CST – Computer Simulation Technology

A Square Loop Antenna Using Hybrid High Impedance Surface and Four Way Wilkinson Power Divider

This article will show a planar Square Loop Antenna (SLA) simulation on a Hybrid High Impedance Surface, including the feeding structure which consists of a 4 way Wilkinson power divider. The planar SLA produces a semi doughnut radiation pattern, which is suitable for vehicular communication when the antenna is deployed on the roof top of the vehicle.

Antenna Configuration

The square loop antenna is printed on a Rogers substrate material with a real permittivity value of 3.4. The substrate material's thickness is 1.52mm. The total length of the square loop antenna has been optimized to 120mm and the track width to 1.5mm, which deliver the required resonance frequency of about 4.8 GHz. Instead of using a conventional metal ground plane, this square loop antenna uses a High Impedance Surface (HIS) with an Electromagnetic Band Gap (EBG) as its ground plane. Whereas the metal ground plane produces 180° reflection phase, the HIS behaves as an Artificial Magnetic Conductor (AMC) with a reflection phase which varies from -180° to +180°. If the resonant frequency of the antenna lies between the ±90° reflection phases of the HIS structure, the antenna can be placed in close proximity to the HIS [1]. This helps to lower the overall thickness of the antenna in comparison with a conventional metal ground plane, which requires a quarter wavelength substrate thickness. In order to supress the side-lobes from the surface waves, grounding vias at the outer patches have been introduced as well....



Figure 1: The Square loop antenna with HIS. The substrate is displayed with transparency for a better view inside the structure

The figure 1 shows the square loop antenna consists four input ports which fed by four 50 ohm SMA connectors using the waveguide ports. Each of the antennas consists of one conducting strip and has a length of 30mm. The fed position is located at the center position of each antenna arm. The figure 2 shows the polarized tilted beam of each input port achieved by exciting one of the corresponding input port and leaving the rest of the ports passive terminated. The polarized tilted beam can be explained as follows, e.g. considering the first port excitation, the current will flow afterwards in the direction of the second- and the fourth antenna arm. Hence, it behaves similarly to an antenna array with two antenna elements separated by the length 30mm. The produced far field beam will be then tilted in the direction of third antenna arm.



Figure 2: Far field pattern of square loop antenna from each input port

A combination of all the far field patterns with the same phase and amplitude at each antenna input port will produce a semi doughnut pattern. Such a simultaneous excitation can be achieved easily using the circuit simulator CST DESIGN STUDIO® (CST DS) and some ideal 3dB power splitters. In figure 3, the CST MWS File block with four pins is connected with the ideal 3dB power splitters, which in turn are fed by the external port which represents the excitation source.



Figure 3: Simultaneous excitation of all input port using ideal 3dB power splitter

The figure 4a shows the return loss of the square loop antenna with simultaneous excitation using the ideal 3dB power splitter. Exciting the antenna simultaneously with the same amplitude and phase will produce a homogenous surface current distribution at all the antenna arms as can be seen from the figure 4b. The additional grounding vias are inserted at the edge of the HIS structure, since the surface current can propage at the edge of the structure, which can introduce the sidelobes in the far field pattern. The resulting semi doughnut far field pattern from the square loop antenna on the top of the HIS is shown in figure 4c.



Figure 4: (a) Return loss of a square loop antenna (b) Near field distribution (c) Far field pattern

4 Way Wilkinson Power Divider



Figure 5: 4 way Wilkinson power divider

Previously an ideal 3dB power divider has been used in order to excite the antenna simultaneously. However, in the practical world, such an ideal power divider is difficult to manufacture. Thus, a four way Wilkinson power divider has been deployed in order to excite the antenna simultaneously with the same amplitude and phase. The four way Wilkinson power splitter has been modeled with a Rogers substrate. The resistors are modelled in the 3D simulation by using a lumped element with the typical value of 2*Zo, where the Zo is the line impedance at the input port. In addition, the SMA connector and the housing have been modeled, since their presence will in reality introduce a slight mismatch.

The output port of the four way Wilkinson power divider provides 6dB insertion loss around 4.8GHz and a low return loss value of better than 20dB at the resonance frequency as well. The resistor ensures that all the output ports are matched and therefore a good isolation between the output ports can be achieved. The isolation between the output ports at the resonance frequency are about 20dB.



Figure 6: Return loss, insertion loss and isolation of 4 way Wilkinson power divider

Cable Connection

For the connection between the power divider and the antenna, simplified coax cables have been simulated. All four cables are identical and have been modelled with the same length, in order to ensure that each of the feeding lines is in phase.



Figure 7: 3D Cable model and the phase information

Complete System

At the final stage, the antenna, the cable connection and the four way Wilkinson power divider will be combined into one complete system. This combination can be applied using the circuit simulator, CST DESIGN STUDIO. As the far field pattern of the complete system is required, a full 3D EM-simulation with CST MWS is necessary. At the initial stage, each of the component blocks will be placed into the CST DS schematic and linked afterwards. Following this, a new simulation project consisting all the three connected component blocks are defined as an S-parameter task. Using the integrated 3D-layout generator and the schematic topology, a complete 3D antenna system model can be automatically generated.



Figure 8: Schematic of a complete system and the corresponding 3D model

The return loss and far field pattern 4.8 GHz of the complete system are illustrated in figure 9. Due to the additional reflection from the cables and the metal housing of the Wilkinson divider, the far field pattern of the complete system becomes slightly asymmetrical. However, a semi doughnut radiation pattern can still be achieved.



Figure 9: Return loss and far field pattern of the complete system

Conclusion

A planar square loop antenna on a hybrid High Impedance Surface including a four way Wilkinson power divider is presented in this article. As the Hybrid High Impedance Surface behaves similarly to an AMC, the antenna can be placed in close proximity to the surface. Hence, the thickness of the antenna can be reduced, which will lead to a smaller and more compact antenna profile. The four way Wilkinson power divider provides an in-phase input signal to each port of the antenna, which generates a semi doughnut radiation pattern. For a practical realization of the antenna, a full antenna system simulation has been demonstrated.

References

[1]. P. Deo, A. Mehta, D. Mirshekar-Syahkal, P.J. Messy and H. Nakano, "Thickness Reduction and Performance Enhancement of Steerable Square Loop Antenna Using High Impedance Surface", IEEE Trans. Antennas Propag., vol 58, pp. 1477-1485, May 2010
[2] A.Pal, A. Mehta, D.Mirshekar-Syahkal, and H.Nakano, “A Square-Loop Antenna With 4-Port Feeding Network Generating Semi-Doughnut Pattern for Vehicular and Wireless Applications”, IEEE Antennas and Wireless Propag. Letter. vol.10, pp. 338-341, Apr, 2011.

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