GB1591449A - Plane wave lens - Google Patents

Description

(54) PLANE WAVE LENS (71) We, PLESSEY INCORPORATED, a New York corporation having a place of business at 320 Long Island Expressway South, Melville, New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a dielectric lens for converting a spherical wavefront of electromagnetic radiation to a plane wave, and to arrangements employing such lenses. The principles of antenna range design are discussed in detail by Hollis et al, "Microwave Antenna Measurements" Scientific-Atlanta Inc. 1969) the following summary of portions of which will assist in understanding the invention.

When testing any radiating devices or device/systems receiving electromagnetic energy, the ideal test environment for determining far-zone performance is to provide a plane wave of uniform amplitude and phase to illuminate the test aperture. Various approaches to simulation of this ideal electromagnetic environment have led to the evolution of two basic types of electromagnetic test facilities: (1) Free-space Ranges (2) Reflection Ranges Free-space ranges are those in which an attempt is made to suppress or remove the effects of all surroundings, including the range surface or surfaces, on the wavefront which illuminates the test antenna. This suppression is sought through one or more of such factors as (a) directivity and sidelobe suppression of the source antenna and test antenna, (b) clearance of the line of sight from the range surface, (c) redirection or adsorption of energy reaching the range surface, and (d) special signal processing techniques such as tagging by modulation of the desired signal or by use of short pulses.

Reflection ranges are designed to make use of energy which is reradiated from the range surface(s) to create constructuve interference with the direct-path signal in the region about the test aperture. The geometry is controlled so that a small, essentially symmetric amplitude taper is produced in the illuminating field. The two major types of reflection ranges in use are the ground reflection range and, for low frequencies, the tapered anechoic chamber.

For either basic type of range, the fundamental electromagnetic design criteria deal with control of five factors: A) Inductive or radiation coupling between antennas; (B) Phase curvature of the illuminating wavefront; C) Amplitude taper of the illuminating wavefront; D) Spatially periodic variations in the illuminating wavefront caused by reflections; and E) Interference from spurious radiating sources.

Items A through D primarily establish the dimensional requirements in the range design, and limiting values of source-antenna directivity. Item E must be considered in the overall design.

At lower microwave frequencies, effects of inductive coupling between the source antenna and the test antenna must be considered. Such effects are usually considered negligible when the criterion R2 lOX (1) is satisfied, where R is the separation between antennas and X is the wavelength. This criterion is based on the field equations for an elemental electric dipole, from which the ratio of the amplitude of the induction field to that of the radiation field is seen to be X Pg = - (2) 2irR At R 2 10 A, Pe g 1/207r and the criterion is seen to be equivalent to the requirement that 20 log (pus) 2-36 decibels (3) The effect of curvature of the incident phase front is a most important one. The principal difficulty is that the generally accepted criteria is that the minimum range length acceptable is determined by the relationship.

2D2 (4) X Where D is the diameter or maximum dimension of the test item (e.g. aperture).

For apertures in excess of twelve (12) inches at X-Band or above > 8 GHz the range length requirement is longer than sixteen (16) feet, which is the maximum length of the most common size rectangular chambers used for measuring low gain antennas. Due to the high cost of absorbing materials, larger chambers are prohibitively expensive, and outdoor ranges are not always conveniently available, either due to lack of space or weather.

The testing of microwave antennas usually requires that the device under test be illuminated by a uniform plane electromagnetic wave. However, the creation of such a wave can be a difficult task. Conventional techniques require that a transmitting antenna be located at a sufficient distance from the test antenna such that its spherical wavefront closely approximates a uniform plane wave incident upon the test device.

Since ranges of several hundred to several thousand feet are often required, to satisfy the 2D2/ X criteria, far zone measurements usually are taken on outdoor installations which are subject to adverse weather conditions and changing range effects. Small antenna or targets may be tested adequately in anechoic chambers, but since large antennas (large in terms of wavelength) require long ranges, the cost of a chamber for such tests becomes prohibitively high.

