In recent years, several papers have been published dealing with enclosed or surface-waveguides of elliptical cross section. Examples have included hollow metal waveguide, enclosed waveguide filled with one or more dielectrics, coaxial transmission lines, dielectric surface waveguides, and waveguides employing anisotropic media. Some of these types have been investigated experimentally, and several have been used in practical microwave-frequency applications, such as in feed lines for antenna installations.
The large aperture Fresnel zone plate antenna has been extensively investigated in recent years for
application at frequencies from the microwave range to the terahertz region. These zone plate antennas
have a focal length (F) and diameter (D) that are comparable (F/D = 0.3 to 2.5). The results of these
investigations show that the phase-correcting zone plate gives performance comparable to a true lens.
There is one limitation, however, in that most cases a secondary focus occurs on-axis at one-third the focal
distance from the zone plate. The intensity is 10 dB or more down from the main focus. A method has now
been developed to essentially eliminate this secondary focus by properly adjusting the amount and location
of the phase-correcting grooves or rings. A revised positioning of the grooves and degree of correction can
reduce the secondary focus, but maintain the integrity of the main focus. Specific examples have been
analyzed and will be presented at the conference.
The Fresnel zone plate lens antenna, which provides advantages compared to a normal paraboloidal or spherical lens,
has been extensively investigated in the millimeter-wave and terahertz regions. The advantages include reduced weight,
volume, and attenuation and simplicity of design. The principal disadvantage is that the zone plate sometimes provides
reduced gain compared to a true lens. Particularly at high millimeter-wave or terahertz frequencies the low loss of the
zone plate more than compensates for the reduced directivity. This paper investigates the gains and far-field patterns for
a number of cases and gives both the analysis and numerical results for the examples. These cases have dealt with
large-angle designs, where the focal length (F) and diameter (D) are comparable (F/D = 0.3 to 2.5), unlike the typical
optical examples. The antenna patterns are found to have beamwidths and first sidelobes that are similar to what one
would obtain with a standard lens, given the same aperture illumination. Appropriate feed designs are also described.
For best aperture efficiency the illumination taper is about 10 dB, and this gives first sidelobe levels of about -24dB for
a circular aperture. Far-out average sidelobes are not as low as for a true lens, and this is where the gain is affected.
Surface-wave propagation on coated or uncoated conducting wires, rods, or tubes has been shown to provide
low attenuation, moderate field extent, low dispersion, and high power-handling at frequencies from 100
GHz to 1 THz. Typical conductors are copper, aluminum, or stainless steel. Uncoated conductors provide
the lowest loss, while conductors coated with a thin layer of low-loss dielectric (such as Teflon, polystyrene,
or polyethylene) have the smallest field extent. The guided mode is the TM01, often referred to as the
Sommerfeld mode. The properties of the guided wave were theoretically analyzed by King and Wiltse in
1962, and measured results were obtained at 105 and 140 GHz. In the last two years the work has been
rediscovered, and now four different research groups have reported new results. While the earlier work was
conducted in the search for long, low-loss transmission lines (100 meters to 1 kilometer), the current
applications are for lengths about 1 meter long, as might be used in probes. The recent results will be
summarized, and an optimized design will be presented, along with general curves of attenuation, field
extent, and power handling capability from 100 GHz to 1 THz.
Atmospheric attenuation is one of the most significant factors in limiting the performance of millimeter-wave
and terahertz systems. Although atmospheric propagation is fairly well understood up to 1 THz, major errors
have been published in numerous locations showing atmospheric propagation at frequencies from 10 GHz to
1 THz. Some of these errors have been reported in the past by the present author. The topic was also
reviewed in an invited plenary presentation by Bruce Wallace at the 2006 SPIE Defense and Security
Symposium in Orlando. Several cases are discussed here, involving clear-air conditions, rain, and fog. In
one example, the attenuation at 4 km elevation has been mislabeled as 9150 m (or 30,000 feet) for the 10 to
400 GHz range. This error has appeared in several journal articles, vendors' catalogs, short-course notes,
and a recently-published book. In another case the attenuation peak near 22.3 GHz (due to water vapor
absorption) has been plotted at 20 GHz. The third case deals with errors pertaining to attenuation in fog for
frequencies between 10 and 1000 GHz. Specific information and corrections will be given for all three
cases. The net result of these errors is that development of sensor and communications applications has been
impeded because the errors usually make atmospheric losses appear to be greater than they really are.
