Coplanar Waveguide Feeds for Phased Array Antennas

G 9

NASA Technical Memorandum 104467 AIAA-91-3422

Rainee N. Simons Sverdrup Technology, Inc. Lewis Research Center Group Brook Park, Ohio and

Richard Q. Lee National Aeronautics and Space Administration Lewis Research Center Cleveland, Ohio

Prepared for the Conference on Advanced Space Exploration Initiative Technologies cosponsored by AIAA, NASA, and OAI Cleveland, Ohio, September 4 -6, 1991

NASA COPLANAR WAVEGUIDE FEEDS FOR PHASED ARRAY ANTENNAS

Rainee N. Simons Sverdrup Technology, Inc. Lewis Research Center Group Brook Park, Ohio 44142

and

Richard Q. Lee National Aeronautics and Space Administration Lewis Research Center Cleveland, Ohio 44135

Ahctrart CPW fed slot antennas which are the complement to printed dipole antennas (Fig. 2(b)) have also been reported. This This paper presents the design and performance of the antenna also radiates in a direction normal to the plane of following CPW microwave distribution networks for linear the substrate. as well as circularly polarized microstrip patches and printed dipole arrays: (1) CPW/Microstrip Line feed, (2) CPW/ This paper presents the design and performance of the Balanced Stripline feed, (3) CPW/Slotline feed, (4) GCPW/ following CPW microwave distribution networks for linear Balanced coplanar stripline feed, and (5) CPW/Slot coupled as well as circularly polarized microstrip patches and printed feed. Typical measured radiation patterns are presented, dipole arrays: (1) CPW/Microstrip Line feed, (2) CPW/ and theri relative advantages and disadvantages are compared. Balanced Stripline feed, (3) CPW/Slotline feed, (4) GCPW/ Balanced coplanar stripline feed, and (5) CPW/Slot coupled Introduction feed. Typical measured radiation patterns are presented, and their relative advantages and disadvantages are compared. Coplanar waveguide (CPW) is a transmission line which consists of a center strip and a semi-infinite ground plane CPW Microwave Distribution Network Design on either side of itl as shown in Fig. 1. This type of waveguide offers several advantages over conventional Coplanar Waveguide/Microstrip Line Feed microstrip line, namely, it facilitates easy shunt as well as series mounting of active and passive devices; it eliminates A CPW to microstrip line feed with post coupler$ is the need for wraparound and via holes, and it has a low shown in Fig. 3. The CPW and the microstrip line share a radiation loss. Another important advantage of CPW which common ground plane that has an aperture. The coupler is has recently emerged is that CPW circuits render them- formed by a metal post which passes through the aperture selves to fast and inexpensive on-wafer characterization at connecting the strip conductors of the CPW and the microstrip frequencies as high as 50 GHz. 2 Lastly, since the RF line. A pair of wire bonds located adjacent to the post tie magnetic fields in the CPW are elliptically polarized,3 the CPW ground planes and the microstrip ground planes nonreciprocal components such as ferrite circulators and to a common potential. The diameters of the aperture and isolators4 can be efficiently integrated with the feed net- the metal post were experimentally optimized to obtain the work. These as well as other advantages make CPW useful best insertion loss characteristics. The advantage of this for a MMIC based microwave distribution network. Grounded type of feed is that the thickness and relative permittivity CPW (GCPW) is a variant of CPW which incorporates an of the dielectric substrates for the feed network and the additional ground plane on the back side of the substrates antenna can be independently chosen. This allows the as shown in Fig. 1. This additional ground plane can serve individual components to be optimized for the best as a heat sink and provide mechanical strength. In addition, performance. the ground plane serves as a shield between stacked antenna boards to improve isolation. Coplanar Waveguide/Balanced Stripline Feed

Several CPW fed antennas have been reported in the A CPW to balanced stripline feed also makes use of a post literature. A GCPW fed coplanar stripline antenna con- coupler9 as illustrated in Fig. 4. The mechanical features structed by widening the center strip of the GCPW to form of the coupler is similar to that described above. This a rectangular patch (Fig. 2(a)) has been reported. 6 This coupler in addition uses a tapered stub to improve the antenna produces a linearly polarized far field radiation bandwidth of the device. The advantage of this feed is pattern in a direction normal to the plane of the substrate. similar to that of the previous example. -

