T/R Lens Amplifier Antenna Arrays for X-band and Ka-band This article describes the principles and applications of quasi-optical techniques for microwave and mm-wave transmit-receive amplifiers
By Zoya Popovic´ University of Colorado, Boulder ntennas integrated with active circuitry are usually referred to as active antenna arrays. The integration can be done at different levels, depending on the goal and application. For example, a low-noise amplifier (LNA) is often integrated directly with the antenna for reduced loss and therefore decreased noise figure. Another example is the integration of power amplifiers with antenna arrays to achieve spatial power combining. The focus of this paper is the integration of bidirectional amplifiers with each element in an antenna ▲ Figure 1. Schematic of a T/R lens amplifier array. The amplifiers are array, for transmit-receive fed with a plane wave radiated by a feed positioned at the focal point (T/R) applications. of the discrete lens. The delay lines between feed-side and radiatingThe schematic of the side antennas vary in length across the array and are designed to proarchitecture is shown in duce lensing. Figure 1. Each element of a directional antenna array is connected to a power amplifier (PA) and LNA. along a focal surface, and the output radiated The elements can be fed with a corporate feed wave is a collimated beam defined by the output network, but such a network is typically lossy antenna array. These active arrays are appropriand complex for a large number of elements. ately referred to as quasi-optical. (A review of Figure 1 shows an array with a free-space feed. quasi-optical active components developed to In this architecture, all elements are fed in date can be found in [1]). phase from a feed placed at a focal point in the In transmission, the advantage of the archinear field. This is accomplished by delay lines of tecture from Figure 1 is that the radiated power varying length between the input (feed-side) from the array is obtained by combining the antenna and output antenna. An array designed individual amplifier output powers [2,3,4,5]. in this way is a planar microwave lens with gain. This allows for lower cost and lower power indiThe lens is fed from a point (or multiple points) vidual elements, and graceful degradation. In
A
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reception, the noise figure of the entire array is the same as that of the single element [6], while the received powers combine coherently. This increases the dynamic range of the receiver and improves its reliability.
X-band and Ka-band lens array examples The design and fabrication of a proof-of-concept X-band T/R lens using discrete devices [7], and a Ka-band lens using MMICs [8] is discussed next, followed by measurements of basic ▲ Figure 2. Photograph of front and back side of hybrid X-band array with 24 elements arrayed properties, and meain a triangular lattice. sured performance related to systems applications. The two lenses are in principle the same, but designed using 65 ohm microstrip lines. The DC bias for demonstrate scaling and are designed with different the PIN diodes (two per switch) is supplied through the antenna and circuit elements. slot antenna feed lines. Both transistors share the same A photograph of an X-band T/R lens amplifier is drain bias line. shown in Figure 2. The amplifier array has 24 elements A photograph of a Ka-band lens amplifier designed at and is fabricated on a single Rogers Duroid substrate the Local Multipoint Distribution System (LMDS) frewith a relative permittivity of 2.2 and 0.508 mm thick- quency around 28 GHz is shown in Figure 4. The array ness (20 mils). The bidirectional amplifiers and switches is a multi-layer structure fabricated on two Rogers are designed in microstrip using discrete devices. TMM10 substrates. The architecture is a hybrid one, An element of the array is shown in Figure 3. The with commercial MMIC power and low-noise amplifiers slot antennas are etched in the microstrip ground and SPDT switches. plane. In transmission, the input antenna is the vertically polarized slot. The wave received from the freespace feed is routed through the delay line and the MESFET power amplifier (PA) by controlling the PIN diode SPDT switches. The PA feeds the horizontally polarized output slot. All the output slots radiate inphase waves, and their powers are coherently combined upon radiation into a beam defined by the array pattern of the output