Multiband frequency selective surface with multiring ... - IEEE Xplore

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 42, NO. 11, NOVEMBER 1994

Multiband Frequency Selective Surface with Multiring Patch Elements Te-Kao Wu, Senior Member, IEEE, and Shung-Wu Lee, Fellow, IEEE

Abstract- Theoretical discussions and experimental verifications are presented for a multiband frequency selective surface (FSS) with perfectly conducting multiring patch elements. It is found that the narrow-ring approximation is valid for a ring width less than 0.025X with X being the wavelength of the FSS’s resonant frequency. A single screen double-ring element FSS is demonstrated for (1) a low-pass FSS that reflects the Ka-band signal while passing the S-, X-, and Ku-band signals, and (2) a tri-band system that reflects the X-band signal while transmitting through the S- and Ku-band signals. In addition, a double screen four-band FSS with non-similar double-ring elements is developed by cascading the above mentioned two single screens. The good agreement obtained between the measured and the computed results verified the computer codes and the approaches of this paper.

K /X

FEED

\ Fig. 1. Proposed Cassini high-gain antenna with a four-frequency FSS

I. INTRODUCTION

F

REQUENCY SELECTIVE SURFACES (FSS) have often been considered for the reflector antenna applications [1]-[7]. Typically, an FSS is employed for the subreflector and the different frequency feeds are optimized independently and placed at the real and virtual foci of the subreflector. Hence, only one single main reflector is required for the multifrequency operation. For example, the Voyager FSS was designed to diplex S and X bands [l]. In that application the S-band feed is placed at the prime focus of the main reflector, and the X-band feed is placed at the Cassegrain focal point. Note that only one main reflector is required for this two band operation. Thus, tremendous reduction in mass, volume and, most important, the cost of the antenna system are achieved with the FSS subreflector. In the past, the cross-dipole patch element FSS was used for the subreflector design in the reflector antennas of Voyager [ 11 for reflecting the X-band waves and passing the S-band waves, and the Tracking and Data Relay Satellite System (TDRSS) for diplexing the S- and Ku-band waves [2]. The characteristics of the cross-dipole element FSS changes drastically as the incident angle is steered from normal to 40”. Thus a large band separation is required to minimize the RF losses for these dual band applications. This is evidenced by the reflection Manuscript received August 30, 1993; revised June 9, 1994. This work was carried out by the Jet Propulsion Laboratory, California Institute of Technology, and supported under contract with the National Aeronautics and Space Administration. T.-K. Wu is with the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91 109 USA. S.-W. Lee is with the University of Illinois, Urbana, IL 61801 USA. IEEE Log Number 9406390.

and transmission band ratio ( f T / f t ) being 7 : 1 for single screen FSS [2] or 4 : 1 for double screen FSS [l] with cross-dipole patch elements. Better elements, such as the multicircular or square loop elements [3]-[lo] are definitely needed to achieve (1) the multiplexing of four frequency bands, (2) smaller frequency-band separations (.ft/fr less than 1.7), and (3) less sensitivity to the incident angle variation and polarizations. Recently, NASA’s Cassini project required the use of multiple microwave frequencies at S-, X-, Ku- and Ka-band for science investigations and data communication links. A single high gain antenna (HGA) with a four-band FSS subreflector, as illustrated in Fig. 1, was proposed. This arrangement allows a Cassegrain configuration at X (7.2 and 8.4 GHz) and Ka (32 and 34.5 GHz) bands and a prime focus configuration at S (2.3 GHz) and Ku (13.8 GHz) bands [5]-[8]. Circular polarizations are required for all frequencies except the Ku-band. To meet the Cassini HGA subsystem’s requirements, two design approaches as shown in Fig. 2, were proposed. In addition, the FSS screen was bonded on to a Kevlar honeycomb panel to meet the mechanical and thermal environmental requirements. The first approach, implementing the two-screen design, uses two FSS grids. The front FSS grid is called Ka-add-on FSS. It reflects Ka-band waves but passes S-, X- and Kuband waves. The back FSS grid is called the three-frequency (or tri-band) FSS. It reflects X-band waves but passes S- and Ku-band waves. The resultant four-band FSS reflects both Xand Ka-band waves but passes both S- and Ku-band waves. The second approach, implementing the single screen design, uses only a single FSS grid to reflect the X- and Ka-band

