From the results in this chapter, we conclude that the amplitudes of the E- and H-fields inside a dielectric body depend both on the depth and on the exact composition of the body. A layered structure gives rise to variations in the E-field due to reflections. The same is true for the H-E-field. The exact E-field that an implanted antenna operates in will thus depend on the thickness of the fat layer, which varies between individuals and with time. The thickness of the muscle layer behind the implant will also influence the wave propagation. This shows that antennas for medical implants must either be insensitive to this kind of varying operating conditions, or be designed with an appropriate margin to operate within the specifications in all instances.
4.6. CONCLUSION 29
Figure 4.9: E- and H-field for a layered cylinder of skin-fat-muscle-lung-muscle-fat-skin. the incident E-field is parallel to the cylinder axis.
Figure 4.10: E- and H-field for a layered cylinder of skin-fat-muscle-lung-muscle-fat-skin. The incident H-field is parallel to the cylinder axis.
30 CHAPTER 4. WAVE PROPAGATION INTO MATTER
Chapter 5
Antenna Design
Antenna design is a mature science today, and an engineering discipline with a large number of design manuals available, e.g. [28][38][39]. All these books have one thing in common: they mainly describe antennas placed in a non-conducting surrounding with a relative permittivity of 1, or close to 1. In other words, they describe antennas placed in vacuum or air. The only structure that is typically found close to the antenna is a radome, which is made of low loss materials with low permittivity. When the antenna is placed inside a human body, we have a completely different situation. The antenna is surrounded by a lossy material with high permittivity. There are two instances in classical antenna applica-tions where similar condiapplica-tions occur: buried antennas and submarine antennas.
Buried antennas are closely related to the beverage antenna, developed by H.
Beverage, C. Rice and E. Kellogg in 1923[40]. The theory of buried antennas was developed in order to cover the applications of submarine communication at VLF, and geophysical prospecting. In addition, the need to communicate from bunkers built during the cold war added interest to the field in the period 1960-1970 [41]. At that time, the main interest was in low frequency applications, and the general simplification was a lossy half-space with the buried antenna, with the other half-space being air. King and Smith wrote the book ”Antennas in Matter” in 1981 which sums up this field[31]. Onward, from 1980, not many articles have been published about ”buried antennas”, ”underwater antennas”
or submarine communication.
Submarine communication at low frequencies uses trailing wire antennas [42]. Other antenna systems for submarines are located in the tower, or sail, and are used when this part of the submarine is above the surface of the water.
Towed buoys with antennas are also used. The design of an efficient underwater antenna, for a frequency band with high information transfer properties, is hard.
This can be seen in that newly tested autonomous underwater vehicles, designed to locate and destroy sea mines, all incorporate a mast in order to keep the antennas, used to communicate with the mother ship, above the water [43].
High frequency antennas dedicated to medical implants are rare in the lit-erature. One well-reported design is shown in [44] and a couple of patents
31
32 CHAPTER 5. ANTENNA DESIGN have been granted, [45][46][47]. Apart from these we have found very little in the literature. If we expand the search to ”biomedical telemetry” there is much more published, but mostly for low frequencies, and utilizing inductive coupling.
However, the design of antennas for biomedical telemetry is not well published either. The systems themselves are described, both in classic texts such as those by Mackay[48] and Caceres[49], and in published articles. The systems described in the books use mainly coil antennas, as they use low frequencies for transmission. Most of the commercially available implantable systems today from Advanced Telemetry Systems [50] use coil antennas, although some use wire antennas similar to the trailing wire antennas for submarines. The wire antennas are often used for aqueous animals. Subcutaneously implanted wire antennas are also used for birds. No information about the design of these wire antennas is given.
5.0.1 What is the antenna?
When we look at the antenna implanted in a lossy and finite body, the defi-nition of the extent of the antenna needs to be discussed. The naïve view is that the antenna is what is attached to the implant, which is then inserted into the patient. This disregards the influence of the implants on the antenna char-acteristics. Furthermore, the analysis of the radio link will have to consider a wave propagating from the antenna through the body into the air and over to the base-station antenna. This propagation is hard to characterize, especially as it is hard to characterize the radiation pattern from the implant itself. The radiation characteristics are influenced by the tissues in the near-field of the antenna, and thus vary between different patients.
If we now look at the system from the outside, we can define the implant antenna characteristics as the sum of the implant antenna, the implant itself and the body. This is what we will see as a radiating structure when the radiating implant is in place. It is of this structure that we can measure the gain and the efficiency. The complication is that we then have to include the body shape and the actual placement of the implant in the analysis. However, this is no real change, since we always have to make sure that the antenna works when placed where it will actually be used. It also leads to the added complexity that the link budget will not have a fixed gain of the implant antenna. The gain, the directivity, and the efficiency will vary with the patient. These variations must be taken into account by adding them to the link budget calculations.