Design and Performance of Frequency Selective ...

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 8, AUGUST 2010

1

Design and Performance of Frequency Selective Surface With Integrated Photodiodes for Photonic Calibration of Phased Array Antennas W. Mark Dorsey, Member, IEEE, Christopher S. McDermitt, Frank Bucholtz, Member, IEEE, and Mark G. Parent

Index Terms—Frequency selective surface (FSS), radome, calibration.

I. INTRODUCTION

HASED array antennas are utilized in applications including radar, communications, and electronic warfare (EW). The performance of these phased arrays can be degraded by environmental effects including temperature, mechanical stresses, and vibration as well as long-term aging of system components [1]. A common calibration technique for large phased array apertures utilizes a calibration signal transmitted from an external source located away from the aperture of the phased array [2]–[4]. The presence of the external sources in the techniques described in [2]–[4] has the disadvantage of requiring a source that is not co-located with the array. The external source is often difficult to realize on vehicle platforms and requires additional real estate that is often not available. Conversely, the calibration technique described in [5], [6] uses an optical method consisting of photodiodes integrated within a frequency selective surface (FSS) that is placed in front of the phased array. This has the advantage of providing a compact calibration system that can be co-located with the phased array, thus minimizing the size requirements for the overall system. The calibration technique described in [5], [6] uses a photonic method consisting of photodiodes integrated within a FSS that is

P

placed in front of the phased array aperture. This technique has the advantage of providing a compact calibration system that can be co-located with the phased array, thus minimizing the size requirements for the overall system. The RF-modulated optical signal is detected by a zero-biased photodiode, and the RF signal excites electrically small dipole antennas (ESDAs) that are integrated in the FSS panel located in front of the phased above array elements. An ESDA element is centered each unit cell of the phased array to minimize the coupling from each ESDA to neighboring array elements. In this design, corresponds to the center frequency of the band pass response for the FSS (3 GHz). An ESDA element is present above each element of the phased-array to provide a calibration signal for each array element. The calibration signal will be monitored for changes that indicate variation in the amplitude and/or phase of the element excitation. Since the technique relies on relative change to a coupled signal, the tolerances on the registration between the FSS and the array are not critical as long as the alignment can be maintained. The performance and stability of the photonic calibration system has been described in [7], and the zero-biased photodiodes have been fully characterized [8]. In order to successfully implement this calibration system, it is essential that the FSS with integrated optical components maintain RF transparency within the operational frequency band of the phased array in order to avoid degrading RF performance. This paper will discuss the integration of a photonic calibration system into a frequency selective surface. The details of the optical calibration system integration will also be provided. This discussion will be followed by a presentation of simulated and measured results on the RF performance of the FSS with integrated optical components. These results will include simulations and measurements of the transmission coefficient through the FSS radome as well as measured radiation patterns for a phased array placed behind the FSS. Performing full calibration of a phased array with accompanying T/R modules was beyond the scope of this study. However, the performance and stability presented in [7] coupled with the performance of the FSS presented in this manuscript provide a high confidence level in the utility of the photonic calibration technique discussed in [5], [6].

IE E W E eb P r Ve oo rs f ion

Abstract—The design, fabrication, and integration of a frequency selective surface (FSS) with integrated photodiodes to allow for photonic calibration of phased array antennas is presented. The design includes embedding electrically short dipole antennas in each unit cell of the FSS, with a zero-biased photodiode placed across the gap of the diode. Fibers from an optical distribution network are passed through the honeycomb core of the frequency selective surface and pigtailed to the photodiodes. The RF performance of the frequency selective surface with integrated optics is investigated via simulations and measurements, and the results show that the structure maintains RF-transparency.

Manuscript received September 28, 2009; revised February 23, 2010; accepted February 23, 2010. Date of publication May 18, 2010; date of current version August 05, 2010. W. M. Dorsey and M. G. Parent are with the Radar Division, U.S. Naval Research Laboratory, Washington, DC 20375 USA (e-mail: [email protected]. navy.mil). C. S. Mcdermitt and F. Bucholtz are with the Optical Sciences Division, U.S. Naval Research Laboratory, Washington, DC 20375 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2010.2050451

II. DESIGN OF A FREQUENCY SELECTIVE SURFACE WITH INTEGRATED PHOTODIODES

A frequency selective surface (FSS) provides minimal attenuation in the passband while becoming increasingly opaque outside of the pass band. This type of structure is often used in the

0018-926X/$26.00 © 2010 IEEE

2

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 8, AUGUST 2010

Fig. 2. Architecture for phased-array calibrator employing an array of zerobiased photodiodes (PD) each driving an electrically short dipole (ESDA). Each PD receives RF-modulated light from a single-mode optical fiber through an optical distribution network (ODN).

