US3605098A

US3605098A - Phased array antenna including impedance matching apparatus

Info

Publication number: US3605098A
Application number: US815566A
Authority: US
Inventors: Peter W Hannan
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1969-04-14
Filing date: 1969-04-14
Publication date: 1971-09-14
Anticipated expiration: 1988-09-14

Abstract

An impedance-matched phased array antenna in which the radiating elements comprise slots or holes in a conductive ground plane. The antenna includes a plurality of impedance-matching members, each consisting either of a thin sheet of high-k dielectric material or thin metal pieces which collectively have dielectric characteristics at the operating wavelengths. Each impedance matching member is supported in laterally spaced relationship with the other impedance matching members by a dielectric support member associated with each opening in the ground plane. Each combination of impedance-matching member and support member provides environmental protection for one of the openings in the ground plane. The lateral spacing between the impedance-matching members prevents accumulation of large sheets of water in front of the radiating elements which could cause severe impedance mismatches. Alternative arrangements are also covered.

Description

United States Patent Peter W. Hannah Centerport, N.Y. 815,566

Apr. 14, 1969 Sept. 14, 1971 Hazeltine Corporation lnventor Appl. No. Filed Patented Assignee PHASED ARRAY ANTENNA INCLUDING IMPEDANCE MATCHING APPARATUS Primary Examiner-Eli Lieberman Attorney-Kenneth P. Robinson ABSTRACT: An impedance-matched phased array antenna in which the radiating elements comprise slots or holes in a conductive ground plane. The antenna includes a plurality of impedance-matching members, each consisting either of a thin sheet of high-k dielectric material or thin metal pieces which collectively have dielectric characteristics at the operating wavelengths. Each impedance matching member is supported in laterally spaced relationship with the other impedance matching members by a dielectric support member associated with each opening in the ground plane. Each combination of impedance-matching member and support member provides environmental protection for one of the openings in the ground plane. The lateral spacing between the impedancematching members prevents accumulation of large sheets of water in front of the radiating elements which could cause severe impedance mismatches. Alternative arrangements are also covered.

PATENIEU SEPI 41971 SHEET 1 [1F 2 TO saGNAL GENERATOR FIG. 2

PEG. I

FIG. 3

PATENTED SEP 1 41971 SHEET 2 BF 2 FIG. 5

FIG. 6

Fl-IASED All! ANTENNA INCLUDING IMPEDANCE MATCHING APFARATIJS BACKGROUND OF THE INVENTlON Phased array antennas are commonly employed to provide a steerable beam of electromagnetic energy. However, a fundamental problem of phased array antennas is the variation of impedance with frequency and with scan angle. In the case of phased array comprising slots or holes in a metal ground plane, this variation of impedance can be reduced by covering the array with a slab of dielectric. When properly designed, the dielectric slab acts like a quarter wavelength transformer which matches the impedance of free space to the relatively low impedance of the array. Because this matching structure is located between the array and free space it provides a good impedance match over a wide frequency band. Because this matching structure has an impedance that varies with scan angle in a manner related to the array variation, it provides a good impedance match over a wide range of scan angles in all planes of scan. A disadvantage of a dielectric slab is that rain falling on it will tend to form a continuous sheet of water, creating an impedance mismatch which can be intolerable at short wavelengths of operation. Another disadvantage is that individual radiating elements cannot be withdrawn for repair or replacement from the front of the array without first removing the slab of dielectric.

Another disadvantage of the dielectric slab is that it permits a TM surface wave to propagate along the array face at a velocity considerably slower than that of a plane wave in free space. This is known to cause a very large mismatch of impedance when the array beam is scanned to a certain angle which is less than the angle at which an end fire-grating lobe occurs. This large mismatch results in a drastic reduction of radiation from the array.

