Patent Publication Number: US-8108992-B2

Title: Method of making a microwave field director structure having V-shaped vane doublets

Description:
FIELD OF THE INVENTION 
     The present invention is directed to a reusable microwave field director assembly for use in a microwave oven. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     Subject matter disclosed herein is disclosed in the following copending applications filed contemporaneously herewith and assigned to the assignee of the present invention: 
     Molded Microwave Field Director Structure (CL-3655); 
     Microwave Field Structure Having Vanes Covered With A Conductive Sheath (CL-4040); 
     Microwave Field Director Structure Having Vanes With Outer Ends Wrapped With A Conductive Wrapper (CL-4055); 
     Microwave Field Director Structure With Vanes Having A Conductive Material Thereon (CL-4060); 
     Microwave Field Director Structure Having V-Shaped Vane Doublets (CL-4062); 
     Microwave Field Director Structure With Laminated Vanes (CL-4037); 
     Microwave Field Director Structure Having Over-Folded Vanes (CL-4064); 
     Method of Making A Microwave Field Director Structure Having Metal Vanes (CL-4078); and 
     Microwave Field Director Structure Having Vanes With Inner Ends Wrapped With A Conductive Wrapper (CL-4081) 
     BACKGROUND OF THE INVENTION 
     Microwave ovens use electromagnetic energy at frequencies that vibrate molecules within a food product to produce heat. The heat so generated warms or cooks the food. To achieve surface browning and crisping of the food a susceptor may be placed adjacent to the surface of the food. A typical susceptor comprises a lossy metallic layer on a paperboard substrate. When exposed to microwave energy the material of the susceptor is heated to a temperature sufficient to cause the food&#39;s surface to brown and crisp. 
     However, variations in the intensity and the directionality of the electromagnetic field energy form relatively hot and cold regions within the microwave oven. These hot and cold regions cause the food to warm or to cook unevenly. If a microwave susceptor material is present the browning and crisping effect is similarly uneven. 
     One expedient to counter these uneven effects is the use of a turntable. The turntable rotates a food product along a circular path within the oven. This action exposes the food to a more uniform level of electromagnetic energy. However, the averaging effect produced by the turntable&#39;s rotation occurs along circumferential paths within the oven and not along radial paths. Thus, even with the use of the turntable bands of uneven heating within the food are still created. 
     This effect may be more fully understood from the diagrammatic illustrations of  FIGS. 1A and 1B . 
       FIG. 1A  is a plan view of the interior of a microwave oven showing five regions (H 1  through H 5 ) of relatively high electric field intensity (“hot regions”) and two regions C 1  and C 2  of relatively low electric field intensity (“cold regions”). A food product F having any arbitrary shape is disposed on a susceptor S which, in turn, is placed on a turntable T. The susceptor S is suggested by the dotted circle while the turntable is represented by the bold solid-line circle. Three representative locations on the surface of the food product F are illustrated by points J, K, and L. The points J, K, and L are respectively located at radial positions P 1 , P 2  and P 3  of the turntable T. As the turntable T rotates each point follows a circular path through the oven, as indicated by the circular dashed lines. 
     As may be appreciated from  FIG. 1A  during one full revolution point J passes through a single hot region H 1 . During the same revolution the point K passes through a single smaller hot region H 5  and one cold region C 1 . The point L experiences three hot regions H 2 , H 3  and H 4  during the same rotation. Rotation of the turntable through one complete revolution thus exposes each of the points J, K, and L to a different total amount of electromagnetic energy. The difference in energy exposure at each of the three points during one full rotation is illustrated by the plot of  FIG. 1B . 
     Owing to the number of hot regions encountered and cold regions avoided points J and L experience considerably more energy exposure than Point K. If the region of the food product in the vicinity of the path of point J is deemed fully cooked, then the region of the food product in the vicinity of the path of point L is likely to be overcooked or excessively browned (if a susceptor is present). On the other hand the region of the food product in the vicinity of the path of point K is likely to be undercooked. 
     Another expedient to counter the undesirable presence of hot and cold regions is to employ a field director structure, either alone or in combination with a susceptor. 
     The field director structure includes one or more vanes, each having an electrically conductive portion on a support of paperboard or other non-conductive material. The electrically conductive portions of the field director structure mitigate the effects of regions of relatively high and low electric field intensity within a microwave oven by redirecting and relocating these regions so that food warms and cooks more uniformly. When used with a susceptor the field director structure causes the food to brown more uniformly. 
     When an electrically conductive portion of a vane of the field director is placed in the vicinity of either an inherently lossy food product or a lossy layer of a susceptor attenuation of certain components of the electric field occurs. This attenuation effect is most pronounced when the distance between the electrically conductive portion of the field director and the lossy element (either the lossy food product or the lossy layer of the susceptor) is less than one-quarter (0.25) wavelength. For a typical microwave oven this distance is about three centimeters (3 cm). This effect is utilized by the prior art field director structure to redirect and relocate the regions of relatively high electric field intensity within a microwave oven. 
       FIG. 1C  is a stylized plan view, generally similar to  FIG. 1A , illustrating the effect of a vane V of a field director as it is carried by a turntable T in the direction of rotation shown by the arrow. The vane V is shown in outline form and its thickness is exaggerated for clarity of explanation. 
     Consider the situation at angular Position  1 , where the vane V first encounters the hot region H 2 . Due to one corollary of Faraday&#39;s Law of Electromagnetism only an electric field vector having an attenuated intensity is permitted to exist in the segment of the hot region H 2  overlaid by the vane V. However, even though only an attenuated field is permitted to exist the energy content of the electric field cannot merely disappear. Instead, the attenuating action in the region adjacent to the conductive portion of the vane manifests itself by causing the electric field energy to relocate from its original location A to a displaced location A′. This energy relocation is illustrated by the displacement arrow D. 
     As the rotational sweep carries the vane V to angular Position  2  a similar result obtains. The attenuating action of the vane V again permits only an attenuated field to exist in the region adjacent to the conductive portion of the vane. The energy in the electric field originally located at location B displaces to location B′, as suggested by the displacement arrow D′. 
     The overall effect of the point-by-point attenuating action produced by the passage of the vane V through the region H 2  is the relocation of that region H 2  to the position indicated by the reference character H 2 ′. Similar energy relocations and redirections occur as the vane V sweeps through all of the regions H 1  through H 5  ( FIG. 1A ) of relatively high electric field intensity. 
       FIG. 1D  is a plot showing total energy exposure for one full rotation of the turntable at each discrete point J, K and L. The corresponding waveform of the plot of  FIG. 1B  is superimposed in  FIG. 1D  as a dotted line thereover. 