IE E W E eb P r Ve oo rs f ion

Fig. 1. Top view (a) and dielectric profile (b) of a frequency selective surface (FSS) with 2.5–2.5 GHz pass band. (a) Top view of FSS unit cell, (b) Dielectric profile of FSS unit cell.

design of a hybrid radome consisting of FSS layers sandwiched between dielectric layers; the goal of the hybrid radome is to reduce the out of band radar cross section (RCS) of an antenna. At frequencies where the FSS is opaque, the incident signal is reflected back in the bi-static direction [9]. At frequencies where the radome appears transparent, minimal RCS improvements are seen. However, if the pass band of the hybrid radome coincides with the operational frequency band of the phased array, the energy incident on the array can be received with minimal attenuation. For this demonstration, a FSS with a pass band covering 2.5–3.5 GHz was designed. In FSS designs, conducting grids appear inductive to incident waves while arrays of conducting patches appear capacitive. In a design proposed by Wahid and Morris [10], a conducting grid and array of conducting patches are superimposed to result in a band pass structure. The design used in this demonstration is based on the ideas and concepts from Wahid and Morris [10]. The unit cell for the band pass FSS shown in Fig. 1(a), is a square with side length of 4.32 cm. The conducting grid consists of a rectangular grid with 0.24 cm thickness, and each corner has a 90-degree arc with a 1.27 cm radius. The center of the unit cell consists of a 1.27 cm radius circular patch that will serve as the capacitive component. The conducting pattern is printed on . cm) RO4350 substrates ( that are present above and below a 1.2 thick honeycomb core. The dielectric profile of the FSS unit cell is shown in Fig. 1(b). The optical calibration approach illustrated in Fig. 2 [5], [6] requires the installation of a zero-biased photodiode (PD) and accompanying electrically short dipole antenna (ESDA) above each element in the antenna array. Within the optical distribution network (ODN), the RF modulated signal is optically amplified and then distributed to an ESDA located within each unit cell of the FSS. Each channel of the photonic calibration network allows static adjustment over both the amplitude and phase. The ODN and fiber pigtails from the FSS photodiodes are connected via a ribbon fiber cable—allowing the ODN control unit to be remotely located on the platform and in principle, hundreds of meters away from the array aperture. After photodetection, the RF signal is placed across the ESDA antenna located at each unit cell in the FSS. The RF signal excites currents on the ESDA, and the dipole will transmit the calibration signal to the corresponding antenna element. The shape of the transmitter is shown in Fig. 3(a), and the gap where the diode will be located is circled for clarity. A photograph of the diode used in this design is

Fig. 3. (a) Location of the photodiode within the ESDA and (b) photograph of the photodiode. (a) Shape of the electrically short dipole antenna (ESDA), (b) external electrical connection pads of the zero-bias photodiode.

provided in Fig. 3(b). The photodiodes used in this design were manufactured by Anadigics (Part Number PD070-001-720) and had a responsivity of 0.91 A/W. The performance of this diode was characterized in [7]. The center of the FSS unit cell is modified to include the ESDA and accompanying photodiode. Conducting solder pads are printed on the bottom side of the microwave substrate and plated through holes are added to provide electrical continuity between the ESDA and the solder pads. The photodiode is installed across the solder pads on the back side of the dielectric substrate. The unit cell of the FSS with integrated optical calibration system is detailed in Fig. 4. A detailed drawing showing the ESDA, solder pads, plated through holes, and photodiode is provided in Fig. 5. In this figure, one side of the ESDA is removed for clarity. The conducting pattern is printed on RO4350 . ) that are present substrates ( . The above and below a 3 cm thick honeycomb core bandpass for the FSS covers the 2.5–3.5 GHz operational bandwidth of the phased array. Fig. 6 shows two views of the constructed FSS panel with integrated photonics calibration system. Fig. 6(a) shows the optical fibers leaving the optical distribution network (ODN), where they pass through a channel in the honeycomb core of the FSS. The channel is machined into the honeycomb core to provide room for the optical fibers and the photodiodes. Fig. 6(b) shows the back side of the microwave substrate that

DORSEY et al.: DESIGN AND PERFORMANCE OF FSS WITH INTEGRATED PHOTODIODES

3

IE E W E eb P r Ve oo rs f ion

Fig. 6. Photographs showing (a) optical fibers leaving the optical distribution network and passing through a channel in the FSS honeycomb core and (b) the optical fiber - ESDA interface. (a) Optical fibers pass through a channel in the honeycomb core of the (FSS), (b) The optical are connected to a photodiode that is integrated across two solder pads.

detailed in Fig. 5. The solder pads are electrically connected to the printed ESDA by plated through holes. The FSS design was characterized in simulations and measurements—both with and without the integrated optical calibration system—to investigate its performance. The results are presented in the following section. III. SIMULATED AND MEASURED RESULTS

Fig. 4. Dimensioned drawing of the frequency selective surface (FSS) unit cell cm). (a) Top view of FSS with integrated dipole probe and photodiode (units unit cell with integrated electically short dipole antenna (ESDA), (b) detailed view of ESDA integrated into FSS unit cell, (c) detailed view of photodiode integration.

