Special Techniques of Printed Antenna
Dr. Dinesh Kumar Singh
C L Bajaj Institute of Technology and Management, Greater Noida
Dr. Canga Prasad Pandey
Pandit Deendayal Petroleum University, Gujarat
Printed antennas are available in various shapes and most widely accepted by researchers, scientists, and engineers for various applications; however, they have bandwidth limitations. Since the inception of printed antennas, a number of techniques have been invented to enhance the bandwidth of these antennas. In this chapter, some of the special techniques of designing printed antennas for reconfiguration, polarization, feeding, etc. are discussed. This chapter is broadly divided into two parts; in the first part, two reconfigurable antennas are presented, while the second part presents a circularly polarized wideband magneto-electric dipole (MED) antenna with a defective semicircular patch for C-band applications (4-8GHz).
The first design presents an antenna with frequency notch characteristics. This section includes two designs of reconfigurable antennas. In the first design, a novel antenna with frequency notch characteristics is presented. The antenna consists of a C-shaped Microstrip antenna with two symmetrical notches and a rectangular parasitic patch. The antenna has tunable property due to the integrated PIN diode. The second design consists of a simple radiating truncated rectangular patch with a crossshaped slit and a ground plane embedded with L-shaped slit. The antenna produces two separate impedance bandwidths with three senses of polarization, namely, right- hand circular polarization, left-hand circular polarization, and linear polarization. The PIN diode is used to reconfigure the L-shaped slit in the ground plane. The antenna generates a dual band behavior with multiple circularly and linearly polarized bands.
The second part presents a circularly polarized wideband magneto-electric dipole (MED) antenna with a defective semicircular patch for C-band applications (4-8 GHz). In the proposed design, to get proper impedance matching and stable gain, a pair of folded vertical patches is shorted between a pair of defected semicircular patches and minimum ground plane. The defected semicircular patches work as electric dipoles, while the vertical patches work as magnetic dipoles.
C-Shaped Recongfiurable Antennas
A reconfigurable antenna is useful in the changing the operating requirements to maximize the antenna performance due to its reconfigurable capabilities. The reconfigurable antenna has a capability to modify its frequency, polarization, and radiation properties in a controlled and reversible manner. Reconfigurable behavior can be obtained by modifying the antenna structure using different mechanisms such as PIN diodes, varactors, RF switches, and tunable materials. These mechanisms enable the intentional redistribution of the surface currents producing reversible modifications of the antenna properties. The reconfigurable antenna is useful in applications where multiple antennas are required. Multiple antennas can be replaced by a single reconfigurable antenna.
C-Shaped Antenna with Switchable Wideband Frequency Notch
The number of bands are increasing day by day with multiple high-speed services. These services need a high bandwidth and need to be separated from each other to avoid interference between channels. There are two primary design requirements for this. The first is an antenna design with frequency notch and the second is an antenna design with high cross-polarization attenuation. The notched frequency band refers to frequency response with a small stopband and cross-polarization refers to the orthonormal orientation of electric fields (vertical and horizontal). The adjacent bands in a satellite are isolated by notched frequency bands and polarization, that is, if the first channel is vertically polarized, the second will be horizontally polarized and the third will again be vertically polarized, and so on. This provides isolation among various channels. The antennas designed for multibands with a stopband in between are known as frequency notched antennas. For instance, three bands are allocated for Wireless Local Area Network (WLAN) 2.4GHz (2,400-2.484MHz). 5.2GHz (5.150-5,350MHz), and 5.8GHz (5,725-5,825MHz). Worldwide, the WiMAX system operates at the frequency bands of 2.5GHz (2,500-2.690MHz), 3.5GHz (3,400-3,690MHz), and 5.8GHz (5,250-5,825MHz). Another high-speed WLAN service of 1 Gbps speed is under development (IEEE 802.1 l.ac), which is to operate around 5GHz.