     It is clear from  FIG. 1D  that the presence of a field director results in a total energy exposure that is substantially uniform. As a result warming and cooking of a food product placed on the field director will be improved over the situation extant in the earlier prior art. Similarly, the use of a field director in conjunction with a susceptor improves uniformity of browning of a food product. 
     The typical prior art field director is designed for minimum cost and is intended for a single (i.e., one-time) use for heating or browning a food product. When used in a microwave oven to heat a food product the field director structure warps and discolors due to the heat generated by the microwave energy. This problem is exacerbated when the field director is used with a susceptor. The warping and discoloration render the field director unsightly and may be of sufficient severity to render the field director unsuitable for a second use. Thus, the typical prior art field director is considered to be unsuitable for multiple uses. 
     In view of the foregoing it is believed advantageous to provide a field director structure that is both physically robust in construction and appropriately configured in arrangement so as to be able to withstand repetitive heating without loss of structural integrity. Such a field director structure could be advantageously used multiple times to heat a food product and, if used each time with a new susceptor, also to brown and crisp that food product. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a self-supporting field director structure for use in heating an article in a microwave oven. 
     The field director structure includes a vane array that itself comprises a plurality of a number N of angularly adjacent vanes. Each vane extends radially outwardly from the central axis of the field director structure. Each vane is formed from a nonconductive substrate material that carries an electrically conductive material. The vane array may be formed from a plurality of individual vanes or from a plurality of vane doublets. 
     In one embodiment the invention is directed to a field director structure in which the materials used to fabricate the vanes of the field director structure are selected with the view to making the field director structure sufficiently physically robust so as to be able to remain self-supporting over multiple uses. In addition, and perhaps more importantly, in most aspects of this embodiment of the field director structure the materials of construction are arranged in a laterally symmetric fashion across the thickness of each vane. Arranging materials in a laterally symmetric fashion across the thickness of each vane equalizes thermal expansion effects due to heating over repetitive exposures to microwave energy, thus reducing the tendency to warp and contributing to the re-usability of the field director structure. One of several forms of vane support structure can be used to enhance the physical robustness of the vane array. 
     In accordance with a second embodiment of the invention the desired physical robustness of the field director structure is imparted by integrally molding or thermoforming individual vanes with a central support member. 
     In a third embodiment of the invention the field director structure is fabricated from a plurality of either totally metallic vanes or substantially metallic vanes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more fully understood from the following detailed description, taken in connection with the accompanying drawings, which form a part of this application and in which: 
         FIG. 1A  is a plan view showing regions of differing electric field intensity within a microwave oven and showing the paths followed by three discrete points J, K, and L located at respective radial positions P 1 , P 2  and P 3  on a turntable; 
         FIG. 1B  is a plot showing total energy exposure for one full rotation of the turntable at each of the discrete points identified in  FIG. 1A ; 
         FIG. 1C  is a plan view, generally similar to  FIG. 1A , showing the effect of the field director structure upon regions of high electric field intensity and again showing the paths followed by three discrete points J, K, and L located at respective radial positions P 1 , P 2  and P 3  on a turntable; 
         FIG. 1D  is a plot, similar to  FIG. 1B , showing total energy exposure for one full rotation of the turntable at each discrete point, with the waveform of  FIG. 1B  superimposed for ease of comparison; 
         FIG. 2A  is a stylized pictorial view of a field director structure assembled from a plurality of individual vanes as generally in accordance with a first embodiment of the present invention, the Figure also illustrating one form of a vane support structure with a portion of the vane support structure being broken away for clarity of illustration; 
         FIG. 2B  is a detailed view of an alternative form of a vane support structure with one of the vanes shown in outline form prior to insertion into the vane support structure; 
         FIG. 2C  is an exploded perspective view illustrating the steps in a method for making a field director structure in accordance with the present invention, the Figure also illustrating a second alternative form of a vane support structure; 
         FIG. 3A  is a plan view illustrating a vane doublet having a pair of vanes each conforming to a first aspect of the embodiment of the invention shown in  FIG. 2A  in which each vane has an inner core formed of an electrically conductive material completely enclosed within a pair of electrically non-conductive outer laminae, with portions of the outer radial regions of the vanes being broken to show the internal construction of the vanes, while  FIGS. 3B and 3C  are a respective front elevational view and a side sectional view taken along respective view lines  3 B- 3 B and  3 C- 3 C in  FIG. 3A , with the side sectional view of  FIG. 3C  illustrating the arrangement of the materials of the vane in a laterally symmetric fashion across the thickness of the vane; 
         FIG. 3D  is a plan view illustrating a vane doublet having a pair of vanes each conforming to a second aspect of the embodiment of the invention shown in  FIG. 2A  in which a non-conductive material is over-folded over the major surfaces of the vane, with portions of the outer radial regions of the vanes being broken to show the internal construction of the vanes, while  FIGS. 3E and 3F  are a respective front elevational view and a side sectional view taken along respective view lines  3 E- 3 E and  3 F- 3 F in  FIG. 3D , with the side sectional view of  FIG. 3F  illustrating the arrangement of the materials of the vane in a laterally symmetric fashion across the thickness of the vane; 
         FIG. 3G  is a plan view illustrating a vane doublet having a pair of vanes each conforming to a third aspect of the embodiment of the invention shown in  FIG. 2A  in which a non-conductive substrate is covered with a sheath of a conductive material, with portions of the outer radial regions of the vanes being broken to show the internal construction of the vanes, while  FIGS. 3H and 3I  are a respective front elevational view and a side sectional view taken along respective view lines  3 H- 3 H and  3 I- 3 I in  FIG. 3G , with the side sectional view of  FIG. 3I  illustrating the arrangement of the materials of the vane in a laterally symmetric fashion across the thickness of the vane; 
         FIG. 3J  is a plan view illustrating a vane doublet having a pair of vanes each conforming to a fourth aspect of the embodiment of the invention shown in  FIG. 2A  in which a non-conductive substrate is end-wrapped with a wrapper of a conductive material, with portions of the outer radial regions of the vanes being broken to show the internal construction of the vanes, while  FIGS. 3K and 3L  are a respective front elevational view and a side sectional view taken along respective view lines  3 K- 3 K and  3 L- 3 L in  FIG. 3J , with the side sectional view of  FIG. 3L  illustrating the arrangement of the materials of the vane in a laterally symmetric fashion across the thickness of the vane; 