=

Fig. 5. Detailed drawing showing the integration of the photodiode with the dipole probe.

contains the printed grid of the FSS and the solder pads that were previously illustrated in Fig. 4. The optical fibers will pass through the channel in the honeycomb core until they reach the photodiodes. One photodiode is present at each unit cell in the FSS, and they are integrated across the solder pads that are

A unit cell of the baseline FSS defined in Fig. 1 was modeled using CST Microwave Studio, a computational electromagnetic (CEM) software package employing the finite integration technique (FIT) [11]. This CEM package contains both a time domain and frequency domain solver. The frequency domain solver was used in these simulations owing to its ability to study off-axis performance for plane wave transmission through a unit-cell of the FSS. The FSS unit cell simulations focused on two orthogonal TEM modes defined as TE (y-polarized) and TM (x-polarized). The transmission coefficient for various scan angles is plane (parshown in Fig. 7. These scans were in the allel to the x-axis). This scan represents an H-plane scan for the TE mode and an E-plane scan for the TM mode. The results indicate excellent transmission in the pass band at all scan angles. The performance of the FSS deteriorates slightly at the 45 scan for both the TE and TM mode. In the TE case, the transmission coefficient degrades slightly at the low end of the pass band. Conversely, the TM case experiences a slight performance drop at the high end when scanned to 45 . However, in both cases the performance drop is small indicating that this design will appear transparent to incident waves in the pass band of 2.5–3.5 GHz. It should be noted that the transmission coefficients displayed in Fig. 7 do not indicate a sharp roll-off at high frequencies. The intention of this study was to illustrate the impact of the optical calibration system on the FSS performance, and not necessarily to show optimized FSS design. If sharper roll-off is desired in the transmission coefficient, cascaded designs consisting of multiple stacked FSS panels can be used as described in [10].

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 8, AUGUST 2010

IE E W E eb P r Ve oo rs f ion

4

Fig. 7. Simulated transmission coefficient of FSS unit cell for multiple scan angles.

Fig. 9. Photographs of the fabricated frequency selective surface with integrated photonic calibration system for phased array antennas. (a) Front view of the bandpass FSS radome containing the photonic calibration system for phased array antennas, (b) Detailed view showing location of the optical distribution network (ODN) and the single-mode fibers that pass through the noneycomb core of the FSS.

Fig. 8. Measured transmission coefficient through FSS panels with various numbers of optical calibration probes compared to a baseline unit cell simulation.

Several FSS panels were constructed to investigate the impact of various ESDA configurations. Three sets of 10 10 panels including various numbers of ESDA elements were constructed. The first panel served as the baseline case, and it contained no ESDA elements. The second panel had ESDA elements poputransmitters total, and the third panel had lating one row ESDAs on all 100 elements. The transmission coefficient for the three panel types was measured using a pair of quad-ridged horn antennas and a pair of spot-focusing lenses. The measurement technique is similar to that described in [12]. Time gating was also used to further improve the fidelity of the measurement system by removing the contributions from environmental reflections that occur outside of the primary coupling. The measured results from Fig. 8 show two important results. The first observation is that the presence of the electrically short dipole probes did not significantly impact the transmission properties of the FSS panels. The second observation is

that the results for all three panels are in good agreement with the simulated results for the unit cell with no probes. The measured FSS show that the transmission efficiency starts to roll off slightly at the high edge of the pass band, but the measured dB across the transmission efficiency is still greater than entire band of interest. The measured transmission efficiency is shifted to slightly lower frequencies than the simulations due to the presence of a honeycomb substrate. Only an approximate value for the honeycomb substrate dielectric constant was avail, and subsequently the exact value could not be able used in simulations. In an actual system implementation, a transmission coefficient closer to 0 dB would be desired in the operational frequency band to minimize reflected energy that would negatively impact the performance of both the array elements and the calibration system. Overall, the measured results indicate that the optical calibration system can be integrated into the FSS panel without significantly degrading transmission performance of the FSS panel. After the transmission coefficient was characterized, the FSS in radome with integrated ESDA elements was placed

DORSEY et al.: DESIGN AND PERFORMANCE OF FSS WITH INTEGRATED PHOTODIODES

5

of the dipole array. The patterns are normalized to the baseline case with no optical calibration elements, and no noticeable gain degradation is seen in the measured data. Minimal impact is seen on wide-angle side lobe levels (SLL) at 2.75 GHz, and some nulls have filled in slightly. However, the impact on SLL and null depth is minimal. Moreover, the active element impedance match was calculated from measured s-parameters. The results—shown in Fig. 11—indicate that the presence of the photonic calibration FSS had minimal impact on the element impedance match.

IE E W E eb P r Ve oo rs f ion

IV. CONCLUSION

Fig. 10. Radiation pattern of a 16-element dipole array with and without the presence of the FSS with integrated optical calibration system components. (a) 2.75 GHz, (b) 3.00 GHz, (c) 3.25 GHz.

This paper presents a method for integrating the components of an optical calibration system into a frequency selective surface (FSS). A zero-biased photodiode is placed across the gap of an integrated electrically short dipole antenna within each unit cell of the FSS. Fibers from an optical distribution network are passed through the honeycomb core of the frequency selective surface and pigtailed to the photodiodes. Measurements and simulations show that the FSS with integrated optical network remains RF-transparent. Furthermore, the radiation pattern of a phased array remained unchanged when the array was placed behind the FSS with integrated optics. Subsequently, this FSS enables placement of the calibration system in front of a phased array antenna without impacting the RF performance of the array.