array. In reception, the horizontally polarized slots become the input antennas, and the signal is routed through the PHEMT LNAs and the delay lines. The vertically polarized slots focus the amplified signals onto a receiving element placed at a focal point. In reception, the signal powers are combined coherently, and the noise incoherently (assuming the individual amplifiers have uncorrelated noise), thereby increasing the dynamic range by 10 log N (dB), where N is the number of elements. The microstrip-fed slot antennas are designed to ▲ Figure 3. Single element active T/R antenna using discrete devices. The antennas are second-resonant slots cenoperate at second resonance, which makes it possible to tered at 10 GHz. The area of the element is one square obtain a 40 percent 2:1 VSWR bandwidth and a 65 ohm free space wavelength. impedance around resonance. The entire circuit is 32 · APPLIED MICROWAVE & WIRELESS
order to determine the losses in the passive part of the arrays, passive arrays were fabricated, identical to the active ones, but with throughlines instead of the amplifiers and switches. The feed and input and output wave polarization for the passive slotantenna X-band array are illustrated in Figure 6. The focal distance to diameter ratio (F/D ratio) of the lens is 1.5, or 27.5 cm. The active lens exhibited gains of 5 dB in transmission and 2 dB in reception. The losses are identified and described in detail in [9]. The 28 GHz lens array was measured to have 2.7 dB loss just due to the passive parts of ▲ Figure 4. Photograph of front and back side of a 28 GHz lens T/R array. The center ele- the arrays (antennas, microment is not populated with MMICs and can be used as a folded feed, which reduces strip lines and slot coupler). The elements were gradually the total (lens and feed) volume by a factor of two. populated with MMICs, and the gain was measured for 5, 8, 11 and 17 transmitting elements, as shown in Figure 7. The layout of a single element is shown in Figure 5. This measurement is relative to the power transmitted Both patch antennas are fed at the nonradiating edge 50 between two copolarized horn antennas. To measure the ohm points. In transmission, the input horizontally lens, one of the antennas is used as the feed, the amplipolarized patch antenna receives the wave radiated from fier is inserted a focal distance away from it, and the a free-space feed. The signal then undergoes a delay that other horn is rotated by 90 degrees and is in the far field depends on the position of the element in the array. The of the array. As can be seen in this power gain measurebidirectional amplifier is fabricated on the other substrate, and a slot-coupler etched in the common ground ment, fewer than 11 elements are not sufficient for the plane enables efficient broadband coupling between the two layers (5 GHz bandwidth centered at 30 GHz, with 0.5 dB loss). Initially, measurements were performed to investigate the lensing and gain properties of the arrays. In
▲ Figure 5. Front and back side of a single element 28 GHz active T/R antenna using MMIC PAs, LNAs and SPDT switches. 34 · APPLIED MICROWAVE & WIRELESS
▲ Figure 6. Free-space focal-point feed of a T/R lens amplifier array. Both transmitted and received waves on the feed side of the array are vertically polarized, while the antennas on the other side of the array transmit and receive horizontally polarized waves.
▲ Figure 7. Transmit-mode power gain through the 28-GHz lens measured for 5, 8, 11 and 17 elements populated with MMIC amplifiers.
array to exhibit real focusing, so the array has no gain. With 17 elements, focusing is achieved, and 7 dB of small signal absolute power gain through the arrays is measured. In these 2-dimensional arrays, the heat is generated by the amplifiers across the entire area. The heat can be taken out only along the perimeter, so the center of the array operates at a higher temperature than the edges. This was verified by the measurements shown in Figure 8. With no cooling, the center of the array is at 108 degrees. When a simple fan is used, due to the temperature drop and more even distribution, the temperature at the center drops to 3 degrees and the overall array gain increased by about 3 dB.
Focusing properties
▲ Figure 8. Measured temperature profile for the Ka-band lens with and without air cooling using a simple fan. The gain drops by about 3 dB when the array is not cooled. (The measurements were done at a DC bias point of 4 V and 2.25 A.)
