A NEW PRINTED QUASI-LANDSTORFER ANTENNA Prepared by Masud Al Aziz

INTRODUCTION A new type of planar high gain antenna. Traditional approach of designing planar high gain antenna is to take Yagi-Uda antenna with driver, directors, and reflectors. A modification of Yagi-Uda antenna was the Quasi-Yagi antenna where the ground plane was used as the reflector and thus the antenna got rid of additional element as the reflector. However, the Quasi-Yagi antenna suffers from low gain. Another development was the Landstorfer antenna where the dipole elements of the traditional Yagi-Uda antenna were replaced by sweeping elements. By the introduction of the sweeping elements, the current was allowed to travel for longer path and eventually leading to higher gain. However, the Landstorfer antenna increases the overall size of the Yagi-Uda antenna.

BACKGROUND REVIEW

THE YAGI-UDA ANTENNA The antenna consists of driver, director, and reflector elements. The driver is excited by the feed transmission line. The current in the other elements is induced by the mutual coupling and therefore, the elements act as parasitic radiators. The parasitic radiators at the rear of the driver are called reflectors and the elements in front of the driver are called directors. The length of the driver element is close to λo where λo is the free space wavelength at the resonant frequency. Reflectors are electrically larger and thus are inductive. Directors are electrically small and thus are capacitive. In reflectors, induced emf leads the current whereas the phase of the current in the directors leads the phase of the induced emf. The Yagi-Uda supports a traveling wave propagation. Fig.1. A wire Yagi-Uda antenna Fig.2. A printed Yagi-Uda antenna

DESIGN GUIDELINE OF THE YAGI-UDA ANTENNA The reflector has an impact on the backward gain and input impedance but little effect on forward gain. The reflector element closest to the driver plays the significant role and therefore only one reflector element is used. The director has effect on forward gain, backward gain, and input impedance and therefore, the director is considered to be the most critical element. Addition of director elements increases the gain. The length of the feedline also affects the input impedance and gain of the antenna. The traditional Yagi-Uda antenna is featured to be a high gain but narrow bandwidth antenna. S11 (dB) Frequency (GHz) Fig.3. S11 of Yagi-Uda antenna

THE QUASI-YAGI ANTENNA The Quasi-Yagi antenna is a modification of the Yagi-Uda antenna. The antenna uses the ground plane as the reflector and thus gets rid of additional element as the reflector. The antenna reduces the overall size of the Yagi-Uda antenna. The antenna suffers from low gain. Fig.5. Schematic of Quasi-Yagi antenna Fig.6. S11 of Quasi-Yagi antenna

THE NEW PRINTED QUASI-LANDSTORFER ANTENNA The antenna uses the sweeping element as the driver and director element instead of dipole in the Quasi-Yagi antenna. The antenna uses the ground plane as the reflector. The antenna reduces the size of the Landstorfer antenna by 44% whereas increases the gain of the Quasi-Landstorfer antenna. The gain of the antenna was realized to be 7 dBi. The gain can be further increased by introducing a partial slot. Fig.7. The schematic of the Quasi-Landstorfer Antenna (v = 26.3 mm, g = 9.0 mm, q = 1.5 mm, c = 9.5 mm, s = 9.5 mm, r = 20.74 mm, b = 10.69 mm, t = 7.2 mm, n = 1.2 mm, d = 3.66 mm, k = 10.2 mm, m = 2.4 mm, H = 82.0 mm, D = 114.35 mm, W = 132.67 mm).

THEORY Let us consider a center fed 3 λo /2 dipole as shown in Fig. 8. The center λo /2 section is divided into λo /4 stub as shown in Fig. 9. The field in the section cancels out each other and the resultant antenna becomes 1λo in-phase dipole antenna. The shape of the elements can be further curved to get more directionall pattern. Fig.8. A dipole with 3 λo /2 Fig.9. A folded dipole with 3 λo /2 Fig.10. Landstorfer antenna

THEORY (CONT) Free-space wave radiation will occur if D < 150/F D = diagonal dimension of the circuit in mm F = Operating frequency in GHz In our case, D = 155.8 mm and 150/F= 61.22. D > 150/ F. Therefore, free-space wave radiation will occur. Surface wave radiation will occur if T < /alpha √(€r -1) T=Thickness of the substrate in mm €r = Relative permittivity of the substrate Alpha= 20 (with ground plane) = 10 (without ground plane) And Frequency (GHz) < 300/ {alpha √(€r -1) x T} T = 0.508 and alpha √(€r -1) = 116.82 And Frequency= 2.45 and 300/ {alpha √(€r -1) x T} = 565.5 Therefore, no surface wave radiation will occur in our design. For microstrip transmission line radiation, fc = 0.4 x zo/ T fc= cutoff frequency 0.4 x zo/ T = 39.37 GHz and our operating frequency is 2.45 GHz. Therefore, the microstrip transmission line will not radiate.

