WO1997044850A1

WO1997044850A1 - Open ground transmission line circuits

Info

Publication number: WO1997044850A1
Application number: PCT/US1997/008410
Authority: WO
Inventors: Zhi-Yuan Shen
Original assignee: E.I. Du Pont De Nemours And Company
Priority date: 1996-05-22
Filing date: 1997-05-16
Publication date: 1997-11-27

Abstract

Open-ground transmission line circuits comprise at least one dielectric substrate having at least one conducting line and a conducting ground plate deposited on opposite sides thereof; the ground plate having an opening therein at a location directly opposite each conducting line, wherein the conducting line and the conducting ground plate preferably comprise high temperature superconductors with a critical temperature greater than 90 °K and selected from YBa2Cu3O7-⊃, Tl2Ba2CaCu2O8 and (Tl,Pb)Sr2Ca2Cu3O9; and wherein at least one connecting link traverses each opening in the ground plate and the opening has a broader width in the area of the connecting link to compensate for discontinuity caused by the connecting link.

Description

TITLE OPEN GROUND TRANSMISSION LINE CIRCUITS

Background Of The Invention The invention relates to transmission line circuits, more particularly to "open ground" transmission line circuits, that is, transmission line circuits having an opening in the ground plates thereof. The open ground tramsmission line circuits of this invention may be in the microstrip or the strip form. The circuits are useful to increase the line width for handling high power at high impedance and have particular application in coupling circuits of high power high temperature superconducting filters, power splitters and other applications.

Transmission line circuits such as microstrip lines and strip lines are commonly used in microwave circuits. Typical designs for microstrip lines are disclosed in Brian C. Wadell, Transmission Line Design Handbook, Artech House Inc.,

Boston, Ma (1991), on pages 2, 93 and 126, for example. Corresponding designs for strip lines are disclosed in K.C. Gupta et al., Computer-Aided Design of Microwave Circuits, Artech House Inc., Boston, MA (1981), page 56. Transmission line circuits generally comprise a transmission line (such as a high temperature superconductor film) deposited on one side of a dielectric substrate and a ground plate deposited on the opposite side of the substrate. Two main requirements of transmission lines are characteristic impedance and power handling capability. For a given transmission line, these characteristics are a function of the geometry and dimensions of the transmission line circuit and of the dielectric constant of the substrate on which the transmission line is deposited.

In some applicatons, the requirement for characteristic impedance and high power handling capability are contradictory. In particular, the power handling capability of a transmission line is mainly determined by the width of the line. The greater the line width, the higher the power handling capability. Characteristic impedance, on the other hand, is determined by the width of the transmission line and by the distance between the transmission line and the ground plate, that is, by the thickness of the substrate. Increasing the line width to provide higher power handling capabilities will result in a decrease in characteristic impedence. While the characteristic impedance can be increased by increasing the thickness of the substrate, that may not be a practical choice, particularly for high temperature superconductor (HTS) circuits which utilize 254 microns or 508 microns thick LaAlθ₃ substrates with a dielectric constant of ε_r = 24. In addition, some microwave circuits, such as multi-stage binary matched power splitters, require very high characteristic impedance (up to several hundred ohms). When thin substrates with high dielectric constants are used for such applications, extremely narrow line widths (microns to sub-micron) are necessary, which are very difficult or impossible to fabricate.

These competing requirements have made it very difficult in the past to obtain the advantage of handling high power at high impedance, which is particularly important for HTS transmission lines due to the limited electrical current carrying capacity of HTS materials and the high dielectric constant of substrates.

Summary Of The Invention

In its broadest embodiment, the invention comprises open-ground transmission line circuits comprising at least one dielectric substrate having at least one conducting line and a conducting ground plate deposited on opposite sides thereof; the ground plate having an opening therein at a location directly opposite each conducting line.

In the microstrip line form, only one substrate is employed. In the strip line form, the invention comprises at least one conducting line sandwiched between two dielectric substrates, each said substrate having a conducting ground plate deposited thereon; wherein each ground plate has an opening therein at a location directly opposite from each conducting line.