         FIG. 3M  is a plan view illustrating a vane doublet having a pair of vanes each conforming to an alternative aspect of the embodiment of the invention shown in  FIG. 2A  in which a conductive material is disposed over a portion of the major surface of the vane, with portions of the outer radial regions of the vanes being broken to show the internal construction of the vanes, while  FIGS. 3N and 30  are a respective front elevational view and a side sectional view taken along respective view lines  3 N- 3 N and  3 O- 3 O in  FIG. 3M , in which a vane support structure is utilized to compensate for the lack of a laterally symmetric arrangement of the materials of the vane; 
         FIG. 4A  is a stylized pictorial view illustrating an integrally molded field director structure generally in accordance with a second embodiment of the present invention and illustrating the disposition of a portion of an optional vane support structure able to used with the integrally molded embodiment; 
         FIG. 4B  is a top sectional view of the integrally molded field director structure of  FIG. 4A  taken along section lines  4 B- 4 B thereon; 
         FIG. 4C  is a side sectional view taken along section lines  4 C- 4 C of  FIG. 4B  showing the positioning of the conductive portion embedded within each vane; 
         FIG. 4D  is a front elevational view taken along view lines  4 D- 4 D in  FIG. 4B ; 
         FIG. 5A  is a stylized pictorial view illustrating a field director structure having metallic vanes generally in accordance with a third embodiment of the present invention, the Figure also illustrating a third alternative vane support structure; 
         FIG. 5B  is a top sectional view of the metallic vane field director structure of  FIG. 5A  taken along section lines  5 B- 5 B thereon; 
         FIG. 5C  is a side sectional view taken along section lines  5 C- 5 C of  FIG. 5B  while  FIG. 5D  is a front elevational view taken along view lines  5 D- 5 D in  FIG. 5B , both views showing one all-metal vane construction; 
         FIG. 5E  is a side sectional view taken along section lines  5 E- 5 E of  FIG. 5B  while  FIG. 5F  is a front elevational view taken along view lines  5 F- 5 F in  FIG. 5B , both views showing an alternative all-metal vane construction; 
         FIG. 5G  is a top view generally similar to the view taken in  FIG. 5B  illustrating an alternative aspect of a metallic vane field director structure in which the radially inner end of a non-conductive substrate is wrapped with a metal wrapper with one of the vanes having an additional wrapping around the radially outer end, with portions of the inner and outer radial regions of one vane and a portion of the inner radial region of the other vane both being broken and shown in section to illustrate the internal construction; and 
         FIGS. 5H and 5I  are respective front elevational views taken along respective view lines  5 H- 5 H and  5 I- 5 I in  FIG. 5G ; and 
         FIG. 5J  is a side sectional view of each vane of  FIG. 5G  taken along section lines  5 J- 5 J therein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout the following detailed description similar reference characters refers to similar elements in all figures of the drawings. 
     With reference to  FIGS. 2A ,  4 A and  5 A shown are pictorial views of alternative embodiments of a reusable self-supporting field director structure, generally indicated by the reference numeral  10 ,  10 ′ and  10 ″ respectively, each in accordance with the present invention. In each case the field director structure  10 ,  10 ′,  10 ″ has a respective reference axis  10 A,  10 ′A and  10 ″A extending through its geometric center. 
     The field director structure  10 ,  10 ′,  10 ″ is, in use, disposed within the resonant cavity on the interior of a microwave oven M. The oven M is suggested only in outline form in  FIGS. 2A ,  4 A and  5 A. In operation, a source in the oven produces an electromagnetic wave having a predetermined wavelength. A typical microwave oven operates at a frequency of 2450 MHz, producing a wave having a wavelength on the order twelve centimeters (12 cm) (about 4.7 inches). The walls W of the microwave oven M impose boundary conditions that cause the distribution of electromagnetic field energy within the volume of the oven to vary. This generates a standing wave energy pattern within the volume of the oven. 
     In the same manner as is explained in the Background of this application the field director structure  10 ,  10 ′,  10 ″ in accordance with the present invention redirects and relocates the regions of high and low electric field intensity of the standing wave pattern within the volume of the oven M. Thus the field director  10 ,  10 ′,  10 ″ may be used to effect more uniform tempering, thawing and cooking of a food product or other article. Tempering is the warming of a food product, typically meat, from a sub-zero temperature (e.g., −40° F.) to about freezing (32° F.). 
     To effect browning or crisping of a food product or other article a conventional susceptor S may be used in conjunction with the self-supporting field director structure  10 ,  10 ′,  10 ″. The susceptor S is illustrated in the  FIGS. 2A ,  4 A and  5 A as being generally planar and circular in outline, although it may exhibit any predetermined desired form consistent with the food product to be browned or crisped within the oven M. Only a segment of the planar susceptor S is suggested in  FIGS. 2A ,  4 A and  5 A. In use, the planar susceptor S is received upon and supported by the field director structure  10 ,  10 ′,  10 ″ in a generally horizontal disposition within the oven M. The food product or other article is typically placed is contact with the planar susceptor S. 
     When the field director structure  10 ,  10 ′ or  10 ″ is mounted on a turntable the positions of the redirected and relocated regions of the electric field change continuously, further improving the uniformity of tempering, thawing, warming or cooking and, if a susceptor S if used, the browning or crisping of a food product placed on the field director structure  10 ,  10 ′,  10 ″. 
     As seen from the circled detail portion of  FIGS. 2A ,  4   a  and  5 A the planar susceptor S comprises a substrate S S  having an electrically lossy layer S C  thereon. The substrate S S  may be made from any of a variety of materials conventionally used for this purpose, such as cardboard, paperboard, fiber glass, other composites, or a polymeric material such as polyethylene terephlate, heat stabilized polyethylene terephlate, polyethylene ester ketone, polyethylene naphthalate, cellophane, polyimides, polyetherimides, polyesterimides, polyarylates, polyamides, polyolefins, polyaramids or polycyclohexylenedimethylene terephthalate. The layer S C  is typically implemented as a coating of vacuum deposited aluminum. 
     In the embodiment of  FIG. 2A  the field director structure  10  is fabricated from a plurality of individual vanes or, more preferably, a plurality of vane doublets.  FIGS. 3A through 3O  illustrate construction details of vanes in accordance with various aspects of this embodiment of the present invention. 
       FIGS. 2A through 2C  also illustrate various alternative forms of vane support structures used in the field director structure  10 ,  10 ′,  10 ″ having any form of individual vanes or vane doublets. An additional alternative vane support structure (limited to use with the field director structure  10 ,  10 ′,  10 ″ having individual vanes) is illustrated in  FIG. 5A . 
     In accordance with the teachings of the present invention the materials used in the field director structure  10  are selected with the view to making the field director structure  10  sufficiently physically robust so as to be able to remain self-supporting over multiple uses. 