A thin sheet of dielectric having a high dielectric constant (k) and spaced in front of the array approximately one-eighth of a wavelength, has an impedance-matching action similar to that of the dielectric slab, but has far less slowing action for a TM surface wave. This is due to the fact that the electric field of the surface wave is nearly perpendicular to the sheet and is effected only slightly when the sheet is thin. As a result, the drastic effect of the surface wave occurs at a scan angle which is substantially greater than the scan angle at which drastic effect is produced by a dielectric slab which is mounted on the array face. In fact, with this thin sheet of dielectric the angle at which the drastic effect due to surface wave occurs can be made relatively close to the maximum scan angle permitted for the array, as determined by the avoidance of grating lobes. Unfonunately, the thin sheet of dielectric retains the same disadvantages of accumulated precipitation across the array and the difficulty of access to the radiating elements that exists for the dielectric slab. In addition, a thin high-k dielectric sheet sufficient to cover a large array is difficult to fabricate and if struck by any foreign object, a stone for example, would be subject to damage that could affect the operation of large portions of the array.

Objects of the present invention therefore are to provide new and improved phased array antenna systems which are impedance matched to the impedance of free space over a wide range of scan angles and over a wide frequency band and which are not adversely effected by the accumulation of precipitation on the impedance-matching member In accordance with the present invention, there is provided a phased array antenna including apparatus for matching the impedance of the array to the impedance of free space which comprises an array of radiating elements including a plurality of openings In a conductive ground plane for propagating a beam of electromagnetic energy over a desired range of wavelengths and a plurality of impedance matching.

For a better understanding of the present invention together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Referring to the drawings:

FIG. 1 which is common to FIGS. 3-6. is a side view of a phased array antenna constructed in accordance with the present invention;

FIG. 2 which is common to FIGS. 3-6 is an exploded partial view of the front of the FIG. 1 antenna looking in the direction of arrow A;

FIG. 3 is a sectional view along section BB of FIG. 2 which in conjunction with FIGS. 1 and 2 illustrate one embodiment of the present invention;

FIG. 4 is an alternate sectional view along section BB of FIG. 2 which in conjunction with FIGS. 1 and 2 illustrates another embodiment of the present invention;

FIG. 5 is a second alternate sectional view along section BB of FIG. 2 which in conjunction with FIGS. 1 and 2 illustrates another embodiment of the present invention; and

FIG. 6 is a third alternate sectional view along section BB of FIG. 2 which in conjunction with FIGS. 1 and 2 illustrates another embodiment of the present invention.

DESCRIPTION OF THE FIG. 3 EMBODIMENT FIGS. 1-3 are an illustration of a phased array antenna 10 constructed in accordance with the present invention. The antenna ill includes an array of radiating elements including a plurality of openings 11 in a conductive ground plane 13. Each of the openings 11 terminates a circular waveguide 12 which couples electromagnetic energy having a desired range of frequencies from a signal generator to the face of the array through the openings 11 in the ground plane 13 via the transmission lines 12'. By properly phasing the electromagnetic energy coupled to each opening 11 a narrow beam of electromagnetic energy can be caused to scan a region in space over the desired range of wavelengths.

The antenna also includes a plurality of impedancematching members 15 individually associated with one of the openings II in said ground plane 13. Each impedancematching member 15, has at least one thin flat section of material, illustrated as dielectric disc 15, substantially parallel to the ground plane 13, said thin flat section collectively having dielectric characteristics over the desired range of wavelengths for providing impedance-matching between the array radiating elements 10 and free space.

The phased array antenna further comprises a plurality of means 14 each associated with one of said impedancematching means 15 for supporting said impedance-matching means in a laterally spaced relationship with respect to the other impedance-matching means 15 so as to provide environmental protection for one of the openings II in said ground plane 13. As illustrated the support member 14 in combination with the impedance-matching member 15 completely encloses the opening 11 in the ground plane I3 preventing foreign matter such as dirt, rocks or precipitation from entering the waveguide 12 from the atmosphere. Substantial environmental protection can also be provided by impedancematching members l5 which are supported and attached to the ground plane 13 by dielectric support means which do not completely enclose the opening 11 in the ground plane 13.

The provision of a separate impedance-matching member 15 with each opening in the ground plane 11 and lateral spacing between said members I5 prevents impedance mismatch due to the accumulation of precipitation and provides easy access to each of the radiating elements.

OPERATION OF THE FIG 3 EMBODIMENT The phased array of FIG 1 can be caused to produce a steerable beam in space by coupling signals of varying phase and amplitude from a signal generator to each of the waveguides 12. As is well known. by the proper control of phase and amplitude of the supplied signals, the outputs from the waveguides 12 through the openings lll in the ground plane 13 are combined to form a single narrow beam of electromagnetic energy. This resultant beam can be caused to scan a region in space by proper variation of the phase of these signals. However, intercoupling between the array elements tends to cause the effective impedance of the array to vary as the scan angle of the beam is changed. Uncompensated variations'in the impedance produce reflections which can cause deterioration of the radiation efiiciency, the radiation pattern and amplifier stability.

