Quasi-optics in radar systems.
Radar systems present exceptionally large range of design challenges for the microwave engineer, in that great variety of disciplines, including antennas, transmitters, signal processing and low noise receivers, are involved. Radar systems in the mm-wave region are still evolving. The significant potential offered by this wavelength range includes the high gain and small beamwidth available from a specified antenna size and the large bandwidth available. Taking advantage of these characteristics is rendered difficult by the fact that components built in conventional propagation media, for example, waveguide and microstrip, become increasingly lossy and are difficult and expensive to fabricate as the operating frequency increases.
It is thus fortunate that quasi-optical propagation, that is, propagation in free space of a beam of radiation that is a relatively small number of wavelengths in transverse extent, becomes increasingly effective at mm-wavelengths. This is because the effects of diffraction diminish as the wavelength decreases. While system design, to a large degree, has been controlled by the availability of components, it is also true that system needs drive the development of new component capability. In the case of quasi-optics, it is the recognition of the medium's potential that has encouraged the development of new components during recnt years, and has led to the actual realization of devices and components that previously had been conceived of only theoretically. The availability of a wide variety of high performance components has in turn led to increased consideration of how quasi-optical technology can fit into radar system design strategies. In this paper, some of the more interesting developments in the area of quasi-optics of particular relevance to radar systems will be reviewed and how their performance opens up new possibilities for mm-wave radar systems will be demonstraded.
Quasi-optics is the term used to describe propagation of a beam of electromagnetic radiation in free space, but one that is a relatively small number of wavelengths in transverse dimensions. The indicated restrction of the beam size is driven by the desire for a relatively compact system (when the relatively large wavelengths even in the mm-wave region of the spectrum, compared to optical wavelengths, are considered). While it is inevitable that diffraction will play a major role in the propagation of beams of small transverse extent, it would be highly desirable to avoid the complexities of diffraction calculations in designing quasi-optical systems. This is one of the key motivations for the use of Gaussian beams and Gaussian optics in quasi-optical systems, since with this simplification, quasi-optical system design is greatly facilitated, while retaining good accuracy if a few basic design rules are satisfied.
In Guassian optics, it is assumed that the beam of radiation has a Gaussian distribution of the electric field perpendicular to the axis of propagation> this is written as, [Mathematical Expression Omitted] where r = the distance from the axis of propagation w = the beam radius, which depends on the position along the axis of propagation
The power distribution is also a Gaussian, but narrower by a factor [square root. The distribution given in Equation 1 is actually the first of a complete set of Gaussian modes in two dimensions, but is by far the most important for quasi-optical propagation. Higher-order Gaussian modes have been given previously. (1-3) The polarization of the Gaussian beam is arbitrary (as long as E is perpendicular to the direction of propagation), a fact which is of great value in quasi-optical systems. The beam radius is smallest at the location along the axis of propagation called the beam waist> having there its minimum value [W. sub. 0]. In order for Gaussian beam theory to be valid, [w. sub. 0] must be [is greater than or equal to]. Away from the beam waist, the beam radius grows in a predictable fashion, given by the relation, [Mathematical Expression Omitted] in which z denotes the distance along the axis of propagation from the beam waist. The growth of a Gaussian beam is shown schematically in Figure 1, which also indicates how the peak value of the electric field strength diminishes away from the beam waist. A second important property of a Gaussian beam is its phase distributio> the surfaces of constant phase are spherical caps, which have a radius of curvature given by, [Mathematical Expression Omitted]
At the beam waist, the equiphase surface is a plane (R = [infinity]). Away from the waist, the radius of curvature first drops, then reaches a minimum value [R. sub. min] = 2[pi]W.sup.2.sub./]/Lamda, and finally increases, achieving the limiting form R(z) alpha Z [is greater than][is greater than][pi]W.sup.2.sub.0/Lamda, which is the geometrical optics limit.
To make effective use of quasi-optical propagation, it is necessary to deal with the fundamental characteristics that a beam of radiation in free space grows in size as it propagates. Quasi-optical design thus involves two basic aspects, allowing adequate clearance for all elements of the system and combatting the effects of beam growth.