Many techniques have been proposed for designing band-notched antennas. The simplest method is to etch slots on the radiating patch of the antenna, such as U-shaped slots , L-shaped slots [2,3], and V-shaped slots . An effective way of achieving a UWB compact frequency notched antenna is the use of a split ring resonator (SRR) [5-7]. A square split ring resonator is used in , while in , a slot-type SRR has been used to design an ultrawideband notched antenna. Multiple notched frequency bands have been implemented with an SRR below feed line in , with simple slots in , and using a half-mode substrate in . A frequency notched printed slot antenna has been implemented in , while a compact antenna is designed using fractal geometry in . Instead of using the conventional fractal geometry such as Koch curves, Sierpinski triangles, and Minkowski fractals, a Koch-curve-shaped slot has been employed to achieve a compact geometry with frequency notch characteristics.
The coaxial probe feed is one of the most popular feeding techniques for electrically thick substrates, but the inductance of the probe creates impedance mismatch and causes low bandwidth. The inductance of the probe is compensated for by cutting slots on the patch, using an L-shaped strip feed  or by introducing a capacitive feed strip [13,14]. But these configurations create theoretical complexity in terms of analysis apart from the mechanical alignment issues while assembling and hence may increase the production cost. Capacitive feed provides a simple and ultrawide impedance matching by compensating for probe inductance with its capacitance.
A C-shaped antenna may be described as an RMSA with rectangular notch at one of the edges . This is a very popular technique to achieve dual band  and wideband  characteristics.
In this design, a new technique has been presented to achieve a frequency notched antenna. A simpler capacitive coupling technique has been employed to get ultra- wideband characteristics. A PIN diode has been used so that the antenna operates in ultrawide band as well as dual band with a frequency notched antenna. The gain and other radiation characteristics are normal, while the cross-polarization attenuation is very high.
Multiband Multipolarized Reconfigurable Circularly Polarized Monopole Antenna with a Simple Biasing Network
In recent years, circularly polarized monopole antennas have attracted a great deal of attention for the current wireless communication system, as circular polarization (CP) plays a very important role in improving the quality of the received signal . These applications include the C-band communication satellite from 5.925 to 6.425 GHz for their uplink and amateur satellite operations in the frequency range of 5.830 to 5.850GHz for downlinks. The X-band uplink frequency band uses 7.9-8.4GHz for military communication systems. The traffic light crossing detector operates at 10.4GHz. In Ireland, Saudi Arabia, and Canada, terrestrial communication uses the bandwidth of 10.15-10.7 GHz. The usage of multiple antennas for achieving different CP radiations will make the system complex. Hence, as per the demand of the next-generation communication systems, single multiband circularly polarized antennas can be used to reduce the complexity of the system. Therefore, designing such antennas have attracted a great deal of attention among the researchers. However, designing such antennas is challenging when the number of operating CP frequency bands increases. Generally, a monopole antenna generates linearly polarized radiation. Hence, it is difficult to radiate CP waves which were produced by two near-degenerated orthogonal resonant modes of equal amplitude with opposite phase differences. In , CP is generated in the triangular patch by using a Koch curve. The axial ratio bandwidth is about 1.3%. A triple proximity-fed microstrip antenna gives the CP with an axial ratio bandwidth of 0.70% for the L-band . In , an elliptical microstrip antenna with proximity coupling is used to excite CP waves with an axial ratio bandwidth of about 0.85%. In , a simple circularly polarized antenna is presented. The CP operation is obtained by using an open slot having an open width at the low'er side of the model. In , a slot antenna with straight feed is presented. An SRR-inspired structure is used to obtain the circular polarization. Wideband high gain circularly polarized antennas are proposed in . The use of feed and parasitic patches makes the structure complex. Many CP monopole antennas were proposed for various applications in wireless communication [25-28]. In , an asymmetrically