A design of radial line slot array antennas using the specification of panel antennas

RLSA antennas were suggested by several researches as Wi-Fi antennas in addition to panel antennas. Therefore, this paper researched the possibility of this suggestion. We used the size of an available in market 16 dBi panel antenna (225 mm 2 ) as the size for our developed RLSA antenna. Based on this size, we developed 60 RLSA models using extreme beamsquint technique and simulated them. We then chose a best model with a best performance. The best model was then fabricated and measured. The simulation and measurement results show that the developed RLSA antenna has better performance compared to the 16 dBi panel antenna in term of gain (0.25 dB higher) and bandwidth (570 MHz wider). The RLSA antenna also tested as antenna for a Wi-Fi device and it showed good performance.


Introduction
RLSA antennas basically were developed for satellite communications [1][2][3][4]. Thereafter, RLSA antennas were developed for other applications such as Wi-Fi [5][6][7][8][9][10]. However, since Wi-Fi devices use small antennas, then RLSA antennas should also developed in small form. The small RLSA antennas have much less number of slots compared to the normal RLSA which is developed for satellite communications. The effect of small number of slots is the capability of slots to radiate power into space is linearly reduced, so that increasing reflected power at the perimeter of RLSA antennas, thus lead to the increase of reflection coefficients.
Several papers tried to overcome the problem of this high reflection coefficient. Hirokawa and Akiyama used the technique of matching slot pair aiming to waste the remaining power at perimeter antennas [11]. Zagriatski used long slots in order to reduce the remaining power [12]. Based on author's observation, the technique uses in these papers are not efficient since the power is wasted so that the power does not contribute to antenas gain. In last decade, there is no innovations in developing techniques to overcome the problem of high reflection coefficient in small RLSA antennas. The researches were only about the theory and the design of RLSA antenna using conventional techniques [13][14][15][16][17][18][19][20] Purnamirza introduced a technique to reduce the reflection coefficient by using two material of cavity, which are polypropylene and FR4. This technique successfully reduces the reflection coefficient without wasting remaining power [21]. Purnamirza also introduced a technique to reduce reflection coefficient by designing RLSA antennas using high beamsquint values. This technique also successfully minimizes the reflection coefficient [22]. Due to this success, Purnamirza recommended their technique to be implemented in designing small RLSA antennas [23][24][25].
Based on this recommendation, we tried to design a small RLSA antenna for Wi-Fi devices at frequency of 5.8 GHz. In order to show the competitively of the RLSA antenna, we design the RLSA antennas with a same size with an antenna usually used as Wi-Fi antenna and widely available in market, which is a 16 dBi panel antenna. In order to test the RLSA antenna in real condition, we developed a test bed system by setting the RLSA antenna as an antenna

