A Triband Planar Inverted-F Antenna with Quadratic Koch Fractal Shaped Slit Along with a Shorted Parasitic Strip

In this paper, a novel compact tri-band planar inverted-F antenna (PIFA) for mobile communication application is proposed. The antenna is capable to cover GSM 900 MHz, DCS 1.8 GHz and WLAN (IEEE 802.11b) 2.45 GHz bands. The proposed PIFA is composed of a quadratic Koch shape slit and a parasitic strip. The PIFA with the fractal shaped slit contributes to the first and second resonance while the shorted strip brings forth the third band. The impedance bandwidths of 84 MHz, 132 MHz and 81 MHz for GSM 900, DCS 1800 and WLAN (IEEE 802.11b) 2450, respectively are achieved. A realized gain of 2.44 dBi, 4.48 dBi and 3.86 dBi is obtained at 0.9 GHz, 1.8 GHz and 2.45 GHz, respectively. The proposed antenna is fabricated and |S11| dB is measured. Reasonable agreement between simulated results as well as measured results is obtained.


Introduction
An indispensable component in a wireless transceiver is an antenna. The shrinking of available antenna placement space has ushered into a new paradigm of transceiver design because of the advent of smart phones, where the designer puts in additional features which are based on different frequency bands. Planar Inverted-F antenna (PIFA) has been a preferred choice of transceiver designers as an internal antenna for mobile handset due to its low profile, small size, built-in structure, ease of fabrication and low manufacturing cost. PIFA antenna structure is a modified form of the conventional quarter wavelength (λ/4) monopole antenna.
Mobile phone internal antenna, which is integrated in the handset, offers certain advantages in comparison to conventional external antennas like a monopole or helix, it is not vulnerable to damages and can offer reduced SAR [1]. There have been a number of PIFA designs that describe different configurations to achieve multiple band operation [2]- [14]. Multiband PIFA was obtained by implementing various shapes and sizes of slot and slit in the radiating patch [2]- [9]. Dual-band PIFAs were introduced with various techniques like, using a capacitive feed and a capacitive load [3], embedding radiating patch with different shapes of slot and slit like U-slot, V-slot, L-slit, Tshaped slit [4]- [9]. In [10], Hilbert curve PIFA geometry was employed for overall size reduction, but matching obtained at higher bands has not been impressive. Adding parasitic patch technique also facilitates additional resonance modes in [11]. Antenna designers also developed the PIFA geometry with more compactness using slots and slits for tri-band application [12]- [14].In [15] another compact structure has been proposed, where a three branch top patch has been used to resonate three frequency bands. A multiband PIFA has been proposed where, instead of top patchgridded ground plane structure with overlapping cells are used to achieve multiband operation [16].
In this paper, a simple and compact PIFA covering three frequency bands: GSM (900 MHz), DCS (1.8 GHz) and WLAN (IEEE 802.11b) 2.45 GHz is proposed invoking a fractal shaped slit on the radiating patch as well as adding a parasitic strip. A quadratic Koch shaped slit is implemented on the radiating patch to get the dual-band resonance frequency as well as to reduce the patch size. The total volume of antenna is 35.65 × 20 × 8 mm 3 . Rest of the paper is described as follows.
Step by step implementation of triband PIFA is given in section 2. This is followed by discussion on results in section 3. Concluding remarks are given in step 4.

Antenna design and parametric studies
The geometry of the proposed antenna is shown in Fig. 1. It illustrates the antenna design that comprises three design steps which results in a PIFA having the ability of radiating at three different bands. The step by step implementation of the PIFA is elucidated in three sub-sections. The radiating part is consiered to be made of copper of thickness 0.25 mm and the ground is realized on a FR-4 substrate of height 1.6 mm, having copper deposit on one side. It has a length of 100 mm, and width of 45mm. The antenna is fed with a 50-Ω SMA connector. For all electromagnetic simulation CST Microwave Studio™ is used.

Initial PIFA design
For the conventional PIFA antenna, the initial radiator dimension is obtained at the desired first resonance frequency of 900 MHz. This is done by using the equation given in (1) [4]. (1) In (1) , f is the resonant frequency, c is the velocity of light in free space, εr is the dielectric constant of the medium in between top patch and ground, here we use air (εr = 1). Lp and Wp are the length and width of the radiating patch. The initial PIFA patch dimension is calculated to be 45 mm in length, and 21 mm in width,with an antenna height of 8mm. The total length of the current path, i.e, the total length (LTotal = Lp+Wp+h) of the radiating patch, which is 74 mm, determines the approximate resonant frequency of the antenna. Where h is the height of antenna. The simulated |S11| dB for the rectangle shaped PIFA indicates a 900 MHz resonance.