Coplanar Waveguide/Slotine Feed trates the integration of a rectangular patch antenna with the feed. A CPW to slotline feed lo which uses a nonplanar CPW T-junction is illustrated in Fig. 5. The circuit is formed The measured return loss (S 11 ) and insertion loss (S21) when a CPW on a separate substrate is placed perpendicular of the CPW/Balanced stripline post coupler is shown in on the CPW of the CPW/Slotline feed network. The advan- Fig. 9. The return loss and insertion loss are better than 17 tage of this type of feed is that it eliminates the need for and 0.25 dB at the design frequency of 2.2875 GHz. Fig- direct connection between the feed line and the feed net- ures 10(a) and (b) present the proof-of-concept model of a work of the array module. seven patch hexagonal circularly polarized (CP) subarray with the above feed system. Grounded Coplanar Wave uide Balanced Coplanar Stripline Feed The above are typical characteristics of the feeds discussed in the previous section and the characteristics of the Figure 6 shows a GCPW to balanced coplanar stripline other feeds will be presented at the conference. feed.'' Since the GCPW is an unbalanced structure, the transition to a balanced coplanar stripline requires a balun.12 Far Field Radiation Patterns of the Antennas At the unbalanced end, currents of equal magnitudes but opposite direction flow in the center strip conductor 2 and The measured far field radiation pattern of the microstrip in the ground planes I and 3 of the GCPW. At the balanced patch antennas shown in Fig. 8 are presented in Fig. 11. end, currents of equal magnitude but opposite in direction The radiation is linearly polarized and is parallel to the flow in the strip conductors 2 and 3 of the coplanar stripline. plane of the antenna. The short circuit placed between conductors 1 and 2 at the balanced end results in an open circuit between the conduc- The measured far field radiation patterns of the CP tors a quarter wavelength away at the unbalanced end subarray shown in Fig. 10 is presented in Fig. 12. These forcing all currents to flow between conductors 2 and 3. patterns were measured at 2.325 GHZ. The on-axis axial ratio for the LHCP is 1.5 and 3 dB beam widths are 36° in In addition, the balun also provides impedance transfor- both planes of the subarray. The gain of the subarray as mation. This transformation ratio is determined by the determined from the beam widths is 13 dB. The measured characteristic impedance of the coplanar stripline formed return loss at the coaxial input port of the subarray is better by conductors 2 and 3. This is because all of the microwave than 10 dB at 2.325 GHz. energy propagates through this section of the transmission line. The advantage of this type of feed is that it allows The above are typical measured radiation patterns of the construction of end fire arrays which are necessary for antennas investigated and more results will be presented at building large two-dimensional phased arrays. The disad- the conference. vantage is that it requires a bond wire to tie conductors 1 and 3 at equal potential which might impact reliability. Conclusions and Discussions

Coplanar Waveguide/Slot Coupled Feed The paper presents the design and performance of several CPW microwave distribution networks which have poten- This circuit is formed by etching a narrow resonant slot tial applications in phased arrays. Typical measured far in the bottom ground plane of a GCPW. The slot is oriented field radiation patterns of several antennas with the above symmetrically and transverse to the center strip conductor feeds have been presented and discussed. of the GCPW. The advantage of this type of feed is that the microstrip patch antennas can be electromagnetically coupled References to the feed through the aperture thus making integration easy. 1. Wen, C.P., "Coplanar Waveguide:A Surface Strip Transmission Line Suitable for Nonreciprocal Gyromagnetic Device Applications," Coplanar Waveguide Feed System Performance and IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-17, No. 12, Dec. 1969, pp. 1087-1090. Antenna Integration 2. Jones, K.E., Strid, E.W., and Gleason, K.R., "mm-Wave Wafer The measured return loss (S 11 ) and insertion loss (S21) Probes Span 0 to 50 GHz," Microwave Journal, Vol. 30, No. 4, of the CPW/Microstrip line post coupler is shown in Fig. 7. Apr. 1987, pp. 177-183. The return loss and insertion loss are better than 10 and 3. Simons, R.N. and Arora, R.K., "Coupled Slot Line Field Compo- 1 dB, respectively, over a wide band of frequencies extend- nents," IEEE Transactions on Microwave Theory and Techniques, ing from 0.045 to 6.5 GHz. Figure 8 schematically illus- Vol. MTT-30, No. 7, July 1982, pp. 1094-1099. 4. Koshiji, K. and Shu, E., "Circulators Using Coplanar Waveguide," 9. Simons, R.N., Lee, R.Q., and Lindamood, G.R., "A New Coplanar Electronics Letters, Vol. 22, No. 19, Sept. 11, 1986, Waveguide/Stripline Feed Network for a Seven Patch Hexagonal pp. 1000- 1002. CP Subarray," Electronics Letters, Vol. 27, No. 6, March 1991, pp. 533-535. 5. Shih, Y.C. and Itoh, T., "Analysis of Conductor-Backed Coplanar Waveguide," Electronics Letters, Vol. 18, No. 12, June 10, 1982, 10. Lee, R.Q. and Simons, R.N., "Electromagnetically Coupled Feed pp. 5 38 - 540. Network for an Array Module of Four Microstrip Elements," 1988 IEEE International Symposium on Antennas and Propagation Sym- 6. Greiser, J.W., "Coplanar Stripline Antenna," Microwave Journal, posium Digest, Vol. 111, IEEE, 1988, pp. 1018-1021. Vol. 19, No. 10, Oct. 1976, pp. 47-49. 11. Simons, R.N., Ponchak, G.E., Lee, R.Q., and Fernandez, N.S., "Coplanar 7. Nesic, A., "Printed Slotted Array Excited by a Coplanar Waveguide," Waveguide Fed Phased Array Antenna," 1990 IEEE International 12th European Microwave Conference Digest, Microwave Exhibi- Symposium onA ntennas and Propagation Symposium Digest, Vol. IV, tions and Publishers, Kent, England, Sept. 1982, pp. 478-482. IEEE, 1990, pp. 1778-1781.

