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Principles and techniques for making antennas small |
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Figure 6.50 Equivalent expression of (a) ILA and (b) IFA on the ground plane.
of these are possible. Enhancement of antenna performance or improvement of antenna characteristics may create novel antennas with small size that cannot practically be realized by conventional antennas, although it might not be as easy as expected.
Although integration techniques are very attractive and useful means to create novel FSAs, there are some serious problems to be taken into consideration. They involve noise and non-linear characteristics in devices and components. Noise produced from an active device deteriorates the FSA performance, while the non-linear characteristics inherently existing in devices, even in passive ones, may generate interference due to its intermodulation performance.
6.3.3FSA of composite structure
Some composite structures have been described in previous sections, some of which are concerned with ESA. They are self-complement structure, conjugate structure, and combination of different types of antennas. This type of FSA can be composed by similar techniques as in ESA: (1) by loading impedance components, which includes conjugate components, (2) with complementary elements, and (3) by using travelling wave structure. These techniques are essentially based on application of IAS (Integrated Antenna Systems [58]) techniques.
6.4Techniques and methods for producing PCSA
PCSA is a Physically Constrained Small Antenna, which has a part of its structure very small as compared with the wavelength as in ESA. PCSA is generally constructed with either low-profile or planar structures. In usual low-profile structures, a low-height antenna is placed on a ground plane and the antenna is treated equivalently as a symmetrical antenna constituted with its image, as shown in Figure 6.50, where (a) depicts an Inverted-L antenna, and (b) an Inverted-F antenna as the examples. Planar structures are treated in the same way as linear antennas. Images of these antennas must be included in the antenna design. The PCSA is an antenna which has very low height that is much smaller than the wavelength; however, other parts of the antenna are not necessarily small, but may have dimensions comparable to the wavelength; for instance, the periphery of a PIFA is about a half wavelength. No particular methods are used for the design of these antennas, excepting cases where proximity effects must be included.
6.4 Techniques and methods for producing PCSA |
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h HIS |
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Figure 6.51 A dipole placed on (a) PEC surface and (b) on HIS.
6.4.1PCSA of low-profile structure
Generally low-profile antennas are mounted on the ground plane and in the evaluation of the antenna performance the image antenna is taken into consideration. When the ground plane is not infinite, but has appreciable size, some currents driven by the antenna flow on it, and the ground plane will act as a part of the antenna system. When the size of the ground plane is larger than several wavelengths, it can be assumed as an almost infinite ground plane. Design of low-profile antennas is not always simple, depending on the size of the ground plane and the environmental conditions. Well-known low-profile linear antennas are ILA, IFA, and a half-loop of either circle or square, while planar ones are parallel plate, MSA, PIFA, printed antenna, and tiny ceramic-chip antennas. Design methods of these antennas differ depending on the type of antenna, but are not necessarily specific, excepting cases where complexity surrounding the antenna system needs serious considerations. Use of a simulation technique may become useful for such cases where ordinary methods are not applicable, as very complicated problems such as inclusion of proximity effects, are involved. However, simulation methods are not a cure-all means, but are only an approximation, although they can provide fairly accurate and precise results as compared with experiments performed in extraordinary environmental conditions. However, it would be recommended that the results obtained by the simulation should better be assured by comparison with experimental results, although for smaller antenna sizes, the implementation of the experiment can become difficult.
6.4.2PCSA employing a high impedance surface
When an ordinary dipole antenna is placed in parallel and very close to the PEC (Perfect Electric Conductor) surface, the performance of the antenna degrades significantly because of the negative influence caused by the image of the dipole, which has opposite phase to that of the primary dipole (Figure 6.51(a)). When, however, the PEC surface is replaced with a High Impedance Surface (HIS) [59–61], the performance of the dipole does not degrade, but may be modified to allow the dipole to be placed very close to the surface (Figure 6.51(b)), and have improved impedance characteristics, efficient radiation, and so forth.
The input impedance Zd of the dipole antenna placed on the HIS is given by [68]
Zd = Z11 + Z12ejθ |
(6.20) |