feed is at the focal point, the beam on the other side of the lens is collimated (and in this case given by the array pattern). If, however, the feed is placed at twice the focal distance (2F) on the optical axis, the lens should focus the beam back down at 2F on the other side of the lens. The discrete microwave amplifier lenses also exhibit this property, as confirmed by measuring the gain for both cases, Figure 10. The figure also shows the power measured when the amplifiers were biased off, indicating good isolation in both cases. In the 2F case, due to a larger separation from the feed and more power diffracted around the edges of the array, the on/off ratio is slightly degraded. These measurements indicate that the lens can be used as a radiating power combiner in applications where an antenna array is required, but also in cases where it may be required to collect the power from
Some interesting properties of lens arrays were confirmed by measurements shown in Figure 9 and Figure 10. When the space between the feed and the array is shielded by an overmoded horn (made out of cardboard and copper tape to fit the X-band array dimensions), very little difference in gain versus frequency was observed (Figure 9). This means that the feed and lens patterns are effectively matched and that such arrays can be mounted in mechanically stable shielding packages without degradation or instabilities. The radiation patterns with and without the shield were essentially the same. ▲ Figure 9. When the space on the feed side of the array is shielded by a overmoded As in an optical lens, if the horn, little change in the gain frequency response is noticed. FEBRUARY 1999 · 35
To reduce the volume of the arrays, another property of the focusing was exploited in the 28 GHz array. Note in Figure 3 that the center element is not populated by MMICs. This element is connected to a line going to the edge of the array and subsequently to a connector. It can be used as a folded feed when a mirror is placed at half of a focal distance and parallel to the plane of the array, as shown in Figure 11a. During the design, it was shown that little degradation results in the center element not being active. This element, if active, would operate at the highest temperature, so this design also improves the temperature distribution uniformity across the array.
Applications The active lenses possess some properties useful for practical systems. These are mostly related to the built-in possibility of beam steering and beam forming without the need for ▲ Figure 10. Measured gain and on/off ratio frequency response for microwave phase shifters. The different practitwo positions of the feed. When the array is fed from a focal point cal advantages of a lens compared to standard (solid line), the radiated beam is collimated (bottom left). When the architectures are illustrated with several differfeed is placed at twice the focal distance (2F), the lens array focus- ent measurements, described below. es the power (dashed line) down to a point located at 2F (bottom Figure 12 shows the measured radiation patright). terns of the X-band lens in transmission, compared with theoretical patterns. The theory was done assuming a uniformly excited output the individual elements into a guiding structure. This array (phase and amplitude) for the array factor, and a kind of free-space power combiner has the advantage measured slot element radiation pattern. The discrepthat the power combining efficiency (PCE) remains con- ancies indicate some phase and amplitude nonuniformistant as the number of elements grows, and has been ty across the array. These measurements were pershown to be above 70 percent (as demonstrated in [10]). formed with the feed placed at the focal point on the
(a)
(b)
▲ Figure 11. (a) A folded feed, as implemented in the 28 GHz array from Figure 3, reduces the overall volume by a factor of two. (b) Measured gain frequency response for a focal-point feed (blue) and a folded feed (red) in reception with 10 elements populated with LNAs. 36 · APPLIED MICROWAVE & WIRELESS
optical axis. The lens, however, has an entire focal surface, and when the feed is positioned offaxis, a linearly-progressing phase is produced across the feed-side array, and preserved upon radiation. This implies that feeds positioned at different angles result in beams steered with respect to the boresight (optical axis) direction, as demonstrat▲ Figure 12. Measured copolar E and H plane radiation patterns of the X-band array in trans- ed by the measurement at mission. The theoretical patterns are calculated assuming a uniform array and using a X-band shown in Figure 13. The power in the main measured slot element pattern. beam was within 1 dB for the three cases. The grating lobes are increased in the steered beams because the period of the array is one free-space wavelength. In applications where a fixed number of discrete beams is sufficient, such in base station transmitter, or in SAR, this approach may have advantages over using a corporate feed with microwave phase shifters. In the lens approach, the beam is steered with low-frequency switching of the appropriate feeds. In addition, since more than one feed can be used at the same time, beam forming is possible. For example, if the two feeds at –20 and +20 degrees (corresponding to the steered beams in Figure 13) were turned on simultaneously, since the array is linear, the resulting pattern would be a superposition of the two, with a null at broadside. Measurements on a similar array showing null steering are reported in [11]. ▲ Figure 13. Measured beam steering of the X-band lens The spatial beam forming can also be used in recepwith three feed positions. The peak power is shown nor- tion [12], as illustrated in Figure 14a, where three malized, but remained within 1 dB. receivers are placed at three positions along a focal arc of the lens. The corresponding measured received IF sig-
(a)
(b)
▲ Figure 14. (a) A lens amplifier receiver with third order angle diversity. (b) The IF signals out of each of the receivers positioned along a focal arc are relatively independent. 38 · APPLIED MICROWAVE & WIRELESS
▲ Figure 15. Test setup (top) for a frequency-reuse experiment using an X-band lens amplifier. The bottom plots show demodulated FSK signals from receivers corresponding to signals incident from 0 and 20 degrees modulated onto the same Xband carrier frequency.