8. Simons, R.N. and Lee, R.Q., "Coplanar Waveguide/Microstrip Probe 12. DeBrecht, R.E., "Coplanar Balun Circuits for GaAs FET High-Power Coupler and Applications to Antennas," Electronics Letters, Vol. 26, Push-Pull Amplifiers," 1973 IEEE G-MTT International Micro- No. 24, Nov. 22, 1990, pp. 1998 2000. wave Symposium Digest, IEEE, 1973, pp. 309-311.

__H i } ^^ H i GROUND PLANES ^

STRIP CONDUCTOR (a)CPW patch antenna.

DIELECTRIC ADDITIONAL SUBSTRATE GROUND ( E r) PLANE Figure 1.—Schematic illustrating the fields along the coplanar waveguide and the optional lower ground plane.

(b) CPW slot antenna. Figure 2.—Coplanar wave guide antenna.

r Coplanar waveguide input/output (port #1) i Coplanar Microstrip ground plane waveguide IWS ^^ (common ground plane) S = 0.045 in. ground plane, W^^ W = 0.01 in. W m = 0.03 in. ^Ti Bond wires D t = 0.04 in. D2 = 0.1 in. T 1 = 0.125 In. EOEr 1 - Er1 =2.21n. T 2 =0.01 in. r` y E r 2 = 2.2 in. T2 ..

Dielectric ,' r ^'^ \ ` substrates —Microstrip line E OEr 2 J y output/input (port #2) Aperture of IW m diameter D 2 in the ' \\ microstrip ground plane — Metal post of r .\ diameter, D1

Figure 3.—Schematic of a CPWrmicrostrip line post coupler feed.

R, 2S RZJ Coplanar waveguide ^L input/output (port it1) N, ^-- Bond wires s ground plane CoCop^l anar wav^9eg uide i (common(commo groundn plane) ground plane w ^ Shorting pin i Tt^

Focr1 %\ '

T2 ground T^ Plane

FoFr2 —L F=^ \ Stripline 7 - output/input Ws Di e lectric I ^ I ^ (Port #2) substrate ^- etalM post of l diameter, D t Aperture of diameter D 2 in the stripline ground plane J s = 0.055 inch D2 = 0.1 inch F,2 = 2.2

w = 0.01 inch T 1 = 0.125 inch R t = 0.055 inch Ws = 0.1 inch Fri = 2.2 R2 =0.065 inch D I = 0.05 inch T2 = 0.062 inch L = 0.235 inch ctu0-51487

Figure 4.—Schematic of a CPW/balanced stripline post coupler.

4 S 11 Log mag S21 Ref. 0.0 dB Ref. 0.0 dB 5.0 dB/ 1.0 dB/

2 Slot b I I —_^ line J_ — — J 7— Coplanar W8Yegulde

1 Figure 5—Schematic of a CPW/slot line feed. Start 0.04500000 GHz Stop 6.50000000 GHz

Figure 7.—Measured return loss ( S 11 ) and insertion loss (S 21 ) of a CPW/microstrip line post coupler feed.

a = 2.07 in. b = 1.745 in.

