2. Review of RLSA antenna developments

Kelly introduced the concept of RLSA antennas in the 1950s [1]. Although Kelly could produce a high-gain RLSA antenna, the structure of the antenna feeder was still complex, leading it to be costly.

In 1988, Ando et al. proposed a RLSA antenna at a frequency of 12 GHz. This antenna was designed using the technique of slot arrangements. The technique aims to produce a uniform-aperture distribution. This antenna has a double-layer cavity and exhibits a good linear polarization. Ando also proposed a beamsquint technique to improve the poor reflection coefficient in linearly polarized RLSA antennas [2, 3]. In the same year, by applying a reflection coefficient suppression and slot coupling technique, Ando successfully designed a LP-RLSA antenna for satellite applications at 12 GHz. This antenna has the efficiency of 76% and the gain of 36 dB [4–6]. Takada et al. introduced a technique to improve the reflection coefficient using a reflection cancelling slot technique. This technique successfully improved the reflection coefficient of RLSA antennas from 2 to 10 dB [7]. Endo et al. designed an optimum thickness of double-layer RLSA antennas in order to realize the mass production of thinner RLSA antennas [6].

In 1990, Ando et al. furthermore introduced a circularly polarized RLSA (CP-RLSA) antenna. This antenna utilizes a single-layer cavity instead of a double-layer cavity. This simpler cavity structure improves the complexity of RLSA fabrications and can achieve the gain of 35.4 dBi and the efficiency of 65%. Ando used two techniques to improve the antenna performance. The first is the technique of varying the slot length and slot spacing used to event out the aperture illuminations of the antenna. The second is the technique of matching spiral used to reduce the reflection of the residual power at the antenna perimeters [8, 9]. In 1991, Takashi et al. proposed the technique of varying the slot length and spacing. Utilizing this technique Takahashi proposed several high-efficiency single-layer RLSA antennas with the diameter of 25–60 cm. These antennas can achieve efficiencies of between 70 and 84% [10]. Furthermore, Takashi et al. produced and marketed a-78% efficiency, 32.6 dB gain and single-layer RLSA [11–13].

Australian researchers started to investigate RLSA in 1995. They reported several investigations to design LP-RLSA antennas for satellite receivers. These investigations used the combination of the theoretical and experimental approach. The availability of low-cost materials (polypropylene) and low-cost fabrication also become a consideration in these researches. In 1997, Davis reported a 60 cm diameter LP-RLSA prototype designed using the reflection cancelling slot technique. This technique can overcome the inherent poor reflection coefficient of LP-RLSA antennas [4]. Davis and Bialkowski also successfully tested a RLSA antenna designed utilizing the reflection cancelling slot technique and a beamsquint value of 20° [14, 15]. Furthermore, Davis and Bialkowski reported an investigation of LP-RLSA antennas utilizing the beamsquint technique for several squint angles. This technique successfully improved the reflection coefficient under 25 dB [16]. Davis integrated the report of [2, 4, 7, 11] to form a beam synthesis algorithm used to calculate the design parameter of LP-RLSA antennas [17].

Due to the successful development of RLSA antennas for satellite applications, researchers tried to bring RLSA antennas into small antenna application for Wi-Fi devices. However, the design of small-RLSA antennas was not easy since small-size RLSA antennas normally performed high reflection coefficient [18–20]. Hirokawa

#### Radial Line Slot Array (RLSA) Antennas DOI: http://dx.doi.org/10.5772/intechopen.87164

radiating element and multiband RLSA antennas. It is hoped that the ideas can

Telecommunication Systems – Principles and Applications of Wireless-Optical Technologies

Kelly introduced the concept of RLSA antennas in the 1950s [1]. Although Kelly could produce a high-gain RLSA antenna, the structure of the antenna feeder was