0018-926W94$04.00 0 1994 IEEE

WU AND LEE: MULTIBAND FREQUENCY SELECTIVE SURFACE

I485

10 MIL DUROID 6010.5 (WITH FSS GRIDS)

12 MIL KEVLAR-49 EPOXYSKIN (3 PLIES)

PERIODIC CELLS

Y

HONEY COMB OR FOAM CORE

SINGLE SCREEN DESIGN

A UNIT CELL: MAX 4 CONCENTRIC METAL RINGS

1

2 MIL KAPTON WITH Ka-ADD-ON FSS GRIDS

12 MIL KEVLAR SKIN (3 PLIES)

‘ 1

2 MIL KAPTON WITH 3-FREQUENCY FSS GRIDS

I

HONEY COMB OR FOAM CORE DOUBLE SCREEN DESIGN SIDE VIEW

Fig. 2.

TOP VIEW

Cassini’s FSS design approaches.

waves and to pass the S- and Ku-band waves. Earlier only the single and narrow ring element FSS was considered for the triband FSS design [6]. However, for the four-band FSS, wide and multiring elements have to be considered. The current expansion functions for both narrow and wide ring elements were presented in [6], but no discussion was given on their convergence rate and limitations. Hence they will be discussed in this paper. In addition, the design and performance of all the above mentioned FSS’s with multiring elements are demonstrated.

11. THEORETICAL DISCUSSIONSOF MULTI-RINGFSS

A modal analysis of a single screen FSS with circular ring patch element was presented by Parker, et al. [9]-[ 131. However, their analysis is limited to narrow rings with dielectric substrates on one side of the metallic screen only. For the present multiband application, wider rings are required. Hence the exact theory [6] is considered for rings with an arbitrary width and multiple layers of dielectrics on both sides of the grid. The theory is similar to the standard modal analysis published by Chen [14], Lee [ 1.51, and by Roberts and McPhedran [16]. The difference is in the ringelement’s current expansion function. The expansion function used to represent the current on the ring element, i.e., [6, (9)-( 1 I)] is related to the modal field in a coaxial waveguide via the Babinet principle. The Fourier transform of this current expansion can be found in the paper by Amitay and Galindo (171.

Some discussions on this current expansion are given below; I ) Each coaxial mode satisfies the boundary condition that the tangential electric field is zero at the inner and outer circular edges of the aperture element. Consequently, all mo’des form a field that satisfies the edge condition at sharp metal rims. Hence the use of coaxial modes as expansion function for the present FSS problem is the “natural” one. It has been proven 1191 that the natural waveguide modes allows rapid convergence and accurate solution in the scattering from periodic structures. 2) The eigenvalues { xlnn and xnLnin 1.6, (IO), ( 1 l)]} must be determined numerically. A small table is available in [ 181, but is not adequate for the present application. Using modern computers, we have compiled a much bigger table which enable us to interpolate the eigenvalues efficiently for most ring size used in the FSS design [20]. 3) In the case when the width of the ring is electrically small (less than O.O25X), a simplified expression, i.e., [6, (12)], can be used for the field expansion function. 4) The aperture field in 16, (9)] is for one ring. A superposition of this equation is needed to represent the complete aperture field for the multiring element. A matrix equation may be formulated by matching the following, boundary conditions at the region that contains the ring elements: (1) the tangential E fields are continuous at the grid plane, and (2) on the metallic rings, the tangential E field := 0. This matrix equation allows the determination


1486

TABLE I CONVERGENCE STUDY FOR THE RESONANT FREQUENCY OF A SINGLE SCREEN

FSS WITH

SINGLE RING PATCH ELEMEN?

