Abstract’ This paper presents the simulation results and analysis of a right triangular microstrip patch antenna (RTPA) with slits for C band communication. In this paper due to the effect of length of SMA connector, the broadband behavior of antenna with circular polarization is achieved. Conventionally the tip of SMA connector is taken underneath of the patch but here the tip of SMA connector is taken up to a height above the patch. In this modified antenna structure an impedance bandwidth (VSWR 2:1) of order of 27% with respect to central frequency 7.0GHz and circularly polarized bandwidth of order of 8.8% with respect to frequency 7.45GHz is observed. The maximum gain and directivity values for proposed structure are 5.06dBi and 5.60dBi respectively.
Keywords- Microstrip antenna; Broadband; Circular polarization; Axial ratio; Radiation patterns
I. INTRODUCTION
In recent times, microstrip antennas initiate widespread applications in different fields due to their striking features. These are not only finding application as individual radiators but also becoming integral part with other components in current communication systems [1]. Extensive work on microstrip patch antennas have been done owing to their advantages such as low-profile, conformability, low-cost fabrication and ease of integration with feed networks. However, conventional microstrip patch antenna suffers from very narrow bandwidth (1’2%) which poses a design challenge for the microstrip antenna designer to meet the broadband requirements [2]. The limitation of the transmitter-to-receiver orientation can be effectively solved when antennas with circular polarization are utilized. Circular polarized antenna can reduce the loss caused by misalignment between the signal and the receiving antenna [3-4]. In this communication triangular patch is taken in consideration since it provide radiation characteristics similar to those of rectangular or circular patches, but have physically smaller area than these geometries. Therefore it is easy to accommodated more triangular patches in a given area that may be proved worthy in array design [5]. Survey of available literature depicts that triangular geometries other than an equilateral triangular microstrip antenna are almost untouched. However the authors of paper have published paper on triangular microstrip antenna [6].
The C band is a name given to certain portions of the electromagnetic spectrum including wavelength microwaves that are used for long-distance radio telecommunications. The IEEE C-band contains the frequency ranges 4.00 GHz to 8..00 GHz that are used for many satellite communications transmissions, some cordless telephones, Wi-Fi devices and weather radar systems. For satellite communications, the microwave frequencies of the C-band perform better under adverse weather conditions in comparison with Ku band (11.2 GHz to 14.5 GHz) microwave frequencies, which are used by other communication satellites[7]. This paper presents the design and analysis of right triangular microstrip antenna with slits, in which the SMA probe is above the patch.
II. ANTENNA GEOMETRY AND SIMULATION RESULTS
In first part the theoretical analysis of a right triangular patch antenna (RTPA) having patch dimensions a = 40.6mm, b = 35.2mm with angle ??1 = 40.2o is carried out by applying cavity model based modal expansion technique [8] as shown in Fig. 1. The analysis of antenna is carried out by considering angles as ??, 90o, 90o-??. The substrate parameters available for glass epoxy FR4 substrate (??r = 4.4, tan?? = 0.025 and height of substrate h = 1.59mm) are applied for both theoretical and simulation analysis.
This antenna resonates at different resonance frequencies corresponding to different modes of excitation as shown in Fig. 2. The mathematical analysis of geometry following [9] revels that out of these frequencies, first frequency f1 = 6.7GHz corresponds to TM23 mode of excitation, second frequency f2 = 7.0GHz corresponds to TM41 mode of excitation and the third frequency f3 = 7.4GHz corresponds to some further higher mode of excitation probably TM33 mode of excitation. The simulation analysis of this antenna is carried out by applying IE3D simulation software [10]. The antenna is fed by inset (probe) feed arrangement with SMA connector having radius 0.62mm as shown in Fig. 1.
Fig. 1 Geometry of RTPA structure with coordinate system
Fig. 2 Variation of reflection coefficient with frequency for RTPA geometry
In the next step in order to increase the impedance bandwidth two parallel slits of dimensions length l1 & l2= 17.0mm and width w1 & w2= 2.0mm along y-axis is inserted in the same antenna geometry taken in previous case as shown in Fig.3. The separation between the slits is optimized and taken 3.0mm.
On simulating this antenna geometry by probe feed technique an impedance bandwidth of 15% with respect to central frequency 7.45GHz is observed as shown in Fig.4, which is marginally improved in comparison to previous case in which impedance bandwidth was 14% with respect to central frequency 7.12 GHz.