The conducting lines and ground plates are preferably made of conductors having a conductivity greater than 3 x 10⁷ per ohm-meter, such as copper, aluminum, gold and silver and the substrates comprise low loss dielectric material with a loss tangent less than 0.001, such as AI₂O₃, fused quartz and fiber glass reinforced fluoropolymers. In a most preferred embodiment, the conducting lines and ground plate comprise high temperature superconductors with a critical temperature greater than 90°K, such as YBa₂Cu₃θ_7-δ, Tl₂Ba₂CaCu₂θg and (Tl,Pb)Sr₂Ca₂Cu₃O₉ and the substrates comprises a dielectric material with low loss and lattice matched with the high temperature superconductor, such as LaAlO₃, MgO, yttria-stabilized zirconia and sapphire.

In another embodiment, the open ground transmission line circuits of this invention further comprise at least one connecting link traversing the at least one opening in the ground plate and wherein the opening, at the location of the connecting link, has a broader width to compensate for discontinuity introduced by the connecting link.

In yet another embodiment, the invention comprises an open ground transmission line circuit as aforesaid and further comprises a metallic case for housing the circuit, the case comprising a body and a lid therefore, wherein at least one of said housing and said lid are provided with at least one groove therein in the area of the at least one opening in the ground plate to prevent short-circuiting of the circuit.

These and other features of the invention will become apparant upon a further reading of this specification, with reference to the drawing figures, and of the appended claims.

Brief Descriptions Of The Drawings

Figure 1(a) - 1(c) illustrate an embodiment of an open ground transmission line of the invention in microstrip line form, wherein Figure 1(a) is a top view, Figure 1(b) is a cross sectional view, and Figure 1(c) is a bottom view.

Figure 2(a) -2(c) illustrate an embodiment of an open ground transmission line of the invention in the srtip line form, wherein Figure 2(a) shows the top view, Figure 2(b) is a cross sectional view, and Figure 2(c) is a bottom view.

Figure 3(a) - 3(c) illiustrate another embodiment of the open-ground microstrip line form of the invention, wherein Figure 3(a) is a top view, Figure 3(b) is a cross sectional view, and Figure 3(c) is a bottom view. Figure 4 illustrates another embodiment of the open-ground strip line of the invention, wherein Figure 4(a) is a top view, Figure 4(b) is a cross sectional view, and Figure 4(c) is a bottom view.

Figure 5(a) - 5(b) illustrates an HTS high power 2-pole filter with octagon shaped resonators and coupling circuits comprising the open-ground microstrip line of this invention, wherein Figure 5(a) is a top view and Figure 5(b) is a bottom view.

Figure 6(a) - 6(d) illustrate an HTS high power 3 -pole filter with rectangular resonators and coupling circuits comprising the open-ground strip line of this invention, wherein Figure 6(a) is a longitudinal cross sectional view, Figure 6(b) is a sectional view along line A-A, Figure 6(c) is a top view, and Figure 6(d) is a bottom view.

Figure 7(a) - 7(b) illustrates a 3-stage power splitter with a matched input and four outputs utilizing the open-ground microstrip transmission line of this invention, wherein Figure 7(a) is a top view and Figure 7(b) is a bottom view.

Figure 8(a) - 8(b) illustrate the open ground transmission line circuits of this invention housed within a case, wherein Figure 8(a) illustrates a cross sectional view of an open-ground microstrip line circuit/case combination and Figure 8(b) illustrates a cross sectional view of an open-ground strip line circuit/case combination.

Figure 9(a) - 9(b) illustrate a 2-pole HTS filter with octagonal resonators and input and output coupling circuits utlizing the open ground transmission line circuits of this invention, wherein Figure 9(a) is a top view and Figure 9(b) is a bottom view.

Figure 10(a) - 10(b) are frequency response curves: S₂₁ vs. frequency, for the 2- pole filter of Figure 9, wherein Figure 10(a) shows the frequency response over a broad band sweep at a fixed power level and Figure 10(b) shows the frequency response over a narrow band sweep at 7 different power levels. Detailed Description Of The Embodiments

With reference first being made to Figure 1(a) - 1(c), the open ground microstrip line circuit illustrated therein comprises a dielectric substrate 1 1 having a center strip line 12 deposited on one side thereof. On the opposite side of the substrtae 1 1 is deposited open, or discontinuous ground plate comprising ground plate 13 a and ground plate 13b, separated by an opening or slot 14. See Figure 1(b) and 1(c). The opening 14, which in the embodiment shown has a longitudinal slot¬ like configuration, is located on the area of the substrate 1 1 directly opposite the center line 12, as best seen in Figure 1(b).