By allowing adequate clearance for all elements of the system, it is ensured that the effects of beam truncation are tolerably small. Cutting off the Gaussian beam, which as suggested by Equation 1 extends to infinity, is a practical necessity that introduces sidelobes and other non-Gaussian components at some level and shifts the positions of the beam waists. Ideally, a clear diameter for a Gaussian beam should be [is greater than or equal to] 4 X W at all positions, corresponding to a relative power density at the edge of -35 dB relative to that on the axis of progation. (4) It is possible to allow the truncation to occur at the -20 dB level (= 3 X W diameter), or somewhat less, and still preserve moderately good beam quality. (5) This obviously results in a more compact system, but one with slightly higher loss, and the system designer must be prepared to deal with tradeoffs of this type.
To combat the effects of beam growth, it is necessary to refocus the propagating Gaussian beam. to first order a focusing element, whether mirror or lens, acts to change the radius of curvature of the Gaussian beam, while leaving the beam radius w unaffected. For a given elemet focal length and input waist at a fixed distance away, the Gaussian beam transformation formulas determine the size and location of the output waist. (6,7) Gaussian beam transformation by a focusing element (represented by a lens) is shown schematically in Figure 2. Alternatively, the focal length required to transform a given input waist radius into a specified output waist can be determined. For a fixed separation between the waists, there are only two possible solutions. (8) Lenses have finite loss due to dielectric absorption and surface reflection (unless anti-reflection coated, which limits their bandwidth), but offer a convenient optical layout with no off-axis aberrations. Mirrors, on the other hand, offer nearly perfect reflectivity in the mm-wave frequency range over essentially arbitrary bandwidths, but do require a more complex geometry along with some problems associated with their off-axis geometry, including cross-polarization generation and coupling to higher order modes. (9) Thus, a Gaussian beam system generally employs a combination of lenses and mirrors to achieve the optimum combination of compactness, low loss, and consistency with the desired overall system configuration.
The major virtues of quasi-optical propagation include there is no source of absorption loss produced in transmitting power from one point to another aside from the very small losses at the focusing elements> the basic Gaussian beam can handle an arbitrary polarization> power in the Gaussian beam is spread out over an area under the control of the system designer, in any case it is [is greater than][is greater than][Lambda].sup.2], so the power-handling capability is far greater than for single mode transmission medium> a sequence of lenses of mirror forming a Gaussian beam waveguide can handle multiple spatial modes simultaneously, thus allowing for the possibility of signal handling imaging systems including monopulse processors> the spatially extended nature of aGaussian beam allows for the possibility of multiple devices coupling coherently into the desired, single quasi-optical mode, which is a feature that has profound implications for power generation with multiple devices, which is of importance for radar systems> and quasi-optical Gaussian beams couple efficiently to guided-wave propagation media, especially by means of scalar feedhorns, devices that produce symmetric radiation radiation patterns of arbitrary polarization, with = 98 percent of the energy in the fundamental Gaussian mode, (10) which means that quasi-optical processing can be efficiently employed with systems using mixers and transmitters built in waveguide.
The ability of a quasi-optical Gaussian beam system to handle a diversity of polarization states has resulted in considerable development of multi-polarization systems. Some of the quasi-optical components that carry ut polarization transformation are discussed briefly.
Quasi-optical linear polarizers are straightforward and consist of an array of conducting strips or wires. As long as the spacing is considerably less than [lambda]/2, any radiation with electric field parallel to the conductors is reflected, while any radiation having E perpendicular to the conductors is transmitted. (11, 12) This process is essentially lossless throughout the mm-wavelength range. As suggested by the above design criterion, quasi-optical polarizers can be made to have extremely good polarization isolation over bandwidths of many octaves and are extremely effective for polarization separation and duplexing.
Linear polarizers can be made free standing by stretching wires across a supporting frame. This technique results in a surprisingly strong structure and one that has minimum loss. At longer wavelengths ([LAMBDA][is greater than is equal to] mm), it is also effective to use strip grids, fabricated by photolithography on a low-loss dielectric substrate. Some absorptive and reflective loss is inevitably introduced by the substrate, but this type of grid has very desirable properties of mechanical ruggedness and low fabrication cost.