     In addition, and perhaps more importantly, for the aspects of the field director  10  shown in  FIGS. 3A through 3L  the materials of construction of the field director  10  are arranged in a laterally symmetric fashion across the thickness of the vane. By “laterally symmetric across the thickness of the vane” (and like terms and phrases) it is meant that materials having substantially equal thermal responses (primarily due to the thermal coefficient of expansion of the material) form the outer major surfaces of the vanes and that these materials sandwich a material having a different thermal response. Arranging materials in a laterally symmetric fashion across the thickness of the vane equalizes thermal expansion effects due to heating over repetitive exposures to microwave energy, thus reducing the tendency to warp and contributing to the re-usability of the field director  10 . 
     In all of its various aspects the embodiment of the field director structure  10  as generally illustrated in  FIG. 2A  includes a vane array generally indicated by the reference character  16 . The vane array  16  itself comprises a plurality of a number N of angularly adjacent vanes  16 - 1  through  16 -N. Each vane extends radially outwardly from the central axis  10 A of the field director structure  10 . Although any convenient number of vanes may be used, in a typical instance as illustrated in the drawings the vane array  16  includes six vanes respectively indicated by reference characters  16 - 1  through  16 - 6 . 
     Each vane has a first major surface  16 F, a second major surface  16 S, a first minor surface  16 M extending along the upper edge  16 U of the vane, a second minor surface  16 N extending along the lower edge  16 G of the vane, an inner end  16 I and an outer end  16 D. Although the details of construction differ among each of the various aspects of this embodiment of the present invention ( FIGS. 3A through 3O ), in each case a vane is formed from a nonconductive substrate material  16 Q that has a radially outer zone  14 Z which carries an electrically conductive material  16 C. The conductive portion  16 C may be formed from a metallic foil having a thickness typically in the range from less than 0.1 millimeter to about 0.6 millimeter. 
     Suitable materials for the nonconductive substrate  16 Q include paperboard, cardboard, fiber glass, other composites, or a polymeric material such as polyethylene terephlate, heat stabilized polyethylene terephlate, polyethylene ester ketone, polyethylene naphthalate, cellophane, polyimides, polyetherimides, polyesterimides, polyarylates, polyamides, polyolefins, polyaramids or polycyclohexylenedimethylene terephthalate. 
     Suitable paperboard materials are those having a thickness in the range of 0.010 inches to 0.040 inches (0.4 to 2 millimeters). Two paperboard materials approved by the Food and Drug Administration (FDA) for use in microwave cooking applications are: Fortress Cup Stock, 17 point (0.017 inches thickness) available from International Paper Company, or Smurfit-Stone  16  point Cup Stock, (0.016 inches thickness) available from Smurfit-Stone Consumer Packaging Division, Montreal (Quebec) Canada. For use in Europe the materials must be “CE compliant” (i.e., comply with the Conformité Européenne). 
     The vanes in the vane array  16  may be attached together at their inner ends  16 I. The point of interattachment is aligned with the axis  10 A of the field director structure  10 . The attachment of the vanes at their inner ends is effected using an adhesive, preferably an adhesive approved for use in situations involving food contact. A suitable adhesive is type BR-3885 available from Basic Adhesives, Inc., Brooklyn, N.Y. Alternative adhesive are the industrial adhesive 45-6120 available from Henkel Adhesives, Elgin, Ill., or the laminating adhesive XBOND 705 available from Bond Tech Industries, Brampton, Ontario, Canada. 
     As noted earlier the various aspects of this embodiment of the invention shown in  FIGS. 3A through 3L  are configured with considerations of both physical robustness and laterally symmetric construction in mind. The physical robustness of the vane array in accordance with these aspects of this embodiment of the invention may be enhanced by the optional inclusion of one form of a vane support structure. 
     In the aspect of the embodiment of the invention illustrated in  FIGS. 3M through 3O , in which the vanes are configured only from the point of view of physical robustness, an additional vane support structure  18 ,  118 ,  218  (or  318  in the case of individual vanes) is required to achieve the desired reusability. It should be understood that the inclusion in the vane array  16  of any form of vane support structure  18 ,  118 ,  218  or  318  may avoid the necessity of attaching the inner ends  16 I of the vanes to each other along the axis  10 A of the field director  10 . 
     The first form of a vane support structure  18  is shown in  FIG. 2A . In this instance the vane support structure  18  is configured from a plurality of bracing members  18 B. Each bracing member  18 B extends between and is attached to the first major surface  16 F of one vane and the second major surface  16 S of an adjacent vane. The attachment of the ends of a bracing member  18 B to the confronting major surfaces of adjacent vanes may be made using one of the same adhesives as identified above. The area of attachment between a bracing member  18 B and the major surface of a vane is indicated by reference character  20 . 
     The bracing members  18 B each have a radially inner surface  18 I and a radially outer surface  18 R thereon. When this form of vane support structure  18  is used some of the electrically conductive portion  16 C of each vane may lie radially inwardly of the radially inner surface  18 I of the bracing members  18 B. 
     Although shown in  FIG. 2A  as being substantially cylindrical with an arcuate edge it should be appreciated that the bracing members  18 B may take any convenient alternative form. For example, a bracing member may be planar with a linear edge or may be comprised of multiple planar segments (each with a linear edge) intersecting along fold lines. 
     The vane support structure  18  may further include a planar bottom  18 M that is connected to the lower edge of each of the bracing members  18 B. One of the same adhesives as identified above may be used for this purpose. The area of interconnection between a bracing member  18 B and the bottom  18 M is indicated at reference character  22 . The bracing members  18 B and the bottom  18 M when so assembled cooperate to define a cup-like vane support structure. The minor surface  16 N extending along the lower edge  16 G of some or all of the vanes may, if desired, be attached to the bottom  18 M by one of the same adhesives. The line of interattachment between a vane and the bottom  18 M is indicated at reference character  24 . 
       FIG. 2B  shows an alternate vane support structure  118  that takes the form of a cylindrical wall-like member  118 W having a top lip  118 T, a bottom lip  118 L, a radially inner surface  118 I and a radially outer surface  118 R thereon. The top lip  118 T of the wall  118 W is interrupted by slots  118 S. As may be appreciated from  FIG. 2B  the slots  118 S extend completely through the thickness of the wall  118 W but end at a point above the bottom lip  118 L thereof. 
     When this form of vane support structure  118  is used the vanes of the vane array  16  are provided with a notch  16 H therein. As suggested in  FIG. 2B  each vane extends radially outwardly through the slot  118 S in the wall  118 W. The notch  16 H on the vane engages with the material of the wall  118 W immediately adjacent the slot  118 S thereby to secure the vane to the wall  118 W. The engaging portions of the vane and the wall may be reinforced using the adhesive mentioned above, as suggested by the thickened line indicated at reference character  120 . 
     If the notched arrangement is used the notch  16 H should be positioned on the vane so that the entire conductive portion  16 C of the vane lies radially outwardly of the radially outer surface  118 R of the wall  118 W. 
