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S-parameters, Return loss and Far field computation for a PIFA Antenna Application


PIFA Antenna - Design and Simulation

The mobile phone industry exert a very high demand to build better and smaller antennas used for mobile communication. Planar Inverted Antenna F, PIFA, are the major classification of antennas for the mobile industry. PIFA antenna is the best choice because the radiated field is away from the user’s head and secondly it has a small size and a quarter wave antenna, compared to other antennas (such as whip, rod, helix).

This article shows the workflow and simulation features in HFWorks, that enable electrical engineers to envision, design, simulate and optimize a dual band PIFA antenna. The antenna simulation in HFWorks was used to simulate a PIFA Antenna for GPS and WiMAX applications.

Figure 1, shows the antenna structure designed with SolidWorks. It consists of driven and parasitic elements. The driven inverted F-shaped element is composed of C-shaped patch and shorting strip. Whereas, the parasitic inverted-L element is a shorted rectangular patch. Both the elements are aligned face to face over a finite ground plane (Lg × Wg) with air as substrate. 

3D model of a PIFA antenna

Figure 1 - 3D model of a PIFA antenna

The schematic diagram of the dual band antenna is depicted in figure2. The structure is excited with a coaxial feed line, with impedance of 50 Ohms. The electrical parameters of a structure like the one in this example can be calculated using ATLASS[1].

Schematic diagram of the PIFA antenna

Figure 2 - Schematic diagram of the PIFA antenna

In order to facilitate the workflow, the geometrical parameters are defined in a file and they will be imported to SolidWorks as equations. These parameters are summarized in a Table 1.

Table 1- Geometric parameters of the PIFA antenna

Variable Value (mm) Variable Value (mm)
L 100 W3 22
W 24 Ls1 9.5
H 9.7 Ws1 5
T 9.3 Ls2 9.5
S 1 Ws2 2.5
G1 2 Lp 27
G2 11 Wp 13
G3 2 Lg 99
L1 71 Wg 24
L2 70 X1 45
L3 15 X2 44
W1 6 Y 1
W2 5    

Simulation Results

Figure 3, shows the simulated results of S-Parameters  of the PIFA antenna, using HFWorks.  The structure operates at two frequencies of 1.595 GHz with a reflection coefficient of -26.90 dB and at 3.363 GHz with a reflection coefficient of  -45.96 dB.

The lower mode has an impedance bandwidth of 90MHz, while for the higher mode has a wider continuous bandwidth of 135 MHz. The obtained bandwidths can sufficiently cover the bandwidth requirements for GPS and WiMAX.

Return loss of the PIFA antenna (dB)

Figure 3 - Return loss of the PIFA antenna (dB)

The far field simulation shows the distribution of the field around the model at a distance from the structure. Figure 4 illustrates the gain pattern in a 3D format at the first resonance frequency (3.363 GHz) and Figure 5 shows the same gain pattern in a 2D format. 

3D plot of the gain pattern at 3.363 GHz

Figure 4 - 3D plot of the gain pattern at 3.363 GHz


2D plot of the gain pattern at 3.363 GHz

Figure 5 - 2D plot of the gain pattern at 3.363 GHz

Parameterization

To know the effect of modulating the area of the metal strip in between branch 1 and branch 2 in  figure2, we need to vary the length L3 and study how this affects the S parameters. HFWorks helps designers to support various scenarios, this way user can ensure optimum performance and to know the impact of a parameter on the final performance. HFWorks is also seamlessly integrated inside SOLIDWORKS, allowing users to run simulations right on the SolidWorks model, without the need for re-importing data with each design modification.
Figure 6, shows that the impedance bandwidth in the GPS band is almost unaffected with decrease in length L3 but it deteriorates the impedance matching performance. With the decrease in length L3 the overlapping area between branches and ground plane of the driven element increases. 
 

Return loss as a function of dimension, L3 of a PIFA antenna

Figure 6 - Return loss as a function of dimension, L3 of a PIFA antenna

This change in area is increasing the capacitive reactance and hence the resonant frequencies in the WiMAX band decreases. It is interestingly noted that the bandwidth of the WiMAX band increases with decrease of length L3 at certain values by keeping higher frequency of such band constant. Further, when the value of L3 is kept less than 15 mm, wide band spectrum vanishes and dual band behavior appears.

Conclusion

In this article, a dual band antenna for mobile applications was designed using Solidworks and simulated using HFWorks. The resonance frequencies were found to be 1.599 GHz and 3.363 GHz with bandwidths which sufficiently cover the bandwidth requirements for GPS and WiMAX.

References

[1] https://www.emworks.com/product/ATLASS
[2] Mayank Agarwal, Rajesh Singh, and Manoj Kumar Meshram " Dual-Band Linearly Polarized Planar Inverted-F Antenna (PIFA) for GPS/WiMAX Applications", presented at Students Conference on Engineering and Systems (SCES), 2013 IEEE.