In 1988, Ando et al. proposed a RLSA antenna at a frequency of 12 GHz. This antenna was designed using the technique of slot arrangements. The technique aims to produce a uniform-aperture distribution. This antenna has a double-layer cavity and exhibits a good linear polarization. Ando also proposed a beamsquint technique to improve the poor reflection coefficient in linearly polarized RLSA antennas [2, 3]. In the same year, by applying a reflection coefficient suppression and slot coupling technique, Ando successfully designed a LP-RLSA antenna for satellite applications at 12 GHz. This antenna has the efficiency of 76% and the gain of 36 dB [4–6]. Takada et al. introduced a technique to improve the reflection coefficient using a reflection cancelling slot technique. This technique successfully improved the reflection coefficient of RLSA antennas from 2 to 10 dB [7]. Endo et al. designed an optimum thickness of double-layer RLSA antennas in order to realize

In 1990, Ando et al. furthermore introduced a circularly polarized RLSA (CP-RLSA) antenna. This antenna utilizes a single-layer cavity instead of a double-layer cavity. This simpler cavity structure improves the complexity of RLSA fabrications and can achieve the gain of 35.4 dBi and the efficiency of 65%. Ando used two techniques to improve the antenna performance. The first is the technique of varying the slot length and slot spacing used to event out the aperture illuminations of the antenna. The second is the technique of matching spiral used to reduce the reflection of the residual power at the antenna perimeters [8, 9]. In 1991, Takashi et al. proposed the technique of varying the slot length and spacing. Utilizing this technique Takahashi proposed several high-efficiency single-layer RLSA antennas with the diameter of 25–60 cm. These antennas can achieve efficiencies of between 70 and 84% [10]. Furthermore, Takashi et al. produced and marketed a-78% effi-

Australian researchers started to investigate RLSA in 1995. They reported several investigations to design LP-RLSA antennas for satellite receivers. These investigations used the combination of the theoretical and experimental approach. The availability of low-cost materials (polypropylene) and low-cost fabrication also become a consideration in these researches. In 1997, Davis reported a 60 cm diameter LP-RLSA prototype designed using the reflection cancelling slot technique. This technique can overcome the inherent poor reflection coefficient of LP-RLSA antennas [4]. Davis and Bialkowski also successfully tested a RLSA antenna

designed utilizing the reflection cancelling slot technique and a beamsquint value of 20° [14, 15]. Furthermore, Davis and Bialkowski reported an investigation of LP-RLSA antennas utilizing the beamsquint technique for several squint angles. This technique successfully improved the reflection coefficient under 25 dB [16]. Davis integrated the report of [2, 4, 7, 11] to form a beam synthesis algorithm used to

Due to the successful development of RLSA antennas for satellite applications, researchers tried to bring RLSA antennas into small antenna application for Wi-Fi devices. However, the design of small-RLSA antennas was not easy since small-size RLSA antennas normally performed high reflection coefficient [18–20]. Hirokawa

inspire researches for the next development of RLSA antennas.

2. Review of RLSA antenna developments

the mass production of thinner RLSA antennas [6].

ciency, 32.6 dB gain and single-layer RLSA [11–13].

calculate the design parameter of LP-RLSA antennas [17].

186

still complex, leading it to be costly.

et al. used a technique for matching slot pair in order to reduce the remaining power at the antenna perimeter of small-aperture RLSA, so that this technique can minimize the reflection coefficient [21, 22]. Akiyama et al. also used the same technique for matching slot pair [23, 24]. However, the technique for matching slot pair is only used to radiate the remaining power at the antenna perimeter and does not contribute to the antenna gain. Reference [25] introduced the use of long slots in order to increase the ability of slots to radiate power, so that it can reduce the remaining power at the perimeter of small-aperture RLSA antennas, thus reducing the reflection coefficient. However, although this method can reduce the reflection coefficient, this method also can decrease the antenna gain. This is because that the long slots cannot radiate a focus power.

