum

radiation is found almost along the x-axis, which is normal to the main PCB; and there also exist two nulls toward the main PCB directions. This is due to the ground plane reflection from the main PCB, and is favorable for cell-phone applications to reduce the EM emission toward user’s head.

Loading Cap. [pF]

Res. Freq [GHz]

Instant BW [%]

Max Gain[dBi]

0.5 2.15 6.5 4.67 1.5 1.77 5.1 3.68 2.5 1.66 5.4 3.39 3.5 1.60 6.3 3.35

Table 1: Single frequency-band reconfigurable PIFA radiation parameters.

Figure 3. Radiation gain patterns for single frequency-band reconfigurable PIFA with LPF-type DC bias: (a) on XZ plane, (b) on YZ plane, and (c) on XY plane.

Cvar

=0.5 pF

Cvar

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Cvar

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Cvar

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Cvar

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Cvar

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Cvar

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Cvar

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Cvar

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Cvar

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Cvar

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Cvar

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(a) (b) (c)

3. Dual-Band Reconfigurable PIFA

The designs of a dual frequency-band reconfigurable PIFA antenna with two varactors loading are also developed and presented in this section. The antenna structure is similar to that of the single-band reconfigurable PIFA as described in the previous section. Its antenna radiation traces are printed on the flexible polyimide tapes, and adhered to the carrier forming a 3D geometry. The high and low-band resonances are generated by the current flows along two arms of the radiator. Hence, in the dual-band reconfigurable design, two slots are cut out from the radiator along those two arms, and two varactors are separately soldered cross the slots. By changing the varactor diode bias DC voltage individually, the high-band and low-band of the antenna can be swept in 1GHz and 2GHz range, respectively. Detailed layouts of the antenna are illustrated in Fig. 4. It is shown that from the

top view in Fig. 4(b), the shorter arm controls the high-band resonance, and is called ``short-arm'' for brevity; also, the longer bended arm controls the low-band resonance, and is called ``long-arm'' for brevity. The geometric parameters of the antenna shown in the picture are as Llant=30mm, Lhant=38mm, Want=5mm, and Lhvar=12mm. The other parameters of this antenna is the same as that of the single band reconfigurable PIFA example, with Tant=5mm, Lboard=80mm, Wboard=50mm, and Tboard=1.58mm. To evaluate the two band reconfigurability, the HFSS full-wave EM simulations have been conducted. The results of the RF-port return loss for varying the “long-arm” and “short-arm” varactor capacitance is shown in Fig. 5(a) and 5(b), respectively. From the simulation results, it can be observed that while varying the “long-arm”' varactor junction capacitance CLVAR from 0.5pF to 3.5pF, and keeping the “short-arm” varactor bias unchanged as 0.5pF, only the low-band of the antenna are swept. The dynamic range for the low-bands in this case is from 1.26GHz to 0.89GHz, or around 34.4%, while the variation at high-bands is from 2.16GHz to 2.01GHz, or around 7.2%. In other case, while varying the “short-arm” varactor junction capacitance CHVAR from 0.5pF to 3.5pF, and keeping the “long-arm” varactor bias unchanged as 0.5pF, it turns out that the high-band can be swept. The dynamic range for this case at the high-band is from 2.16GHz to 1.71GHz (23.2%), and the variation at the low-band is from 1.27GHz to 1.25GHz (1.6%). These results prove that with this configuration, individual high-band or low-band frequency reconfigurations are obtained, while keeping the other band almost unchanged.

Figure 5: Return losses of dual frequency-band reconfigurable PIFA for (a) low-band tuning and (b) high-band tuning.

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CLVAR

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CLVAR

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CLVAR

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CLVAR

=3.6 pF

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Wboard

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oa

rd

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nt

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nt

Figure 4: (a) Prospective view, (b) top view of dual frequency-band reconfigurable PIFA.

(a) (b)

Radiator

Varactors

DC Biases

x Y

Z

Feed Pin

Shorting Pin

The antenna gain radiation patterns of the dual-band reconfigurable PIFA are provided in Fig. 6, for the low-band or high-band tuning cases. Since there is a RF ground plane on the main PCB underneath the radiator, the antenna patterns at high-band are similar to the single varactor-loaded PIFA example, with maximum radiation along the x-axis perpendicular to main PCB, and nulls towards the main PCB. At the low-band, thepattern shapes are more similar to a tilted donuts, with nulls around z-axis. It is because when frequency decreases, the electrical wavelength becomes longer, and the antenna radiation body together with the relative small RF ground plane forms a dipole-like structure, and generates donut-like radiation patterns.

4. Conclusion

In summary, in this paper, varactor-loaded frequency reconfigurable flexible PIFA have been designed, both for single-band and dual-band applications. By changing the varactor’s junction capacitance, the operation frequency of antennas can be tuned over a wide range of spectrum. With the aid of LPF-type bias circuit, good isolations between antenna and DC control source are achieved. Detailed antenna geometries, tuning varactor specification, DC bias circuit configuration and prototype radiation measurements will be presented in the conference.

5. References

1. A. C. K. Mak, C. R. Rowell, R. D. Murch, and Chi-Lun Mak, “Reconfigurable Multiband Antenna Designs for Wireless Communication Devices,” IEEE Transactions on Antennas and Propagation, vol.55, no.7, pp.1919-1928, 2007. 2. C. Rowell and R. Murch, “A capacitively loaded PIFA for compact mobile telephone handsets,” IEEE Transactions on Antennas and Propagation, vol. 45, no. 5, pp. 837-842, 1997. 3. Ansoft Corporation, High Frequency Structure Simulator (HFSS), Pittsburgh, USA.

CHVAR

=0.5 pF

CHVAR

=1.5 pF

CHVAR

=2.5 pF

CHVAR

=3.5 pF

CLVAR

=0.5 pF

CLVAR

=1.5 pF

CLVAR

=2.5 pF

CLVAR

=3.5 pF

CLVAR

=0.5 pF

CLVAR

=1.5 pF

CLVAR

=2.5 pF

CLVAR

=3.5 pF

CLVAR

=0.5 pF

CLVAR

=1.5 pF

CLVAR

=2.5 pF

CLVAR

=3.5 pF

CHVAR

=0.5 pF

CHVAR

=1.5 pF

CHVAR

=2.5 pF

CHVAR

=3.5 pF

CHVAR

=0.5 pF

CHVAR

=1.5 pF

CHVAR

=2.5 pF

CHVAR

=3.5 pF

(a) (b) (c)

(e) (f) (d) Figure 6: Radiation gain patterns for dual frequency-band reconfigurable PIFA: (a) on XZ plane, (b) on YZ plane, and (c) on XY plane at low-band; and (d) on XZ plane, (e) on YZ plane, and (f) on XY plane at high-band.