     Similar to the situation described in connection with  FIG. 2A  a planar bottom  118 M may be connected to the bottom lip  118 L of the cylindrical wall  118 W, again using one of the same adhesives as identified above, as suggested by the thickened line indicated at reference character  122 . The minor surface  16 N extending along the lower edge  16 G of some or all of the vanes may be, if desired, attached to the bottom  118 M by one of the same adhesives, as suggested by the thickened line indicated at reference character  124 . 
       FIG. 2C  shows a field director structure  10  in which the vane array  16  is fabricated using an alternative form of construction. A second alternative vane support structure  218  is also illustrated in this Figure. 
     The vane support structure  218  takes the form of an integrally molded cup-like member  218 C having an annular wall  218 W and an integral bottom  218 M. The wall  218 W has a radially inner surface  218 I and a radially outer surface  218 R. Through slots  218 S extend along the full height of the wall  218 W. 
     Instead of individual vanes attached at their inner ends of the vane array  16  (as in  FIGS. 2A and 2B ) the vane array  16  of the field director structure  10  of  FIG. 2C  is formed from a plurality of generally V-shaped vane doublets  17 . Each vane doublet  17  comprises a first vane  16 A and a second vane  16 B. The vanes  16 A,  16 B in each doublet  17  are integrally attached at a vertex  16 V of the “V”. 
     As suggested in  FIG. 2C  each vane doublet  17  is itself formed from a vane blank  14 . The particular arrangement of vane blank used to form a doublet for each of the various vane configurations shown in  FIGS. 3A through 3O  is discussed in connection with those respective Figure groupings. However, generally speaking, each finished vane blank  14  is an elongated member formed using the selected substrate material  14 Q. The blank  14  has two spaced-apart radially outer zones  14 Z that carry a conductive material  14 C. The finished vane blank  14  has a long axis  14 A extending longitudinally through the blank. The long axis  14 A extends through the spaced regions  14 C of conductive material. The arrangement of a vane blank that serves as the precursor to a vane doublet  17  depends upon the particular form of vane construction being deployed in the given vane array. 
     Once a vane blank  14  is finished the V-shaped vane doublet  17  is created by folding the elongated vane blank  14  along a central fold line  14 F perpendicular to the long axis  14 A, as indicated by the dashed arrows in  FIG. 2C . The fold defines the vertex  16 V of the doublet  17  and subdivides the doublet  17  into two vanes  16 A,  16 B. The appropriately shaped conductive material  14 C on the outer zones  14 C of the vane blank  14  each define the respective conductive portion  16 C of each vane  16 A,  16 B. It is noted that the conductive regions on both the vane blank and on the vanes  16 A,  16 B of the doublet  17  are shown in full for clarity of illustration. 
     Each vane doublet  17  so formed is inserted into the cup-like support member  218 C so that each vane  16 A,  16 B in each vane doublet  17  extends through an adjacent slot  218 S in the wall  218 W of the cup  218 C. 
     The plurality of vane doublets  17  may be attached to each other at their vertices  16 V (e.g., using one of the same adhesives as discussed) either before or after insertion into the cup  218 C. Additionally or alternatively, each of the vanes may be attached to the wall  218 W of the cup  218 C at the point where the vane passes through the slot  218 S. The engaging portions of the vanes and the wall  218 W may be secured using one of the adhesives mentioned above. The lower edge  16 G of each vane may additionally or alternatively be attached to the integral bottom  218 M of the cup  218 C. 
     The attachment of the vane doublets at their vertices and/or the attachment of the individual vanes of the doublets to the wall of the cup define the vane array  16 . The paired vanes  16 A,  16 B of each doublet  17  thus become adjacent numbered vanes in the vane array  16 . 
       FIGS. 3A through 3O  are various plan, elevational and sectional views illustrating alternative configurations of vanes used in the field director structure  10 . As noted, although a vane array may be configured from a plurality N of individual vanes, in the preferred instance of this embodiment of the invention the vane array is formed from a plurality of vane doublets  17  (e.g.,  FIG. 2C ). It is noted that throughout these Figures references to features relating to the vane blank used to form the vane doublet for each of these aspects of the invention are indicated with dashed lead lines. The outer radial regions in the plan views of the vanes are broken to show the internal construction of the vanes. The laterally symmetric vane configurations are believed best illustrated in the side sectional views of  FIGS. 3C ,  3 F,  3 I and  3 L. Electrically non-conductive material of the vanes is illustrated in the sectional views by stipled hatching. Electrically conductive material of the vanes is illustrated in the sectional views by diagonal hatching. 
       FIG. 3A  is a plan view illustrating a vane doublet  17  having a pair of vanes  16 A,  16 B each conforming to a first aspect of the embodiment of the invention shown in  FIGS. 2A through 2C . 
     In accordance with this aspect the electrically conductive portion  16 C of each vane defines an inner core that is completely enclosed by layers of electrically non-conductive material  16 Q that form a pair of electrically non-conductive outer laminae  16 Y 1 ,  16 Y 2 . 
     Any of the substrate materials discussed earlier are suitable for the outer laminae  16 Y 1 ,  16 Y 2 . The conductive portion  16 C is formed from a metallic foil typically less than 0.1 millimeter in thickness. Each vane has a predetermined thickness dimension  16 T ( FIG. 3C ). 
     The conductive portions  16 C are shaped and positioned to exhibit various predetermined dimensional constraints that contribute to the prevention of arcing and overheating in the event the field director is used in an unloaded oven (i.e., an oven without a food product present). 
     The electrically conductive core  16 C on each vane  16 A,  16 B is disposed at least a predetermined close distance  16 E ( FIGS. 3B and 3C ) from both the upper edge  16 U and the lower edge  16 G of each vane. The predetermined close distance  16 E lies in the range from about 0.025 times the wavelength of the microwave energy to about 0.1 times the wavelength. With a vane so constructed the occurrence of arcing in the vicinity of the electrically conductive material  16 C is prevented when the field director structure  10  is used in an unloaded microwave oven. 
     The electrically conductive material  16 C on each vane has a predetermined width dimension  16 W ( FIG. 3B ). The width dimension  16 W is about 0.1 to about 0.5 times the wavelength of the microwave energy. Each corner of the electrically conductive material  16 C is rounded at a radius  16 R ( FIG. 3B ) up to and including one half of the width dimension  16 W, again so that the occurrence of arcing in the vicinity of the electrically conductive material is prevented when the field director structure  10  is used in an unloaded microwave oven. 
     The electrically conductive core  16 C on each vane has a predetermined length dimension  16 L ( FIG. 3B ). The length dimension  16 L is about 0.25 to about 2 times the wavelength of the microwave energy. 
