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CST – Computer Simulation Technology

High-Impedance Electromagnetic Surfaces

The high frequency community is currently demonstrating a growing interest in a new type of metallic structure which offers fascinating electromagnetic properties. Although it is made of conductive material, it behaves, for a certain frequency band, as an isolator while maintaining DC currents. Within this gap, propagating surface waves are not supported and image currents are in phase rather than phase reversed. The concept of suppressing surface waves has been used for decades in geometries such as corrugated slabs and bumps, but is now applied to thin 2D-structures described by band structure concepts, refered to as photonic bandgap structures (PBG). The repetitive structure consists of mainly 2D slabs such as patches and vias; they are often called metallo-dielectric PBGs. Embedded in a substrate, they are inexpensive to fabricate. The following example is a hexagonal PBG structure taken from [1]. Two ways of numerical analysis are explained:...

1. Using periodic boundary conditions all characteristics of the PBG - especially the phase diagram - can be extracted by considering only one element of the PBG structure. The results are obtained in a couple of very small and quick simulation runs.

2. Due to the memory efficiency of a transient simulator the full PBG structure including a patch antenna and all edge effects can be simulated and analyzed.



Figure 1: The full PBG structure to analyze

In order to determine the phase diagram of the PBG surface, only a single cell in combination with periodic boundary conditions (zero phase shift) is considered. The unit cell set-up implicitly assumes an infinite size of the periodic structure, at the same time the model size is kept very small.



Figure 2: The unit cell set-up including periodic boundaries and a field probe

Evaluating simulation runs with and without the resonant structure finally gives the required reflection phase diagram shown in Figure 3. Each simulation run only takess 90 sec.The bandgap is defined within the range of +/- 90 degrees, the centre frequency is designed to be located at 13 GHz.



Figure 3: The reflection phase diagram for the given hexagonal lattice

The full effect of the PBG structure can best be shown by really embedding an antenna in a high impedance surface. A 6.8x6.8 mm patch antenna is placed an a ground plane that is 84x84 mm large. The considered frequency range 0 to 25 GHz. Although the resulting model (see Figure 1 and Figure 4) gets quite complex, it can easily be simulated with the transient solver of CST MICROWAVE STUDIO® (CST MWS). The total simualtion time is only about an hour. As a comparison, the same patch antenna is simulated on a simple ground plane.



Figure 4: Coaxial fed patch antenna embedded into a 6 by 6 lambda hexagonal PBG structure

Embedding the patch antenna in a high-impedance surface (green line, with triangles) leads to an improvement of the reflection factor of -10 dB compared to the patch antenna without PBG surface (red line, with dots).



Figure 5: Return loss for patch antenna on two different ground planes. The radiation patterns were compared at 13 GHz

The farfield pattern were produced at a frequency where the antenna has the same return loss, in this case at 13 GHz. The radiation patterns of the two antennas are shown in the following pictures. The patch on the ordinary surface shows significant radiation in the backward direction and ripples in the forward direction.



Figure 6: Farfield plot for the PBG structure


Figure 7: E- and H-Plane pattern of the patch on the two different ground planes

The advantages displayed by photonic bandgap surfaces make them especially interesting for portable communication devices, where the radiation into the users body should be minimized.

References

[1] D. Sievenpiper, L. Zhang, F. J. Broas, N.G. Alexopolous, E. Yablonovitch:
“High Impedance Electromagnetic Surfaces with a Forbidden Frequency Band”
IEEE Trans. On Microwave Theory and Tech., Vol.47, No.11, Nov. 1999, pp. 2059-2074

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