8.3Design and practice of PSA (Physically Small Antennas)

(a)

Figure 8.100 (a) Small-sized MS-DBE layout on a 2 × 2 (in inches) substrate and (b) tangential electric field magnitude on the top surface ([83], copyright C 2009 IEEE).

communications, but also for control, sensor, data transmission, identification, remote sensing, wireless power transmission, and so forth, require small antennas to fit into the units of small pieces of equipment for those systems. Typical wireless applications are such systems as various short-range communications, for instance NFC (Near Field Communications), including RFID (Radio Frequency Identification), where numerous applications have been deployed practically in the recent decade, and radio watches/clocks, which operate very precisely with nearly standard time, being synchronized automatically by receiving time signals from long-wave standard-time broadcasting stations.

Types of antennas are not necessarily specific to these applications, but generally they are conventional types. However, most of them are specifically designed to fit into the small equipment used in the various applications and yet satisfy the requirements for each system. Antennas are not necessarily ESAs; however, antennas in almost all these cases need to be miniaturized and ESA techniques are employed as almost inevitable means. The techniques include application of slow-wave concepts, lowering resonance frequency, filling space with radiation elements, increase of radiation modes, and so forth, as covered previously in this chapter.

As the antenna dimensions become smaller, evaluation of antenna performances tends to become harder to obtain correct results. Special techniques to evaluate antenna performances often are required. However, the appropriate electromagnetic simulations may gain greater importance for replacing the measurements when evaluating antenna performances. The simulation can be used even for design of small antennas.

Another important problem is impedance matching of antenna to the load, conventionally a resistance of 50 . In cases of very small RFID equipment, for example, the antenna is often directly connected to the RF circuits, which has impedance not of 50, but usually a much higher impedance. Further, connection of a type of balanced antenna with unbalanced circuits or vice versa may often be encountered. Without good