Radar systems often require operation in circular polarization. Quasi-optical transformation of linear to circular polarization (and vice versa) can be carried out by several means, of which the most common is the waveplate. In this device, the speed of propagation is different for componets of the electric field along two orthogonal directions perpendicular to the direction of propagation of the beam, called the fast and the slow axis. If the resulting relative phase shift for a beam passing through the waveplate is equal to [PI]/2 or 90[degrees], a quarter-waveplate exists. A linearly polarized beam having its electric field at 45[degrees] between the fast and the slow axis will emerge circularly polarized after propagating through a quarter-waveplate and vice versa. Waveplates are extremely compact and have quite low loss. The actual anisotropy in the propagation speed can be obtained by using material for which the index of refraction and thus the wave propagation speed is naturally anisotropic, such as crystal quartz and sapphire. In this situation, it is possible to anti-reflection coat the surfaces of the waveplate by a layer of material of the appropriate index of refraction.
Alternatively, an inherently isotropic material can be made to propagate anisotropically by machining grooves that result in a different index of refraction for the two directions of electric field polarization.  This is shown schematically in Figure 3, in which the principal directions are denoted h and v. The indices of refraction are given by,
[Mathematical Expression Omitted] 
where [summation over pebbles p] = the diabetic constant of the material ([summation over pebbles p] = [n.sup.2])
The condition for operation of any quarter-waveplate relying on an index of refraction difference is,
[Mathematical Expression Omitted] 
and for this type, [delta]n = [n.sub.h] - [n.sub.v]. Since it is generally not possible to anti-reflection coat this type of waveplate, it is desirable to keep the indices of refraction relatively similar and close to unity by using low-index plastics and moderate grooving. This does mean that waveplates of this design are typically thicker than those made from naturally anisotropic material, but their loss is extremely low due to the inherently low absorption of these dielectrics.
A half-waveplate has just twice the differential phase shift of a quarter-waveplate> this results in a 180[degrees] phase delay of he component along the direction of slower propagation relative to that parallel to the fast direction, which reflects the direction of polarization about the fast direction. If an incident beam is linearly polarized with polarization direction at angle [alpha] with respect to the fast direction, its plane of polarization is thus rotated by an angle 2[alpha], making a half-waveplate an effective polarization rotator. Half-waveplates canbe fabricated in the same way as quarter-waveplates, being essentially twice as thick.
Another method of making waveplates is by reflection from different planes, as shown in Figure 4. A device consisting of a wire grid and a total reflector is arranged so that the plane of reflection, and hence the phase, will depend on the polarization direction of the incident radiation. For vertically and horizontally linearly polarized radiation, the path difference is given by,
[Mathematical Expression Omitted] 
so that the phase shift is (4 [pi]s/[lamda]) X cos [THETA]. The choice of grid-mirror spacing and incidence angle determines the differential phase shift and thus, the functioning of the device. The reflective waveplate has several important advantages for radar systems, of which the most important is very high power-handling capability, due both to the ability to spread the beam out over a large surface, and the high inherent reflectivity of the wire grid and the plane mirror. The operation of grid-mirror waveplates as polarization rotators has been analyzed previously.  An important advantage of quasi-optical propagation is that the basic elements, whether lenses or mirrors, propagate any polarization state, which is a critical advantage for implementation of polarization diverse radar systems.
Quasi-Optical Ferrite Devices
An important component of radar systems is the transmit and receive duplexer, which directs the transmitter signal to the antenna and then to the target, while routing the return signal to the receiver. This function is typically carried out by a non-reciprocal device made from magnetized ferrite material. The quasi-optical ferrite devices that can function as duplexers are based on the principle of Faraday rotation, in which the direction of the electric field of a linearly polarized wave traveling parallel to the magnetic field sufers a certain rotation per unit distance.  This is shown schematically in Figure 4 as the difference between the initial angle [[THETA].sub.1] and the final angle [[THETA].sub.2]. Faraday rotations is due to the precession of the atomic spins of the ferrite material and has the critical property that the sense of rotation is fixed relative to the magnetic field direction. Thus, if a wave traveling to the right in Figure 5a has its field direction rotated by an angle [delta][THETA] in the sense of [[THETA].sub.1] to [[THETA].sub.2] as it passes through the ferrite, a wave traveling to the left would experience the same rotation of polarization in the same sense. If the thickness d of the ferrite material is chosen so that a rotation angle of 45[degrees] exists, and the ferrite and its magnet are combined with a half-waveplate set up to produce a 45[degrees] rotation, the situation shown in Figure 5b is obtained. For a wave traveling to the right, the rotations of the ferrite and the quarter-waveplate add together to make a 90[degrees] rotation, while for a wave traveling in the opposite direction, they cancel, yielding zero net rotation. This is the essence of the nonreciprocal performance of the magnetized ferrite that gives it the potential to act as a circulator, isolator or duplexer.