shaped radiator fed by a microstrip line and a limited ground plane is presented. In , the CP is achieved by loading four cylinders that are perpendicular to the substrate of the microstrip antenna near the edge of the circular patch. In , by using a rectangular dual-loop technology and tuning the separation between the ground plane and antenna, the monopole antenna produces a CP. In , an asymmetric antenna geometry is used to obtain the wideband CP. However, CP monopole antennas mentioned above focus on single CP band operation. Dual band CP has been investigated in [29-33]. The antenna presented in  composed of a partial ground plane and a Y-shaped radiating patch that consists of two unequal monopole arms and a modified circle. The dual CP is obtained w'ith two unequal monopole arms and a modified circle wdth an axial ratio bandwudth of 3.8% and 6.8%. In , dual CP is obtained by embedding an inverted-L slit in the ground plane. A halved falcate-shaped dual-broadband CP printed monopole antenna is proposed in . To generate dual CP, two halved falcate-shaped antennas were used to generate orthogonal modes and three stubs in the ground plane were used to give 90 degree phase difference. A novel monopole antenna w'ith dual CP consisting of a radiating patch composed of an annular-ring linked by a square ring over the corner and a ground plane with embedded rectangular slit was proposed in . In , the dual band CP operations are realized by using two parallel monopoles - one curved monopole and one fork-shaped monopole - and a crane-shaped strip is placed on the ground plane. A dual-feed, dual-band-stacked, CP patch antenna system is presented in . The use of dual feed and stacking gives the structure complex. Triple band CP radiations are presented in [35,36]. In , a hexagonal slot antenna with L-shaped slits is presented with a narrow 3dB axial ratio bandwidth of 1.7%, 3.86%, and 5.23%, while 3dB axial ratio bandwidths of 9.8%, 4.6%, and 2.8% have been achieved in . The complexity of these two designs [35,36] may hinder the integration of antennas in different applications. In , an inverted U-shaped radiator rotated by 45° around the horizontal axis is used to generate the triple band CP.
Recently, reconfigurable CP antennas have attracted significant attention. The PIN diodes are switched in different states to obtain the tunable property of the antenna ; this technique is also used to generate a reconfigurable CP microstrip antenna . The authors in  proposed a dual feed microstrip patch antenna with frequency and polarization reconfigurability. The frequency and polarization reconfigurability are achieved by using six PIN diodes. The design is complex because of the six PIN diodes and complex biasing circuits. A frequency- and polarization- reconfigurable antenna using PIN diodes is designed in . PIN diodes are commonly used as switching devices for RF and microwave application systems, as they have the advantages of low insertion loss, good isolation, and low cost . It has been observed from literature reviews that mainly two considerable problems are often encountered in reconfigurable CP antenna designs. The excessive diodes will lead to a complex dc-bias network [19,43], providing independent bias for each diode by some special mechanism, such as using capacitors .
In this design, a very simple monopole antenna with reconfigurable multiband CP operations is presented. The antenna has overcome the abovementioned two problems of the reconfigurable CP antenna. The antenna generates right-hand circular polarization (RHCP), left-hand circular polarization (LHCP), and linear polarization (LP) in different bands with a simple biasing network without making the system complex. By removing the triangular portion at the lower edges of the rectangular radiating patch and embedding the cross-shaped slit at the right lower edge and L-shaped slit in the ground plane, the antenna provides two CP bands with three LP bands in the OFF state and three CP bands with three LP bands in the ON state of the PIN diode. The reconfigurable CP is achieved by using a PIN diode on the L-shaped slit in the ground plane. The multiband CP operation is obtained by controlling the ON/OFF state of the diode. The antenna is suitable for many applications such as traffic light crossing detectors at 10.4 GHz, C-band communication satellites for their uplink from 5.925 to 6.425 GHz, amateur satellite operations for downlinks from 5.830 to 5.850GHz, terrestrial communication from 10.15 to 10.7 GHz, and X-band uplink frequency band from 7.9 to 8.4GHz for military communication systems.