CHAPTER 5 PRINTED FLARED DIPOLE ANTENNA 5.1 INTRODUCTION This chapter deals with the design of L-band printed dipole antenna (operating frequency of 1060 MHz). A study is carried out to obtain 40 % impedance bandwidth with increased directivity of 2.5 dbi by optimizing width of the radial strip and further flaring the radial. The complete simulation is carried out in IE3D Software. A novel technique of further bandwidth enhancement of printed dipole to better than 50% by angle orientation is also investigated. The same concept is translated to S-band and X-band frequencies. A practical dipole is of length, which is comparable to or larger than the wavelength. A half wavelength dipole is fairly common in practice. Due to finite length of dipole, the current is not uniform along the length but exists in the form of current standing waves. Depending upon the length of the dipole the current can vary from many standing wave cycles along the length. It can be seen that as the dipole length increases upto a wavelength, the radiation pattern becomes more directive without showing any significant change in the shape of the pattern. As the length becomes larger than wavelength all of a sudden multiple beams are formed. A longer antenna therefore can provide higher radiation but at the same time may generate undesired radiation pattern. 119
Increasing the length therefore is not a solution to increase in directivity of an antenna. The typical configuration for dipoles is a straight, center fed, one half wavelength antenna. They are often utilized in conjunction with a dielectric coating to enhance radiation characteristics. The dipole antenna is fed by a two wire transmission line, where the two currents in the conductors are of sinusoidal distribution and equal in amplitude, but opposite in direction. Hence due to canceling effects, no radiation occurs from the transmission line. As shown in Figure 5.1, the currents in the arms of the dipole are in the same direction and they produce radiation in the horizontal direction. Thus for a vertical orientation, the dipole radiates in the horizontal direction and for horizontal orientation, the dipole radiates in the vertical direction. I / 2 z I Fig 5.1. Dipole Antenna x y x The typical gain of the dipole is 2dB and it has a bandwidth of about 10%. The half power beamwidth is about 78 degrees in the E plane and its directivity is 1.64 (2.15dB) with a radiation resistance of 73 ohm. 120
A microstrip dipole consists of two radials on one side of the dielectric substrate which has a ground plane on the other side. A printed dipole in its configuration consists of an active radial on one side of dielectric substrate and ground radial on the other side. Generally microstrip dipoles provide a bandwidth of 1-5 % and printed dipoles better than 10 % [71-75]. 5.2 FLARED DIPOLE WITH IMPROVED BANDWIDTH AND DIRECTIVITY In order to obtain an optimum performance from a dipole, it is desirable to design it as resonant dipole. This requires the dipole to be slightly less a half a wavelength long. A very good approximation would be 0.47 times the wavelength in dielectric. A resonant printed dipole for the frequency of 1060 MHz is designed using a Taconic make 60 mil (1.524 mm) dielectric substrate with r = 3.5. The trace width for the radials has been set to 1 mm initially. The length of the resonant printed dipole on the proposed printed circuit board (PCB) is calculated as 85.05 mm. 5.2.1 SIMULATION AND ANALYSIS The printed dipole consist of two radials namely the active and ground radial, printed on either side of the dielectric substrate. A microstrip line based quarter wave transformer is connected to the active radial to excite the printed dipole. Hence a ground plane is introduced for this microstrip line and also to reproduce the effect of electric circuitry on the PCB. The ground radial is 121
connected to this ground plane with a tapered transition. A stub is introduced in the feed line for tuning. The dipole along with the quarter wave transformer feed and tuning stub is modeled and simulated in IE3D. The printed dipole is shown in Figure 5.2. Active Radial Ground Radial Feed Line Tuning Stub Fig 5.2. Printed dipole During simulation, the resonant length is optimized to 106 mm in order to obtain the desired operating frequency. It gives a strong resonant response with bandwidth of about 7.5 % and a directivity of 2.1 dbi. The return loss plot is shown in Figure 5.3. Fig 5.3. Return Loss Plot 122
Simulation is carried out with different strip widths of the radial to obtain better impedance bandwidth. The results are given in Table 5.1. Also the simulated return loss plots for the printed dipole with different strip widths are given in Figure 5.4. Strip Width(mm) Return loss at centre frequency (db) Bandwidth (MHz) Directivity (dbi) Bandwidth (%) 1-33 74 2.17 6.98 2-24 83 2.18 7.83 3-22 94 2.22 8.86 4-25 96 2.36 9.06 6-21 101 2.39 9.52 8-27 88 2.40 8.30 10-19.5 87 2.45 8.21 Table 5.1 Bandwidth with different strip width Fig 5.4. Return Loss Plot with diff. Strip width 123
