Figure 6.52 Mushroom structure to represent EBG surface; (a) Mushroom type EBG implementation, (b) a cross section of several segments, (c) a cross section of one segment, and

(d) equivalent circuit of one segment.

where Z11 is the impedance of the primary dipole, Z12 is the mutual impedance between the primary and the image dipole, and θ is the reflection phase angle, which is 180 degrees for the PEC surface. The θ can be determined by the surface media parameters. One of the typical HISs is EBG (Electromagnetic Band Gap) media [57], in which the Mushroom type [58] is the most popular one. Figure 6.52 illustrates (a) implementation of the Mushroom type EBG, (b) a cross section of the Mushroom structure, (c) a cross section of the one segment, and (d) an equivalent representation of the one segment with an L-C circuit. The capacitance C is due to the gap between the two surface planes and the inductance L is produced by the current flowing through the surface plane, vias, and the ground plane. Applications of the Mushroom type media are found in various situations; one is for lowering antenna height on the ground plane [64], another is to reduce mutual coupling between two antennas [65–66]. The first one contributes to creating a very low profile antenna, and the second one allows antennas to be located closely in a limited area. Thus, in a small wireless unit, in which more than one antenna is required to be installed in a small limited space, mounting several antenna systems simultaneously, for instance, placing such antennas as multiband antennas, MIMO antennas, and multi-purpose antennas in a limited area, becomes feasible. The HIS media can be applied to other significant purposes; reduction of surface wave, enhancement of radiation [67, 68], beam scanning [69, 70], and bandwidth enhancement [71], are some examples. The design and practices will be described in Chapters 7 and 8.

6.5Techniques and methods for making PSA

A PSA is a Physically Small Antenna which has such small general physical dimensions that the antenna can be put on a human palm. High-frequency antennas in microwave (MW) and millimeter wave (MMW) regions are typical examples, as they have inherently small dimensions according to their frequency. Lower-frequency antennas, which have simply small dimensions, are also included in the PSA. Lately, various types of PSAs have been developed and employed in small terminals or units used in recently deployed wireless systems such as NFC (Near Field Communication) systems, including RFID (Radio Frequency Identification) systems, UWB (Ultra Wide Band) systems, and so forth.

Matching problems are often encountered in these systems, as antennas used for these systems are too small to satisfy appropriate matching conditions. Serious issues are

found particularly in application to the RFID systems, where equipment is so small that the space to install an antenna is strictly limited to a very narrow area, and thus the size of the antenna is forced to be very small, and satisfying matching conditions becomes extremely difficult. Antennas used unfittingly in such situations without proper matching conditions often suffer insufficient system performance; for example, low sensitivity so that the service is limited to small areas.

6.5.1PSA in microwave (MW) and millimeter-wave (MMW) regions

Antennas for MW and MMW regions are inherently small size because they are designed for high operating frequencies. Since they are physically very small, many types of them can be classified into ESA in addition to PSA, but they are not necessarily restrained by the limitations imposed on small antennas as the usual ESA. The basic structure of these antennas is the same as those used in lower-frequency regions; however, since aperture antennas such as horn, reflector types, and waveguide require generally some physical volume, they are not usually used in lower-frequency applications. But, thanks to their small size in MW and MMW regions, they are useful for application in various systems, especially in integrated RF devices with small antennas. Particularly, planar antennas are employed in such systems. Antennas integrated with RF circuits play very important roles in small mobile wireless systems, where small, compact, and yet functional antenna systems are required. Some of these antennas are thus classified into FSA.

Recently demands on antennas in MW or MMW regions have gradually increased, especially for signal transmission of high data rate, video, and multimedia applications. One example is an application to mobile phones, to which a small 60-GHz wireless module is attached, that is used for the purpose of transmission of high-rate data information or TV images at home. Small chip antennas or planar antennas integrated into RF parts constitute the front end of the wireless systems.

Design of these antennas differs depending on where to apply, how to use, the type of antenna, and so forth. Use of simulation is particularly helpful for design of antenna systems having complicated or extraordinary structure.

6.5.2Simple PSA

There are wide varieties of PSAs. Some of them are ordinary types, while others are extraordinary types constituted with complicated or composite structure. Typical ordinary antennas are small wire antennas such as dipole, monopole, loop, IFA, meander line, helices, fractal structure, and so forth, whereas planar antennas are PIFA, MSA, printed antennas, and related types. The wire elements can be printed on the surface of a planar substrate to comprise a planar antenna. They can be implanted into a small bulk substructure to form a tiny chip structure. Meanwhile, many of the extraordinary types of antennas are composed by integrating impedance components, materials, or other types of antenna elements into an antenna structure.

Realization of antennas with physically small size is in other aspects to achieve either wider bandwidth or higher efficiency with the antenna size being kept unchanged.