The phase-correcting Fresnel zone plate provides lens-like focusing and imaging of electromagnetic waves by means of diffraction instead of refraction, often referred to as diffraction optics. The zone plate has seen extensive investigation and use at microwave and millimeter-wave frequencies, and recently has been applied in the terahertz region. These cases have dealt principally with large-angle designs, where the focal length (F) and diameter (D) are comparable (F/D = 0.3 to 2.5), unlike the typical optical examples. The planar zone plate, in particular, offers the advantages of simplicity of design and construction, low loss, low weight, and low cost, while giving performance similar to that of a refractive lens. As one goes to terahertz frequencies, ease of construction becomes more difficult. The attenuation in conventional low-loss materials increases at higher frequencies, and dimensional tolerances become smaller, making fabrication more difficult. Although earlier designs employing polystyrene have been built and tested at frequencies up to 280 GHz, higher frequency designs are simpler to fabricate and have lower loss if low dielectric constant materials are used. This investigation addressed designs for terahertz frequencies. The optimization of the zone plate has also been examined, and improvement has been found for radial compression, where the radii of the zone boundaries are slightly shortened.
Conventional lenses are important components for many terahertz applications, but ordinary lenses are very difficult to fabricate for short-focal lengths. Multi-level phase-corrected zoned lens antennas have been investigated with particular application at terahertz wavelengths. These zoned lenses (or diffractive optics) give better performance than ordinary lenses, and because of their planar construction are easier and cheaper to fabricate. The depths of cut needed for a grooved zone plate are quite small, even when materials with low dielectric constants are used. Zoned lenses have been built and tested at various frequencies from 100 GHz to 1.5 THz, with phase correction levels of half-wave, quarter-wave, or eighth-wavelength. The inherent losses in transparent materials increase monotonically over this frequency range. Typical low-loss materials include polystyrene, polyethylene, Teflon, polycarbonate, polystyrene foam, foamed polyethylene, low density polytetrafluoroethylene (PTFE), TPX, quartz, sapphire, and silicon. Low dielectric-constant materials are normally preferred to reduce reflection and attenuation losses. Techniques for cutting or milling the materials to small dimensions are important, because at 1.0 THz an eighth-wavelength correction for silicon is only 15 μm. Another characteristic of zoned diffraction optics is their frequency behavior. Previous investigations have considered their bandwidth dependence and quasi-periodic extended frequency response for a specified focal length. As frequency changes, the focal point moves along the axis of the zoned lens. An analysis is given to explain this effect.
At terahertz frequencies the attenuation of conventional lenses can be very high. In addition, the fabrication tolerances for standard curved lenses (e.g., hyperboloid in shape) make construction difficult and costly. In contrast, zone plate lenses are thin (typically on the order of a wavelength) and therefore have low loss. In addition, zone plates are planar, and the flat surfaces are easy to fabricate. They are also much lower in weight than a true lens. These considerations have led to the development of several designs at frequencies to 600 GHz. All typical lens materials show an increase in loss tangent at frequencies above 100 GHz, and for some the loss increases at an undesirable rate. For example, polystyrene has much greater loss than Teflon at 600 GHz. Thus, material choices are a consideration, and dielectric constant is also a part of this consideration. At 300 GHz the wavelength is 1 millimeter. A zone plate with this thickness poses support problems, but by the use of material with a dielectric constant of 1.05, the structure becomes about 10 times thicker and therefore more structurally feasible. These options are described, and design cases will be given. Various examples have already been designed, built, and tested at frequencies near 100, 140, 210, 235 and 280 GHz, and their characteristics will be summarized. The result is a compilation of recommendations for usable designs, with information on losses, beamwidths, feed arrangements, and construction considerations, as well as comparisons to conventional lenses.
During the past several years many new results have been obtained for Fresnel zone plate antennas having a focal length to diameter ratio (F/D) near unity. Although zone plates have been used for many years at optical wavelengths, the typical configuration has employed F/D values from 20 to 100, sometimes greater than that. This results in a small angle (a few degrees or less) at the focal region, whereas the focal angles for F/D near unity (between 0.5 and 2.0) range from 90° to 28.1°. The small-angle optical conditions permitted making approximate analytical solutions which do not apply to the large-angle case. The recent results for large-angle applications will be summarized for microwave and millimeter-wave examples, although they are valid at any wavelength. In addition, a description will be given of optimization of feed methods (typically corrugated horn antennas) to improve overall efficiency (including aperture efficiency and diffraction efficiency), while also improving (lowering) first-sidelobe levels. This involves solving the case of tapered amplitude illumination across the zone plate aperture, whereas most of the previous investigations have assumed uniform illumination. A truncated Gaussian amplitude distribution with a 10 dB taper at the edge of the aperture provides the optimum aperture efficiency, as well as low sidelobe levels. The utilization of zone plate antennas at terahertz frequencies will also be described. At these frequencies the thin structure of the planar zone plate offers much lower loss than that of a conventional lens. All transparent materials increase in attenuation (loss tangent) as one moves from microwave frequencies to the terahertz range, and conventional lenses have high attenuation. This makes the use of the zone plate lens a preferred choice, for certain configurations.
