Coupled Sectorial Loop Antenna (CSLA) for Ultra Wideband Applications N. Behdad and K. Sarabandi Presented by Nader Behdad at Antenna Application Symposium, Monticello, IL, Sep 2004 Email: behdad@ieee.org
Presentation Overview Applications of UWB technology Coupled Sectorial Loop Antenna: Antenna design and optimization for UWB operation Measurement results Modified CSLAs with reduced overall area M1-CSLA and M2-CSLA Time domain characteristics of a system that uses two CSLAs Conclusions
Applications of UWB systems 1. Home networking environments Short range data rates > 250 Mbps (streaming video, data, etc.) 2. RFID systems Paperless ticketing with ranges > 25m Localizers for inventory control. 3. Secure military communications Spread spectrum systems for secure military communications 4. Vehicular radar systems 5. Ground penetrating radars and In many of these applications, small size is a necessity therefore a COMPACT UWB antenna is needed. i.e. Logperiodic, self complementary, or traveling antennas are not good choices.
Wideband Loop Antenna Wideband operation can be achieved by minimizing the spectral variations of the input impedance A wire, circular loop antenna has a sharp anti-resonance for C 0.5λ but for C > 0.5λ, other resonances and anti resonances are not very sharp. Spectral variations of Z in of a loop antenna with thick wires are much lower than the thin ones It should be easier to minimize the spectral variations of such an element, since it does not vary strongly to begin with
Parallel, Coupled Loop Antennas I I M + _ M Two identical loop antennas are excited with a single source and in parallel. V V 1 2 = = Z Z 11 21 I 1 I 1 d + + Z Z 12 22 V V = V and I = I = 1 = 2 1 2 I 2 I 2 Z = V /( I + I ) = 1/ 2( Z + Z ) in 1 2 11 12 I Input impedance, as seen from the source, is a function of the input impedance of each loop and the mutual coupling between the two loops.
Alternative Topologies Axis of symmetry (PMC) Axis of antisymmetry (PEC) Instead of a circular loop, a half-circle is used to use the overall area more efficiently. Because of symmetry, the two loops can be connected along the vertical axis of symmetry and no horizontal current flows in the vertical arm.
Alternative Topologies Axis of symmetry (PMC) Axis of antisymmetry (PEC) α Instead of separating the two antennas to control the mutual coupling, we can change the effective area of each loop. This way, the overall area of the antenna does not change.
Coupled Sectorial Loop Antenna Ant. 2 R in R out α The input impedance of the antenna is a function of R in, R out, and α Lowest frequency of operation determines the average radius R av =(R in +R out )/2 The antenna response is only a function of α and τ=r out -R in I 2 I 1 I 1+I 2 Ant. 1 f l = ( π α + 2) 2c ε eff ( R in + R out )
FDTD Calculation of Z 11 and Z 12 600 200 Real Imaginary 400 R 11 100 X 11 200 0 0-100 -200 R in R 12-200 X 12 X in -400-300 4 6 8 10 12 14 16 18 4 6 8 10 12 14 16 18 Frequency [GHz] Frequency [GHz] α=20 300 100 X 11 Real 200 R Imaginary 11 50 0 100-50 0-100 R -100 in R 12-150 X X 12 in -200-200 4 6 8 10 12 14 16 18 4 6 8 10 12 14 16 18 Frequency [GHz] α=40 Frequency [GHz]
FDTD calculation of Z 11 and Z 12 300 100 Real R Imaginary 200 11 50 X 11 0 100-50 0 X -100 in R -100 in R 12 X 12-150 4 6 8 10 12 14 16 18 4 6 8 10 12 14 16 18 Frequency [GHz] Frequency [GHz] α=60 200 R 11 100 Real 50 Imaginary X 11 100 0 0-50 -100-100 R R X in 12 in -150 X 12-200 -200 4 6 8 10 12 14 16 18 4 6 8 10 12 14 16 18 Frequency [GHz] α=80 Frequency [GHz]
CSLA: Experimental Optimization A number of different CSLAs are fabricated: 1. R av and τ are fixed and optimum α value is found 2. R av and α opt are fixed and optimum τ is found. In order to avoid using a wideband BALUN, half of the structure above a ground plane is used and the antenna is fed with a coaxial cable. Ground Plane Antenna x φ z θ Half CSLA above the ground plane y α Probe Feed
Experimental optimization: Optimum α value Nine CSLAs with R av =13.5 mm, τ=1 mm and α=5-80 are fabricated and their S 11 responses are measured. Optimum α values is determined to be α=60, which results in the maximum possible BW for constant τ values. VSWR < 2 BW from 3.70 GHz- 10.34 GHz (2.8:1) Measured S 11 results of a number of CSLAs with different α values.
Experimental optimization: Optimum τ value Three CSLAs with R av = 13.5 mm, α = 60 and τ = 0.4, 1, and 1.6 mm are fabricated and their S 11 responses are measured It is observed that as τ decreases the antenna BW increases τ = 0.4 mm, has a VSWR < 2 BW from 3.56 GHz- 11.44 GHz (3.2:1) Measured S 11 results of a number of CSLAs with different τ values.