     The electrically conductive core  16 C on each vane is disposed at least a predetermined separation distance  16 X ( FIG. 3B ) from the axis  10 A. The separation distance  16 X is at least 0.05 times the wavelength of the microwave energy. This arrangement prevents the occurrence of overheating of the field director structure when used in an unloaded microwave oven. 
     The blank for the vane doublet  17  for the vanes of  FIGS. 3A through 3C  is itself formed by positioning electrically conductive material on the radially outer zones  14 Z of the substrate material  14 Q that becomes the first lamina  16 Y 1 . The conductive material placed on the zones becomes the conductive material  16 C of the vane. The layer of substrate material that becomes the second lamina  16 Y 2  is then placed over the conducting material on the substrate material of the first lamina  16 Y and adhered thereto. The layers of substrate material are adhered to each at the border regions to finish the blank. The finished blank is then folded along the fold line  14 F ( FIG. 3A ) to define the vanes  16 A,  16 B of the doublet  17 . 
     As seen from  FIG. 3C  the structure of each vane is both physically robust and arranged in a laterally symmetric fashion across the thickness  16 T of the vane so that thermal expansion effects due to heating over repetitive exposures to microwave energy are equalized. The physical robustness of a vane array in accordance with this aspect of the invention may be enhanced by the optional use of one of the support structures as discussed earlier. 
       FIGS. 3D ,  3 E and  3 F show a vane doublet  17  for a field director structure  10  in which the non-conductive substrate material  16 Q is folded over the electrically conductive material  16 C of each vane  16 A,  16 B. The electrically conductive material  16 C is substantially completely enclosed within an electrically non-conductive outer jacket  16 J so that each vane is laterally symmetric across its thickness dimension  16 T ( FIG. 3F ). 
     Any of the substrate materials discussed earlier are suitable for the outer jacket  16 J. The conductive portion  16 C is formed from a metallic foil typically less than 0.1 millimeter in thickness. 
     As suggested in  FIG. 3F  the vane doublet  17  for the vanes of these Figures is formed by folding a blank  14  along a fold line  14 G ( FIGS. 3E ,  3 F) that extends parallel to the long axis  14 A of the blank so that a leaf of the fold overlies the electrically conductive material on the blank. The leaves are adhered to the conductive material  16 C to form the outer jacket  16 J. The finished vane blank is then folded along the fold line  14 F ( FIGS. 3D and 3E ) to define the doublet  17  having the vanes  16 A,  16 B. 
     Each vane in the vane array in accordance with this aspect of the invention is both physically robust and arranged in a laterally symmetric fashion across the thickness  16 T of the vane so that thermal expansion effects due to heating over repetitive exposures to microwave energy are equalized. The vanes are thus able to withstand multiple exposures to microwave energy without the necessity of any additional vane support structure. However, the optional use of one of the vane support structure as discussed earlier would enhance the physical robustness of a vane array in accordance with this aspect of the invention. 
     The various dimensional parameters regarding the preferred limits on the close distance  16 E, the width dimension  16 W, the radius  16 R of the rounded corners, the length dimension  16 L and the separation distance  16 X as discussed in connection with the vane construction shown in  FIGS. 3A through 3C  apply to the vane construction of  FIGS. 3D through 3F . 
       FIG. 3G  is a plan view illustrating a vane doublet  17  having a pair of vanes each conforming to yet another aspect of the embodiment of the invention shown in  FIG. 2A . Each vane includes a substrate  16 Q made of an electrically non-conductive material. Any of the substrate materials discussed earlier is suitable for the vane substrate  16 Q. 
     In accordance with this aspect a portion of the electrically non-conductive substrate  16 Q of each vane is encased within a sheath  16 K of metallic foil. The major surfaces  16 F,  16 S and the minor surfaces  16 M,  16 N of each vane are thus electrically conductive. The thickness  16 Z ( FIG. 3I ) of the foil used to form the sheath  16 K is preferably on the order of 0.5 millimeters, greater than the thickness of the foil used to form the conductive portion in the vane of  FIG. 3C  or  3 F. 
     The blank for the vane doublet  17  for the vanes of  FIGS. 3G through 3I  is itself formed by wrapping an electrically conductive foil about the two spaced zones  14 Z near the radially outer ends of the electrically non-conductive substrate  14 Q that becomes the substrate  16 Q. The central region of the substrate  14 Q is left uncovered. The blank is then folded along the fold line  14 F ( FIGS. 3G and 3I ) to define the vanes  16 A,  16 B. 
     Each vane in the vane array in accordance with this aspect of the invention is both physically robust and arranged in a laterally symmetric fashion across the thickness  16 T of the vane so that thermal expansion effects due to heating over repetitive exposures to microwave energy are equalized. The vanes are thus able to withstand multiple exposures to microwave energy without the necessity of any additional vane support structure. However, the optional use of one of the vane support structure as discussed earlier would enhance the physical robustness of a vane array in accordance with this aspect of the invention. 
     Because the conductive sheath  16 K covers the major surfaces  16 F,  16 S and the minor surfaces  16 M,  16 N of the vane the dimensional consideration regarding the close distance  16 E does not apply to this aspect of the vane construction. However, the considerations regarding the preferred limits on the radius  16 R of the rounded corners, the width dimension  16 W and the length dimension  16 L as discussed in connection with the vane constructions shown in  FIGS. 3A through 3F  apply to the vane construction of  FIGS. 3G through 3I . However, for this vane construction, the separation distance  16 X should be at least 0.16 times the wavelength of the microwave energy to prevent overheating. 
     The thicker foil material used for the conductive sheath  16 K results in an increased thickness dimension  16 T for the vane over those vane structures earlier discussed. Accordingly, the concentration of the electric field in the vicinity of the upper edge  16 U and the lower edge  16 G is reduced, thus preventing the occurrence of arcing in the vicinity of the conductive sheath when the field director structure is used in an unloaded microwave oven. 
       FIG. 3J  is a plan view illustrating a vane doublet  17  having a pair of vanes each conforming to a fourth aspect of the embodiment of the invention shown in  FIG. 2A . In this aspect of the invention the same foil as used in the vane construction of  FIG. 3G  may be used to form a wrapper  16 P of a conductive material around a portion of the non-conductive substrate  16 Q of each vane. Any of the substrate materials discussed earlier is suitable for the vane substrate  16 Q. In this aspect of the invention the wrapper  16 P covers both major surfaces  16 F,  16 S and wraps around the outer end  16 D of the vane. However, the minor surfaces  16 M,  16 N of the vanes are left uncovered. 