Hence, it is observed that the strip width of 6mm gives the maximum bandwidth of about 9.52 %, after which it starts decreasing. The simulated azimuth and elevation pattern are shown below in Figure 5.5 and Figure 5.6 respectively. Fig 5.5. Simulated Azimuth Pattern Fig 5.6. Simulated Elevation Pattern Simulation is carried out for different flare angles as shown in Figure 5.7 with the radial strip width of 6mm in order to achieve maximum impedance bandwidth. The simulation results are given in Table 5.2. 124
Fig 5.7. Flaring of the printed dipole radial The simulated return loss plot for the printed dipole with different flare angles is given in Figure 5.8. It is also interesting to observe that there is an increase in directivity with the flare as given in Table 5.2. Flare Angle (Deg) Return loss at centre frequency (db) Bandwidth (MHz) Directivity Bandwidth (%) 0-21 101 2.39 9.52 1-27 122 2.45 11.51 3-22 154 2.46 14.53 5-28 216 2.48 20.38 7-24 221 2.48 20.85 10-24 396 2.49 37.36 12-23 398 2.48 37.54 15-20 405 2.49 38.30 17-17 392 2.48 36.98 Table 5. 2. Simulated results for different flare angles 125
Fig 5.8. Return Loss with difference Flare angle The Smith chart for the 6mm wide radial strip with flare angles of 0 and 10 are given in Figure 5.9 and Figure 5.10 respectively. It is observed that in case of the dipole with 0 flare the impedance is predominantly inductive and also the resonant loop appears towards one side of the circle. For a flare angle of 10, the resonant loop moves near the center of the circle and also a good impedance match is obtained. 126
Fig 5.9. Smith chart for 0 flare Fig 5.10. Smith chart for 10 flare Hence, it is observed that a flare angle of 10 to 15 degrees gives a good bandwidth of about 38 % with optimum antenna performance. Beyond this point, the bandwidth remains almost constant, but the tuning frequency starts shifting. Also, there is a possible interaction between the radial and the ground plane. The physical geometry of this printed dipole with flare angle of 10 is shown in Figure 5.11. 127
Fig 5.11. Printed Dipole with a flare of 10 The simulated azimuth and elevation patterns are shown below in Figure 5.12 and Figure 5.13 respectively. It is observed that the radiation pattern is not affected by change in the shape of the printed dipole. Fig 5.12. Simulated Azimuth Pattern 128
Fig 5.13. Simulated Elevation Pattern 5.2.2 FABRICATION AND MEASUREMENTS The printed dipole antenna is fabricated using the Taconic material RF-35 of 60 mil thickness and the realized flared printed dipole antenna photograph is shown in Figure 5.14. (i) Rear view (ii) Front view Fig 5.14. Flared Printed dipole antenna The measured return loss and radiation pattern is shown in Figure 5.15 and Figure 5.16. The radiation pattern is omni directional in azimuth plane whereas it is a figure-of-eight in the elevation plane. 129
Fig 5.15. Measured Return Loss Plot (i) Azimuth Pattern (ii) Elevation Pattern Fig 5.16. Measured Radiation Pattern 130
5.3 BANDWIDTH ENHANCEMENT OF FLARED DIPOLE BY ANGLE ORIENTATION Further studies and investigations have been carried out on the printed flared dipole used above for bandwidth enhancement by changing the orientation of the dipole angle, that is by changing the angle a as shown in Figure 5.17. Oriented flared dipole Oriented angle of the flared dipole a Straight flared dipole Fig 5.17. Angle Orientation of the flared printed dipole radial Axis of the straight flared dipole The printed dipole with a radial of 6mm strip width and flared to 15 is found to be giving the best bandwidth as per Table 5.2. Hence, this particular dipole element is selected for further studies. The orientation angle is changed from 0 to 30 in steps for both the radials of dipole. Modeling and simulation is carried out in IE3D software and results for different orientation angle a are given in Table 5. 3. 131
Orientation angle a (Deg) Return loss (db) Bandwidth (MHz) (%) 0-20 405 38.3 3-25 408 38.5 5-22 413 39 10-22.5 426 40.2 15-21.5 444 41.9 20-22.5 469 44.2 25-23 498 47 30-25 526 49.6 Table 5.3 Simulation results for different a respectively. The Smith chart for a =3 and a =30 is given in Figure 5.18 and 5.19 Fig. 5.18. Smith chart for a =3 132
Fig. 5.19. Smith chart for a =30 It is observed that the reactive component of the impedance reduces with the increase in the orientation angle and also the resonance loop gets closes to the center of the circle. The simulated return loss plot for different orientation angles of the axis of the printed flared dipole are given in Figure 5.20. Fig 5.20. Simulated return loss plot for various orientation angles 133