The stepped conical zoned lens antenna has better overall efficiency than a true lens, and provides an excellent antenna pattern. It also exhibits somewhat different bandwidth characteristics than the Fresnel zone plate antenna. This paper examines the frequency behavior in detail, particularly for microwave and millimeter-wave applications. For the usual zone plate antenna employed at microwave or millimeter wavelengths, path length adjustment (i.e., phase correction) is accomplished by cutting different depths (grooves) in a dielectric plate or by using two or more dielectrics having different dielectric constants. The new design uses a tilted cut in a flat dielectric plate, which more accurately matches the shape of a true lens and produces much lower phase error. The construction is still linear (i.e. spherical or hyperboloidal curves do not have to be cut), and can be made, for example, by a milling machine with a tilted bit. For a circular zone plate, the lens is a stepped conical shape. The phase correction steps are small, usually a few degrees, which is much smaller than for the typical Fresnel zone plate. The bandwidth characteristics are calculated for specific cases.
The Fresnel zone plate lens was invented and developed for optical frequencies. However, fabrication difficulties at the short optical wavelengths have prevented obtain good efficiencies. At longer microwave or millimeter-wavelengths fabrication is easier and phase correcting zone plate antennas have been used to obtain good efficiencies. This paper describes a new type of phase correcting zone plate having even better efficiency, namely a diffraction efficiency of 99 percent compared to a true lens, and an overall efficiency much better than a true lens. For the usual zone plate antenna employed at microwave or millimeter wavelengths, path length adjustment is accomplished by cutting different depths in a dielectric plate or by using two or more dielectrics having different dielectric constants. The new design uses a tilted cut in a dielectric plate, which more accurately matches the shape of a true lens and produces much lower phase error. The construction is still near and can be made for example, by a milling machine with a tilted bit. For a circular zone plate, the lens is a stepped conical or tapered shape. Because the phase steps are small, the far-field antenna pattern is excellent and sidelobe-levels are very low. Analysis of typical configurations will be given, showing that phase errors are small, lower than those for an eighth-wave corrected phase zone plate.
The phase-correcting Fresnel zone plate antenna has been studied extensively at microwave and millimeter-wave frequencies in the past few years, in part because it offers advantages of low weight, cost, and loss. Scores of articles have appeared describing fundamental characteristics and methods of improving the efficiency, gain, and multiple- frequency band performance. As a result of these studies, one can now optimize the design, and an optimized zone plate antenna can provide performance superior to a standard lens or paraboloid antenna, especially at millimeter wavelengths. There are many differences between performance at optical wavelengths and microwaves/millimeter-waves, and these differences are described. A new planar design with non- parallel surfaces is discussed. This configuration offers further improvement in diffraction efficiency.
A well-designed phase correcting Fresnel zone plate antenna can provide performance superior to a lens or, in some cases, a paraboloid antenna, particularly at millimeter wavelengths. This paper discusses design considerations and includes approaches to give improved characteristics, such as greater efficiency or higher gain. The approaches include the use of quarter-wave or better correction, thickness designs that permit the central zone and other zones to be air dielectric (for lower losses), and the use of low dielectric constant materials to reduce surface reflections and multiple reflections. At higher millimeter-wave or sub- millimeter wavelengths low loss materials are important. More sophisticated zoning is described, as well as the use of a compromise thickness to compensate for the fact that refraction of waves at the surfaces causes the path lengths through the zone plate to be different at different angles of incidence. Multiple-band zone plates are discussed.
The Fresnel zone plate antenna is an example of an optical analogy that has been transferred to microwave/millimeter wavelength use. The latter case has seen extensive research and application, and in the past dozen years more than seventy relevant papers have been published on a worldwide basis. These studies have dealt with either lens or reflector designs, and have quantified many parameters, such as gain, antenna patterns, efficiency, bandwidth, and structural options. The most recent designs have dealt with high efficiency or dual band configurations. This report will summarize the many advances of the past few years, and will provide some parametric design tradeoffs.
Factors that affect the efficiency of a phase-correcting Fresnel zone plate (FZP) lens antenna are analyzed. These include attenuation, diffraction, and reflection losses. The FZP efficiency is compared to that for a standard lens, and the FZP is shown to be better at millimeter wavelengths, where attenuation is an important factor.
A description is given of a method of designing phase- correcting Fresnel zone plate lens antennas for operation at two or more frequency bands. A new analysis is derived for apportioning the amount of phase correction to give the desired performance at the chosen frequencies. The result permits the development of narrow-beam, high-gain behavior at two or more bands, with reduced gain at other frequencies.