Measurements will full-size antennas or fair size targets may be made on indoor 'compact ranges', with range reflector and a special feed system close to the test device, to produce incident plane waves with far-zone results. A properly focused parabolic-type reflector collimates the rays and thus produces a plane wave across its aperture. This wave is not uniform due to the illumination taper of the feed horn, and due to space-attentuation effects, However, a properly selected feed will generate a wave which is approximately uniform over an acceptable area. It is this area of an approximately uniform plane wave that is used on compact ranges to illuminate the antenna under test. There have been prior efforts to produce plane wave illumination with lenses, but unsatisfactory results were obtained due to amplitude distortion caused by random and uncontrolled reflections.

Dielectric lenses and cones are knownper se, and have been used in microwave and other transmission systems for many years. For example, conical dielectric horns are used to improve the efficiency of microwave reflector and horn antennas.

Understanding of the invention will be facilitated by considering the following analysis of two simple lens types: the planoconvex lens and the meniscus lens.

For a planoconvex lens (from FIGURE 2): for (1-ncos0) (5) 1-n where

relative permittivity of the lens, also defined as the refractive index, and f = focal length of the lens.

For a meniscus lens (see FIGURE 3): (n-cos0) f= (6) n - 1 The focal length of the lens is the distance from the phase center of the illuminating antenna to the center of the closest point on the planoconvex lens and the center of the lens farthest from the phase center of the meniscus lens.

To generate the lens geometry given the focal length and the relative permittivity, the equations are rearranged as follows: Planoconvex Lens: f(1-n) (7) (1-ncos0) Meniscus Lens: f(n-1) (8) (n-cos 0) To be useful, the lens of the invention must not degrade the performance of the test region consistent with the following guide lines.

The lens operates only on the propagation constant in the wave equation. That is, the wavefront is delayed more at the center of the lens than at the edges. Thus, a spherical wavefront striking the lens delays the center of the wavefront greater in the center thus causing the emerging wavefront to be all in line or uniform as it is called in the antenna testing field. This approximate uniform field is then used to illuminate a test device which is typically an antenna. The result is that the test antenna "sees" far field test conditions (uniform amplitude and phase) and thus behaves as if it were one. Thus, by inserting a lens between a source antenna and a test antenna the distance can be shortened because the lens provides the delay needed to cause a planar wavefront to be created.

When the wave passes through the lens it encounters the boundaries between the lens and free space. Due to the difference in the dielectric constant of air and the lens, a portion of the wave is reflected at each boundary. These do not occur uniformly, since the lens geometry varies in thickness; thus the uniformity, of the wave actually reaching the test region is distorted by the reflections at the lens boundaries. This distortion appears as ripples on the amplitude properties of the wave in the test region. If the dielectric constant is low enough, however, then this ripple is well within acceptable values.

Another phenomenon occurs when the electromagnetic wave encounters the lens; this phenomenon is called diffraction and occurs whenever an electromagnetic wave encounters an obstruction. The effect of the diffraction is to cause energy to 'spill over' the edges of the lens and cause distortion in the amplitude uniformity of the field behind the lens. This distortion is a function of the operating wave length, size of the lens (obstruction) and dielectric constant (the electrical property of the obstruction) of the lens. This is a direct function of the operating frequency and the dielectric constant.

Another design consideration is the uniformity of homogenity of the lens. If sufficient uniformity of the form density (estimated to be + 6%of the nominal value) is not maintained then the phase front will be distroted and the usefulness of the lens will be negated.

Unfortunately, solid dielectrics traditionally used in such applications (ethyl cellulose, polyethylene, polystyrene, polyisobutylene or methyl methacrylate) all have a dielectric constant that is much too high to be of use. It is believed that efforts or prior workers to create plane wave conditions with lenses could have been unsatisfactory because of a failure to recognize (1) the need to use a lens of very low dielectric constant (1) the need to use a lens of very low dielectric constant and (2) the need to independently minimize amplitude distortions, (e.g., reflections) by using absorbent materials and proper selection of the source antenna in the test range.

This invention seeks to provide a dielectric lens in which the above mentioned disadvantages of known lenses are mitigated.