As noted, this invention has particularly utility in HTS high power circuits due to their limited power handling capability and because of the use of high dielectric constant substrates in such applications. Accordingly, the materials of choice for the transmission lines and ground plates are HTS films. Likewise, the substrates of choice are those having a lattice match to the HTS films.

In practice, the open ground transmission line circuits of this invention, such as the embodiment shown in Figure 1(a) - 1(c) provide means for tailoring the power handling capabilities and the characteristic impedance of the circuit to the particular needs of the desired application. More particularly, using the embodiment of Figure 1(a) - 1(c) as an example, a suitable substrate 1 1 is first selected, the thickness and dielectric constant of which is known. Next, the width of the center line 12 is selected based on the desired power handling capability. Finally, an opening 14 is created in the ground plate 13a, 13b directly opposite the center line 12. The width of the opening 14 is selected to provide the desired characteristic impedance of the circuit. The wider the opening 14, the greater the distance between the center line 12 and the ground plate 13a, 13b, and the higher the charateristic impedance.

Those skilled in the art will appreciate that the function of the opening in the ground plate effectively increases the distance between the transmission line and the ground plate, without increasing the thickness of the substrate. Furthermore, although the functional aspects of the invention just described was with particular reference to the microstrip line form depicted in Figure 1(a) - 1(c), those skilled in the art will appreciate that the discussion is eaqually applicable to the other embodiments described in detail below.

With reference now being had to Figure 2(a) - 2(c), illustrated therein is another embodiment of the open ground transmission line circuits of this invention. In this embodiment, which is in the strip line form, a center line 26 is sandwiched between two substrate 25a and 25b. A discontinuous ground plate, comprising ground plates 27a, 27b separated by opening 29a is provided on substrate 25a directly opposite the location of center line 26. See Figure 2(a) and 2(b). Similarly, an open or discontinuous ground plate, comprising ground plates 28a, 28b separated by opening 29b, is provide on substrate 25b directly opposite the location of center line 26. See Figure 2(b) and 2(c).

The open-ground transmission line circuits of this invention have discontinuous ground plates, that is effcetively two ground plates separated from one another by an opening. Accordingly, these circuits support more than one mode. Among these modes, only the even mode with symmetrical electrical fields with respect to the center of the line in the cross section is the useful and desirble operating mode. Generally speaking, the desireable even mode is supported by having symmetry in the two parts of the open ground plate with respect to the center line. Accordingly, as seen in the embodiments depicted in Figures 1 (a) - 1 (c) and Figure 2(a) - 2(c), respectively, the open ground transmission line circuits have a cross-sectional geometry with left-right mirror symmetry relative to the center line 12, 26 which supports the desirable even mode with an electrical potential polarity of - (left ground), + (center line), - (right ground).

However, if the symmetry at any place along the center line is broken, the circuits will also support odd modes, which have an electrical potential polarity of + (left ground), - (center line left edge), + (center line right edge), - (right ground). The odd mode is undesireable because it causes interference to the even operating mode. Since the odd modes are only supported by reason of having, in effect, two ground plates, such as in the case of a discontinuous ground plate as shown in Figures 1(a)- 1(c) and Figure 2(a) -2(c), the odd modes can be supressed by introducing a conducting link across the opening in the discontinuous ground plate, in effect, a bridge connecting the two halves of the open ground plate. An example of such a connecting link is shown in Figure 3. The transmission line circuit shown in Figures 3(a)- 3(c) comprises a microstrip transmission line comprising a center line 112 (see Figures 3(a) and 3(b)) deposited on one side of a dielectric substrate 111, and an open ground plate, comprising ground plates 113a and 113b separated by an opening 114 (see Figure 3(b) and 3(c)). The opening is located directly opposite the center line 112, as best seen in Figure 3(b). A conducting link 115, as seen in Figure 3(c), traverses the opening 114 and connects ground plate 113a to 113b. The conducting link 1 15 effectively "short circuits" the ground plates 113a and 1 13b and forces ground plates 1 13a and 113b to share the same electrical potential polarity and, thus effectively suppresses the odd mode.