In 2002, Malaysian and Australian researchers started to investigate the application of RLSA antennas for wireless LANs. Tharek and Ayu successfully fabricated a low-profile RLSA antenna at a frequency of 5.5 GHz with a broad radiation pattern of 60° used for indoor wireless LANs [26]. Bialkowski and Zagriatski investigated the design of RLSA antennas for wireless LANs and successfully fabricated a dualband 2.4/5.2 GHz antenna [27, 28]. Furthermore, Imran et al. reported the design and test of RLSA antennas for outdoor point-to-point applications at the frequency of 5.8 GHz [29–31]. However, this design utilized a beamsquint technique that is similar with the technique used to design RLSA antennas for satellite applications. Hence, the diameter of this antenna is still considered large with a diameter of 650 mm, so that it is not applicable for small Wi-Fi devices. Islam reported the utilization of low-cost FR4 materials to fabricate RLSA antennas at the frequency of 5.8 GHz for wireless LANs. This invention is quite innovative since FR4 materials are a low-cost material and easy to be fabricated [32, 33]. However, there are some drawbacks in designing this antenna, such as a design of overlap slots, a loss cavity due to the use of several FR4 boards and the use of material loss of FR4. These all lead to low gain (only 8 dB) and low bandwidth (75 MGhz).

Purnamirza et al., in 2012, introduced a technique called extreme beamsquint technique in order to overcome the problem of high reflection in small-RLSA antennas [34]. This technique uses the beamsquint values higher than 60°. The theory of how the high values of beamsquint can significantly minimize the reflection coefficient is explained. Purnamirza also developed RLSA antennas that mimic the specification of other types of antenna that is available in markets [35–38].

## 3. Basic theory of RLSA antennas

This section discusses the theory of RLSA antennas including the structure, the theory of how RLSA antennas work as well as several formulas to design RLSA antennas.

#### 3.1 Structure of RLSA antennas

Figure 1 shows the illustration of the structure of a RLSA antenna. The figure shows the structure of RLSA antennas consisting of a radiating element, a cavity, a background and a feeder. The radiating element usually is a circular plate made of metals, such as aluminium, copper or brass. The radiating element consists of many slot pairs. One slot pair acts as one antenna element so that all the slot pairs form an array antenna. The background is a metal plate just like the radiating element, but the background does not have slots. The cavity is a dielectric material that has the form of a tube. Together with the radiating element and the background, the cavity operates as a circular waveguide that guides the signal from the feeder to propagate in radial direction. The feeder is a part of RLSA antennas used to feed signals from a transmission line into the antenna.

3.3 Polarizations

Radial Line Slot Array (RLSA) Antennas DOI: http://dx.doi.org/10.5772/intechopen.87164

Figure 3.

Figure 4.

189

Illustration of the power escaping from the slot pairs [39].

A slot pair, which represents a signal source in RLSA antennas, is located in the top surface of the radiating element of a RLSA antenna. A linear polarization in the RLSA antenna can be produced by combining two signals from the slot pair. Figure 4a shows the illustration of the slot pair. The signal from Slot 1 and the signal from Slot 2 have a phase difference of 180° or phi radians since Slot 1 and Slot 2 have the distance of half wavelength (0.5λgÞ to each other. Since the orientation of Slot 1 and Slot 2 is perpendicular to each other, the signals from Slot 1 (at y axis) and Slot 2 (at x axis) are also perpendicular to each other, as shown in Figure 4b.

Figure 4b shows that when Signal 1 is increasing in positive values, Signal 2 is decreasing in negative values. Since their position is perpendicular to each other, the resulting wave becomes a line in Quadrant II. When Signal 1 is decreasing towards zero and Signal 2 is increasing towards zero, the resulting signal will be a line in Quadrant II but with a shorter length compared to the line in the previous case. When Signal 1 is decreasing in negative values and Signal 2 is increasing in positive values, then the resulting signal will be a line in Quadrant IV. When Signal 1 is increasing towards zero and Signal 2 is decreasing towards zero, then the resulting signal will be a line in Quadrant IV but with the shorter length compared to the line in the previous case. Now, we can understand that the resulting signal of Signal 1 and Signal 2 results in a signal that looks like a straight line where the

Polarization establishment in a linearly polarized RLSA [39]. (a) slot pair position (b) signal of each slot.