Dielectric: substrate

X 4 balun Printed CPW port 1 dipole (a) Direct feed.

3 a=2.07 in. E ,T3,Y b=1.745 in. OEr3^, T 3 0.01 in. I I Era=2.2 I I Al^ ^^ •^ F— Coplanar Coplanar twin strip waveguide Proximity Coupled /(unbalanced) transmission line (balanced) J ` CPW port Figure 6.—Schematic of a CPW/balanced coplanar stripline feed. (b) Proximity coupled feed. Figure 8.—Schematic illustrating the integration of a patch antenna with CPW/microstrip feed.

-10 Log -20 magnitude, Sit, -30 db -40 -50 0 Log magnitude, - . 5

db -1.0 f — -1.5 045 3.0 Frequency, GHz Figure 9.—Measured return loss (S11) and insertion loss (S21 ) of a CPW/balanced stripline coupler feed.

.00 ".4 Abb— 4ftp^

142 (a) Antenna layer.

C-90-14443 (b) CPW power divider layer. Figure 10.—Proof-0f-concept model of a seven patch hexagonal CP subarray.

E-plane E-plane

H-plane H-plane Feed Feed Frequency = 2.2 GHz Frequency = 2.2 GHz

p ^ E-plane E-plane

–10 - a –20 r(a) I I I I 1 (b) ti

E 0 H-plane H-plane aE -^- –10 ^ -20 1 1 1 1 –90 60 –30 0 30 60 90 –90 60 –30 0 30 60 90 Angle, deg (a) and (c) Antena excited directly (b) and (d) Antena excited by by CPW/microstrip feed. proximity coupled CPW/microstrip feed. Figure 11.—Measured far field radiation pattern of the microstrip patch antenna.

Amplitude, 11^vY I'll J dB –20--I' 't^ Y.^ ff V M ^I r l –30 ^ ^ I I Yl, I II I I –40 I V^ (a)(b = 0` plane.

r –to-- IIVn'i',' h

Ampllitude,it –20 ' ' I Nei, I , ^, V^

–30

I I 1 –40 1 I I 1 –90 –60 –30 0 30 60 90 Angle, deg (b)d) = 90° plane. Figure 12.—Measure far field radiation pattern of the subarray.

7 NASA National Aeronautics and Report Documentation Page 1 Space Administration 1. Report No. NASA TM-104467 2. Government Accession No. 3. Recipient's Catalog No. AIAA-91-3422 4. Title and Subtitle 5. Report Date Coplanar Waveguide Feeds for Phased Array Antennas

6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No. Rainee N. Simons and Richard Q. Lee E-6315

10. Work Unit No. 506-44-2C 9. Performing Organization Name and Address National Aeronautics and Space Administration 11. Contract or Grant No. Lewis Research Center Cleveland, Ohio 44135 - 3191 13. Type of Report and Period Covered

12. Sponsoring Agency Name and Address Technical Memorandum National Aeronautics and Space Administration Washington, D.C. 20546-000 1 14. Sponsoring Agency Code

15. Supplementary Notes Prepared for the Conference on Advanced Space Exploration Initiative Technologies cosponsored by AIAA, NASA, and OAI, Cleveland, Ohio, September 4-6, 1991. Rainee N. Simons, Sverdrup Technology, Inc., Lewis Research Center Group, 2001 Aerospace Parkway, Brook Park, Ohio 44142. Richard Q. Lee, NASA Lewis Research Center. Responsible person, Rainee N. Simons, (216) 433-3462.

16. Abstract This paper presents the design and performance of the following CPW microwave distribution networks for linear as well as circularly polarized microstrip patches and printed dipole arrays: (1) CPW/Microstrip Line feed, (2) CPW/Balanced Stripline feed, (3) CPW/Slotline feed, (4) GCPW/Balanced coplanar stripline feed, and (5) CPW/Slot coupled feed. Typical measured radiation patterns are presented, and their relative advantages and disadvantages are compared.

17. Key Words (Suggested by Author(s)) 18. Distribution Statement Phased arrays Unclassified - Unlimited Microstrip antennas Subject Category 33

19. Security Classif. (of the report) 20. Security Classif. (of this page) 21. No. of pages 22. Price' Unclassified Unclassified 4 A03 NASA FORM 1626 OCT 86 *For sale by the National Technical Information Service, Springfield, Virginia 22161