According to this invention in its broadest aspect there is provided a dielectric lens for converting a spherical wavefront of electromagnetic radiation to a plane wave comprising a convex or meniscus shape formed of a single body of uniformly foamed plastics material having a single dielectric constant in the range of 1.05 to 1.5 and a density of no more than 20 pounds per cubic foot.

According to a second aspect of this invention there is provided a dielectric lens arrange ment comprising a dielectric lens in accordance with a first aspect of this invention, a movable supporting structure; an electromagnetic radiation antenna mounted on said structure said lens being mounted on said structure in a fixed spaced relation to said antenna, and radiation absorbing material surrounding the radiation path between said antenna and said lens.

According to a third aspect of this invention there is provided an electromagnetic radiation testing system comprising a dielectric lens in accordance with a first aspect of this invention a source of electromagnetic radiation; a test device wherein the lens is disposed between the source and the device and positioned to direct a plane wave to the device, there being provided radiation absorbing means along the path between the source and the device for minimising reflections and amplitude distortions.

In accordance with a fourth aspect of this invention there is provided a method of testing electromagnetic radiation devices under far-field conditions including the steps of generating a test signal having a spherical wavefront at a first point, intercepting said signal at a second point and converting same to a planar wavefront by means of a dielectric lens in accordance with a first aspect of the invention, the distance between the first and second points being substantially equal to the focal length of the lens, intercepting the converted wavefront with a device to be tested, providing absorbing means adjacent the path of said signal for minimising amplitude distortion and reflection, and measuring predetermined properties of said device.

This invention will now be further described by way of example with reference to the accompanying drawings, wherein: FIGURE 1 is a simplified schematic drawing of an electromagnetic testing system in accordance with an aspect of this invention.

FIGURE 2 and FIGURE 3 are cross sectional elevations of a planoconvex lens and a meniscus lens, respectively, with various lens parameters noted thereon; FIGURE 4 is a lens design curve, diameter vs. thickness, for foam plastic dielectrics in accordance with the present invention; FIGURES 5 and 6 are charts showing phase and amplitude probe data, respectively, with and without a lens; FIGURES 7 and 8 are charts showing other tests results with the invention; and FIGURE 9 is a simplified schematic drawing of a test fixture employing the invention.

The invention is based, at least in part, on the realization that the dielectric constant in foamed plastics is inversely related to the foam density, and the latter parameter can be controlled during manufacture. The desired dielectric constant in the lens of the invention is one that is large enough to shape the phase curvature, but not so high as to cause a larger surface reflection from the lens and block the wave, which happens at about Er = 1.7. The dielectric constants of the solid dielectrics noted hereinabove are all 2.1 or greater. By using foamed plastics, the dielectric constant can be held in the range of about 1.05 to 1.5, which has been determined experimentally to be preferred for purposes of the invention. For polyurethan foams, the 1.5 upper limit corresponds to a density of about 20 pounds per cubic foot. In another aspect, the invention is based on the realization that amplitude distortions and reflections can be eliminated by use of a small source antenna as the source antenna, and by always using appropriate radiation absorbing material to define a radiation path between the source and the lens and, in most cases, between the lens and test aperture as well.

As illustrated in FIGURE 1, an anechoic chamber 10 has a point source antenna 12 and a test antenna 14. Dielectric lens 16 is placed a distance equal to 4 times its diameter from antenna 12 (e.g. its focal length). Spherical wave fronts 18 radiating from antenna 12 are converted to plane wave fronts one of which is shown at 20, after passing through lens 16.

Typically, chamber 10 may be 20 feet long and have a range length of 15 feet. For a foamed plastic of dielectric constant Er = 1.25, it can be deduced from FIGURE 4 that a 36 inch diameter lens should be 10 inches thick. Thus, test antenna 14 will see a virtually flat wave front, as quantitifed hereinbelow. FIGURE 4 includes curves for dielectrics having Er throughout the preferred range, and were calculated from equation 7. The conditions assumed were those typical for chambers in many laboratories, e.g. R = 168 inches.