The dielectric discs 55 provide an impedance-matching structure which produces a good match over a wide range of scan angles in all planes of scan. The impedance-matching provided by the combined effects of the discs 15 approximate the impedance-matching which is provided by a thin sheet of high-k dielectric positioned one-eighth of a wavelength in front of the array Surface waves are thereby minimized by using the thin high-k dielectric material which is spaced above the ground plane 13. The FIG. 3 impedance-matching structure has the further advantage that each of the dielectric discs 15 and the corresponding support member I4 is associated with one of the openings Ill in the ground plane 13. Each of the discs 15 and corresponding support member 114 can therefore be readily removed in order to replace it or in order to provide access to the corresponding waveguide 112. F urthermore, small discs of high-k dielectric illustrated for the FIG. 3 impedance-matching device are much easier to fabricate than a large continuous sheet of high-k dielectric. Also objects striking the face of the array will damage only a few of the dielectric discs 15, whereas an object striking a thin sheet of highk dielectric may necessitate replacement of the entire sheet.

The design of the impedance-matching structure illustrated in FIGS. 1-3 depends to some degree on other antenna design criteria; for example, the size and spacing between the openings 11 in the ground plane 13. Generally speaking, however, the dielectric discs are separated from the ground plane I3 by one-eighth of the free space wavelength of the center frequency of the operating range (A Further, the impedance-matching effect provided is related to the dielectric constant of the dielectric discs 15, the thickness of the discs 15 and the lateral spacing 16 between the discs 15. The required thickness of the discs 15 is inversely related to the dielectric constant. It is desirable to have the discs 15 as thin as possible to minimize surface wave propagation. However, the higher the k of the material, the more costly and the more fragile the material usually is.

It is convenient to have the cross section of each impedance-matching member 15 correspond to the cross section of the opening 11 in the ground plane l3 with which it is associated. FIGS. 1-3 illustrate a phased array having circular openings 11 in the ground plane 113 and therefore the impedance-matching members 15 are circular discs. In another embodiment the radiating elements may include elongated slots in the ground plane and the impedance-matching members would most conveniently consist of elongated pieces of high-k dielectric having corresponding cross sections.

The lateral spacing 16 between the discs 15 must be sufficient to permit precipitation to flow between dielectric sup ports I4. On the other hand, if the discs 15 are too widely spaced the desired impedance-matching cannot be achieved. Generally speaking, the lateral spacing 16 between the discs 15 will be appreciably less than 0.5)

An impedance-matching structure as illustrated in FIG. 3 has been designed and successfully tested on an array simulator. The device tested provided wide-band and wide-angle impedance matching similar to the best results obtained on a continuous thin sheet. The following table lists the dimensions of the device constructed and tested (A the free space wavelength at midband of the operating wavelength).

Reference Dimension Numeral Value DESCRIPTION OF FIG. 4

FIG. 4 is an alternate sectional view along section BB of FIG. 2 and illustrates another embodiment of the present invention. In this Figure and FIGS. 5 and 6 the elements that are difierent from but correspond to elements illustrated in FIG. 3 are indicated by three digit reference numerals with the last two digits of said reference numerals being the same as the corresponding element in FIG. 3.

FIG. 4 illustrates a plurality of thin flat pieces of conductive material 415 positioned parallel to and separated from the ground plane 113. The conductive pieces 415 collectively have dielectric characteristics over the desired range of wavelengths for providing impedance matching between the array of radiating elements 10 and free space.

The FIG. 4 embodiment further includes a plurality of dielectric support means 414 individually associated with one of the openings 1 l in the ground plane 13 for individually supporting one or more of the thin pieces of conductive material 415 in laterally spaced relationship with the thin pieces of conductive material supported by the other support members so as to provide environmental protection for one of the openings 1 l in the ground plane 13.