I Floguet Mode Number ' 2.33 35

24.3(GHz)

23.8

23.1

23.1

23.0

22.0

22.0

22.0

21.6

21.6

4.0

of the unknown coefficients for the current J , on the metallic rings. Once the current is found, the incident and reflection coefficients at each dielectric interface can be calculated. Finally, the reflection and transmission coefficients (Rpqand Tpq) of the overall FSS can also be calculated, which will lead to the determination of the reflected and transmitted fields. Two computer codes were developed based on the above theory. MRINGC code is limited for narrow ring element, since it is based on the thin wire approximation, i.e., the ring current has no radial variation nor components. However, MRING2C code is not limited, since it is based on the exact waveguide modal fields. To check the convergence rate and the accuracy of the codes, the resonant frequency of the single thick polyester was computed ring element FSS on a 0.075" using the MRINGC code and compared to the measured results of [ll]. The geometric dimensions of this FSS grid are (see Fig. 2) a = b = 4.9 mm, c = 1.95 mm, d = 2.25 mm. Table I summarizes the number of Floquet modes needed for different dielectric constant of the substrate. Here the current expansion mode number is seven. Note that the results converge, as 625 Floquet modes are used. Furthermore, in [l 11, it is stated that the same FSS etched on a polyester substrate (with 2.33 dielectric constant) is resonant at 22 GHz. However, from Table I, in order to get the same resonant frequency at 22 GHz as in [I 11, the dielectric constant must be 3.5 instead of 2.33. For the MRING2C code, seven terms in the current expansion are sufficient to obtain the same result. 111. SINGLESCREEN LOW-PASS (KA-ADD-ON)FSS

To design the required Ka-add-on FSS as mentioned in Section I, a single screen FSS with one to four-ring as the element were studied. The goal is to determine a high-Q ringelement FSS with maximum transmission loss for frequencies higher than 32 GHz and minimum transmission loss for frequencies lower than 14 GHz. In general, the element's circumference of a free standing single ring element FSS should be a wavelength long at the resonant frequency, i.e., 33 GHz for this low-pass FSS. However, due to dielectric loading this wavelength (or resonant frequency) is increased (or decreased) when the FSS grid is etched on a dielectric substrate. The amount of frequency shift is proportional to the dielectric constant of the substrate. Thus the element's

D = 169 r l = ,047" r 2 = ,028"

COMPUTED -10

-TM 45" TE 45" TE 30"

MEASURED

-30

0

5

10

15

20

25

30

35

FREQUENCY (GHz)

Fig. 3. Design and transmission performance of the Ka-add-on FSS

circumference should be reduced accordingly to reset the resonant frequency back to 33 GHz. Comparing to single ring element FSS's, the double ring (DR) element FSS was found to give a much higher Q performance [7], since it exhibits two resonances, i.e., one at a lower frequency (caused by the larger ring) and the other at a higher and closely separated frequency (caused by the smaller ring). Fig. 3 gives the design and computed transmission performance of a DR FSS on the Kevlar honeycomb panel as in Fig. 2, for incident angles steered from normal to 45'. Note the double-ring patch element array is etched on a 0.002-in thick Kapton sheet with 0.029 ounce/ft2 copper and then bonded to the Kevlar honeycomb panel. Only representative measured data at 30" incidence is given here to demonstrate the good agreement between the computed and measured data. The computed results were obtained using the MlUNG2C code, because the narrow-ring code predicts a wrong resonant frequency. This also implies that the narrow ring code is not valid for ring width greater than 0.028 A, i.e., the wavelength at the resonant frequency of the FSS. Elements with more


INSERTION LOSS

CI

1487

TABLE I1 (dB) SUMMARYOF THE THREE-FREQUENCY FSS

I

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than two rings were also considered. But they were discarded because of the fabrication complexity and no performance improvement.

