A Small RLSA Antenna Utilizing the Specification of Back Fires 17 dBi LAN Antennas

This research developed a small RLSA antenna that mimics the specification of a Wi-Fi antenna that is available in markets, which is a Back Fires 17 dBi LAN antenna. This research used the size of the back fires antenna as the size for the RLSA antenna. Base on this size, we designed and simulated 71 RLSA antenna models. Among them, we chose a best model and fabricated its prototype. We measured the prototype and found that the measurement result fits the simulation result, thus verifying the correctness of the antenna model. Furthermore, we analysed that with the same size, our RLSA antenna has better performance compared to the back fires antenna, in term of gain (0,53 dB higher), and in term of bandwidth (1075 MHz wider). We also found that our RLSA antenna is lighter and thinner compared to the back fires antenna. We also test the RLSA antenna in real condition for indoor and outdoor communication link. The test showed that the RLSA antenna can performs properly.


Introduction
Researches in developing small RLSA antennas have been carried out for last decades, such as Tharek [1], Islam [2,3], Imran [4,5] in Malaysia, Bialkowski [6][7][8] in Australia, Hirokawa [9] and Akiyama [10] In Japan.However, the developments of small RLSA antennas were still facing the problem of high reflection coefficients, so that it obstructed the development of small RLSA antennas.In 2012, Purnamirza introduced a technique that able to overcome the problem in small RLSA antenna.The techniques are named the FR4 technique and extreme beamsquint technique.In FR4 technique, RLSA antennas are designed by adding material of FR4 in addition to polypropylene which is normally utilized as material for RLSA cavity.The use of FR4 aims to change wavelength of reflected signal, then the reflected signals are eliminated by other reflected signal from their neighbours slots, thus decrease the reflections [11].In extreme beamsquint technique, RLSA antennas are designed by using high values of beamsquint angle.The use of high beamsquint values in designing the slot pairs positions will concentrate the slot pairs positions in certain area, so that this will cause most of power from feeder flow to this area and escaped.Since most of power escape from concentrated slot pairs, then the reflection in slot can be reduced [12].
In 2016, Purnamirza has successfully designed a prototype of small RLSA antenna using the extreme beamsquint technique developed in previous research [13,14].The prototype was designed to meet the market needs.The result shows that the prototype meets the market needs and performed well in real conditions.Purnamirza continued his research by developing a small RLSA antenna that mimic the specification of patch antennas [15].This research tried to find out whether RLSA antenna ables to perform as well as patch antenna.This research concludes that with same size, the RLSA antenna can perform better than the patch antenna, especially in term of bandwidth and gain.Referring to this result, this paper investigates the performance of a small RLSA antenna which has same size with other type of antenna that is Back Fires 17 dBi LAN antenna that is also available in markets.

Antena Design Steps
The design of the antenna was conducted as follows: a).Determine the antenna specifications.
We chose one of antenna that commonly available in markets, that is a Back fires 17 dBi LAN antenna as shown in Figure 1.The specification of the Back Fires antenna are the bandwidth of 125 MHz, the gain of 17 dBi, the beamwidth of 250, and the VSWR of 1.5, as shown in Table 1.These specifications were used as the specification of the RLSA antenna that will be achieved.We used the diameter of back fires antenna (listed in Table 1), that is 246 mm, as the diameter of the developed RLSA antenna.The structure of RLSA antennas consists of 3 main parts.Those are the radiating element, the background element, the cavity, as shown in Figure 2. We chose polypropylene with the permittivity of 2.33 as the cavity material.We also chose copper as the radiating element and the background material.We chose these materials based on the successful of the utilization of these materials in several previous researches [1,[6][7][8][11][12][13][14][15].We referred to Imran [16][17][18] for the thickness of cavity which is 8 mm.c).Design the feeder of the RLSA antenna.
In this research, we referred to Imran [16][17][18] for the feeder design as specified in Table 2.We used a conventional SMA feeder and modified it to fulfil the need of RLSA antenna.We placed a copper disc with the height of 3 mm at the core bar of the SMA feeder, as shown in Figure 3 (a), the definition of feeder specification is shown in Figure 3 (b).The modification of SMA feeder aims to convert the mode signal within the cavity, from TEM coaxial mode within the coaxial cable into TEM cavity mode within the cavity.Hence, signal radiated by feeder will flow radially within the cavity, as shown at Figure 3 (c).We developed a computer program using Visual Basic Applications (VBA) language in order to calculate the slots positions and orientations.Based on this calculation, the program will automatically draw the slots on the radiating elements of RLSA antennas.This program also draws the RLSA structure along with the feeder.Hence, using this program, we can draw many antenna models within a short time.In this research, we drew 71 antenna models.Each of 71 models has their own unique slot configurations as the result of different design parameters.The parameters are the number of slots in first ring (n varies from 12 to 16) and the beamsquint (Φ varies from 60 0 to 90 0 ).The other parameters values are listed in Table 3.We simulated the 71 antenna models using simulation software and took their performance parameter, such as gain, radiation pattern, beamwidth, bandwidth, and coefficient reflection.We chose a best model which has the best performance.In this research the best model was the model that was designed using n = 14 and Φ = 70