▲ Figure 16. Measurement setup and results of a fading experiment with a lens array. In the bottom right plot, nulls as deep as 50 dB are seen with no amplifier in place and for certain mirror orientations. When the lens is put in the path, no fading nulls below 5 dB are measured.
nals, after downconversion, are shown in Figure 14b. For these measurements, the IF frequency was around 500 MHz. The received IF signals from different directions are seen to be relatively independent. The implication on mobile and indoor communications, with slowfading (Rayleigh) fading, is that it is unlikely that 40 · APPLIED MICROWAVE & WIRELESS
incoming signals would simultaneously destructively interfere at all three receivers. This architecture has built-in angle diversity, in this case of third order (three incoming incident angles). As an example, for a binary phase shift keying (BPSK) modulation, the probability of error for a given signal to noise ratio (SNR) decreases as Pe ~ (3/SNR)3 [13]. The same built-in spatial beam forming can be utilized for a frequency reuse communication link, as shown in Figure 15. The two receivers, A and B, receive signals from two different users, A´ and B´, with the same X-band carrier. As an example, measurements were performed with two FSK signals (50 kHz and 150 kHz). The measured received and demodulated signals are shown in Figure 15. The received power of an interfering signal originating in a direction 20 degrees off the desired signal direction has a measured relative power of about –10 dB. A measurement that shows reduction of the effects of multipath fading with a lens and single receiver is shown in Figure 16. A metallic mirror is placed parallel to the optical axis in front of the Xband array, and is translated in steps perpendicularly to the optical axis. At each step, the mirror is rotated through a set of angles. The received power is measured for all mirror positions with and without the lens in the direct path between two antennas. The results are shown in Figure 16. It is seen that maximum fading nulls were improved by 45 dB in this case by using a lens array, due to the directivity of the array. (Note that different scales are used on the two plots.)
Discussion In summary, this paper presents an overview of T/R active antenna lens arrays. The fundamental advantages of this architecture are: power combining in transmission with high reliability and graceful degradation; increased dynamic range in reception; and the built-in
optical switch exhibits several gigahertz of bandwidth with less than 1 dB insertion loss and over 30 dB isolation. The speed is proportional to the optical power, and this is currently under investigation.
Acknowledgements I would like to thank all my graduate students for their hard work, motivation and good humor, especially Dr. Stein Hollung (now at Chalmers University in Sweden), whose thesis work I borrowed from extensively for ▲ Figure 17. The multimode optical fibers that control the T/R X-band active antenna this paper. I appreciate the supare aligned to the photodiodes through holes machined in an FR4 low-cost fixture, port of Dr. Jim Harvey at the shown on the left photograph. The fixture for the optical control circuit is aligned to U.S. Army Research Office an FR4 RF fixture holding the active antenna element, shown on the right. through numerous programs over the years, and Dr. Bill Miceli from the Office of Naval spatial beam steering and beam forming capability. The Research for his support through a MURI program in active array is amenable to monolithic integration, RF photonics. ■ although the increased availability of MMICs into the Ka-band range justifies a hybrid approach. However, References there is a disadvantage of using MMICs — the designer 1. R.A. York, Z. Popovic, eds., Active and Quasiis constrained to a 50 ohm environment, which is often Optical Amplifier Arrays for Solid-State Power not the best choice for circuits integrated with antennas, Combining, New York: Wiley, 1997. and also the bias pads are located so that an array bias2. M. Kim, E. A. Sovero, J. B. Hacker, M. P. DeLisio, J. ing network is hard to