LAYOUT The overall structure comprises of a feeding part, a driven part, and a reflector part with metallization on two layers. The top layer consists of the feeding part and the driven part. In the feeding part, two arms of the microstrip line constitute the transition part that transmits energy from the microstrip line to the coplanar stripline (CPS). The driven part consists of two sweeping elements, of which the larger element is the driven element fed by CPS. The second sweeping element is the director or parasitic element that guides electromagnetic energy in the endfire direction Fig.11. A photograph of Quasi-Landstorfer antenna

MEASUREMENT AND SIMULATION RESULTS The antenna has a simulated reflection co-efficient of -16 dB at 2.45 GHz. The measured return loss is -43 dB at 2.44 GHz. The 10 dB bandwidth is measured to be 60 MHz. Fig.12. S11 of Quasi-Landstorfer antenna

MEASUREMENT AND SIMULATION RESULTS (CONT) Both of the simulated and measured results confirm the antenna to have an end-fire radiation pattern. In the y-z plane, the measurements were conducted twice from 0o to 180o to get the 360o pattern. Therefore, there is an overlapping of the measured data in the y-z plane at θ = 90o. Fig.13. Radiation pattern of Quasi-Landstorfer antenna in xz plane Fig.14. Radiation pattern of Quasi-Landstorfer antenna in yz plane

MEASUREMENT AND SIMULATION RESULTS (CONT) The gain of the Quasi-Landstorfer antenna depends on the sweeping angles of the elements. The gain also depends on careful optimization of the length of the director element, the driver element, and the reflector element. The spacing between the driver and the director elements as well as spacing between the driver and the reflector element also affect the gain of the antenna. The maximum gain of the antenna was simulated to be 7.1 dBi and measured to be 6.6 dBi. Fig.15. Simulated and measured gain of Quasi-Landstorfer antenna

MEASUREMENT AND SIMULATION RESULTS (CONT) The length of the driven element is chosen to be close to 3λ0 /2, where λ0 is the free space wavelength. The fields in the curved portion of the driven element cancel each other. Fields in the flattened portion of the driven element add to each other. The distance between the driven element and the parasitic element is optimized to induce current at the same phase from the driven element to the director. Fig.16. Current distribution of the Quasi-Landstorfer antenna

DISCUSSION In our proposed Quasi-Landstorfer antenna, the ground plane was used as a reflector and no additional reflector element was required. The overall dimension of the antenna is 133 mm × 82 mm × 0.5 mm that is 44% smaller than the planar Landstorfer antenna. The size reduction is achieved by eliminating the reflector element and utilizing the ground plane as a reflector. The removal of the reflector element also simplifies the overall design. The gain of the antenna was simulated to be 7.2 dBi that can be further enhanced by introducing a partial slot. The gain with partial slot was simulated to be 8.1 dBi. We also incorporated the balun in a single layer transmission line. Therefore, this design eliminates the need of any balun through via that simplifies the overall design. Fig.17. Measurement of the Quasi-Landstorfer antenna in an anechoic chamber

DESIGN GUIDELENE The length of the driver. The spacing between the driver and the director. The length of the director. The spacing between the driver and the ground plane.

DESIGN GUIDELENE (CONT) Fig.18. Simulated S11 of the Quasi-Landstorfer antenna for different driver length The optimum value for 160 mm of driver length was chosen.

DESIGN GUIDELENE (CONT) Fig.19. Simulated S11 and gain of the Quasi-Landstorfer antenna for different driver to director spacing The optimum value of 9.6 mm for driver to director spacing was chosen.

DESIGN GUIDELENE (CONT) Fig.20. Simulated S11 and gain of the Quasi-Landstorfer antenna for director length The optimum value of 65 mm for director length was chosen.

DESIGN GUIDELENE (CONT) Fig.21. Simulated S11 and gain of the Quasi-Landstorfer antenna for different driver to reflector spacing The optimum value of 5 mm for driver to reflector spacing was chosen.

GAIN ENHANCEMENT DESIGN A partial slot can be introduced in the Quasi-Landstorfer antenna to realize a higher gain. The overall dimension of the antenna remains same. The partial slot acts as further reflector from broadside to endfire direction. Fig. 22: The schematic of the Quasi-Landstorfer Antenna with partial slot (v = 26.3 mm, g = 9.0 mm, q = 1.5 mm, c = 9.5 mm, s = 9.5 mm, r = 20.74 mm, b = 10.69 mm, t = 7.2 mm, n = 1.2 mm, d = 3.66 mm, k = 10.2 mm, m = 2.4 mm, H = 82.0 mm, D = 114.35 mm, W = 132.67 mm, u= 1 mm).

SIMULATION RESULTS OF QUASI-LANDSTORFER ANTENNA WITH PARTIAL SLOT (b) (c) Fig.23. Simulated radiation pattern and gain of the Quasi-Landstorfer antenna with partial slot (a) radiation pattern in xz plane (b) radiation pattern in yz plane and (c) gain The radiation pattern of the antenna is still at the endfire direction. The gain increases from 7 dBi to 8 dBi with the introduction of slot.

CONCLUSION the design of a compact Quasi-Landstorfer antenna that is based on the Quasi-Yagi antenna and the Landstorfer antenna has been presented. The antenna achieves 7 dBi gain with a well-defined end-fire direction pattern. The antenna also exhibits a good match at 2.45 GHz with a bandwidth of 60 MHz. A gain enhanced design of the antenna has also been presented with the overall dimension remaining the same. With the small size and moderately high gain, the antenna can be widely used in WLAN applications, such as wireless communication, phased arrays, and millimeter wave applications.