Therefore, at the location of the link 15, the characteristic impedance is lower than that of the rest of the line, which serves as a discontinuity and causes mis-match. The discontinuity can be equivalently represented by a shunt capacitance, C, attached to the line. To address this problem two additional discontinuities 1 16a and 1 16b, located adjacent to and on either side of the connecting link 115, are introduced. These additional discontinuities 116a, 1 16b, are in the form of a broadened width area within opening 114, as seen in Figure 3(c). The two discontinuities, 116a and 116b, can be represented by two series inductances, L, located on each side of the capacitance, C. If the guided wavelength of the signal in the microstrip line is much large than the total length of 115, 116a, and 116b along the longitudinal direction, the three discontinuities can be treated as lumpy circuits of a T-type L-C-L low pass filter, in which the two inducdances L, compensates the capacitance to eliminate the mis-match within the passing band. If the open-ground microstrip line is long compared to the signal's guided wavelength in the line, a number of such links and compensate structures should be used. In such cases, the distance between adjacent structures must be carefully chosen to minimize the overall reflection.

Figure 4(a) - 4(c) illustrate another embodiment of the open ground plate with connecting link in a strip line form. This embodiment, similar to that of Figure 2, comprises a center line 126 sandwiched between substrates 125a and 125b. See Figure 4(b). Substrate 125a is also provided with an open ground plate 127a, 127b, separated by an opening 129a. See Figure 4(a). The opening 129a is located directly opposite cemter line 126, as best seen in Figure 4(b). With reference again to Figure 4(a), the separated halves of open ground plate 127a, 127b are connected by a connecting link 130a which traverses the opening 129a which comprises. To compensate for the reduced impedance at the location of the connecting link 130a, two discontinuities 131a and 131b are provided which comprise an expanded width area in the opening 129a on either side of the connecting link 130a.

Similarly, as seen in Figure 4(c), substrate 125b is provided with an open ground plate 128a, 128b, separated by an opening 129b located directly opposite the center line 126 (see Figure 4(b)). A connecting link 130b, flanked by discontinuities 132a, 132b, is also provided. The combination of 130a, 131a, 131b, and 130b, 132a, 132b forms a T-type L-C-L lowpass filter, which minimize the adverse effect of the discontinuity caused by the connecting links, 130a and 130b.

There are two primary applications where the open ground transmission line circuits of this invention have the maximum utility: (1) high power HTS transmission lines and (2) high impedance transmission lines. Figure 5(a)- 5(b) illustrate a 2-pole HTS bandpass filter comprising a dielectric substrate 41 having two TMoi mode octagon-shape HTS resonators, 40a, 40b, (see Figure 5(a)) which serve as the frequency selecting element of the filter. An HTS ground plate 44 is deposited on the back side of 41, as seen in Figure 5(b). Open-ground microstrip lines in accordance with the invention are used for input and output coupling circuits, as well as the inter-resonators coupling circuit.

With reference again being made to Figure 5(a), the input coupling circuit comprises a center line 42a and a T-shape branch line 43a. The output coupling circuit consists of a center line 42b and a T-shape branch line 43b. The inter- resonator coupling circuit consists of a short section of center line 50 located between resonators 40a and 40b. On the substrate, directly opposite from the input coupling lines 42a, 43a; the inter-resonator coupling line 50; and the output coupling lines 42b, 43b, are openings 45a, 46 and 45b, respectively, in the ground plate, whereby the input coupling circuit, the inter-resonator circuit and the output line circuit comprise open ground circuits in accordance with the invention.

Figure 6(a) - 6(d) illustrate a 3-pole HTS bandpass filter. With reference to Figure 6(a), the filter comprises two substrates 91 ,92, each having an open ground plate 93, 94, respectively, on one side thereof. Sandwiched between the substrates 91, 92 (as seen in Figure 6(b)) are three rectangular-shape TMoi mode resonators 93a, 93b, 93c, comprising the frequency selecting element of the filter, along with input coupling line 94a and T-shape branch line 95a (comprising the input transmission lines); inter-resonator coupling lines 96a, 96b and output coupling line 94b and T-shaped branch line 95b (comprising the output transmission lines). With reference to Figure 6(c) and 6(d), identical open ground plates 93, 94 comprise HTS ground plates with openings 97a, 99a, respectively, located directly opposite the substrate from the input transmission lines; openings 98a, 90a located directly opposite from the inter-coupling line 96a; openings 98b, 90b located directly opposite inter-coupling transmission line 96b; and openings 97b, 99b located directly opposite from the output transmission lines.