While anechoic chambers perhaps represent the broadest use of the invention, others are apparent. Assume a test fixture is needed to test out an antenna system, located in the nose of an airplane. Assume it is a two foot dish at X-Band. Obviously it would be advantageous to leave the antenna installed on the plane during the test. Heretofore, when applying the 2D2/ X curvature, the use of a 100 foot antenna range would be required, and the best site would have to be elevated to 15 feet or more above ground level. Obviously it is not practical to place the aircraft in the air just to check antennas, so the next best thing is done: remove the antennas, put them on the range, make the necessary adjustments and reinstall the antenna.

As shown in FIGURE 9, using the lens 16 of the present invention, a test fixture including a radiation source 22 lens 16 and an absorbent cone 24 is mounted on a dolly 26 which could be rolled up to the nose of the aircraft. Since the aircraft's antenna system was being illuminated by a plane wave front of the proper type, it would perform normally and the equipment could be checked out as installed, functioning as it would in the air. More particularly, a parabolic dish (test) antenna 28 will produce a substantially plane wave front seen by antenna 22. By use of suitable time delay means (not shown) the incident signal is re-broadcast by antenna 22, through lens 16 and antenna 28 will 'see' a simulated reflected plane wave signal of a known delay. By varying the delay, accuracy of the test set in ranging from a few hundred yards to transmission limits can be tested.

Manufacture of lenses in accordance with the invention is not critical, but blowing of the foams should be strictly in accordance with the manufacturer's directions, so as to avoid unfoamed pockets that would affect overall dielectric properties.

For the manufacture of a large number of identical lenses, it is worthwhile to invest in a mold of the desired shape, and by carefully regulating amounts of prepolymer and blowing agent, molding in situ lenses of the desired density and dielectric constant.

More commonly, a lens is desired for a specific environment, and machining a preformed block is preferred to molding for economic reasons. Such blocks are available commercially and, with them, the problems and hazards of blowing, (e.g. unfilled areas and toluene diisocyanate fumes) can be avoided. Since the machining should be as accurate as possible to obtain the desired figure of revolution, a tape or direct numerical control tracer mill with 3-axis control is preferred.

Most rigid and flexible foams are suitable for use with the invention, though it will be appreciated that rigid foams are preferred for ease of handling and to remove any problem with distortion. Set forth below in Table 1 are some of the foams which are suitbale, with the dielectric constant or range thereof available.

Tablet Foamed Plastics Dielectric Constant, E, Cellulose Acetate 1.12 Epoxy, rigid closed cell 1.08 - 1.19 Phenolics 1.19-1.20 Polyethylene, low and inter mediate density 1.05 - 1.15 Polyethylene, cross linked 1.1 -1.55 Polyurethane, rigid 1.05-1.5 Polyurethane, flexible 1.0 -1.5 Silicone, open cell 1.2 Rigid, closed-pore foams are preferred, so that humidity changes will not affect the lens; hydrophilic foams should be avoided. However, in principle any foamed material could be used to achieve the desired test conditions. These foams are not temperature sensitive from an operational viewpoint.

Thus, for a planoconvex lens the necessary data is used in equation 7 and the lens design is established. While the technique is the same as for conventional lenses, the lenses themselves have a novel configuration due to the material employed. Meniscus lenses, while satisfactory from an operational viewpoint, involve more complex mold or machining requirements.

They are preferred for high production lenses, due to savings of material.

Lenses in accordance with the invention are useful as indicated hereinabove, and also in tapered anechoic chambers, where they will permit use of the full region at higher frequencies, large (e.g. 30' x 30' x 60') rectangular or tapered chambers, where they permit use of the full 6-8 ft. test region, and in general purpose test fixtures, one of which has been described.

Understanding of the invention will be further facilitated by reference to the following specific example, and accompanying FIGURES 5 - 8.

Uniform amplitude characteristics of the source radiation were achieved by proper source antenna size selection.