Dual-band PIFA with fractal shaped slit
On the designed PIFA, a slit is implemented to introduce a plurarism in antenna current distribution profile that translates into multifrequency resonances. Implementing slit in the radiating patch facilitates multiband operation which includes 1800 MHz (DCS band) frquency band. Fractal geometry has advantages of self similarity, space filling and larger perimeter within the fixed surface area so we choose to implement quadratic Koch fractal shaped slit. A quadratic Koch slit of slit length (Ls) 56 mm and 1.00 mm slit width (s), has been etched and tunned using CST Microwave Studio 2011™ as shown in Figure. 1. Optimization and simulation results in a dual-band PIFA structure having compact volume of 32 × 20 × 8 mm 3 resonating at GSM (900 MHz) and DCS band (1800 MHz).The DCS (1800 MHz) band can be excited by lengthening the resonant path to 42 mm by slit implementation, which is about-one quarter of the wavelength at 1800 MHz. Fig. 2 clearly shows the addition of resonance frequency with the introduction ofthe slit. The |S11| dB variation with frequency for different fractal slit length (Ls) is shown in Fig. 3 and variation of resonance for differnet slit width (s) is shown in Fig. 4

Inclusion of parasitic patch for introducing the third band in PIFA
A parasitic patch is added to finally obtain a tri-band PIFA with resonance frequency at WLAN (IEEE 802.11b) band 2.45 GHz. Parasitic patch of dimension 20 mm × 2.4 mm is added parellel to the radiating patch, this is obtained by using (1)  There is a coupling beteween main patch and parasitic patch, so it is placed at a balanced coupled gap (g) of 1.25 mm from the main patch since it gives the best impedance matching at 2.45 GHz.The addition of third resonance frequency by adding parasitic patch can be observed clearly in Fig. 2. A compact PIFA structure is obtained for triple band application having overall optimized dimension (Lp+Li)× Wp× h of 35.65 mm × 20 mm× 8mm, with a fractal slit of peripheral length (Ls) 56mm. As indicated in Fig. 1, the proposed antenna is fed by 50 Ω SMA connector at position (xp, yp) = (3 mm, 2.5 mm) on the main patch. The influence of the gap (g) on resonance characteristics of the tri-band PIFA is given in Fig. 5. The third mode resonating frequency decreases with increasing the gap as coupling gap capacitance increases with the gap values. The antenna height parametric analysis is shown in Fig. 6. In figure 6, it is seen that substrate height has effect on resonance frequency at third band but has no effect on first band. It is due to the value of L/h ratio of parasitic patch, which is <1. So, with increase in h the fringing effect is also increased which decreases the resonance frequency at third mode.Also increase in height lengthens the resonant path, i.e, decreases the resonance frequency. The final optimized value of 8mm for height is used in design.

Results and discussion
In order to verify the performance of the proposed antenna design a prototype is developed and measured using R & S ZVA 40 vector network analyzer. Fig. 7 plots the simulated and measured |S11| dB curve of the proposed antenna. Acceptable agreement has been achieved between the simulated and measured results. The -6 dB (3:1 VSWR) measured frequency range at each band is obtained as 880-964, 1741-1873 and 2407-2488 MHz, which corresponds to 9.12%, 7.33% and 3.30% impedance bandwidth, respectively. To have a better insight into the tri-band resonance behaviour of the proposed antenna, the surface current distribution of the proposed antenna at 0.9 GHz, 1.8 GHz and 2.45 GHz is shown in Fig. 8. At first resonating mode, a quarter wavelength resonance at 0.9 GHz is excited with current null which is occurred in the main patch. At 1.8 GHz resonant mode, current maxima is seen to be happened in the Koch fractal slit having a resonant path corresponding to the quarter wavelength at 1.8GHz DSC band and a halfwavelength resonance at 2.45 GHz is excited with the maximum current distribution on the face of parasitic patch for WLAN(IEEE 802.11b) band, where another current maxima has been occurred on the slit end.
The measured and simulated radiation patterns of the antenna are shows in Fig. 9, Fig.10 and Fig. 11

Conclusion
A compact tri-band PIFA is proposed and studied for the application in mobile handsets. Fractal slit is implemented on the main radiating element for dual-band operation. With the addition of a parasitic patch at an optimum distance from main patch, a third frequency band is obtained. The creation of the slit first reduces the dimension of the PIFA due to reduced capacitance and again with the inclusion of the parasitic strip, that cases capacitive loading results in enhancement of overall size in order to tune the resonances to usable frequencies. The overall antenna dimension is 35.65 mm × 20 mm with a height of 8 mm above the ground plane. The measured antenna resonances are obtained at the 0.92 GHz, 1.8 GHz and 2.44 GHz. The proposed antenna has been designed and fabricated. The radiation characteristics have been observed which ensure its suitability for cellular communication.