FSS IV. SINGLESCREENTRI-BAND(THREE-FREQUENCY) Similarly, the three-frequency FSS design and performance are given in Fig. 4. Again the double-ring patch element array is etched on a 0.002-in thick Kapton sheet with 0.029 oz/ft2 copper and then bonded to the Kevlar honeycomb panel of Fig. 2. Since the width of the rings are small compared to the radii, the computed results may be obtained by either the narrow-ring or the wide-ring FSS code. Only representative measured data at 0" TE incidence is given here to demonstrate the agreement between the computed and measured data. As can be observed from this figure, the resonant frequency is very close to the designated 8.4 GHz for both TE and TM polarizations even when the incident angle is changed from normal to 45'. The FSS's insertion losses at these three frequency bands are summarized in Table 11. Note that the losses at 2.3 and 13.8 GHz are transmission losses, while they are reflection losses at the other frequencies. The crosspolarized components are found to be more than 20 dB below the co-polarized components.

FSS V. SINGLESCREENFOUR-FREQUENCY As mentioned in Section 11, the double-ring element FSS provides two resonances, i.e., one at a lower frequency (caused by the larger ring) and the other at a higher frequency (caused by the smaller ring). Therefore, one might be able to design a single screen DR FSS for the Cassini four-band FSS. Namely, only one DR FSS grid might be needed for reflecting the Xand Ka-band while passing the S- and Ku-band waves. To avoid the grating lobe occurrence at Ka-band, the single screen FSS is designed with a high dielectric constant (er = 11) Duroid 6010.5 substrate. To provide a pass-band at Ku-band, the width of the inner ring must be large. Fig. 5 shows the geometry and configuration of this DR FSS. The computed transmission performance of this DR FSS is illustrated in Fig. 6 at S-, X- and Ku-band for incident angle steered from

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Fig. 4. Design and transmission performance of the three-frequency FSS.

normal to 45". Representative comparison data between the measured and computed results are also shown in Fig. 6 for 30" incidence only. The MRING2C code was used to compute the transmission performance of this single screen DR FSS, since the inner ring width is much greater than 0.028X with X being the wavelength of the second resonant frequency (i.e., 33 GHz). Predicted Ka-band performances are illustrated in Fig. 7 along with the representative measured results at 30' incidence. Fig. 7 also shows that at Ka-band no common reflection band can be found for both the TM and TE polarizations. This indicated that the single screen DR FSS is good only for a 3-frequency

1488


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TOP VIEW

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TM 30" D = 0.22" r1 = 0.1" r2 = 0.035"

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w2 = 0.034''

26

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27

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29

31

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32

l

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36

FREQUENCY, GHz

Fig. 7. Ka-band transmission performance of the FSS in Fig. 5 . SIDE VIEW

Namely, the FSS element designs, i.e., the element geometrical dimensions (especially the periodicity and the lattice types), are different for the two screens. As illustrated in Fig. 2(b), 10 MIL this double screen four-frequency FSS consists of a Ka-add-on DUROID 6010.5 FSS (element design shown in Fig. 3) on the top side and a LAMITE single screen tri-band FSS (element design shown in Fig. 4) Fig. 5 . Configuration of a double-ring FSS on a Duroid 6010.5 substrate at the bottom side of the honeycomb sandwich. The exact cascading analysis of two non-similar FSS screens is rather involved [21]. However, using the following systematic procedures, a single-mode cascading approach may readily be employed to get a first order assessment of this double-screen four-frequency FSS's performance. First, one divides the FSS at the mid-plane of the Kevlar honeycomb. Upwards from this mid-plane is considered the first FSS screen cn v, -20 section, and below this plane is the second FSS screen section. COMPUTED MEASURED 0 0 0 30"TE z Each FSS section can be accurately modeled by the single -30"TE o o o 30" TM a screen FSS analysis described earlier. Since the dividing plane e --- 45"TE -30 is electrically far from the FSS grid and the FSS element V 45" TM spacing is less than a half wavelength, one can assume that all the modes are decaying except the 0th order mode. Thus 0 5 10 15 one may next cascade the two sections by converting the FREQUENCY (GHz) scattering matrix [s] from each section to a transmission matrix [t]and multiplying the resulting [t]matrices. The converting Fig. 6. S-, X-, and Ku-band transmission performance of the FSS in Fig. 5 . from [s] matrix to [t] matrix is the same as that described in [22]. The final [t]matrix product is then converted back FSS application. For four-frequency FSS applications, the two- to a scattering matrix, which yields the transmission and screen design should be implemented with this double ring reflection coefficients for the double screen FSS. Fig. 8 shows element FSS. the comparison of the computed and measured transmission performance for this double screen FSS. The good agreement VI. CASCADING Two NON-SIMILARSCREENS verified this efficient cascading approach. The double-screen In [6], equations were derived to analyze single screen FSS performance at all the Cassini frequencies is summarized or double screen FSS with multiring patch elements. This in Table 111. The cross-polarized components are also found to analysis can be further extended to analyze an integrated be more than 20 dB below the co-polarized components. Note that this single mode approximation is similar to the double screen FSS by employing the symmetry property and the technique of image theory [ 6 ] . In another words, transmission-line cascading approach of [ 191. The feature of the second FSS screen is the exact duplicate of the first this simplified approach is that the cascading analysis avoids FSS screen. However, Cassini's four-band FSS (S/X/Ku/Ka the specific element geometrical dimensions of the individual bands) needs double screen FSS with non-similar design [ 8 ] . FSS screen. In other words, the single-mode cascading analysis