     The blank for the vane doublet  17  for the vanes of  FIGS. 3J through 3L  is itself formed by wrapping an electrically conductive foil about the two spaced zones  14 Z near the longitudinal ends of an electrically non-conductive substrate  14 Q so that the central region of the substrate is left uncovered by conductive material. The blank so formed is then folded along fold line  14 F ( FIG. 3J ) to define vanes  16 A,  16 B of the doublet  17 . 
     Each vane in the vane array in accordance with this aspect of the invention is both physically robust and arranged in a laterally symmetric fashion across the thickness  16 T of the vane so that thermal expansion effects due to heating over repetitive exposures to microwave energy are equalized. The vanes are thus able to withstand multiple exposures to microwave energy without the necessity of any additional vane support structure. However, the optional use of one of the vane support structure as discussed earlier would enhance the physical robustness of a vane array in accordance with this aspect of the invention. 
     All of the same considerations regarding the preferred limits on the close distance  16 E, the width dimension  16 W, the radius  16 R of the rounded corners, the length dimension  16 L and the separation distance  16 X as discussed in connection with the vane construction shown in  FIGS. 3G through 3I  apply to the vane construction of  FIGS. 3J through 3L . Since the vanes are end-wrapped, rounded corners having the radius  16 R appear only adjacent to the inner end of the vane. 
     With reference now to  FIGS. 3M through 3O  illustrated is an alternative aspect of the embodiment of the invention shown in  FIG. 2A . In this aspect of the invention the vanes are configured based only upon considerations regarding the physical robustness of the vane. Lateral symmetry across the thickness of the vane is not present. For this reason the vane support structure is required to achieve the desired reusability. 
     Any of the substrate materials mentioned earlier may be used to form the blank for the vane doublet for this aspect of the invention. A conductive foil is disposed in each of the spaced zones  14 Z at the radially outer ends of a substrate  14 Q. The finished blank is then folded along the fold line  14 F to form the doublet  17 . 
     The same considerations regarding the preferred limits on the close distance  16 E, the width dimension  16 W, the radius  16 R of the rounded corners, the length dimension  16 L and the separation distance  16 X as discussed in connection with the vane construction shown in  FIGS. 3G through 3I  apply to the vane construction of  FIGS. 3M through 3O . 
       FIGS. 4A through 4D  depict a second embodiment of the field director structure  10 ′ in which the vane array  16 ′ is integrally molded or thermoformed from an electrically non-conductive heat-resistant material  16 ′Q.  FIG. 4A  shows a field director structure  10 ′ having six vanes although it is understood that any number of vanes greater than two will result in a self-supporting structure. 
     To form this second embodiment of the field director  10 ′ each of a plurality of suitably shaped thin foils of electrical conductive material is appropriately positioned within a suitable mold. The foils define the conductive portions  16 ′C of each vane of the vane array  16 ′. 
     By “suitably shaped” it is meant that the conductive portions  16 ′C of the vanes of the vane array  16 ′ exhibit the various preferred limits on the width dimension  16 ′W, the radius  16 ′R of the rounded corners, and the length dimension  16 ′L as described above. By “appropriately positioned” it is meant that the foils are placed on the mold surfaces corresponding to the major surfaces of the vanes to be formed such that the conductive portions  16 ′C of the vanes of the vane array  16 ′ lie within the close distance  16 ′E of the upper and lower edges of the vane and are positioned at the separation distance  16 ′X from the axis  10 ′A, both as also discussed in connection with  FIGS. 3G through 3M  above. These relationships are illustrated in  FIG. 4D . 
     If integrally molded, a suitable thermoplastic or thermoset polymeric resin material or a non-conductive composite material is injected into the mold using conventional injection molding techniques and allowed to set. 
     Thermoplastic polymeric resin materials suitable for the integrally molded embodiment of the field director  10 ′ include: polyolefins; polyesters such as poly(ethylene terephthalate) and poly(ethylene 2,6-napthalate); polyamides such as nylon-6,6 and a polyamide derived from hexamethylene diamine and isophthalic acid; polyethers such as poly(phenylene oxides); poly(ether-sulfones); poly(ether-imides); polysulfides such as poly(p-phenylene sulfide); liquid crystalline polymers (LCPs) such as aromatic polyesters, poly(ester-imides), and poly(ester-amides); poly(ether-ether-ketones); poly(ether-ketones); fluoropolymers such as polytetrafluoroethylene, a copolymer of tetrafluoroethylene and perfluoro(methyl vinyl ether), and a copolymer of tetrafluoroethylene and hexafluoropropylene; and mixtures and blends thereof. 
     A suitable thermoset polymeric resin is a high temperature epoxy resin or a bis(maleimide)triazine resin. 
     If a non-conductive composite material (i.e., a non-conductive polymeric resin containing a non-conductive reinforcing matrix) is used, this composite material may either include the thermoplastic polymeric resin materials or a thermoset polymeric resin material (both as listed above) as long as the resin is approved for use in situations involving food contact. 
     If thermoformed, suitable thermoplastic sheet may be converted into a three-dimensional shape by heating it to a temperature to render it soft and flowable and then applying differential pressure to conform the sheet to the shape of the mold, cooling it until it sets. Thermoforming may also be accomplished using solid or corrugated paperboard material, as is commonly used for commercial and industrial packaging. 
     Materials useful in the present invention should preferably have sufficient thermal tolerance so that they will not melt or flow when exposed to microwave energy in a microwave oven with food or another article present. More preferably, the materials should have sufficient thermal tolerance so that they will not melt or flow when exposed to microwave energy in an unloaded microwave oven (i.e., without food or another article present). 
     The molded field director  10 ′ may optionally include an annular vane support structure  18 ′ integrally molded with the vanes of the vane array  16 ′. The vane support structure  18 ′ illustrated in  FIG. 4A  is similar in form and function to the annular vane support structure  18  described in connection with  FIG. 2A . Integrally molded versions of the vane support structures  118 ,  218  may alternatively be used. 
     The vane support structure  18 ′ may be molded with the vane array  16 ′ of the field director  10 ′ in a single molding step or may be added to the vane array  16 ′ in a second molding step. As such the vane support structure  18 ′ includes bracing members  18 ′B extending between the first and second major surfaces of adjacent vanes of the vane array  16 ′. For clarity of illustration the optional vane support structure  18 ′ is only partially illustrated in  FIG. 4A  and is shown in dotted outline in  FIG. 4B . Although not illustrated the vane support structure  18 ′ may be provided with a closed bottom. 
     The molded field director  10 ′ must be sufficiently robust to permit its use multiple times to heat a food product without excessive warping or without losing its ability to support the food product. The thickness of the vanes is dependent upon the particular electrically non-conductive material from which the field director  10 ′ is molded. Typically the thickness  16 ′T is on the order of two to five millimeters. 