A 32 inch diameter lens was made from expanded polyurethane foam which had a measured dielectric constant of 1.16. Using the 1.16 dielectric constant and a 32 inch diameter the lens required was 12 inches thick (cf. FIGURE. 4). This foam was purchased and shaped in a parabolic shaped lens per the design equation 7. The lens was then placed in the chamber and the resultant fields probed to determine their uniformity.

FIGURE 5 is a plot of the phase characteristics of the field with and without the lens.

Results were most encouraging. Indeed, when the lens was inserted in the field, the phase variation was considerably less, and quite flat, over a good portion of the plot.

FIGURE 6 shows the amplitude characteristics obtained with and without the lens inserted in the field. Note that with the lens the amplitude characteristics show a substantial portion that is essentially flat. This is the area immediately behind the lens probe in parallel with the lens face. This shows that the field variation is less than 5 dB to peak which is quite acceptable for most test purposes. The steep skirts on either side are due to diffraction effects around the edges of the lens. This is typical of an obstruction placed in an electromagnetic field. This edge effect is reduced by making the lens larger than the test region.

Subsequently a 2 foot by 2 foot plate was placed in the chamber and measured as a reflector with and without the lens in place. Such plates are used as reference standards to measure the performance of absorption materials. Results are shown in FIGURE 7. Note that the beam without the lens starts to break up on the peak (this is typical of what happens to antennas tested in the near field; i.e., the phase deviation is so great that the pattern breaks up).

However, the pattern with the lens in place is very clear -- this is a picture of an antenna that has been measured in the far field (R > 2D/2 A). Thus, the lens achieves exactly the results desired.

The system was then used to test some sheet absorbing material. The results of the test are shown in FIGURE 8. Curve 1 is the same as the "with lens" curve in FIGURE 7. Curve 2, with an absorber covering the plate, gives a direct reading in dB of the absorber compared to the reference plate. In this case a reduction of 20 dB means a 1/100 power ratio for reflected radiation. Thus, the goal of performing absorber tests inside a small anechoic chamber, rather than outside on a 100 foot antenna range, was met.

Various changes in the details, steps, materials and arrangements of parts, which have been herein described and illustrated to explain the nature of the invention, may be made by those skilled in the art within the scope of the invention as defined in the appended claims.

WHAT WE CLAIM IS: 1. A dielectric lens for converting a spherical wavefront of electromagnetic radiation to a plane wave comprising a convex or meniscus shape formed of a single body of uniformly foamed plastics material having a single dielectric constant in the range of 1.05 to 1.5 and a density of no more than 20 pounds per cubic foot.

2. A dielectric lens arrangement comprising a dielectric lens as claimed in claim 1 a movable supporting structure, an electromagnetic radiation antenna mounted on said structure, said lens being mounted on said structure in a fixed spaced relation to said antenna and radiation absorbing material surrounding the radiation path between said antenna and said lens.

3. An arrangement as claimed in claim 2 in which the antenna is a transmitting antenna and the lens is oriented to transmit a planar wavefront into free-space.

4. An arrangement as claimed in claim 3 in which the distance between the antenna and the lens is equal to the focal length of the lens.

5. An arrangement as claimed in claim 2 in which the antenna is a receiving antenna and has lens isbriented to transmit a planar wavefront to the antenna.

6. An electromagnetic radiation testing system comprising a dielectric lens as claimed in claim 1, a source of electromagnetic radiation, a test device, wherein the lens is disposed between the source and the device and positioned to direct a plane wave to the device, there being provided radiation absorbing means along the path between the source and the device for minimising reflections and amplitude distortions.

7. A system as claimed in claim 6 in which the distance between the source and the lens is equal to the focal length of the lens.

8. A system as claimed in claim 6 or 7 in which the absorbing means comprises an enclosure for the source, test device and lens and in which the enclosure has interior surfaces adapted to absorb electromagnetic radiation and forms an anechoic chamber.