t


1489

TABLE III INSERTION LOSS (dB) SUMMARY OF THE INTEGRATED DOUBLE SCREEN FSS MODEL

components are being considered. Furthermore, the 0th order scattering matrix of each individual FSS screen is computed accurately with all the higher order modes included in the analysis.

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VII. CONCLUSION

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Theoretical discussions, design and test results are presented for the multiband FSS with multiring patch elements. Good agreement between the measured and computed results verifies the various design approaches described in this paper. It is found that the narrow-ring approximation is valid for a ring width less than 0.025X with X being the wavelength of the FSS’s resonant frequency. To multiplex the Cassini four frequency bands (S/X/Ku/Ka) only the double screen FSS approach1 can give satisfactory results. It is also demonstrated that this double screen FSS can be designed by the efficient single cascading mode approach described in Section VI. ACKNOWLEDGMENT

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The authors wish to thank Dr. Ken Woo for the managerial support, Mr. G. Hicky for fabricating the honeycomb sandwiches, and Mr. Cosme Chavez for performing the FSS measurement.

15

Frequency (GHz)

Fig. 8. Transmission characteristics of the integrated double screen FSS with 0.75-in thick Kevlar honeycomb sandwich.

depends only on the spacer between the two FSS screens and the 0th order scattering matrix of each individual screen. It should be pointed out that this single-mode cascading analysis, however, is better than the transmission-line approach due to the fact that both CO- and cross-polarization (i.e., TE and TM)

REFERENCES [ l ] G . H. Schennum, “Frequency-selective surfaces for multiple frequency antennas,” Microwave J . , vol. 16, pp. 55-57, May 1973. [2] V. D. Agrawal and W. A. Imbriale, “Design of a dichroic Cassegrain subreflector,” IEEE Trans. Antennas Propagat., vol. AP-21, pp. 466-473, July 1979. [3] K. Ueno, et al., “Characteristics of FSS for a multiband communication satellite,” in Inr. IEEE AP-S Symp. Dig., Ontario, Canada, June 1991, pp. 735-738. [4]S. W. Lee, et al., “Designs for the ATDRSS Tri-band reflector antenna,” in Int. IEEEAP-S Symp. Dig.,Ontario, Canada, June 1991, pp. 666669.