The impedance-matching structure illustrated in FIG. 4 provides a good impedance match over a wide range of scan angles in substantially the same manner as the impedancematching structure of FIG. 3 with the substantial difference that the FIG. 4 structure does not require high-k dielectric which tends to be costly and fragile. The high-k dielectric discs are replaced by thin discs of conductive metal supported by the low-k dielectric support members 414. These thin metal discs are less fragile, easier to construct and less expensive.

In FIG. 4 each support member supports a single metal disc. An artificial dielectric may also be constructed in which the metal discs have a smaller diameter and each support member 414 supports two or more coplanar laterally separated metal discs.

The cross-sectional shape of the discs may be other than circular. If the cross section of the openings in the ground plane 13 were elongated slots, for example, it would be desirable to have the metal pieces have a similar cross section.

The design criteria of the FIG. 4 impedance-matching structure is substantially the same as the FIG. 3 device. The metal discs should be very thin with respect to a wavelength in order to minimize the surface wave propagation. In addition, the I same criteria relating to the spacing between the discs and the distance from the discs to the ground plane stated in conjunction with the FIG. 3 structure apply to the FIG. 4 embodiment.

An impedance-matching structure as illustrated in FIG. 4 has been designed and successfully tested in an array simulator. The device tested provided wide-band and wide-angle impedance matching similar to the best results obtained with a continuous thin sheet of high-k dielectric. The following table lists the dimensions of the device constructed and tested.

DESCRlP'TllON FIG.

H6. 5 is an alternate sectional view along section BB of 'FlG. 2 and illustrates another impedance-matching structure constructed in accordance with the present invention. The FIG. 5 structure includes a plurality of thin flat pieces of high- It dielectric material 515 positioned parallel to and separated from the ground plane 113 for providing impedance matching between the radiating elements and free space.

The FlG. 5 structure further includes a plurality of low-k dielectric support members individually associated with one of the openings ill in the ground plane 13 for individually supporting a plurality of said thin high-k dielectric sections. The thin high-k dielectric sections supported by each support member 5% are separated from each other in the direction perpendicular to the ground plane l3, with the distance from the ground plane l3 to any of the high-k dielectric sections SllS being no greater than one-fourth of any of the free space wavelengths within said desired range of wavelengths. Each dielectric support member 5M supports the plurality of impedance-matching members 515 in laterally spaced relationship with the thin pieces of high-k dielectric material supported by the other support members in a manner which provides environmental protection for each of the openings ll in the ground plane 13.

The impedance matching provided by the combined effects of the high-k dielectric discs 15 approximates the impedance matching which is provided by a quarter wave transformer slab of dielectric with the very significant difference that each dielectric disc is much thinner than the required quarter wave slab would be, thereby minimizing the propagation of surface waves.

The sections of high-k dielectric material that are supported by one of the support members 5M have a combined thickness approximately equal to the thickness of a single highJc dielectric sheet illustrated in iFllG. 3, for dielectric material having an equivalent dielectric constant. The distance between the dielectric discs 5l5 which are supported within the same support member 5114 is determined by several factors including the dielectric constant, the thickness of the discs and other design criteria. However, generally speaking, the discs 5ll5 must be closely spaced with respect to a wavelength and the distance from the ground plane to the furthest disc 515 will be no greater than a quarter of the free space wavelength. Similarly. lateral spacing between the discs which are supported by different support members 514 is dependent upon other antenna design cn'tena. re the size and spacing between the openings ll in the ground plane l3 They must be spaced sufficiently far apart to permit water to flow between them but sufficiently close to provide the desired impedance matching.

Three dielectric discs 5H5 have been shown associated with each support member 5M. However, it should be apparent that fewer or a greater number of thin high-k dielectric sections may be stacked up perpendicular to the ground plane 13.