Descriptions are given of major errors which appear in several published curves showing atmospheric attenuation at frequency intervals between 10 and 1000 GHz. Three cases are discussed, two for clear-air conditions and one for fog. In one example, the attenuation at 4 km elevation has been mislabeled as 9150 km (or 30,000 feet) for the 10 to 400 GHz range. This error has appeared in several journal articles, vendors' catalogs, short-course notes, and a recently-published book. In a second case the attenuation peak near 22.3 GHz (due to water vapor absorption) has been plotted at 20 GHz. The third case deals with an error pertaining to attenuation in fog for frequencies between 10 and 1000 GHz. Specific information and corrections are given for all three cases. The net result of these errors is that development of sensor and communications applications has been impeded because the errors usually make atmospheric losses appear to be greater than they really are.
Fresnel zone plate antennas have seen extensive research in recent years. Both transmission and reflection types have been analyzed and tested, and found to give excellent performance. Parameters such as far-field patterns, gain, efficiency, frequency dependence, aberrations, scan capability, focal region properties, and off-axis performance have been examined. While most zone plates are planar, curved (spherical or paraboloidal) versions have been found to offer certain advantages, such as sharpness of focus or larger field of view. In general, millimeter-wave results are emphasized.
The state-of-the-art has now matured to the point where millimeter wave systems are often a viable replacement and improvement over older microwave (or infrared) systems. However, the decline in defense spending, the sluggish global economy, and increased competition dictate that to be successful a combination of hard and soft system analysis approaches must be applied to the system engineering methodology. This paper surveys recent developments in millimeter wave technology, system engineering methodology, and an overview of recent applications.
Phase-correcting zone plates may be used with off-axis feeds. For angles up to about 20Ã‚Â° off-set, the performance is well-behaved, with minor degradation. The design is given for a transmission type and an equivalent reflecting version having a 17Ã‚Â° feed offset. An investigation has been carried out for angles larger than 20Ã‚Â°, apd the results indicate that the annular zones on the zone plate should not be circular. A ray- tracing analysis shows that the zones need to be egg-shaped, and these results will be described. The analysis has been completed for both transmission or reflection-type zone plates.
The feasibility of developing a speed measurement technique employing passive microwave sensors has been investigated. The system would employ dual radiometric receivers, located at two positions in the direction of travel, to detect microwave energy radiated or reflected from the terrain (grass, crops, plowed fields, etc.). As the system moves along, varying but similar signal levels are detected in each receiver. The receiver outputs would be cross-correlated and the time delay between the signals would permit the calculation of speed. No signals would be radiated (transmitted), thus avoiding problems relating to emission of radio frequencies and the attendant requirements for frequency assignment and licensing. For this investigation it is assumed that the system is to be carried on a ground vehicle, such as a tractor or a tank.
The Fresnel zone plate antenna has seen further development in the recent past. Analytical results and measured data have been obtained at various frequencies to describe the efficiency, bandwidth, far-field pattern, and focal-region and off-axis behavior. Both transmission (i.e., lens-like) and reflective configurations have been used [1,2]. Horn antennas have typically been used as feeds, but recent measurements at 220 GHz have utilized a dipole (or an array of dipoles) for a feed [3,4]. Off-axis feed methods have recently been developed for reflective zone plates (of elliptical cross-section). This arrangement can produce a better far-field pattern (lower sidelobes), because the feed blockage is reduced.
Several major millimeter-wave system developments are proceeding in the United States. The systems include satellite communications, missile terminal guidance, airborne synthetic aperture radar, and airborne radiometric imaging equipment. These are supported by significant technology thrusts, including the MIMIC (Microwave and Millimeter Wave Monolithic Integrated Circuits) program, and a major effort to measure target and background radar returns at several wavelengths.
Recent investigations have been conducted to extend the knowledge on phase-correcting Fresnel zone plate antennas for applications at microwave and millimeter wavelengths. Analyses and/or measurements will be summarized for parameters including efficiency, aberrations (related to scanning and multiple feed applications), bandwidth, far-field antenna patterns, surface reflections, and specialized applications. The recent results will be compared with those of earlier investigations.
SC782: Zone Plate Antennas for Millimeter-Wave and Terahertz Frequencies
The field of Fresnel zone plate antennas has seen extensive research and development in the last several years. Scores of articles and three books have been published recently. Zone plate antennas are being used in applications such as communications, radar, radiometry, and guidance, where they offer lower weight and volume, as well as simplicity of design. This course provides attendees with the basic principles of phase-correcting Fresnel zone plate antennas at millimeter-wave and terahertz frequencies. The course concentrates on describing characteristics such as antenna gain, bandwidth, frequency dependence, far-field patterns, efficiency, aberrations, off-axis performance, focal behavior, and feed considerations for zone plate designs having focal lengths and diameters that are similar in size. Their performance is comparable or sometimes superior to a standard lens. Many practical and useful examples are included. Attendees will become fluent with how one designs zone plate antennas for varying applications. The effects of going to the higher frequencies will be described.