Quasi-optical propagation in a magnetized ferrite is actually much simple to analyze than in a waveguide or other guided wave structure because as long as the diameter of the ferrite is considerably greater than the beam radius w of the Gaussian beam traversing it, it effectively operates like an infinite medium, without the complexities introduced by boundaries of guiding structures. The rotation angle per unit distance through the ferrite is just that derived from basic ferrite properties.
To make a complete quasi-optical circulator or duplexer, only a wire grid polarizer needs to be added. For example, a wire grid on the left hand side of Figure 5b can be oriented to reflect vertically polarized energy into the ferrite> this is called the 0[degrees] direction. After passing through the ferrite and half-waveplate, it will have its polarization rotated by 90[degrees]. Any radiation reflected in the same polarization will retain this same 90[degrees] directed polarization after passing through the half-waveplate and ferrite in the reverse direction, and so will be transmitted through the wire grid, and would then go to the receiver.
An example of such a system is illustrated in Figure 6, which suggests some of the other advantages of quasi-optical processing that might be utilized. These include the ability to propagate multiple spatial modes. In this case, by making the ferrite only slightly larger than required for a single Gaussian beam, it can handle the group of slightly offset beams necessary to make a monopulse feed. The single quasi-optical ferrite can function as the duplexer for all of the beams in question. In addition, it would be possible to add a second polarization grid in front of the ferrite, having the same orientation as the one shown, which would separate out the return signal polarized perpendicular to the plane of the paper, and send it to a second receiver (or second monopulse array receiver).
Impressive performance has been reported recently for quasi-optical ferrite devices operating throughout the mm-wavelength range. Isolation exceeding 40 dB for a Faraday rotation isolator operating at 35 Ghz has been reported.  The ferrite material is 1 cm thick and is anti-reflection coated with quartz matching layers. The insertion loss is extraordinary low, less than 0.1 dB over the 33 to 39 GHz frequency range. A quasi-optical Faraday rotation isolator operating at 285 GHz has been developed.  The loss is typically 2 dB and the isolation 18 dB, but both these quantities show considerable fine-scale frequency variation suggesting that the impedance matching is still less than perfect. The implication is that the absorptive loss in the quasi-optical ferrite rotator is small, and that impressive performance should be attainable even at these very high frequencies.
The lack of high power sources suitable for transmitters has been a significant obstacle to the development of many types of radar systems at mm-wavelengths. While development of solid-state sources and frequency multipliers has extended the availability of low power sources through the mm-wave frequency range, and high power gyrotrons have proven effective for tasks such as plasma heating, there remains a need for high power, spectrally pure coherent sources that can be modulated with complex waveforms, as required for use in modern radar systems.
At first, it may seem surprising that quasi-optics would have any useful connection with power combining, since solid-state power generating devices (for example, Gunn diodes and FETs) have dimensions much smaller than a wavelength, while quasi-optical systems are considerably larger than a wavelength. The need for power combined sources at mm-wavelengths arises from the fact that the output of a single device is limited by thermal dissipation, so that the most direct way to increase the power output is to use multiple devices. In guided-wave combiners,  several active devices (most commonly Gunn diodes at mm-wavelengths) are introduced into a single resonant cavity. The very fact that the cavity is small, limits the number of devices that can be utilized> too many devices will reduce the Q of the cavity. Also, since the devices are very close to one another, heat removal from the assembly can still be a significant problem given that the device temperature rise must be limited. While it is possible to have devices well-separated in individual resonant cavities, this leads to an extremely bulky arrangement that typically will require extremely complex arrangements to ensure that all devices are oscillating in phase.
The key to quais-optical power combining is the recognition that a quasi-optical cavity can be large in terms of number of wavelengths and has an extremely well-defined resonant field configuration, which can couple to a very large number of active devices.  A quasi-optical cavity is based on two key considerations. The first key consideration is that a reflector inserted into a quasi-optical beam will reflect the energy in the same Gaussian beam mode as that of the incident radiation if the radius of curvature of the mirror is equal to that of the Gaussian beam, given by Equation 3. This applies to either partial or complete reflectors. The second key consideration is that the resonance condition applies to the on-axis phase of the Gaussian beam, which for the fundamental mode is given by [theta]([zeta]) = -2 [PIZETA/lambda] + arctan ([lambdazeta/PIW.sup.2.sub.0). the first term is the phase of a plane wave, while the second is the differential phase shift due to the Gaussian nature of the mode.