DESCRIPTION OF FIG. 6

FIG. 6 is an alternate sectional view along section BB of FIG 2 and in conjunction with FIGS 1 and 2 illustrates another impedance-matching structure constructed in accordance with the present invention. The FIG. 6 structure provides impedance matching in substantially the same manner as the FIG. 5 structure. However, in the FIG. 6 structure, the high-k dielectric discs 515 of FIG. 5 are replaced by thin pieces of conductive material 615, attached to the dielectric support member 614. which collectively have dielectric characteristics over the desired range of wavelengths. These artificial dielectric discs 615 are constructed in substantially the same manner as the artificial dielectric discs of FIG. 4. They should be as thin as possible in order to minimize surface waves and as in FIG. 4 they may include a plurality of coplanar metal discs supported by each dielectric support member 6114. The other design criteria of the FIG. 6 embodiment are substantially the same as the FIG. 5 embodiment.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made thereon without departing from the invention and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim is:

l. A phased array antenna including apparatus for matching the impedance of the array to the impedance of free space, comprising:

an array of radiating elements including a plurality of openings in a conductive ground plane for propagating a beam of electromagnetic energy over a desired range of wavelengths;

a plurality of impedance-matching means each individually associated with a different one of the openings in said ground plane, and each having at least one thin flat section of material substantially parallel to said ground plane, said thin flat sections collectively having dielectric characteristics over the desired range of wavelengths for providing impedance matching between the array of radiating elements and free space;

a plurality of means each associated with one of said im pedance-matching means for supporting that impedancematching means above said ground plane and in a laterally spaced relationship with respect to the other impedance-matching means so as to provide environmental protection for one of the openings in said ground plane;

whereby the provision of a separate impedance-matching means, associated with each opening in the ground plane and the spacing between said means, prevents impedance mismatch due to the accumulation of precipitation and provides easy access to each of the radiating elements and impedance matching over said desired range of wavelengths.

2. A phased array antenna including apparatus for matching the impedance of the array to the impedance of free space, comprising:

an array of radiating elements including a plurality of openings in a conductive ground plane, for propagating a beam of electromagnetic energy over a desired range of wavelengths;

a plurality of thin flat pieces of conductive material parallel to and separated from sa1d ground plane, said conductive pieces collectively having dielectric characteristics over the desired range of wavelengths for providing impedance matching between the array of radiating elements and free space;

a plurality of dielectric support means each individually associated with a different one of the openings in said ground plane for individually supporting one or more of said thin pieces of conductive material above said ground plane and in laterally spaced relationship with the thin pieces of conductive material supported by the other support means so as to provide environmental protection for one of the openings in said ground plane;

whereby the provision of separate impedance-matching material. associated with each opening in said ground plane and the lateral spaces between said material, prevents impedance mismatch due to the accumulation of precipitation and provides easy access to each of the radiating elements an impedance matching over said range of wavelengths.

3. A phased array antenna as specified in claim 2 in which each dielectric support means supports one or more coplanar thin pieces of conductive material separated from the ground plane by approximately one-eighth of a free space wavelength.

4. A phased array antenna as specified in claim 3 in which each dielectric support means comprises a member of low-k dielectric material which completely encloses the corresponding opening in said ground plane.

5. A phased array antenna as specifiedin claim 2 in which each support means supports a plurality of thin pieces of conductive material at least two of which are separated from each other in the direction perpendicular to the ground plane, the distance from said ground plane to any of said conductive pieces being no greater than one-fourth of any of the free space wavelengths within said desired range of wavelengths.

6. A phased array antenna as specified in claim 5 in which each support means comprises a member of low-k dielectric material which completely encloses the corresponding opening in said ground plane.

7. A phased array antenna including apparatus for matching the impedance of the array to the impedance of free space, comprising:

a plurality of thin flat pieces of high-k dielectric material parallel to and separated from said ground plane for providing impedance matching between the array of radiating elements and free space;

a plurality of low-k dielectric support members each individually associated with a different one of the openings in said ground plane for individually supporting one or more of said thin pieces of high-k dielectric material in laterally spaced relationship with the thin pieces of high-k dielectric material supported by the other support members so as to provide environmental protection for one of the openings in said ground plane;

whereby the provision of a separate impedance matching material associated with each opening in said ground plane and the lateral spaces between said members prevent impedance mismatch due to the accumulation of precipitation and provides easy access to each of the radiating elements.

8. A phased array antenna as specified in claim 7 in which each low-k dielectric support member supports a thin piece of high-k electric material separated from the ground plane by approximately one-eighth of the free space wavelength.

9. A phased array antenna as specified in claim 8 in which each low-k dielectric support member in combination with said thin piece of high-k dielectric material completely encloses the corresponding opening in said ground plane and the cross section of each thin piece of high-k dielectric material is substantially equal to the cross section of the corresponding opening in said ground plane.

