Microstrip antenna with reflector and air gap for short range communication in 900 MHz band

ABSTRACT


INTRODUCTION
In short range communication (SRC), the performance of a wireless communication is depending on the several requirements such as communication range (longer distance), transmit power (low power), and antenna gains (high gain) [1], [2].It should have a low capability in causing harmful interference towards the other radio equipment as well.In the antenna design, by making the antenna more directional (increasing the antenna gain), RF signals will be more focused either to increase the communication range or to reduce the transmit power (due to power amplifier) in the SRC system, as required in any regulatory body [3].On the other hand, there is a lot of extensive research that has been going on to improve the performance of microstrip antennas in order to meet the current technologies such as 5G [4] and IoT [5].
Therefore, in this paper, a microstrip antenna with air gap and reflector is proposed for SRC in 900 MHz band.This antenna was made of a microstrip fed slot with a complimentary stub on a single dielectric medium.A high gain was produced by using a reflector with air gap and a compact size was achieved if compared to convensional high gain antenna design such as Yagi Uda [6], [7].This antenna performance was validated by integrating with the SRC transmitter and receiver for the field test, a similar method applied in [8], [9].

THEORY AND DESIGN
There is a requirement of high gain performance of antenna in order to increase signal strength on certain directions and in the same time increases the communication distance.Therefore, in order to increase gain, certain element need to be added to the antenna but unfortunately will increase the size of the antenna.Meanwhile, there are several research works on the compact and high gain in microstrip antenna design as reported in [10]- [16].Therefore, it can be used in any portable device such as in the application of SRC.Certain regulatories has limited the maximum effective isotropic radiated power (EIRP) for the SRC [3].Therefore, the use of high gain antenna can reduce the power consumption in the SRC transmitter as depicted in Figure 1.This can be calculated from (1): where PT is the transmit power from SRC transmitter, LC is the losses from cable and connector and GA is the SRC antenna gain.Thus, several research works were done by other researchers on how the high gain antenna can reduce power consumption as reported in [17], [18].The proposed microstrip antenna was made of a microstrip fed slot with a complimentary stub on a single dielectric medium that was modified based on the design in [19].This is shown in Figure 2(a) for the top layer and Figure 2(b) for the bottom layer.This design was selected due to a directive and high gain of the antenna and compact in size.The fed slot with a complementary stub (wx, lx, we and le) is a slot radiating element (SRE) that was reported in [20].
As depicted in Figure 3, a reflecting ground plane was employed to achieve a unidirectional radiation pattern and also to increase the gain.This reflector was placed at the bottom of microstrip antenna as shown in Figure 4 with the air gap height (hair).There are several research works on the study of air gap between antenna and reflector for the high gain antenna [21]- [25].Thus, some of these parameters in Figures 2 and 3  The microstrip antenna part was fabricated as shown in Figure 5(a) for top layer and Figure 5(b) for bottom layer using FR4 substrate (dielectric constant=4.7 and thickness=1.6mm).The reflector (using FR4) was just cut in size and assembled with the fabricated microstrip antenna and SMA connector as depicted in Figure 6.The fabricated and assembled antenna was then performed for the measurement for verification with simulation results.The return loss (S11) for different height of the air gap was investigated in order to see the effect of the air gap towards the antenna's return loss.Therefore, the height of air gap was varied from 0, 13.33, 23, 26.67, and 40 mm and the return loss simulation result is plotted in Figure 7.It showed that the S11 was shifted to the lower frequencies by increasing the height of air gap.This is due to the increasing of capacitance between microstrip antenna and the reflector.On the other hand, without any modification of the microstrip antenna dimensions, the height of 26.67 mm is the best for the return loss.This means that the microstrip antenna is the best match with 50 Ω impedance at this height value.However, since the targeted antenna design is centered at 915 MHz, the best solution is to choose 23 mm which is very close to the 915 MHz frequency and was selected for the antenna design and fabricated for performance verification.