Fig.3 Geometry of RTPA structure with Slits
Fig. 4 Variation of reflection coefficient with frequency for RTPA with Slits
In both the cases the probe length is taken as same as the height of substrate, that probe remain underneath the patch and no part of probe remains in air above the patch. However in order to enhance the impedance bandwidth more with constant radiation properties, we have increases the length of probe beyond the patch. In our case, we have taken the probe pin length such that after passing from the patch its 9.41mm length remains in air. However, in general, the probe length is taken as same as the height of substrate and no part of probe remains in air.
Fig. 5 Geometry of RTPA structure with Slits
(Here Probe length is taken over the Patch)
On analyzing the simulation result of such modified antenna geometry reveals that antenna shows a broadband behavior with circular polarization. The variation of reflection coefficient of proposed structure is shown in Fig. 6, which depicts that antenna shows a broad impedance bandwidth nearly 27% with respect to central frequency. This bandwidth value is almost doubled in comparison to that obtained in the previous case.
Fig. 6 Variation of reflection coefficient with frequency for proposed antenna geometry
The total field directivity, efficiency and maximum field gain variation with frequency for proposed antenna are shown in Fig. 7-9 respectively. Fig. 7 depicts that maximum directivity value is around 5.6 dBi and it is more or less unaffected within the frequency range. Fig. 9 depicts that maximum field gain value is around 5.06 dBi and it is also flat within the desired frequency range.
Fig. 7 Variation of total field Directivity with frequency for proposed antenna geometry
Fig. 8 Variation of Antenna and Radiation efficiency with frequency for proposed antenna geometry
Fig. 9 Variation of total field Gain with frequency for proposed antenna geometry
Fig. 10 Variation of Axial ratio with frequency for proposed antenna geometry
The circular polarization behavior of antenna is realized through variation of axial ratio with frequency as depicts in Fig. 10. With conventional patch geometry, large impedance bandwidth and large axial ratio bandwidth may not be achieved simultaneously. Hence for achieving high axial ratio bandwidth the modification is done by increasing the probe length above the patch. The axial ratio bandwidth within 3dB range is about 8.8% with respect to central frequency 7.45GHz. The axial ratio attains minimum value close to 1.3dB at resonance frequency 7.65 GHz.
The simulated two dimensional radiation patterns of antenna at two frequencies 6.0GHz and 8.0GHz corresponding to -10dB value of reflection coefficient and at resonance frequency 7.0GHz are shown in Fig. 11(a- c). These simulated patterns at all three frequencies are more or less identical in shape.
In general for radiation pattern of patch antenna, the direction of maximum radiations is observed normal to the patch geometry for most of the application. However in our case the direction of maximum intensity has shifted from the direction of normal which perhaps due to non symmetry of patch or may be due to the generation of higher modes. This work is still in preliminary stage of progress, and we are searching the possibilities of beam-steering by making more modifications in this patch geometry.
Fig. 11 (a) Simulated elevation pattern at the frequency 6.0 GHz
Fig. 11 (b) Simulated elevation pattern at the frequency 7.0 GHz
Fig. 11 (c) Simulated elevation pattern at the frequency 8.0 GHz
III. DISCUSSION AND CONCLUSIONS
This paper presents the design and radiation performance of a single feed circularly polarized broad band triangular microstrip antenna with slits for C Band (4.0GHz -8.0GHz) on glass epoxy FR-4 substrate. The remarkable feature of this antenna is that it is not only a broadband antenna but it is also gives a better axial ratio bandwidth which is not observed in general cases. The designed antenna presents 27.7% impedance bandwidth along with 8.8% of axial ratio bandwidth. The gain and directivity values are almost constant (flat) in the desired range of frequency. The radiation patterns through out in the impedance bandwidth are identical in shape, however the direction of maximum intensity is not directed towards normal to patch geometry. This may be used as a beam steering antenna by optimizing the antenna parameters.
This paper presents only the simulation results of RTMA geometry with slits. At present these antennas are ready for testing and hopefully by the time of conference, the experimental results will be available for the validation of simulation results.
ACKNOWLEDGMENT
Authors express their sincere thanks to TEQIP II for supporting the research work.
REFERENCES
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