A Radial Line Slot Array (RLSA) Antenna with the Specifications of 16 dBi Outdoor patch Antenna

It is recommended by several researches that RLSA antennas have possibility as an option for Wi-Fi devices antennas. Therefore, to dig deeper this possibility, we designed a RLSA antenna that mimics the specification of an antenna usually found in market, that is 16 dBi outdoor patch antenna. We carried out a parameterization to get a best RLSA antenna model. The model was then fabricated and measured. The measurement results are quite agrees with the simulation results. We found that with the same size of 0.05 m2, our RLSA antennas has better performance in term of gain (2 dB higher), S 11 (7 dB lower), and beamwidth (90 0 wider) compared to the patch antenna. A significant result is that RLSA antenna has much wider bandwidth (815 MHz wider) compared to the patch antenna. A test to our RLSA antenna as an antenna for Wi-Fi devices shows that it works properly.


Introduction
Efforts to bring small RLSA antennas as the antenna for small devices have been doing since the beginning of 1990 [1][2]. However, due to the small size, small RLSA antennas have not enough slots to radiate power within antenna cavities, resulting high power at the perimeter of antennas, thus causing high reflection from the antenna perimeter into the antenna feeders [3]. This problem of high reflection coefficient has been a main obstacle for the development of small RLSA antenna for decades.
Based on author's observation, the using of extreme beamsquint technique is the best technique since it can obtain small RLSA antenna with high gain and good reflection coefficient response [14]. This is achieved by concentrating the antenna slots in certain area of radiating antenna. This causes most of the signal within the cavity flows into concentrated slots, and then radiated. Because most of the signal is able to be radiated, so that remain power at the antenna perimeter can be reduced significantly, thus minimize the reflection coefficients.
Based on this result [14], this research investigated the capability of RLSA antenna as antenna for Wi-Fi devices. Therefore, in this paper, we designed an RLSA antenna that mimic the specification of 16 dBi outdoor patch antenna, which is one of antenna that usually available in markets.

Research Method
The steps to get the result of this research are as follow:  [3,13]. Normally, the structure of RLSA antennas consist of three layer, which are radiating element, cavity, and background element, as shown by Figure 1(a). Figure 1(b) shows the result of our design. The design parameter of the antenna structure and its material are shown in Table 2 and Table 3, respectively.   The antenna feeder is an SMA feeder that is usually found in market. This SMA Feeder was modified by adding a copper head, as shown in Figure 2. The head functions to transform TEM coaxial mode signal within coaxial cable into TEM cavity mode signal within the cavity structure, so that the signal from the feeder will propagate in radial directions within the cavity of the antenna, as shown by Figure 2(a). Table 4 show design parameters of the feeder and Figure 2  e. Conduct parameterizations: The design antenna resulted from step (d) was then simulated to get the characteristic of the antenna in term of gain, radiation pattern, beamwidth, bandwidth, and coefficient reflection. In order to get a best antenna model, We carried out parameterizations by tuning several antenna parameters such as the number of slots in first ring (n varies from 10 to 16)

Results and Analysis
The prototype of RLSA antenna resulted in previous section, was measured in a Anachaoic Chamber to measure the antenna radiation pattern and the gain, and also using Network Analyzer to measure the reflection coefficient. The activities of measurement is shown by Figure 4.  From Figure 5 and Figure 6, We can notice that the simulation result is agree with the measurement result. The difference between them is due to the imperfections during the fabrication process of the antenna model. The imperfections are caused by: firstly, the radiating 51 element, the cavity and the background are separated elements, so when they were combined during the fabrication process, there was a slight shift from the correct position. Secondly, the permittivity of the cavity is slightly increases due to the use of glue to stick the radiating element and the background to the cavity. Thirdly, there is an imperfection in soldering the head disc at the SMA feeder at the correct position A testbed systems was developed in order to show the performance of the antenna in real situations, as shown in Figure 7. The testbed consists of two 5.8 GHz transceivers, the RLSA antenna connected to one of the transceivers, and a panel ant enna connected to the other transceiver, and two computers connected to the two tranceivers.

Figure 7. Testbed System
Using this testbed, We tested the performance of the RLSA antenna by setting up connection between the two computers. The test result showed that the connection could be established just like usual, then this verify the good performance of the RLSA antenna in real conditions.