design. C. Chiao, S. J. Li, J. J. Rosenberg, D. B. Rutledge, “A 100The two arrays discussed in this paper use switches to Element HBT Grid Amplifier,” IEEE Trans. Microwave separate the transmit from the receive part. This is Theory Tech., vol. 41, pp. 1762-1771, Oct.1993. appropriate for radar applications and some communi3. N. Shetch, T. Ivanov, A. Balasubramaniyan, A. cations, but full duplex operation also needs to be exam- Mortazawi, “A Nine HEMT Spatial Amplifier,” IEEE ined. The reliability and degradation characteristics also MTT-S Int. Microwave Symp. Dig., San Diego, CA, June need to be studied systematically, although it is relative- 1994, pp. 1239-1242. ly obvious that a distributed architecture would have 4. J. Hubert, J. Schoenberg, Z. Popovic, “A Ka-band good reliability. For example, if 10 percent of the ele- Quasi-optical Amplifier," IEEE MTT-S Int. Microwave ments in the Ka-band array fail, the power in the main Symp. Dig., Orlando, FL, May 1995, pp. 585-588. beam would drop by only about 1 dB, and the sidelobes 5. H. S. Tsai, M. J. Rodwell, R. A. York, “Planar would increase by 3 dB. If 30 percent of the elements fail Amplifier Array with Improved Bandwidth Using Folded randomly across the array, the main beam power drops Slots,” IEEE Microwave Guided Wave Lett., vol. 4, pp. by about 3 dB, and the sidelobes increase by about 4 dB, 112-114, Apr. 1994. compared to the fully functional case [8]. 6. J. Schoenberg, T. Mader, B. Shaw, Z. Popovic, The switches in the two arrays discussed here are “Quasi-optical Antenna Array Amplifiers,” IEEE MTThybrid in one case, and MMIC in the other. The MMIC S Int. Microwave Symp. Digest, Orlando, FL, May 1995, switches are fast (few nanoseconds), but are all con- pp. 605-608. trolled in parallel. This means that their capacitances 7. S. Hollung, A. Cox, Z. Popovic, “A Bi-directional add up, as well as the resistive loss, thereby increasing Quasi-optical Lens Amplifier,” IEEE Trans. Microwave the time constant. A photonic approach to individual Theory Tech., vol. 45, no. 12, pp. 2352-2357, Dec. 1997. 8. Z. Popovic, A. Mortazawi, "Quasi-optical control of each array element is now being demonstratTransmit/Receive Front Ends," IEEE Trans. Microwave ed [14]. Figure 17 shows a single element of an opticalTheory Tech., vol. 46, no. 12, pp. 1964-1975, Dec.1998. ly-controlled X-band unit cell, where a pair of multimode fibers control the microwave switch bias. The 9. Stein Hollung, Transmit/ Receive Lens Amplifier 42 · APPLIED MICROWAVE & WIRELESS
Front Ends, Ph.D. dissertation, Department of Electrical ad Computer Engineering, Univ. of Colorado, Boulder, 1998. 10. T. Berg, S. Hollung, J. Lee, Z. Popovic, “A Two-stage Amplifier Lens,” 28th European Microwave Conference Dig., Amsterdam, October 1998, pp. 178-182. 11. J. Schoenberg, S. Bundy, Z. Popovic, “Two-level Power Combining Using a Lens Amplifier,” IEEE Trans. Microwave Theory Tech., vol. 42, no. 12, pp. 2480-2485, Dec.1994. 12. W. Shiroma, E. Bryerton, S. Hollung, Z. Popovic, “A Quasi-optical Receiver with Angle Diversity,” IEEE MTT-S Int. Microwave Symp. Dig., San Francisco, CA, June 1996, pp. 1131-1134. 13. Prof. Mahesh Varanasi, University of Colorado, private communication. 14. Jim Vian, University of Colorado, private communication.
Author information Zoya Popovic´ is an Associate Professor in the Department of Electrical and Computer Engineering at the University of Colorado at Boulder, Boulder, Co 80309-0425. She received her Dipl.Ing. degree from the University of Belgrade, Serbia, Yugoslavia, and her Ph.D. from Caltech, Pasadena (1990). Her research interest include microwave and millimeter-wave active antenna arrays and quasi-optical techniques, high-efficiency microwave circuits, RF photonics, and antennas and receivers for radioastronomy. She is the recipient of the URSI Young Investigator Award, the NSF Presidential Faculty Fellow Award, The International URSI Isaac Koga Gold Medal, and the IEEE MTT Microwave Prize for pioneering work in quasi-optical grid oscillators. Dr. Popovic´ can be reached by fax at 303-492-5323 or by e-mail at:
[email protected]
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