Hence, it is observed that at a = 30, a bandwidth close to 50% is achieved with optimum antenna performance. Any further increase in the angle leads to change the concept of printed dipole antenna towards microstrip dipole. The simulated azimuth and elevation pattern for a = 30 is shown below in Figure 5.21 and Figure 5.22 respectively. It is observed that the radiation pattern is not affected by the change in the shape of the printed dipole. Fig 5.21. Simulated Azimuth Pattern Fig 5.22. Simulated Elevation Pattern 134
The physical geometry of the printed dipole with a = 30 is shown in Figure 5.23. Fig 5.23. Printed Dipole with a = 30 In order to establish this technique, the same concept has been investigated for printed dipoles with operating frequency in S-band and X-band. The printed dipole element shown below has a flare angle of 5 and tuned to 3.3 GHz with a 10 db bandwidth of 1300 MHz (39.3%). The physical geometry of the S-band printed dipole used is shown below in Figure 5.24. Fig 5.24. S Band Printed Flared Dipole 135
The simulated return loss plot, Smith chart and radiation patterns are given in Figure 5.25, Figure 5.26, Figure 5.27 and Figure 5.28 respectively. Fig 5.25. Simulated return loss plot Fig 5.26. Smith Chart 136
Fig 5.27. Simulated Azimuth Pattern Fig 5.28. Simulated Elevation Pattern The same methodology as done in the L-band printed flared dipoleis adopted to S band dipole. The orientation angle a of the S-band dipole is changed from 0 to 30 in steps of 5. The simulation results are given in Table 5.4. Orientation angle (Deg) Return loss (db) Bandwidth 137
(MHz) (%) 0-28 1300 39.3 5-29 1352 41 10-23 1456 44.1 15-19 1557 47.2 20-18 1678 50.8 25-18.5 1760 53.3 30-23 1890 57.2 Table 5.4 Simulated results for different a The return loss plots for different orientation angles are given in Figure 5.29. Fig 5.29 Return loss plot for different a 138
A bandwidth of about 51% is achieved at a = 20 with increase in the orientation angle, the trend in bandwidth increases which can be observed from Table 5.4. However, there is a shift in the tuning frequency towards higher side. The Smith chart for a = 5 and a = 30 are given in Figure 5.30 and Figure 5.31 for comparison and it can be observed that there is a resonant loop formation in both the cases. However in case of a = 30, the resonant loop is broader, also the reactive values are very low comparatively. This accounts for the broad impedance bandwidth achieved with the angle orientation of the dipole. Fig 5.30. Smith chart for a = 5 139
Fig 5.31. Smith chart for a = 30 Simulated azimuth and elevation patterns for the S-band flared printed dipole with orientation of a = 30 is shown in Figure 5.32 and Figure 5.33 respectively. It is observed that the radiation pattern is not affected by the change in the shape of the printed dipole. Fig 5.32. Azimuth Pattern at a = 30 140
Fig 5.33. Elevation Pattern at a = 30 The physical geometry of this printed dipole with a = 30 is shown in Figure 5.34. Fig 5.34. S-band Printed Dipole with a = 30 141
Hence, the trend is established that an impedance bandwidth of the order of 50% and better can be achieved in case of this particular printed dipole configuration without affecting the antenna performance. However, there is a shift in the tuning frequency with the increase in the orientation angle that can be compensated by the dipole geometries. The same concept is translated to X-band for a centre frequency of 9.375 GHz. The printed dipole used is with a flare angle of 15 and tuned to 9.375 GHz with a bandwidth of 1450 MHz (15.5%). The orientation angle a is changed from 0 to 20 in steps of 5. The geometry of the X-band printed dipole a = 20 is shown in Figure 5.35. Fig 5.35. X-band Printed Dipole with a = 20 From the Smith charts shown in Figure 5.36 and Figure 5.37, it is observed that the trace follows the 50 curve in both the cases. However, for a = 0, the resistive and reactive components increase but in case of a = 20, there is a loop 142
formation due to perfect resistive match. Hence, a broader impedance bandwidth is obtained in case of a = 20. Fig 5.36. Smith chart for a = 0 Fig 5.37. Smith chart for a = 20 The return loss plot for different orientation angles are given in Figure 5.38. 143
Fig 5.38. Return Loss plot for different a It is observed that a bandwidth of 4833 MHz that corresponds to 51.6% is achieved. The radiation pattern is unaffected and is same as that of the L-band or S-band dipole. 5.4 CONCLUSION A two step novel approach is followed to enhance the bandwidth, first by flaring the dipole radial and second by changing the angle orientation of the flared dipole. With this concept, the 10 db (VSWR 2) impedance bandwidth achieved is better than 50% and better. The same technique is translated to three different frequency bands and it is found that the concept is well established. 144