10. A phased array antenna as specified in claim 7 in which each dielectric support member supports a plurality of thin high-k dielectric sections which are separated from each other in the direction perpendicular to the ground plane, the distance from said ground plane to any of said high-k dielec tric sections being no greater than one-fourth of any of the free space wavelengths within said desired range of wavelengths.

1 l. A phased array antenna as specified in claim 10 in which the cross section of each thin piece of high-k dielectric material is substantially equal to the cross section of the corresponding opening in said ground plane and said low-k dielectric support member in combination with said thin high-k dielectric pieces completely encloses the corresponding opening in said ground plane.

Claims

1. A phased array antenna including apparatus for matching the impedance of the array to the impedance of free space, comprising: an array of radiating elements including a plurality of openings in a conductive ground plane for propagating a beam of electromagnetic energy over a desired range of wavelengths; a plurality of impedance-matching means each individually associated with a different one of the openings in said ground plane, and each having at least one thin flat section of material substantially parallel to said ground plane, said thin flat sections collectively having dielectric characteristics over the desired range of wavelengths for providing impedance matching between the array of radiating elements and free space; a plurality of means each associated with one of said impedancematching means for supporting that impedance-matching means above said ground plane and in a laterally spaced relationship with respect to the other impedance-matching means so as to provide environmental protection for one of the openings in said ground plane; whereby the provision of a separate impedance-matching means, associated with each opening in the ground plane and the spacing between said means, Prevents impedance mismatch due to the accumulation of precipitation and provides easy access to each of the radiating elements and impedance matching over said desired range of wavelengths.

2. A phased array antenna including apparatus for matching the impedance of the array to the impedance of free space, comprising: an array of radiating elements including a plurality of openings in a conductive ground plane, for propagating a beam of electromagnetic energy over a desired range of wavelengths; a plurality of thin flat pieces of conductive material parallel to and separated from said ground plane, said conductive pieces collectively having dielectric characteristics over the desired range of wavelengths for providing impedance matching between the array of radiating elements and free space; a plurality of dielectric support means each individually associated with a different one of the openings in said ground plane for individually supporting one or more of said thin pieces of conductive material above said ground plane and in laterally spaced relationship with the thin pieces of conductive material supported by the other support means so as to provide environmental protection for one of the openings in said ground plane; whereby the provision of separate impedance-matching material, associated with each opening in said ground plane and the lateral spaces between said material, prevents impedance mismatch due to the accumulation of precipitation and provides easy access to each of the radiating elements an impedance matching over said range of wavelengths.

5. A phased array antenna as specified in claim 2 in which each support means supports a plurality of thin pieces of conductive material at least two of which are separated from each other in the direction perpendicular to the ground plane, the distance from said ground plane to any of said conductive pieces being no greater than one-fourth of any of the free space wavelengths within said desired range of wavelengths.

7. A phased array antenna including apparatus for matching the impedance of the array to the impedance of free space, comprising: an array of radiating elements including a plurality of openings in a conductive ground plane for propagating a beam of electromagnetic energy over a desired range of wavelengths; a plurality of thin flat pieces of high-k dielectric material parallel to and separated from said ground plane for providing impedance matching between the array of radiating elements and free space; a plurality of low-k dielectric support members each individually associated with a different one of the openings in said ground plane for individually supporting one or more of said thin pieces of high-k dielectric material in laterally spaced relationship with the thin pieces of high-k dielectric material supported by the other support members so as to provide environmental protection for one of the openings in said ground plane; whereby the provision of a separate impedance matching material associated with each opening in said ground plane and the lateral spaces between said members prevent impedance mismatch due to the accumulation of precipitation and provides easy access to each of the radiating elements.

10. A phased array antenna as specified in claim 7 in which each dielectric support member supports a plurality of thin high-k dielectric sections which are separated from each other in the direction perpendicular to the ground plane, the distance from said ground plane to any of said high-k dielectric sections being no greater than one-fourth of any of the free space wavelengths within said desired range of wavelengths.

11. A phased array antenna as specified in claim 10 in which the cross section of each thin piece of high-k dielectric material is substantially equal to the cross section of the corresponding opening in said ground plane and said low-k dielectric support member in combination with said thin high-k dielectric pieces completely encloses the corresponding opening in said ground plane.