As mentioned, another primary application for the open ground transmission line circuits of this invention is when very high impedance transmission lines are required or desired. For instance, a 3-stage 50-ohm input binary power splitter consists of a 50-ohm input line in the first stage, a 100-ohm line in the second stage, and a 200-ohm line in the third stage. To shrink the circuit size and to reduce the insertion loss of such power splitter, it is beneficial to make it from HTS materials. As mentioned previously, HTS thin films are usually deposited on 254 micron or 508 micron thick LaAlO₃ substrates. For the HTS microstrip lines on 508-micron thick LaAlθ₃ substrate, the 50-ohm line requires a line width of 170 microns, the 100-ohm line requires a line width of 10 microns, and the 200-ohm line requires a line width of less than 0.02 microns. Obviously, such extremely narrow line widths are very difficult, if not impossible, to fabricate.

Figure 7(a) -7(b) show an example of an HTS 3-stage power splitter. The first stage comprises a 50-ohm HTS thin film input line 62, which comprises a traditional microstrip transmisions circuit with a closed ground plate 61 (see Figure 7b), deposited on opposite sides of a substrate 60, such as a 508 micron thick LaAlU3 substrate. In the second stage, the 100-ohm lines 63a, 63b have the same line width as that of line 62. To increase the characteristic impedance in the second stage, the ground plate opposite lines 63a, 63b is provided with a slot-like opening 66 (see Figure 7(b)) of the appropriate width required by the 100-ohm characteristic impedance. In the third stage, the 200-ohm lines 64a, 64b, 65a, 65b have a narrower line width than that of line 62. Slot-like openings 67 and 68 (see Figure 7(b)) are provided in the ground plate directly opposite lines 64a, 64b and 64c, 64d, respectively. The openings 67, 68 are of appropriate width to provide the 200-ohm characteristic impedance.

Because the binary splitter has the two branch line in parallel at the junction of each stage, the 3-stage power splitter with 50-ohm, 100-ohm, and 200-ohm characteristic impedances in the first, second, and third stages, respectively, are impedance matched for the input. However, as mentioned previously, the open- ground microstrip lines used for the second and third stages can support the undesirable odd modes, which will act as interferences for the operating even mode causing mis-match and other performance degradation. Accordingly, two HTS connecting links, 69a, 69b are introduced in the second stage to connect the ground plate across opening 66 and the width of the opening 66 is increased at the location of the connecting links 69a, 69b to provide discontinuities which off-set the mis-match caused by the connecting links. Similarly, in the third stage, four HTS links, 69c, 69d, and 69e, 69f are introduced to connect the ground plates on either side of openings 67 and 68, respectively, and the openings are broadened in the area of the connecting links 69c, 69d, 69e, and 69f. Such HTS power splitter can split the input signal equally into four output with only the normal 6-dB splitting loss and very small additional ohmic loss. The small size and the matched input make the invented power splitter very broad band resulting in a very small signal distortion and is particularly useful for signal processing.

Figure 8(a) and 8(b) illustrate two examples of the open ground transmission line circuits of this invention in specially designed housings. Due to the opening in the ground plates, special housings are needed for the circuits of this invention. More particularly, the housings must provide sufficient space in the area of the opening(s) in the ground plate to prevent the metallic housing from short-ciruiting at the opening in the ground plate. Figure 8(a) shows a cross sectional view of a specially designed metallic case for circuits comprising open-ground microstrip lines. The case comprises a body 70 and a lid 71. The open-ground microstrip line 73 comprises the center line 74 and open ground plate, comprising ground plates 75a,75b separated by opening 77, deposited on opposite sides of substrate 76. A groove 72 is cut into the case body 70 adjacent to opening 77 to provide space for the opening. The width of groove 72 must not less than the width of the slot 77. The depth of the groove 72 must be sufficient to prevent affecting the electrical field distribution underneath the center line, which affects the characteristic impedance of the open-ground microstrip line.

Figure 8(b) shows a cross sectional view of a metallic case for circuits comprising open-ground strip lines. The case comprises a body portion 80 and a lid 81. The open-ground strip line 82 comprises the center line 85 sandwiched by two substrates 87a, 87b. The ground plates 86a, 86b, and 86c 86d are deposited on the opposite side of substrates 87a and 87b, respectively, with the openings 88a and 88b in the ground plates above and underneath the center line 85. Two grooves 83 and 84 are cut into the metallic body 80 and the lid 81 , respectively, to provide spaces adjacent to openings 88b, 88a. The width of grooves 83 and 84 must not less than the width of the open slots 88a, 88b and the depth of the grooves 83, 84 must be sufficient to prevent affecting the electrical field distribution around the center line, which affects the characteristic impedance of the open-ground strip line.