9. A system as claimed in any one of claims 6 to 8 in which the test device is a receiving antenna.

10. A system as claimed in claim 9 in which the diameter of the lens is larger than the size of the antenna.

11. A method of testing electromagnetic radiation devices under far field conditions including the steps of generating a test signal having a spherical wavefront at a first point, intercepting said signal at a second point and converting same to a planar wavefront by means of a dielectric lens as claimed in claim 1 the distance between the first and second point being substantially equal to the focal length of the lens, intercepting the converted wavefront with a device to be tested; providing absorbing means adjacent the path of said signal for minimising amplitude distortion and reflection, and measuring predetermined properties of said device.

12. A dielectric lens as claimed in claim 1 and substantially as herein described.

13. A dielectric lens arrangement substantially as herein described with reference to and as illustrated in Figure 9 of the accompanying drawings.

14. An electromagnetic testing system substantially as herein described with reference to and as illustrated in Figure 1 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (14)

**WARNING** start of CLMS field may overlap end of DESC **. has been measured in the far field (R > 2D/2 A). Thus, the lens achieves exactly the results desired. The system was then used to test some sheet absorbing material. The results of the test are shown in FIGURE 8. Curve 1 is the same as the "with lens" curve in FIGURE 7. Curve 2, with an absorber covering the plate, gives a direct reading in dB of the absorber compared to the reference plate. In this case a reduction of 20 dB means a 1/100 power ratio for reflected radiation. Thus, the goal of performing absorber tests inside a small anechoic chamber, rather than outside on a 100 foot antenna range, was met. Various changes in the details, steps, materials and arrangements of parts, which have been herein described and illustrated to explain the nature of the invention, may be made by those skilled in the art within the scope of the invention as defined in the appended claims. WHAT WE CLAIM IS:

1. A dielectric lens for converting a spherical wavefront of electromagnetic radiation to a plane wave comprising a convex or meniscus shape formed of a single body of uniformly foamed plastics material having a single dielectric constant in the range of 1.05 to 1.5 and a density of no more than 20 pounds per cubic foot.

2. A dielectric lens arrangement comprising a dielectric lens as claimed in claim 1 a movable supporting structure, an electromagnetic radiation antenna mounted on said structure, said lens being mounted on said structure in a fixed spaced relation to said antenna and radiation absorbing material surrounding the radiation path between said antenna and said lens.

3. An arrangement as claimed in claim 2 in which the antenna is a transmitting antenna and the lens is oriented to transmit a planar wavefront into free-space.

4. An arrangement as claimed in claim 3 in which the distance between the antenna and the lens is equal to the focal length of the lens.

5. An arrangement as claimed in claim 2 in which the antenna is a receiving antenna and has lens isbriented to transmit a planar wavefront to the antenna.

6. An electromagnetic radiation testing system comprising a dielectric lens as claimed in claim 1, a source of electromagnetic radiation, a test device, wherein the lens is disposed between the source and the device and positioned to direct a plane wave to the device, there being provided radiation absorbing means along the path between the source and the device for minimising reflections and amplitude distortions.

7. A system as claimed in claim 6 in which the distance between the source and the lens is equal to the focal length of the lens.

8. A system as claimed in claim 6 or 7 in which the absorbing means comprises an enclosure for the source, test device and lens and in which the enclosure has interior surfaces adapted to absorb electromagnetic radiation and forms an anechoic chamber.

9. A system as claimed in any one of claims 6 to 8 in which the test device is a receiving antenna.

10. A system as claimed in claim 9 in which the diameter of the lens is larger than the size of the antenna.

11. A method of testing electromagnetic radiation devices under far field conditions including the steps of generating a test signal having a spherical wavefront at a first point, intercepting said signal at a second point and converting same to a planar wavefront by means of a dielectric lens as claimed in claim 1 the distance between the first and second point being substantially equal to the focal length of the lens, intercepting the converted wavefront with a device to be tested; providing absorbing means adjacent the path of said signal for minimising amplitude distortion and reflection, and measuring predetermined properties of said device.

12. A dielectric lens as claimed in claim 1 and substantially as herein described.

13. A dielectric lens arrangement substantially as herein described with reference to and as illustrated in Figure 9 of the accompanying drawings.

14. An electromagnetic testing system substantially as herein described with reference to and as illustrated in Figure 1 of the accompanying drawings.