     Composite materials, because they contain a reinforcing matrix, offer enhanced stiffness and may provide the required robustness with vanes having a smaller thickness dimension  16 ′T. Typically the thickness  16 ′T of a composite vane is on the order of 1.5 to four millimeters. 
     If used with a susceptor S it is understood that the field director  10 ′ would typically be used with a new susceptor S for each food product to be browned or crisped. 
     In the embodiment of  FIG. 5A  a field director structure  10 ″ is fabricated from a plurality of individual vanes  16 ″ (six vanes  16 - 1 ″ through  16 - 6 ″ are shown). The vane doublet arrangement is not used with this embodiment. Totally metallic vanes in accordance with various aspects of this embodiment of the invention are shown in  FIGS. 5B through 5F  and various configurations of substantially metallic vanes are shown in  FIGS. 5G  though  5 J. 
     Since the vanes shown in  FIGS. 5B through 5F  are totally metallic and the vanes shown in  FIGS. 5G through 5J  are substantially metallic, the vanes must be disposed at least a predetermined separation distance  16 ″X ( FIGS. 5B ,  5 D,  5 F and  FIGS. 5H ,  5 I) from the axis  10 ″A. The separation distance  16 ″X is at least 0.16 times the wavelength of the microwave energy. This arrangement prevents the occurrence of overheating of the field director structure when used in an unloaded microwave oven. 
     The vanes are supported at the desired separation distance by a vane support structure  318  having a plurality of slots  318 S. The slotted central vane support structure  318  may be solid in form (as shown in full lines) or may have a hollow center (as suggested by the center circular opening  318 Y shown in dotted outline). The slotted central vane support structure  318  may be fabricated from any non-conductive material suitable for use with food. 
     A first aspect of the metallic vane construction, in which the vanes are completely metal, is shown in  FIGS. 5B ,  5 C and  5 D. This aspect of this embodiment of the invention provides the physically most robust construction. Preferably, the vanes are cut from aluminum sheet stock, although other metals, such as stainless steel, may be used. The vanes are approximately one to three millimeters (1 to 3 mm) in thickness, with a vane thickness greater than 1.25 millimeters being preferred. The vanes are machined to produce the desired rounded corner and rounded edge configurations. One suitable expedient to manufacture a field director in accordance with this aspect of the embodiment of the invention shown in  FIGS. 5B through 5D  is inserting individual metal vanes into position in a mold and injecting a non-conductive material to form the central vane support structure. 
     A second aspect of the metallic vane construction, in which the vanes are also completely metal, is shown in  FIGS. 5E and 5F . Preferably, the vanes are cut from thinner aluminum sheet stock, although other metals, such as stainless steel, may be used. The sheet stock used to form the vanes of  FIGS. 5E and 5F  is approximately 0.5 millimeters in thickness. The edges of the vanes are rolled to produce a rolled upper and lower edges and rolled-edged rounded corner configurations. When so rolled the vanes exhibits a predetermined maximum effective thickness dimension (indicated in  FIG. 5E  by the reference character  16 ″T) of at least 1.25 millimeters. The individual metal vanes so constructed are inserted into position in a mold and a non-conductive material injecting to form the central vane support structure. This aspect of this embodiment of the invention also provides a physically robust construction while reducing the amount of metal required for vane construction. 
     The occurrence of arcing in the vicinity of the electrically conductive material  16 ″C is prevented when the field director structure  10 ″ having vanes constructed as shown in  FIGS. 5B through 5F  is used in an unloaded microwave oven. 
     A third and a fourth alternative aspect of this embodiment of the invention using substantially metallic vanes  16 ″A and  16 ″B are shown in  FIGS. 5G through 5J . Both vanes  16 ″A and  16 ″B exhibit a configuration that is laterally symmetric across the thickness of the vane, as in the vane constructions discussed in connection with  FIGS. 3A through 3L . The vanes  16 ″A and  16 ″B are also generally similar to the vane construction discussed in connection with  FIGS. 3J through 3L  in that a conductive metallic material extends over substantially both of the major surfaces  16 ″F,  16 ″S of the vanes  16 ″A,  16 ″B. The minor surfaces  16 ″M,  16 ″N of both of the vanes  16 ″A,  16 ″B are left uncovered. 
     The vanes  16 ″A and  16 ″B differ from the vane shown in  FIGS. 3J through 3L  in that the inner radial end  16 ″I of the vane is wrapped with metal. The vane  16 ″B differs from the vane  16 ″A in that the radially outer end  16 ″D of the vane  16 ″B is also wrapped by the metal wrapper. 
     The blank for the vane shown in  FIGS. 5G and 5J  is itself formed by wrapping an electrically conductive foil about an electrically non-conductive substrate  14 ″Q so that a region near a longitudinal end of the substrate is left uncovered by conductive material. Both major surfaces  16 ″F,  16 ″S of the substrate  16 ″Q are covered and the inner longitudinal end  16 ″I is wrapped by conductive material. As noted the minor surfaces  16 ″M,  16 ″N of the vanes are left uncovered. 
     The blank for the vane shown in  FIGS. 5I and 5J  is itself formed by wrapping an electrically conductive foil longitudinally about both longitudinal ends of an electrically non-conductive substrate  14 ″Q so that both major surfaces  16 ″F,  16 ″S are covered and both longitudinal ends  16 ″I,  16 ″D of the substrate  16 ″Q are wrapped by conductive material. The minor surfaces  16 ″M,  16 ″N of the vanes are again left uncovered. 
     In both the third and the fourth alternative aspects the electrically conductive wrapper  16 ″P on each vane  16 ″A,  16 ′B is disposed at least a predetermined close distance  16 ″E ( FIGS. 5H and 5I ) from both the upper edge  16 ″U and the lower edge  16 ″G of each vane. The predetermined close distance  16 ″E lies in the range from about 0.025 times the wavelength of the microwave energy to about 0.1 times the wavelength. With a vane so constructed the occurrence of arcing in the vicinity of the electrically conductive material  16 ″C is prevented when the field director structure  10 ″ is used in an unloaded microwave oven. 
     Each vane in the vane array in accordance with the third and the fourth alternative aspects of this embodiment of the invention is both physically robust and arranged in a laterally symmetric fashion across the thickness  16 ″T of the vane so that thermal expansion effects due to heating over repetitive exposures to microwave energy are equalized. The vanes are thus able to withstand multiple exposures to microwave energy without the necessity of any additional vane support structure. 
     If used with a susceptor S it is understood that the field director  10 ″ would typically be used with a new susceptor S for each food product to be browned or crisped. 
     Those skilled in the art, having the benefit of the teachings of the present invention may impart various modifications thereto. Such modifications are to be construed as lying within the contemplation of the present invention.