REFERENCES

Fig. 11. Measured active element VSWR for an array element with and without the presence of a frequency selective surface (FSS) with integrated photonic calibration network.

front of a 20 20-element dipole array as shown in Fig. 9. The radiation pattern of a 16-element row of the array with integrated optical calibration network and FSS was measured in a near-field scanning facility, and the results were compared to the baseline array patterns with no FSS present. This test was completed to see if the components of the optical calibration system would interact with the antenna array elements and impact the transparency of the FSS. The measured results displayed in Fig. 10 show that the FSS with integrated optical calibration network had minimal impact on the radiation pattern

[1] P. Hughes, J. Choe, and J. Zopler, “Advanced multifunction RF system,” in GOMAC Digest, 2000, pp. 194–107. [2] W. Yao, Y. Wang, and T. Itoh, “A self-calibration antenna array system with moving apertures,” in Proc. IEEE MTT-S Int. Microwave Symp. Digest, Jun. 8–13, 2003, vol. 3, pp. 1541–1544. [3] R. X. Meyer, “Electronic compensation for structural deformations of large space antennas,” in Proc. Astrodynamics Conf., Vail, CO, Aug. 12–15, 1985, pp. 277–285. [4] E.-A. Lee and C. N. Dorny, “A broadcast reference technique for self-calibrating of large antenna phased arrays,” IEEE Trans. Antennas Propag., vol. 37, no. 8, pp. 1003–1010, Aug. 1989. [5] E. G. Paek, M. G. Parent, and J. Y. Choe, “Photonic in-situ calibration of a phased array antenna using planar lightwave circuit,” in Proc. Int. Topical Meetings on Microwave Photonics (MWP 2005), Oct. 12–14, 2005, pp. 351–354. [6] M. G. Parent, E. G. Paek, F. Bucholtz, C. McDermitt, and P. Knapp, “Phased-array calibration using radome embedded optical transducers,” in Proc. Antenna Application Symp., Monticello, IL, Sept. 21–23, 2005, pp. 345–360. [7] C. S. McDermitt, W. M. Dorsey, M. E. Godinez, F. Bucholtz, and M. G. Parent, “Performance of 16-channel, photonic, phased-array antenna calibration system,” Electron. Lett., vol. 45, no. 24, pp. 1249–1250, Nov. 19, 2009. [8] M. E. Godinez, C. S. McDermitt, A. S. Hastings, M. G. Parent, and F. Bucholtz, “RF characterization of zero-biased photodiodes,” IEEE J. Lightw. Technol., vol. 26, no. 24, pp. 3829–3834, Dec. 15, 2008. [9] B. A. Munk, Frequency Selective Surfaces: Theory and Design. New York: Wiley, 2000, pp. 14–16. [10] M. Wahid and S. B. Morris, “Band pass radomes for reduced RCS,” in Proc. Inst. Elect. Eng. Colloq. on Antenna Radar Cross Section, May 7, 1991, pp. 4/1–4/9. [11] CST Microwave Studio, v.2008.04 Feb. 25, 2008. [12] D. K. Ghodgaonkar, V. V. Varadan, and V. K. Varadan, “A free-space method for measurement of dielectric constants and loss tangents at microwave frequencies,” IEEE Trans. Instrum. Meas., vol. 37, no. 3, pp. 789–793, Jun. 1989.

6

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 8, AUGUST 2010

Frank Bucholtz (M’82) was born in Detroit, MI, on April 4, 1953. He received the B.S. degree in physics and mathematics from Wayne State University, Detroit, MI, in 1975, and the M.S. and Ph.D. degrees in physics from Brown University, Providence, R1, in 1977 and 1981, respectively. From 1981 to 1983, he was an NRC Postdoctoral Research Associate at the U.S. Naval Research Laboratory where he conducted research in the area of ferrimagnetic devices for microwave signal processing. He is currently a member of the Optical Sciences Division at the U.S. Naval Research Laboratory, Washington, DC. His research interests include fiber-optic sensors, hyperspectral imaging, and analog microwave photonics.

Christopher S. McDermitt was born in Harrisburg, PA, on November 29, 1977. He received the B.S. degree in physics from Bloomsburg University, Bloomsburg, PA, in 2000. From 2001 to present, he has been a Research Physicist in the Optical Sciences Division, U.S. Naval Research Laboratory, Washington, DC. His research interests include photonics signal processing, fiber-optic interferometric systems, and enhancement of microwave photonic links.

Mark G. Parent received the B.S. degree in electrical engineering from Michigan Technological University in 1982 and the M.S. degree in physics from Michigan Technological University, Houghton, in 1985. In 1985, he joined the U.S. Naval Research Laboratory, Washington, DC, where he is currently Section Head in the Radar Analysis Branch of the Radar Division, Naval Research Laboratory. His research interests include phased array antenna design, optical beamforming techniques, radar cross section measurements and advanced microwave/optical system concepts.

IE E W E eb P r Ve oo rs f ion

W. Mark Dorsey (M’09) received the B.S. and M.S. in electrical engineering with a focus in electromagnetics from the University of Maryland, College Park, in 2002 and 2005, respectively, and the Ph.D. degree in electrical engineering from Virginia Polytechnic Institute and State University, Blacksburg, in 2009. As a doctorate student, he researched dual-band, dual-polarized antenna elements and arrays. He has worked on antenna design, measurement, and integration for the Radar Division of the U.S. Naval Research Laboratory since 1999. His primary research interests include reconfigurable and multifunction antenna design, ultrawideband (UWB) antenna design, antenna isolation studies, antenna measurement, and calibration.

