Wide Band and High Gain Planar Array with a Suspended Stripline Feeding Network N. Daviduvitz, U. Zohar and R. Shavit Dept. of Electrical and Computer Engineering Ben Gurion University i of the Negev, Israel ANTENNAS MINI SYMPOSIUM ANTENNAS MINI SYMPOSIUM July 30, 2012 Faculty of Engineering, Tel Aviv University
Outline Motivation Suspended Substrate Stripline Radiating Element Design Feeding Network Design Antenna Array Analysis Comparison to other feeding networks Conclusions
Motivation and Previous Work Motivation In many wireless applications, single element antennas are unable to meet the gain and radiation pattern requirements and an antenna array is necessary. The feeding network complexity and length, decrease the array efficiency i and gain for medium and large size arrays. Afeedingnetworkwithlowlossesisanimportantfactorinthe enhancement of the gain and radiation efficiency. Previous Work on the subject R.N. Simons, "A Millimeter Wave Cavity Backed Suspended Substrate Stripline Antenna, IEEE Antennas and Prop. Inter. Symp. Dig.,vol 3, pp. 2110 2113 2113, 1999.
Suspended Substrate Stripline Physical Layout The suspended substrate stripline consists of a strip conductor of width w and thickness h printed on a dielectric substrate of thickness d. The substrate is suspended inside a grooved metal channel of width a and height b. Cross section of the suspended substrate stripline Suspended Stripline Main Advantages Low losses due to thin dielectric sheet r =3.66,tan =0.004. Wide band support due to TEM mode propagation. Small dimensions compared to standard waveguides, which allows compact design. No coupling between suspended stripline adjacent lines. No radiating losses from the feeding network. No surface wave losses.
Radiating Element Physical Layout The radiating element consists of a shorted cylindrical waveguide fed by a metallic strip supported by a thin dielectric substrate thus forming a probe. The probe is the extension of the suspended stripline and is inserted into the waveguide through a rectangular opening in the waveguide wall. Design Guidelines The waveguide supports only TE 11 mode propagation. Impedance matching is achieved by optimization of the suspended ddstripline feeding probe parameters L and w and its distance, Bt from the short. EM simulation performed using HFSS commercial software. Radiating element
Radiating Element Return loss of the radiating element as a function of frequency Description Physical Size Waveguide radius R 10 mm Probe length L 5.34 mm Probe width w 0.6 mm Distance to aperture Tp 26.5 mm Distance to short Bt 6.05 mm Optimized physical dimensions of the radiating element Radiating element
Radiating Element Radiating element gain as a function of frequency Copol and Xpol of the radiating element at the center frequency Parameter Value of 10GHz Gain (10GHz) 8.63 dbi Main lobe direction 0 deg. 94 deg. 62 deg. Rad. efficiency (10GHz) 0.9934 Single element radiation parameters
Suspended Substrate Stripline and Microstrip Comparison Microstrip i Mi Main Disadvantages 1. High transmission losses as frequency goes up 2. Radiation losses 3. Mutual coupling to adjacent lines Cross section of a microstrip line Comparison of the insertion loss of a 1cm suspended stripline and a microstrip line
Suspended Substrate Stripline Parameter Study Characteristic impedance of SSL as a function of the metal strip width at the center frequency of 10GHz Cross section of suspended substrate stripline Insertion loss at the central frequency of 10GHz as a function of the metal strip width
Electric field distribution tion in a cross section of the transmission line Description Physical Size Transmission line width a 1.6 mm Transmission line height b 0.8 mm Metal strip width w 1 mm Metal strip thickness h 0.03 mm Dielectric sheet thickness d 0.2 mm Feeding Network Design Guidelines Low loss operation based on the suspended stripline analysis. Injected power (amplitude and phase) divides evenly among the 16 radiating elements. Main lines characteristic impedance Z 0 =41 and element impedance 22. Feeding network
Feeding Network Analysis Feeding network HFSS design Phase difference of the two output ports of the T junctions as a function of frequency Amplitude difference of the two output ports of the T junctions as a function of frequency
Mutual Coupling Between Array Elements Type 2 Elements Type 1 Elements Type of elements due to mutual coupling considerations Real and imaginary parts of type 1 elements Real and imaginary parts of type 2 elements