Research
Steps a. Determine the RLSA antenna specifications: We determined the antenna specification by choosing one commonly Wi-Fi antenna out of several antennas available in market. In this research we chose the specification of 16 dBi panel antenna, which is listed in Table 1. b. Determine the antenna material: In this research, as shown by Figure 1 (a), we chose a material of copper as the radiating element and background of the antenna. We also chose polypropylenes as the dielectric material for the antenna. The material of copper and polypropylenes were chosen based on the successfully of their using in previous researches [21][22][23][24][25][26]. The values of all the material of antenna are listed in Table 2. c. Determine the antenna size: The size of 16 dBi panel antenna is 190 mm x 190 mm = 36100 mm2. Using this size of 36100 mm2, the diameter of RLSA antenna can be calculated using simple circle equation, which is (36100/phi) = 107 mm, as shown by Figure 1 (b).  Figure 1. e. Design the antenna feeder: We modified an ordinary SMA feeder by adding a header (as shown by Figure 2 (a)). This header functions to convert the TEM coaxial mode into TEM cavity mode, so that signals will spread radially within the cavity material, as shown by Figure 2 (b). The definition of feeder structure parameters is shown by Figure 2 (a). The material and the values of all feeder structure parameters are listed in Tables 3 and 4, respectively.    f. Drawing the antenna: Since the structure of RLSA antenna is complex with hundreds of slots, so it is time consuming to draw this antenna manually in microwave simulation software. Hence in order to speed the drawing process, we develop a VBA (Visual Basic Applications) program. This program is embedded within the microwave simulation software. This program is able to draw an RLSA antenna within seconds while it takes days with manual drawing. Using this program we drew 150 models of RLSA antenna which vary in values of beamsquint (Φ) and the number of slots in first ring (n). g. Simulate the antenna. We simulated the 150 antenna models and we took the simulation results of several performance parameters such as gain, bandwidth, S11 response, beamwidth, efficiency and radiation pattern. A best antenna in term of performance was chosen to be fabricated. The parameters of this antenna are n = 14 and Φ = 60 0 . The best model is shown by Figure 1 (b). h. Fabricate the best antenna model and the feeder. The best antenna model and header chosen in previous step were fabricated as shown in Figure 3. i. Measure the fabricated prototype. We measured several performance parameters including gain, radiation pattern, beamwidth, bandwidth and S11. The measurement was conducted using an anechoic chamber and a network analyzer as shown by Figure 4. If they have good agreement, then meaning the design is the correct ones. k. Test the prototype. We set a test bed in order to show the ability of the prototype in working in a real communication systems. Figure 5 shows the S11 response of the antenna both in simulation and measurement. It is observed that the antenna has a bandwidth of about 720 MHz at the center of 5.8 GHz.

Results and Analysis
We can draw conclusion that the bandwidth is more than enough for Wi-Fi communications which is only 150 MHz. The interesting result is the bandwidth is also much wider compared to the bandwidth of the 16 dBi panel antenna which is only 125 MHz. Figure 6 shows the radiation pattern of the antenna both in simulation and measurement. It is observed that the antenna has beamwidth of about 28 0 and point to about 52 0 from boresight direction. We also got the antenna gain of 16.25 dBi which is 0.25 dB higher than the panel antenna gain (16 dBi).
From Figures 5 and 6, we can observe that the simulation result matches the measurement result, thus verifying the validity of our design. Slight differences between the simulation and measurement result are due to the shift of cavity, radiating element and background elements from the correct positions during the fabrication process. The differences are also due to the inaccuracies during the installation of header at the correct position in SMA Feeder. Based on above analysis, we summarized the specifications of RLSA antenna and the panel antenna as shown by Table 5. From this table, we can conclude that the RLSA antenna has better performance compared to panel antenna in term of gain and bandwidth. Especially in term of bandwidth, the RLSA antenna has much wider bandwidth compared to the panel antenna.  We tested the RLSA antenna as the antenna for Wi-Fi Devices. We set up a test bed as shown in Figure 7. The test bed consists of two transceivers. The first transceiver shown in Figure 7 (a) consists of a laptop and a Wi-Fi access point connected using an Unshielded Twisted Pair (UTP) cable. In the second transceiver shown in Figure 7 (b), we connected the RLSA antenna to a Wi-Fi access point using a pigtail cable RG-147 SMA male to RP-SMA male. The access point is connected to a laptop using a UTP cable. We carried out a Wi-Fi communication (data, picture, audio and video streaming) between the two transceivers in several conditions, those are for outdoor, indoor and different weather. The test result showed that the communication link could be established well, thus verifying the good performance of the RLSA antenna in actual implementation.

Conclusions
We have designed, simulated, fabricated, and measured the RLSA antenna based on the specification of a panel antenna that available in markets. The simulation and measurement result shows that with same size, the RLSA antenna has better performance compared to the panel antenna, especially in term of bandwidth. We also have tested the RLSA antenna as an antenna for Wi-Fi devices. The test shows that the RLSA antenna performed a good performance.