CSLA: Measurement Results Lowest frequency of operation determines R av and for maximum possible BW, τ must be minimized and α must be equal to 60 VSWR 3.8 3.4 3.0 2.6 2.2 1.8 1.4 1.0 A CSLA antenna with R in =27.8 mm, R out =28 mm and α=60 is fabricated and has VSWR < 2 from 1.8-14.3 GHz (~ 8:1 ratio) 1 3 5 7 9 11 13 Frequency [GHz] 15 17 VSWR of a CSLA with R in =27.8 mm, R out =28 mm, and α=60 20 19
Measured Radiation Patterns of the CSLA z x y Measured radiation patterns of the CSLA antenna in the azimuth (x-y) plane
Measured Radiation Patterns of the CSLA z x y Measured radiation pattern of a CSLA in the elevation (x-z) plane.
Measured Radiation Patterns of the CSLA z x y Measured radiation pattern of a CSLA in the elevation (y-z) plane.
Optimizing the CSLA Topology It is possible to scale the dimensions of the CSLA and use it at lower frequencies. This results in a very large and heavy antenna with large wind resistance. It is desirable to reduce the overall metallic surface of the antenna. This reduces both the antenna weight and its wind resistance. In order to do this, however, the current distribution on the antenna surface must carefully be examined.
Height and Weight Reduction of CSLA: Examining Electric Current Distribution θ=30 θ=30 Magnitude of the electric current on the surface of the antenna at different frequencies simulated in IE3D. (Right) Magnitude of the electric current is very small in the pie-slice section of -30 < θ <30. Therefore, removing this section will not significantly change the response of the antenna.
Height and Weight Reduction of CSLA: M1-CSLA Design Antenna x 50Ω SMA Connector Topology of the M1-CSLA 0.37 λ m z 30 60 Ground plane Probe feed VSWR 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 Measured VSWR of the original and M1-CSLAs 1 3 M1 CSLA 5 7 9 11 Original CSLA 13 Frequency [GHz] Original CSLA BW (VSWR < 2.2): 1.76-14.55 GHz M1-CSLA BW (VSWR < 2.2): 1.94-14.80 GHz 0.16 λ m 15 17 20 19
Height and Weight Reduction of M1-CSLA: Electric Current Distribution Magnitude of the electric current on the surface of the M1-CSLA at different frequencies simulated in IE3D. (Right) Again, we need to examine the electric current density across the antenna surface. It is seen that magnitude of the electric current is very small in the pie-slice section of 40 < θ <50 and 130 < θ <140. Therefore, removing this section will not significantly change the response of the antenna.
Height and Weight Reduction of M1-CSLA: M2-CSLA Design Simulating the electric current distribution on the M1-CSLA shows that the magnitude of the electric current is much larger close to the edges. Therefore, two other pie-slice sections are removed and the M2-CSLA is obtained. The main advantage of the M2-CSLA over the regular CSLA is its reduced copper surface and wind resistance while maintaining the same large bandwidth Antenna x 50Ω SMA Connector Topology of the M2 CSLA z 0.37 λ m 30 Ground plane 10 10 10 Probe feed 0.16 λ m
Comparison of the Original, M1-, and M2- CSLAs Comparison of the measured gains of the original, M1-, and M2-CSLAs. The antenna gains are measured at θ=φ=90. Comparison of the measured S 11 of Original, M1-, and M2- CSLAs ( ). It is observed that the antenna bandwidths are very similar.
CSLAs Time Domain Response Time domain capabilities of HP 8720D Vector Network Analyzer (VNA) are used to measure the impulse response of a system containing two CSLAs. Vector Network Analyzer d The two antennas are d=30cm apart and are aligned along their bore-sights (θ=φ=90 ) Measurements are performed for two original CSLAs, two M1-CSLAs, and two M2-CSLAs.
CSLAs Time Domain Response Time Domain Reflection Coefficient [db] 0-20 -40-60 Time [nsec] CSLA M1-CSLA M2-CSLA -80 0.0 0.5 1.0 1.5 2.0 2.5 Normalized Impulse Response [db] 0-10 -20-30 -40-50 -60 0 1 2 3 4 5 Time [nsec] CSLA M1-CSLA M2-CSLA Cable Time domain reflection coefficients of the CSLA antennas Time domain impulse response (transmission coefficients) of the system containing two CSLAs.
Conclusions A new techique for designing UWB antennas is introduced that results in antennas with: 1. Small overall occupied area: 0.37λ m 0.16λ m 2. Very large impedance bandwidth (8:1) with consistent radiation patterns in more than two octaves (4.5:1 ) 3. Good time domain response and low cross polarization levels Furthermore modified versions of CSLAs with reduced overall metallic surface are also designed and fabricated. It is shown that these antennas are more compact and have similar wideband characteristics.
M2-CSLA: Measured Radiation Patterns z x y Measured radiation pattern of the M2 CSLA in the azimuth (x-y) plane.
M2-CSLA: Measured Radiation Patterns z x y Measured radiation pattern of the M2 CSLA in the elevation (x-z) plane.
M2-CSLA: Measured Radiation Patterns z x y Measured radiation pattern of the M2 CSLA in the elevation (y-z) plane.