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 8, AUGUST 2010

1

Design and Performance of Frequency Selective Surface With Integrated Photodiodes for Photonic Calibration of Phased Array Antennas W. Mark Dorsey, Member, IEEE, Christopher S. McDermitt, Frank Bucholtz, Member, IEEE, and Mark G. Parent

Index Terms—Frequency selective surface (FSS), radome, calibration.

I. INTRODUCTION

HASED array antennas are utilized in applications including radar, communications, and electronic warfare (EW). The performance of these phased arrays can be degraded by environmental effects including temperature, mechanical stresses, and vibration as well as long-term aging of system components [1]. A common calibration technique for large phased array apertures utilizes a calibration signal transmitted from an external source located away from the aperture of the phased array [2]–[4]. The presence of the external sources in the techniques described in [2]–[4] has the disadvantage of requiring a source that is not co-located with the array. The external source is often difficult to realize on vehicle platforms and requires additional real estate that is often not available. Conversely, the calibration technique described in [5], [6] uses an optical method consisting of photodiodes integrated within a frequency selective surface (FSS) that is placed in front of the phased array. This has the advantage of providing a compact calibration system that can be co-located with the phased array, thus minimizing the size requirements for the overall system. The calibration technique described in [5], [6] uses a photonic method consisting of photodiodes integrated within a FSS that is

P

placed in front of the phased array aperture. This technique has the advantage of providing a compact calibration system that can be co-located with the phased array, thus minimizing the size requirements for the overall system. The RF-modulated optical signal is detected by a zero-biased photodiode, and the RF signal excites electrically small dipole antennas (ESDAs) that are integrated in the FSS panel located in front of the phased above array elements. An ESDA element is centered each unit cell of the phased array to minimize the coupling from each ESDA to neighboring array elements. In this design, corresponds to the center frequency of the band pass response for the FSS (3 GHz). An ESDA element is present above each element of the phased-array to provide a calibration signal for each array element. The calibration signal will be monitored for changes that indicate variation in the amplitude and/or phase of the element excitation. Since the technique relies on relative change to a coupled signal, the tolerances on the registration between the FSS and the array are not critical as long as the alignment can be maintained. The performance and stability of the photonic calibration system has been described in [7], and the zero-biased photodiodes have been fully characterized [8]. In order to successfully implement this calibration system, it is essential that the FSS with integrated optical components maintain RF transparency within the operational frequency band of the phased array in order to avoid degrading RF performance. This paper will discuss the integration of a photonic calibration system into a frequency selective surface. The details of the optical calibration system integration will also be provided. This discussion will be followed by a presentation of simulated and measured results on the RF performance of the FSS with integrated optical components. These results will include simulations and measurements of the transmission coefficient through the FSS radome as well as measured radiation patterns for a phased array placed behind the FSS. Performing full calibration of a phased array with accompanying T/R modules was beyond the scope of this study. However, the performance and stability presented in [7] coupled with the performance of the FSS presented in this manuscript provide a high confidence level in the utility of the photonic calibration technique discussed in [5], [6].

IE E Pr E int P r Ve oo rs f ion

Abstract—The design, fabrication, and integration of a frequency selective surface (FSS) with integrated photodiodes to allow for photonic calibration of phased array antennas is presented. The design includes embedding electrically short dipole antennas in each unit cell of the FSS, with a zero-biased photodiode placed across the gap of the diode. Fibers from an optical distribution network are passed through the honeycomb core of the frequency selective surface and pigtailed to the photodiodes. The RF performance of the frequency selective surface with integrated optics is investigated via simulations and measurements, and the results show that the structure maintains RF-transparency.

Manuscript received September 28, 2009; revised February 23, 2010; accepted February 23, 2010. Date of publication May 18, 2010; date of current version August 05, 2010. W. M. Dorsey and M. G. Parent are with the Radar Division, U.S. Naval Research Laboratory, Washington, DC 20375 USA (e-mail: [email protected]. navy.mil). C. S. Mcdermitt and F. Bucholtz are with the Optical Sciences Division, U.S. Naval Research Laboratory, Washington, DC 20375 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2010.2050451

II. DESIGN OF A FREQUENCY SELECTIVE SURFACE WITH INTEGRATED PHOTODIODES

A frequency selective surface (FSS) provides minimal attenuation in the passband while becoming increasingly opaque outside of the pass band. This type of structure is often used in the

0018-926X/$26.00 © 2010 IEEE

2

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 8, AUGUST 2010

Fig. 2. Architecture for phased-array calibrator employing an array of zerobiased photodiodes (PD) each driving an electrically short dipole (ESDA). Each PD receives RF-modulated light from a single-mode optical fiber through an optical distribution network (ODN).

