Patch antenna

The patch antenna, at first, seems to be the perfect match for the medical implant. It is compact, small compared with the wavelength, and can be placed on the side of the implant. Unfortunately, we have found that it is difficult to make it to work properly inside a lossy material. Its use as an implant antenna has been patented [45]. In this patent application, there is a reported gain of approximately -32dBi, but it is not clear to us whether this is a calculated, simulated or measured value. In the patent it is reported that the patch antenna has 8 dB better gain than wire and loop antennas, but how these were realized is not reported. The gain is reported as calculated with the body loss subtracted.

That method is not useful, as is shown in this thesis.

Our simulations show that a patch antenna placed in a lossy medium does not work well as an antenna. The patch antenna is a resonant structure, with a plate placed over a ground plane. The plate has such dimensions that there will be a standing wave across the patch. It is fed at a position that excites the resonating modes and has a matching impedance to the antenna feed. The space between the patch and the ground plane is often loaded with a dielectric substrate in order to reduce the wavelength and the size of the patch. In our experiments the dielectric has covered the entire ground plane. The patch antenna used in our simulations had a size of 37.4 mm by 28.8 mm on a 1.55 mm thick FR-4 substrate. It was fed by a probe-feed at a point with an impedance of 50 Ω when the patch was in the air.

58 CHAPTER 5. ANTENNA DESIGN

Figure 5.23: The CAD drawing of the circumference PIFA antenna. The thin circle is the radial extent of the isolation

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Figure 5.24: Real and imaginary part of the impedance of the PIFA circumfer-ence antenna in simulated muscle tissue.

Figure 5.25: SWR of the PIFA circumference antenna. Z0= 20Ω

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Figure 5.26: The impedance of the bare patch antenna in air.

If we place the simple patch antenna in a conducting medium the reflections at the ends of the patch are reduced, since the patch surface is essentially ex-tended infinitely by the lossy matter. The wave propagating between the patch and the ground plane is a surface wave. It is not reflected at the edges of the patch but continues to propagate as an attenuated surface wave. The radiation from this is very small. We thus lose the resonant structure of the patch. The wave propagating into space gives rise to a far-field, but if the wave impedance of the space between the patch and the ground plane does not equal the wave impedance of the matter the efficiency of this ”horn”-like antenna will be low.

Figure 5.26 shows the simulated impedance of the patch antenna in air.

The antenna is designed to be resonant at 2.45 GHz. This is confirmed by the simulation and measurements. Figure 5.27 shows the same patch antenna in the muscle tissue. There are no resonance phenomena taking place anymore.

This has also been confirmed by measurements. Figure 5.28 shows the simulated impedance when we have covered the patch and dielectric with a thin insulation, h=1.5mm. No useful resonance is present. If we cover the antenna with a thicker insulation, h=5 mm, we get the result shown in Figure 5.29. We now have a resonance, but we have no benefit of the high permittivity in the tissue. The patch has a resonance when the length equals half a wave-length. Care must be taken to calculate the correct effective permittivity, as the electric field will be both above and beneath the patch[28]. If we have a thick insulation, we can use the permittivity of the insulation as a rough estimate, in this case ε_r= 4.

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Figure 5.27: The bare patch antenna in the muscle tissue.

Figure 5.28: The patch antenna with thin isolation in muscle tissue.

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Figure 5.29: The patch antenna with thick (5mm) isolation in muscle tissue.

Figure 5.30: The patch antenna with thick isolation in the muscle tissue. The dielectric has a high permittivity: εr= 50.

5.3. IMPLANTABLE ANTENNAS 63 That gives a length of the patch at 403.5 MHz of 18 cm, which is too large to be useful in a medical implant. If we increase the permittivity of the dielectric to εr = 50, we get the result in Figure 5.30. Here we have a reduction of the resonant frequency. There are a couple of drawbacks with this solution. One is that it requires high epsilon materials, a second is that the antenna makes the implant thicker. Most implants today are designed to be flat rather than compact. There are examples of patch antennas being used for hyperthermia applications in the literature [60], and they use a thick insulation between the patch and the lossy tissue.

It is possible to get a compact patch design if the dielectric is confined to the space beneath the patch, and the muscle tissue is allowed to flow around and down to the ground plane. This requires a dielectricum with a higher permittivity than the muscle tissue, or εr> 60. This patch will have a very low efficiency, since the lossy muscle tissue is in contact with the near-field of the radiating sides of the patch.