A quasi-optical resonator can be made from two mirrors matching the radius of curvature of the Gau beam at any two positions along its axis of propagation. One of these positions will be the beam waist where the beam phase front is plane> this is obviously convenient but not essential. If one of the mirrors of the resonator is only partially reflecting, the possibility of coupling lower into and out of the cavity exists. The emergent energy will naturally have the Gaussian mode distribution of the beam inside the resonator itself. A Gaussian beam resonant cavity power combiner is shown if Figure 7.
The critical point is that all active devices at a particular distance along the axis of propagation of the Gaussian beam couple to this beam-mode of the cavity. For example, all devices coincident with the beam waist will be forced to oscillate in phase since the Gaussian beam at its waist has a planar equiphase surface. The coupling between the active devices is via the mode of the cavity, which naturally forces the devices to be coherent, a characteristic witnessed by the enhanced spectral purity of a multi-device cavity relative to a single oscillating device. [21,22] It is also the case that the spectral purity of quasi-optical oscillators can be higher than that of waveguide cavity oscillators using the same devices. 
There are several methods of incorporating active devices into a quasi-optical resonator. One commonly used technique is to mount devices on a plane surface that defines the beam waist. [22,23] Several quasi-optical resonator power-combined sources have ben constructed by placing the active devices within the cavity. [24,25] It is also possible to couple output power through an aperture in the spherical mirror.  This method is particularly appropriate where a very high cavity Q is desired. The use of a partially reflecting spherical mirror (as provided, for example, by an appropriately curved dielectric or metal mesh) will give a larger output power coupling and also a better-defined output beam configuration.
There have already been several quasi-optical power-combined sources developed. Work at microwave frequencies has utilized FETs as the active elements. 36 MESFET devices in a quasi-optical oscillator operating at 3 GHz were used,  which exhibited a reasonably clean output beam pattern and which showed graceful degradation under individual device failure. 100 MESFETs have been used in a 5 GHz oscillator,  which exhibits a DC-to-RF conversion efficiency of 20 percent. In this oscillator, all the active devices share a common DC bias, greatly simplifying operation. The radiation pattern showed some non-Gaussian characteristics, but the basic radiation pattern is relatively clean with a directivity of 16 dB.
Gunn diodes have been used  at 11 GHz to make an oscillator with very good spectral purity. The diodes are mounted between grooves in a metal plate that forms the plane mirror of the resonator, and the output is coupled through an aperture in the spherical mirror. This approach has been significantly extended  by constructing quasi-optical power-combined oscillators using six FETs at 12 GHz, and up to 15 gunn diodes in the 50 GHz range. The power distribution within the resonator follows that of the expected fundamental gaussian very closely, and the output spectrum is considerably narrower than that obtained using a waveguide cavity. a 4X4 array of Gunn diodes at 9.6 GHz has been used,  in which the power is radiated from each device by a planar patch antenna (different aspects of using patch antennas in conjunction with quasi-optical techniques have been studied.) [28-30] The total power radiated is inferred to be 321 mW, consistent with efficient power combining, although individual diode biases are used. The radiation pattern is clean, but 20 percent wider than predicted by theory.
Resonant cavity quasi-optical combiners have great potential for use in radar systems because they should be able to achieve a high output power in a well-defined Gaussian beam pattern having very good spectral characteristics. These advantages should be achievable with monolithic technology for the array of active devices, resulting in good reliability and low cost.
Another promising area in which quasi-optical techniques are contributing to developing sources for radar systems is the use of frequency-multiplying grids. In this application, the great advantage of quasi-optics is the relatively low, but controllable power density and the ability to limit power dissipated by each device. Frequency doubler diode grids have been developed. [31,32] Using a 760 diode monolothic diode grid, a power output of 0.5 W at 66 GHz and an efficiency of 9.5 percent using a pulsed input source were obtained. There is great potential for this approach, in terms of improvements due to better devices, scaling to even larger arrays as higher powered pump sources become available, and the further development of monolithic circuits resulting in a significant cost reduction.