IE E Pr E int P r Ve oo rs f ion

Fig. 1. Top view (a) and dielectric profile (b) of a frequency selective surface (FSS) with 2.5–2.5 GHz pass band. (a) Top view of FSS unit cell, (b) Dielectric profile of FSS unit cell.

design of a hybrid radome consisting of FSS layers sandwiched between dielectric layers; the goal of the hybrid radome is to reduce the out of band radar cross section (RCS) of an antenna. At frequencies where the FSS is opaque, the incident signal is reflected back in the bi-static direction [9]. At frequencies where the radome appears transparent, minimal RCS improvements are seen. However, if the pass band of the hybrid radome coincides with the operational frequency band of the phased array, the energy incident on the array can be received with minimal attenuation. For this demonstration, a FSS with a pass band covering 2.5–3.5 GHz was designed. In FSS designs, conducting grids appear inductive to incident waves while arrays of conducting patches appear capacitive. In a design proposed by Wahid and Morris [10], a conducting grid and array of conducting patches are superimposed to result in a band pass structure. The design used in this demonstration is based on the ideas and concepts from Wahid and Morris [10]. The unit cell for the band pass FSS shown in Fig. 1(a), is a square with side length of 4.32 cm. The conducting grid consists of a rectangular grid with 0.24 cm thickness, and each corner has a 90-degree arc with a 1.27 cm radius. The center of the unit cell consists of a 1.27 cm radius circular patch that will serve as the capacitive component. The conducting pattern is printed on . cm) RO4350 substrates ( that are present above and below a 1.2 thick honeycomb core. The dielectric profile of the FSS unit cell is shown in Fig. 1(b). The optical calibration approach illustrated in Fig. 2 [5], [6] requires the installation of a zero-biased photodiode (PD) and accompanying electrically short dipole antenna (ESDA) above each element in the antenna array. Within the optical distribution network (ODN), the RF modulated signal is optically amplified and then distributed to an ESDA located within each unit cell of the FSS. Each channel of the photonic calibration network allows static adjustment over both the amplitude and phase. The ODN and fiber pigtails from the FSS photodiodes are connected via a ribbon fiber cable—allowing the ODN control unit to be remotely located on the platform and in principle, hundreds of meters away from the array aperture. After photodetection, the RF signal is placed across the ESDA antenna located at each unit cell in the FSS. The RF signal excites currents on the ESDA, and the dipole will transmit the calibration signal to the corresponding antenna element. The shape of the transmitter is shown in Fig. 3(a), and the gap where the diode will be located is circled for clarity. A photograph of the diode used in this design is

Fig. 3. (a) Location of the photodiode within the ESDA and (b) photograph of the photodiode. (a) Shape of the electrically short dipole antenna (ESDA), (b) external electrical connection pads of the zero-bias photodiode.

provided in Fig. 3(b). The photodiodes used in this design were manufactured by Anadigics (Part Number PD070-001-720) and had a responsivity of 0.91 A/W. The performance of this diode was characterized in [7]. The center of the FSS unit cell is modified to include the ESDA and accompanying photodiode. Conducting solder pads are printed on the bottom side of the microwave substrate and plated through holes are added to provide electrical continuity between the ESDA and the solder pads. The photodiode is installed across the solder pads on the back side of the dielectric substrate. The unit cell of the FSS with integrated optical calibration system is detailed in Fig. 4. A detailed drawing showing the ESDA, solder pads, plated through holes, and photodiode is provided in Fig. 5. In this figure, one side of the ESDA is removed for clarity. The conducting pattern is printed on RO4350 . ) that are present substrates ( . The above and below a 3 cm thick honeycomb core bandpass for the FSS covers the 2.5–3.5 GHz operational bandwidth of the phased array. Fig. 6 shows two views of the constructed FSS panel with integrated photonics calibration system. Fig. 6(a) shows the optical fibers leaving the optical distribution network (ODN), where they pass through a channel in the honeycomb core of the FSS. The channel is machined into the honeycomb core to provide room for the optical fibers and the photodiodes. Fig. 6(b) shows the back side of the microwave substrate that

DORSEY et al.: DESIGN AND PERFORMANCE OF FSS WITH INTEGRATED PHOTODIODES

3

IE E Pr E int P r Ve oo rs f ion

Fig. 6. Photographs showing (a) optical fibers leaving the optical distribution network and passing through a channel in the FSS honeycomb core and (b) the optical fiber - ESDA interface. (a) Optical fibers pass through a channel in the honeycomb core of the (FSS), (b) The optical are connected to a photodiode that is integrated across two solder pads.

detailed in Fig. 5. The solder pads are electrically connected to the printed ESDA by plated through holes. The FSS design was characterized in simulations and measurements—both with and without the integrated optical calibration system—to investigate its performance. The results are presented in the following section. III. SIMULATED AND MEASURED RESULTS

Fig. 4. Dimensioned drawing of the frequency selective surface (FSS) unit cell cm). (a) Top view of FSS with integrated dipole probe and photodiode (units unit cell with integrated electically short dipole antenna (ESDA), (b) detailed view of ESDA integrated into FSS unit cell, (c) detailed view of photodiode integration.