Antenna Array 4x4 element array geometry Description Physical Size Parameter Array length 9.4 cm Gain (10GHz) Array width dh 9.4 cm Main Lobe Direction Antenna height 3.3 cm Spacing between 2.35 cm ( ~ 0.8 λ0 ) elements center Antenna array physical and electrical parameters Value 20.41 dbi 0 deg. 16 deg. 16 deg. -12.8 db
Antenna Array Array gain as a function of frequency Copol and Xpol arrayradiation radiation patterns at the center frequency of 10GHz Array radiation efficiency as a function of frequency Array return loss as a function of frequency
Boxed Stripline Feeding Network Boxed Stripline Design Guidelines The boxed stripline consists of a strip conductor boxed inside a waveguide. In order to check the SSL feeding network efficiency, a similar array was designed. Junction and element matching principals were kept in order to efficiently split the energy among the elements. Cross section of a boxed stripline designed with HFSS
Hybrid Waveguide Suspended Stripline Feeding Network The hybrid feeding network The inner slits in the power divider were designed to help distribute the phase evenly among the 4 output ports. Hybrid array top view 1 to 4 power divider top view Description Physical Size Type 1 slit depth 12.1 mm Type 1 slit width 0.05 mm Type 1 slit distance to input 21.05 mm Type 2 slit depth 5.36 mm Type 2 slit width 0.1 mm Type 2 slit distance to input 4.5 mm 1 to 4 power divider physical parameters
Hybrid Waveguide Suspended Stripline Feeding Network Coax to Suspended Stripline Transition Due to physical size limitations, the 4 group element T junctionwas redesigned andthe coaxial line is matched to excite each 4 group element evenly. 4 Group Junction HFSS Design Description Coax inner radius Physical Size 0.32 mm Coax outer radius 0.78 mm Coaxial line physical parameters Phase difference of the two output ports of the T junction as a function of frequency Amplitude difference of the two output ports of the T junction as a function of frequency
Hybrid Waveguide Suspended Stripline Feeding Network Coax to Waveguide Transition The waveguide aegidephysical size was selected to support single mode propagation at a frequency of 10 GHz. The coaxial line was matched to the shorted waveguide to decrease losses as much as possible. Description Waveguide height b 7 mm Physical Size Waveguide width a 19.3 mm Coax inner radius extension L1 1.1 mm Matching section size L2 4 mm Matching section radius R2 0.625 mm Distance from short d 3.9 mm(~0 0.13 λ0) Coaxial to waveguide transition physical parameters Cross section of the waveguide and the coaxial line Return loss as a function of frequency
Hybrid Waveguide Suspended Stripline Feeding Network 4 to 1 Power Divider The waveguide width physical size forced feeding network to become a 4 to 1 power divider. The power divider is excited througha coaxial line inserted through the bottom and distributes the power evenly, amplitude and phase, to the 4 output ports. 4 to 1 power divider side view Phase difference between the 4 ports of the power divider as a function of frequency Amplitude distribution to the 4 ports of the power divider as a function of frequency
Comparison of Arrays Parameters with Different Feeding Networks Return loss of the arrays as a function of frequency Array gain as a function of frequency
Comparison of Arrays Parameters with Different Feeding Networks Parameter Suspended Stripline Hybrid Array Boxed Stripline Array Microstrip Line Feeding Network Array with Patch Element Array Max Gain (dbi) 20.41 20.46 19.53 18.04 Rad. Efficiency (10GHz) 0.923 0.927 0.76 0.55 Height (mm) 33 49 33 ~ 3.6 Manufacturing complexity Comparison Conclusions Moderate High Moderate Easy Array 4x4 parameters comparison The hybrid arrayisthe mostefficient in comparison to other arrays, but the manufacturing complexity and the added height is a major disadvantage. The microstrip line array is the simplest array to manufacture, but it lacks in radiation efficiency. The suspended stripline array is a good compromise in manufacturing complexity and radiation efficiency.
Conclusions The performance of a 4x4 array with SSL feeding network was analyzed and optimized to obtain: Radiation efficiency of 91% in the frequency band 9.6 10.7GHz. Gain of 20.41 dbi at the central frequency (10 GHz). 10dB return loss bandwidth of 1.08GHz (11% bandwidth) around the central frequency (10 GHz). An improvement of 15% in radiation efficiency and 0.8 db in gain was obtained compared to a similar array based on boxed stripline feeding network. The SSL feeding network is simpler to fabricate and provides better radiation efficiency in comparison to the boxed stripline and the hybrid waveguide feeding networks.