Results and Analysis
We simulated the best antenna model and measured its prototype.The measurement was conducted utilizing an Anechoic Chamber to measure radiation patterns and gains, as well as utilizing a Network Analyser to measure reflection coefficients.The measurement activities are shown in Figure 6.The left shows the activity of the measurement in Anechoic Chamber and the right shows the activity of the measurement using Network Analyzer.Figure 7 shows the radiation pattern of the RLSA antenna both for simulation and measurement.From this Figure, we can observe that the simulation fits the measurement result.We get the beamwidth of 26 0 at the main beam of 70 Figure 9 shows the respond of reflection coefficient both for measurement and simulation results.This Figure shows that the RLSA antenna has the wide bandwidth of about 1.2 GHz.In Figure 7 and Figure 9, we can observe that the measurement results are quite agreed with the simulation results.The slight differences between them are due to some imperfections in fabricating the antenna and the feeder.As mentioned earlier in section 2, the antenna materials are not compact material, but they are separated materials which are combined by sticking them together as shown in Figure 2. Hence, When they were combined, there was a slightly shift from the correct position.The slightly shift also occurred when soldering the head disc into the feeder as shown by Figure 3 (a).The imperfection is also caused by the slight change of cavity permittivity value due to the utilization of glue to stick the radiating element and the cavity.From Table 4, we can observe that the RLSA antenna has higher gain of 0.53 dB than the Back Fires antenna.The RLSA antenna also has much wider bandwidth (1200 MHz) compared to the Backfires Antenna (125 MHz).From Table 3, we also analysis that in term of dimension, with same diameter, RLSA antenna is much thinner and lighter than the Back fires antenna.In order to test the performance of RLSA antenna in real condition, we built a testbed system.The testbed consists of two transceiver as shown in Figure 11.At transceiver A, The RLSA antenna is connected to an Access Point using a Pigtail cable RG-147 SMA male to RP-SMA male.The access point is connected to a computer using UTP cable.At transceiver B, a 5.8 GHz wireless access point is connected to a computer using a UTP cable.We conducted a communication between transceiver A and transceiver B for indoor and outdoor.The test showed that the communication could establish as usual, thus verify the good performance of RLSA antenna.

Conclusion
We have developed a RLSA antenna with the size and the specification of a back fires antenna that usually found in market.We designed and simulated 71 antenna models and chose a best model.The best model then fabricated and measured.We conclude that the performance of RLSA antenna is better in term of gain and bandwidth.We also found that the RLSA antenna is much thinner and lighter compared to the back fires antenna.We also test the RLSA antenna in real condition and found that the RLSA antenna can perform its function well.

Figure 3 .
Figure 3. (a) Installation of disc into SMA feeder (b) Definitions of feeder specifications (c) Illustration of the TEM cavity mode and the TEM coaxial mode [15]

0.
The best model design is shown in Figure4.

Figure 10
Figure 10 (a).Direction of antenna rotation (b) Plot of received signal

Table 1 .
Back Fires Antenna Specifications b). Determine the material for the RLSA antenna.

Table 4 .
Comparison between RLSA and Back Fires