=

Fig. 5. Detailed drawing showing the integration of the photodiode with the dipole probe.

contains the printed grid of the FSS and the solder pads that were previously illustrated in Fig. 4. The optical fibers will pass through the channel in the honeycomb core until they reach the photodiodes. One photodiode is present at each unit cell in the FSS, and they are integrated across the solder pads that are

A unit cell of the baseline FSS defined in Fig. 1 was modeled using CST Microwave Studio, a computational electromagnetic (CEM) software package employing the finite integration technique (FIT) [11]. This CEM package contains both a time domain and frequency domain solver. The frequency domain solver was used in these simulations owing to its ability to study off-axis performance for plane wave transmission through a unit-cell of the FSS. The FSS unit cell simulations focused on two orthogonal TEM modes defined as TE (y-polarized) and TM (x-polarized). The transmission coefficient for various scan angles is plane (parshown in Fig. 7. These scans were in the allel to the x-axis). This scan represents an H-plane scan for the TE mode and an E-plane scan for the TM mode. The results indicate excellent transmission in the pass band at all scan angles. The performance of the FSS deteriorates slightly at the 45 scan for both the TE and TM mode. In the TE case, the transmission coefficient degrades slightly at the low end of the pass band. Conversely, the TM case experiences a slight performance drop at the high end when scanned to 45 . However, in both cases the performance drop is small indicating that this design will appear transparent to incident waves in the pass band of 2.5–3.5 GHz. It should be noted that the transmission coefficients displayed in Fig. 7 do not indicate a sharp roll-off at high frequencies. The intention of this study was to illustrate the impact of the optical calibration system on the FSS performance, and not necessarily to show optimized FSS design. If sharper roll-off is desired in the transmission coefficient, cascaded designs consisting of multiple stacked FSS panels can be used as described in [10].

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 8, AUGUST 2010

IE E Pr E int P r Ve oo rs f ion

4

Fig. 7. Simulated transmission coefficient of FSS unit cell for multiple scan angles.

Fig. 9. Photographs of the fabricated frequency selective surface with integrated photonic calibration system for phased array antennas. (a) Front view of the bandpass FSS radome containing the photonic calibration system for phased array antennas, (b) Detailed view showing location of the optical distribution network (ODN) and the single-mode fibers that pass through the noneycomb core of the FSS.

Fig. 8. Measured transmission coefficient through FSS panels with various numbers of optical calibration probes compared to a baseline unit cell simulation.

Several FSS panels were constructed to investigate the impact of various ESDA configurations. Three sets of 10 10 panels including various numbers of ESDA elements were constructed. The first panel served as the baseline case, and it contained no ESDA elements. The second panel had ESDA elements poputransmitters total, and the third panel had lating one row ESDAs on all 100 elements. The transmission coefficient for the three panel types was measured using a pair of quad-ridged horn antennas and a pair of spot-focusing lenses. The measurement technique is similar to that described in [12]. Time gating was also used to further improve the fidelity of the measurement system by removing the contributions from environmental reflections that occur outside of the primary coupling. The measured results from Fig. 8 show two important results. The first observation is that the presence of the electrically short dipole probes did not significantly impact the transmission properties of the FSS panels. The second observation is

that the results for all three panels are in good agreement with the simulated results for the unit cell with no probes. The measured FSS show that the transmission efficiency starts to roll off slightly at the high edge of the pass band, but the measured dB across the transmission efficiency is still greater than entire band of interest. The measured transmission efficiency is shifted to slightly lower frequencies than the simulations due to the presence of a honeycomb substrate. Only an approximate value for the honeycomb substrate dielectric constant was avail, and subsequently the exact value could not be able used in simulations. In an actual system implementation, a transmission coefficient closer to 0 dB would be desired in the operational frequency band to minimize reflected energy that would negatively impact the performance of both the array elements and the calibration system. Overall, the measured results indicate that the optical calibration system can be integrated into the FSS panel without significantly degrading transmission performance of the FSS panel. After the transmission coefficient was characterized, the FSS in radome with integrated ESDA elements was placed

DORSEY et al.: DESIGN AND PERFORMANCE OF FSS WITH INTEGRATED PHOTODIODES

5

of the dipole array. The patterns are normalized to the baseline case with no optical calibration elements, and no noticeable gain degradation is seen in the measured data. Minimal impact is seen on wide-angle side lobe levels (SLL) at 2.75 GHz, and some nulls have filled in slightly. However, the impact on SLL and null depth is minimal. Moreover, the active element impedance match was calculated from measured s-parameters. The results—shown in Fig. 11—indicate that the presence of the photonic calibration FSS had minimal impact on the element impedance match.

IE E Pr E int P r Ve oo rs f ion

IV. CONCLUSION

Fig. 10. Radiation pattern of a 16-element dipole array with and without the presence of the FSS with integrated optical calibration system components. (a) 2.75 GHz, (b) 3.00 GHz, (c) 3.25 GHz.

This paper presents a method for integrating the components of an optical calibration system into a frequency selective surface (FSS). A zero-biased photodiode is placed across the gap of an integrated electrically short dipole antenna within each unit cell of the FSS. Fibers from an optical distribution network are passed through the honeycomb core of the frequency selective surface and pigtailed to the photodiodes. Measurements and simulations show that the FSS with integrated optical network remains RF-transparent. Furthermore, the radiation pattern of a phased array remained unchanged when the array was placed behind the FSS with integrated optics. Subsequently, this FSS enables placement of the calibration system in front of a phased array antenna without impacting the RF performance of the array.