Effect of length and width of reflector
The effect on the reflector size towards the return loss (S11) of the microstrip antenna was investigated.Therefore, different lengths and widths of the reflector were simulated for the analysis.First, twelve different values of length were selected and simulated as shown in Figure 8.It showed that the reflector length of 250 mm was the best S11 of -15.18 dB than the other lengths.The S11 result showed that less than -10 dB for the length lower than 206.67 mm.That means, antenna matching is not good for the length lower than that value.Therefore, more than 206.67 mm length should be selected for a good impedance matching between the antenna and the reflector.Second, the different width of the reflector was simulated where five different width values were selected as shown in Figure 9.It can be seen that not much difference in the return loss performance when increasing the width of the reflector.The starting width of 90 mm was selected due to the width of the antenna substrate.Furthermore, detailed results showed that the width of 90 mm is the best return loss of -13.33 dB compared to other width sizes.Therefore, in the final design, 90 mm width was selected for the reflector size and the length of 220 mm was found to be the best between return loss and size.Take note that, a larger size of reflector is not recommended for any portable device.

Radiation pattern, antenna gain and measured return loss
The simulated three-dimensional radiation pattern of the optimized microstrip antenna for the front view is shown in Figure 10(a) and the side view is shown in Figure 10(b).As can be seen to the red color of the radiation pattern, the antenna radiation was directed to the z-axis which is toward the front view of the microstrip antenna.Beside that, it is proven that the reflector changes the microstrip antenna to be a directive gain.However, there is a small side lobe and back lobe that would be an unwanted radiation pattern for the directive antenna.
Figure 11 shows the two-dimensional view of the radiation pattern for the microstrip antenna.This is another way of plotting and analyzing the radiation pattern of the antenna.The plotted radiation pattern is in a realized gain (dB).As can be seen, the realized gain of the main lobe is higher than 0 dB and the side and back lobes are lower than 0 dB.The microstrip antenna produced 7.32 dB realized gain and 7.44 dBi directive gain.It was proven that the reflector changed the normal microstrip antenna to be a directive gain.
Figure 12 shows a simulated and measured result of the S11 of microstrip antenna in the frequency of 900 MHz band.It can be seen that the measured return loss is comparable with the simulated result.The simulated S11 was -12.84 dB.Meanwhile the measured return loss was -10.79 dB with the difference of 2.05 dB.However, there was unwanted return loss at 0.63 GHz for another resonance of the antenna in the measurement result.This could be an unwanted parasitic parallel inductance and capacitance that created this resonance.Further investigation needs to be done to find the source of this unwanted return loss.On the other hand, this unwanted return loss is operated out of 900 MHz band, thus any received RF signal at 0.63 GHz, will be easily attenuated using bandpass filter in the SRC system.13.It showed that at 5 m distance, the received signal was -25 dBm and then gradually decreased due to signal fading.At 50 m, the received signal was -35 dBm and indicated that the signal was still strong at 50 m distance.Based on these results, it can be predicted that at 100 m, the received signal would be around 60 to 70 dBm.Take note that, for simple communication (small data), the received information is still can be captured for a signal as low as -100 dBm.Therefore, in this field test results, it can be concluded that more than 100 m distance can be reached for the proposed microstrip antenna in the SRC application.Finally, the overall performances of the antenna are listed in Table 1.

CONCLUSION
From the initial design, optimization and verification through measurement and field tests, the microstrip antenna with the air gap and reflector was successfully developed for SRC application in 900 MHz band.Results showed that with the antenna size of 13,770 mm 2 , the measured return loss was -10.79 dB and the directivity gain was 7.44 dBi.It was predicted that at 100 m for the EIRP of 7.44 dBm, the received signal would be around 60 to 70 dBm.Thus, for the overall performance, this antenna works well in 900 MHz band by integrating it with the SRC system.Therefore, it is suitable for communication devices such as the SRC application.

Figure 1 .
Figure 1.Diagram for the EIRP of SRC

Figure 2 .Figure 3 .Figure 4 .
Figure 2. The dimension of the microstrip antenna, (a) top layer and (b) bottom layer

Figure 7 .
Figure 7. Simulation result for return loss with the effect of different air gap heights

Figure 8 .Figure 9 .
Figure 8. Simulation result for return loss with the effect of different lengths of the reflector

Figure 10 .
Figure 10.Simulated result for the 3-dimensional of radiation pattern, (a) front view and (b) side view

Figure 12 .
Figure 12.Measured return loss of the microstrip antenna and comparison with the simulated result

Table 1 .
Summarize of microstrip antenna performances