Nonresonant spatial power combiners have long been a method of making higher powered sources. Recent work has focused on harmonic combiners, where the input is in waveguide or microstrip, and the second harmonic fram an array of radiating elements is combined. [33-35] The major thrust will have to be improvement of radiation patterns, to allow the combined output to be effectively utilized.
Other Quasi-Optical Components
for Radar Systems
The high level of activity in quasi-optical components and systems precludes discussion of all of the developments of relevance to radar systems. Some of the work in the areas of quasi-optical ferrites and resonant power combiners have been discussed. However, the general advantages of quasi-optical propagation enhance the performance of almost any type of component, and a wide variety of new designs for filters, diplexers, and other system components have emerged recently. A design for an entirely quasi-optical monopulse comparator has been developed,  which has good beam quality and null depth in two axes. It is based on a slab of dielectric material that functions as a four-port hybrid junction. A dielectric slab hybrid is shown schematically in Figure 8> this device has a 90[degrees] relative phase shift between the reflected and transmitted beams, [theta](r) - [theta](t) = 90[degrees].  This phase difference is independent of frequency, so that extremely broadband devices can be made. Using n = 2 material, a 3 dB power division with less than 0.3 dB variation over an octave bandwidth can be obtained. 
A new type of Fabry-Perot filter based only on wire grids that would be effective for high power systems has been described.  A filter combining dielectrics and wire grids, which is designed to have good performance for multiple beams as found in imaging systems, has been developed.  A system that includes several quasi-optical cavities intended to serve as the frequency-determining element in a submm-wavelength oscillator has been developed. 
Consederable work also has taken place in the area of interfacing planar components and quasi-optical systems. The use of horn antennas made directly from silicon substrates  now has been extended to dual-polarization devices that exhibit better than 20 dB cross polarization isolation.  Figure 9 shows this device schematically> it has a relatively symmetric radiation pattern, which can be well-represented by a Gaussian distribution, and hence is suitable for coupling to quasi-optical systems. This general area includes the important topic of quasi-optical mixers [43, 44] that show considerable promise for low cost and high performance.
Quasi-optical techniques have great potential for advancing the state of the art in radar systems. Quasi-optical propagation offers several major advantages including low loss, high power capacity, and multiple polarization and spatial mode capability. Some of the most critical areas, including polarization processing, receive and transmit duplexers and high power sources, are at the center of current research in the area of quasi-optical components, with impressive improvements having been achieved in the past few years. Other types of components that benefit from the low loss of quasi-optical transmission include filters and diplexers. The development of quasi-optical components designed to be used in conjunction with monolithically fabricated active devices is certain to have a major impact on power-combined transmitters and imaging radar systems.
The author wishes to thank K. Stephan, G.R. Huguenin and G. Carroll for their contributions to this paper. R. Compton, G. Rebeiz and D. Rutledge generously supplied information on current developments.
(*1) Invited paper
(*2) Research for this article was concluded at the University of Massachusetts, Department of Physics and Astronomy, Five College Radio Astronomy Observatory, Amherst, MA.
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Paul Goldsmith carried out his PhD research developing a sensitive heterodyne receiver for the 1.3 mm-wavelength range and using it to carry out some of the earliest observations of the J = 2-1 transition of carbon monoxide and its isotopic variants. This research into the structure of molecular clouds continued at Bell Laboratories, where he also was involved in designing the quasi-optical mm-wave feed system for the 7 m offset Cassegrain antenna. While at the University of Massachusetts' Five College Radio Astronomy Observatory, Goldsmith initiated the development of cryogenic mixer receivers, which had exceptionally low noise and also good calibration accuracy as a result of the extensive use of quasi-optical technology for single sideband filtering and input switching. In 1981, he led a team that carried out the first submillimeter astronomical observations with a laser local oscillator heterodyne system. Goldsmith is one of the co-investigators for the submm-wave astronomy satellite (SWAS). Hic technologies research has focused on Gaussian optics, quasi-optical system design and imaging systems. In 1982, Goldsmith was one of the founders of Millitech Corp., where he contiues to work on a part time basis, primarily in the area of mm-wavelength antennas and quasi-optical component and system design. He became a senior member of the IEEE in 1985.
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|Title Annotation:||use of optical techniques in narrow-beam radars|
|Author:||Goldsmith, Paul F.|
|Date:||Jan 1, 1991|
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