REFERENCES

Fig. 11. Measured active element VSWR for an array element with and without the presence of a frequency selective surface (FSS) with integrated photonic calibration network.

front of a 20 20-element dipole array as shown in Fig. 9. The radiation pattern of a 16-element row of the array with integrated optical calibration network and FSS was measured in a near-field scanning facility, and the results were compared to the baseline array patterns with no FSS present. This test was completed to see if the components of the optical calibration system would interact with the antenna array elements and impact the transparency of the FSS. The measured results displayed in Fig. 10 show that the FSS with integrated optical calibration network had minimal impact on the radiation pattern

[1] P. Hughes, J. Choe, and J. Zopler, “Advanced multifunction RF system,” in GOMAC Digest, 2000, pp. 194–107. [2] W. Yao, Y. Wang, and T. Itoh, “A self-calibration antenna array system with moving apertures,” in Proc. IEEE MTT-S Int. Microwave Symp. Digest, Jun. 8–13, 2003, vol. 3, pp. 1541–1544. [3] R. X. Meyer, “Electronic compensation for structural deformations of large space antennas,” in Proc. Astrodynamics Conf., Vail, CO, Aug. 12–15, 1985, pp. 277–285. [4] E.-A. Lee and C. N. Dorny, “A broadcast reference technique for self-calibrating of large antenna phased arrays,” IEEE Trans. Antennas Propag., vol. 37, no. 8, pp. 1003–1010, Aug. 1989. [5] E. G. Paek, M. G. Parent, and J. Y. Choe, “Photonic in-situ calibration of a phased array antenna using planar lightwave circuit,” in Proc. Int. Topical Meetings on Microwave Photonics (MWP 2005), Oct. 12–14, 2005, pp. 351–354. [6] M. G. Parent, E. G. Paek, F. Bucholtz, C. McDermitt, and P. Knapp, “Phased-array calibration using radome embedded optical transducers,” in Proc. Antenna Application Symp., Monticello, IL, Sept. 21–23, 2005, pp. 345–360. [7] C. S. McDermitt, W. M. Dorsey, M. E. Godinez, F. Bucholtz, and M. G. Parent, “Performance of 16-channel, photonic, phased-array antenna calibration system,” Electron. Lett., vol. 45, no. 24, pp. 1249–1250, Nov. 19, 2009. [8] M. E. Godinez, C. S. McDermitt, A. S. Hastings, M. G. Parent, and F. Bucholtz, “RF characterization of zero-biased photodiodes,” IEEE J. Lightw. Technol., vol. 26, no. 24, pp. 3829–3834, Dec. 15, 2008. [9] B. A. Munk, Frequency Selective Surfaces: Theory and Design. New York: Wiley, 2000, pp. 14–16. [10] M. Wahid and S. B. Morris, “Band pass radomes for reduced RCS,” in Proc. Inst. Elect. Eng. Colloq. on Antenna Radar Cross Section, May 7, 1991, pp. 4/1–4/9. [11] CST Microwave Studio, v.2008.04 Feb. 25, 2008. [12] D. K. Ghodgaonkar, V. V. Varadan, and V. K. Varadan, “A free-space method for measurement of dielectric constants and loss tangents at microwave frequencies,” IEEE Trans. Instrum. Meas., vol. 37, no. 3, pp. 789–793, Jun. 1989.

6

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 8, AUGUST 2010

Frank Bucholtz (M’82) was born in Detroit, MI, on April 4, 1953. He received the B.S. degree in physics and mathematics from Wayne State University, Detroit, MI, in 1975, and the M.S. and Ph.D. degrees in physics from Brown University, Providence, R1, in 1977 and 1981, respectively. From 1981 to 1983, he was an NRC Postdoctoral Research Associate at the U.S. Naval Research Laboratory where he conducted research in the area of ferrimagnetic devices for microwave signal processing. He is currently a member of the Optical Sciences Division at the U.S. Naval Research Laboratory, Washington, DC. His research interests include fiber-optic sensors, hyperspectral imaging, and analog microwave photonics.

Christopher S. McDermitt was born in Harrisburg, PA, on November 29, 1977. He received the B.S. degree in physics from Bloomsburg University, Bloomsburg, PA, in 2000. From 2001 to present, he has been a Research Physicist in the Optical Sciences Division, U.S. Naval Research Laboratory, Washington, DC. His research interests include photonics signal processing, fiber-optic interferometric systems, and enhancement of microwave photonic links.

Mark G. Parent received the B.S. degree in electrical engineering from Michigan Technological University in 1982 and the M.S. degree in physics from Michigan Technological University, Houghton, in 1985. In 1985, he joined the U.S. Naval Research Laboratory, Washington, DC, where he is currently Section Head in the Radar Analysis Branch of the Radar Division, Naval Research Laboratory. His research interests include phased array antenna design, optical beamforming techniques, radar cross section measurements and advanced microwave/optical system concepts.

IE E Pr E int P r Ve oo rs f ion

W. Mark Dorsey (M’09) received the B.S. and M.S. in electrical engineering with a focus in electromagnetics from the University of Maryland, College Park, in 2002 and 2005, respectively, and the Ph.D. degree in electrical engineering from Virginia Polytechnic Institute and State University, Blacksburg, in 2009. As a doctorate student, he researched dual-band, dual-polarized antenna elements and arrays. He has worked on antenna design, measurement, and integration for the Radar Division of the U.S. Naval Research Laboratory since 1999. His primary research interests include reconfigurable and multifunction antenna design, ultrawideband (UWB) antenna design, antenna isolation studies, antenna measurement, and calibration.