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[5] T. K. Wu, “Double-square-loop FSS for multiplexing four ( S / X / I i t i / I C a ) bands,” in In?. IEEE AP-S Symp. Dig., Ontario, Canada, June 1991, pp. 1885-1888. r61 . _J. Huang. T. K. Wu, and S. W. Lee, “Tri-band FSS with circular ring elements,” fEEE Trans. Antennas Propagat.. vol. 42, pp. 166-175, Feb. 1994. [7] T. K. Wu, et a / . , “Multiring element FSS for multiband applications,” in fnt. IEEEAP-SSympo. Dig.. Chicago, IL, July 1992, pp. 1775-1778. [E] T. K. Wu. M. Zimmertnan, and S. W. Lee, “Evaluation of frequency selective reflector antenna svstems.” Microwave and Ootical Technol. Lett., vol. 6, no. 3. pp. 1751179, Mar. 1993. E. A. Parker and S. M. A. Hamdy, “Rings as elements for FSS,” Elect. Lett., pp. 612-614, Aug. 1981. E. Parker, S. M. A. Hamdy, and R. Langley, “Arrays of Concentric Rings as Frequency Selective Surfaces,’’ Elect. Lett., vol. 17, no. 23, 1981, p. 880. S. M. A. Hamdy. “Modal analysis of periodic arrays of simple and concentric rings as elements for FSS,” Proc. Nar. Rad. Sci. Synzp. EM Waves, Cairo, 1983, p. 115. E. A. Parker and J. C. Vardaxoglou, “Plane-wave illumination of concentric-ring frequency-selective surfaces,” Inst. Elect. Eng. Proc., vol. 132, pt. H, p. 176, June 1985. -. “Influence of single and multiple-layer dielectric substrates on the band spacings available from a concentric ring frequency-selective surface.” Int. J . Elect., vol. 61, pp. 291-297, 1986. C. C. Chen. “Scattering be a two-dimensional periodic array of conducting plates.” IEEE Trans. Antennas Propagat., vol. AP-18, pp. 660-665, Sept. 1970. S. W. Lee, “Scattering by dielectric loaded screen,” IEEE Trans. Anrenncis Propagat., vol. AP-19, pp. 656-665, Sept. 1971. A. Roberts and R. McPhedran, “Bandpass grids and annular apertures,” IEEE Trans. Antennas Propagat.. vol. AP-36. p. 607, May 1988. N. Amitay and V. Galindo, “On the scalar product of certain circular and cartehian wave functions,” IEEE Trans. Microwave Theory Tech., vol. M’I”IIT-16,pp. 265-266, 1968. N. Marcuvitz, Waveguide Handbook. Boston, MA: Technical Publishers, 1964, pp. 72-80. C. C. Chen, “Diffraction of electromagnetic waves by a conducting screen perforated with circular holes,” IEEE Trans. Microwave Theory Tech., vol. MTT-19, pp. 475-481, May 1971. S. W. Lee, Y. M. Wang, S. Ni, and M. Christensen, “Analysis of frequency selective surface with thick rings,” EM Lab Rept. SW 92-7, Univ. of Illinois, Urbana, IL, Oct. 1992. I

[21] J. C. Vardaxoglou, et al., “Scattering from two-layer FSS with dissimilar lattice geometries,” Insr. Elect. Eng. Proc., vol. 140, pt. H, no. 1, pp. 59-61, Feb. 1993. [221 S. Ramo et aL, Fields and Waves in Communication Electronics. New York: Wiley, 1965, p. 605. ~~

T. K. Wu received the B.S. degree in electrical engineering from National Taiwan University, Taipei, Taiwan in 1970 and the M.S. and Ph.D. degrees from the University of Mississippi in 1973 and 1976, respectively. From 1976 to 1978, he did two years postdoctoral research in electromagnetics and antennas at the University of Mississippi. Since 1978 he has been working on various satellite communication and radar antenna systems in the Antenna Industry. At present, he is in the Spacecraft Antenna Research Group of Jet Propulsion Laboratory, California Institute of Technology. His research interests are FSS, antennas, electromagnetics, numerical and measurement techniques. He has been granted nine US patents and ten NASA certificates of recognition. Dr. Wu is a member of Sigma Xi, Eta Kappa Nu, and Phi Kappa Phi.

Shung-Wu Lee received the B.S. degree from Cheng Kung University in Taiwan in 1961, and the Ph.D. from the University of Illinois in 1966, both in electrical engineering. He is a Professor in the Electrical and Computer Engineering Department at the University of Illinois, Urbana. His current interest is writing application computer software in electromagnetics.