**3. Evolution of forward directional antennas & limitations: A literature scenario**

In this section we will focus on the development of multi-band antenna designing process. In last few years, there are a lot of antennas multi-band antennas has been designed, but most of them are omni-directional in radiation manner. But still some novel structures can be found in recent researches, that provide multi-band operation with directional radiation characteristics (Li et al. 2012, Mobashsher et al. 2010, Sabran et al. 2011). However, all of these antennas use a big metallic ground plane in order to reflect the radiation from the patch. Hence the actual applied antenna profile is bigger than the patch alone. So these an‐ tennas are not suitable for portable RFID applications. Another technique is widely applied in antenna domain in order to achieve directional radiation patters- the utilization of surface waves (also called trapper waves) (Zucker 1993).

In literature, surface-wave or end-fire antennas are mostly used to produce front-directional radiation patterns. Folded dipole (Fan et al. 2009) and folded (Yang et al. 2010) antennas are reported to have this type of radiation. Although these antennas produce good directional patterns, they are inappropriate for compact multi-band applications as they are printed in both sides of the substrate and resonate only in one operating frequency. The reported con‐ formal (Dong & Huang 2011) and plate (Yao et al. 2011) end-fire antenna with good radia‐ tion patterns have a bulky profile and are unsuitable for a portable use. It is worth to mention that for handheld compact applications, uni-planar antennas are more beneficial than double-sided microstrip in terms of compactness and the integration capability with solid-state active and passive components. The uni-planar compact yagi antenna, reported in (Nikitin & Rao 2010), is difficult to be incorporated with the circuitry due to its construc‐ tion.

by using mathematical models. The design is simulated and its performance is examined and optimized until satisfactory result is obtained. When the optimized antenna is achieved, it is time to enhance the performances by using some techniques. This section is discussed more in the next sections. The last but not the least step is the fabrication process where the prototype is going to be built and finally the measurement of the antenna parameters for validation and comparison with the simulated results. At the end of this process the desired antenna with the given specification is attained. However, if any step fails to achieve its ob‐

Advancements and Prospects of Forward Directional Antennas for Compact Handheld RFID Readers

http://dx.doi.org/10.5772/53283

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Start

Specifications & Literature Review

Determination of Appropriate Model & Design Dimensions

Simulation in EM Simulator& Optimization of the Model

Fabrication & Measurement

End

The dual-band operation of the any antenna is an additional advantage. The design proce‐ dure of the dual band antenna is slightly different from the previously discussed the single

In order to meet the requirements of dual-band RFID reader antenna specifications, first the lowest cutoff frequency of the operation should be met. It is because of the fact that at the lowest frequency means the biggest wavelength in comparison to other higher frequencies and hence the respective current path should be the longest. Thus, the basic dimension of the antenna is defined by the lowest operating frequency. Electromagnetic simulation is an advanced technology to yield high accuracy analysis and design of complicated microwave and RF printed circuit, antennas and other electronic components. In antenna designing process, after the determination of the antenna dimensions with suitable model, the next step is to simulate the design in suitable electromagnetic software. The modeling and formu‐

In simulation there may be some disagreements with calculated designed dimensions. In that case, the antenna should be optimized varying the parameters. When the lowest fre‐

**Figure 2.** Flow chart design and fabrication process of a desired single band RFID antenna

band antenna. The design process is illustrated in the Fig. 3.

lation are mainly derived through the use of Green's functions.

jectives, it is repeated again until the aims are met.

Several quasi-Yagi (Kan et al. 2007) and bow-tie (Eldek et al. 2005) antenna provides wide bandwidth, but they do not give flexibility to choose specific frequencies of operation, thus in turn increases interference with neighboring operating bands. The frequency reconfigura‐ ble planar quasi-Yagi antennas (Qin et al. 2010) are also unsuitable for its complex feeding structure. A printed dipole (He et al. 2008) with etched rectangle apertures on surface has reported to have dual-band characteristics; but it suffers mostly in the consistency of the ra‐ diation patterns. Again, these are mostly double sided planar antennas. A multi-band Qua‐ si-Yagi-type antenna is reported in (Ding et al. 2011). However, the feeding transition takes a wide area which in turn increases the antenna size significantly. It is obvious that the front directionality of the antennas will provide the handheld RFID readers a compacter solution and there is a huge research interest in this area in recent years to achieve optimum solution for the practical application.