Example With reference to Figure 9(a) - 9(b), a 2-pole HTS filter was prepared by depositing double-sided Tl₂Ba₂CaCu₂O₈ HTS films on a 33.2 mm x 17.2 mm x 0.508 mm LaAlO₃ substrate 50. Using standard bi-level photolithography process and ion milling, two octagonal shaped TM₀₁₀ mode resonators 51a, 51b, were provided on one side of the substrate 50 (see Figure 9(a)) which serve as the frequency selecting element for the filter. Also provided on the top side of the filter (Figure 9(a)) are an input coupling circuit (comprising center line 52a and a T-type branch line 53a) and an output coupling circuit (comprising center line 52b and T-type branch line 53b). The width of center lines 52a, 52b was 2mm to provide for greater power handling capability. On the bottom side of the substrate 50, again using standard bi-level photolithography process and ion milling, an opening 55a was provided in the ground plate 54 directly opposite the input coupling circuit, and an opening 55b was provided directly opposite the output coupling circuit. The openings 55a and 55b were sized to provide a characteristic impedance of 50-ohm. Opening 57 and center line 58 were provided as an inter-resonator coupling circuit.

The filter was then enclosed in a coppper case similar to that shown in Figure 8(a) having open spaces adjacent to the openings in the ground plate. SMA compatible connectors were used the input and output interfaces. The filtrer was then tested at 77°K. Figure 10(a) shows the frequency response curve of the filter at 40.7 watts transmitting power over a frequancy range of 5.74 GHz to 6.34 GHz. The data shows the two poles clearly with a 0.1 dB ripple within a 1% bandwisth and a negligible in-band insertion loss. Figure 10(b) shows a frequency response curve of the filter over a range of 5.98 GHz to 6.05 GHz at transmitting power levels of 2 watts, 28 watts, 45 watts, 55 watts, 78 watts, 100 watts and 115 watts. The data show that the filter can handle at least 1 15 watts of transmitting power without performance degredation.

Claims

WHAT IS CLAIMED IS:

1. An open-ground transmission line circuit comprising at least one dielectric substrate having at least one conducting line and a conducting ground plate deposited on opposite sides thereof; the ground plate having an opening therein at a location directly opposite each conducting line.

2. The open ground transmission line circuit of claim 1, comprising at least one conducting line sandwiched between two dielectric substrates, each said substrate having a conducting ground plate deposited thereon; wherein each said ground plate has an opening therein at a location directly opposite from each conducting line.

3. The open ground transmission line circuit of claim 1 , wherein the at least one conducting line and the conducting ground plate independently comprise conductors having a conductivity greater than 3 x 10⁷ per ohm-meter.

4. The open ground transmission line circuit of claim 3, wherein the conductors are independently selected from the group consisting of copper, aluminum, gold and silver.

5. The open ground transmission line circuit of claim 3, wherein the at least one dielectric substrate comprises a low loss dielectric material with a loss tangent less than 0.001.

6. The open ground transmission line circuit of claim 5, wherein the dielectric material is selected from the group consisting of AI₂O₃, fused quartz and fiber glass reinforced fluoropolymers.

7. The open ground transmission line circuit of claim 1 , wherein the at least one conducting line and the conducting ground plate independently comprise high temperature superconductors with a critical temperature greater than 90°K.

8. The open ground transmission line circuit of claim 7, wherein the high temperature superconductors are selected from the group consisting of YBa₂Cu₃O_7-δ, Tl₂Ba₂CaCu₂O₈ and (Tl,Pb)Sr₂Ca₂Cu₃θ₉.

9. The open ground transmission line circuit of claim 7, wherein the at least one dielectric substrate comprises a dielectric material with low loss and lattice matched with the high temperature superconductor.

10. The open ground transmission line circuit of claim 9, wherein the at least one dielectric substrate is independently selected from the group consisting of

LaAlθ₃, MgO, yttria-stabilized zirconia and sapphire.

1 1. The open ground transmission line circuit of claim 1 , further comprising at least one connecting link traversing the at least one opening in the ground plate and wherein the opening, at the location of said at least one connecting link, has a broader width to compensate for discontinuity introduced by the at least one connecting link.

12. The open ground transmission line circuit of claim 1 , further comprising a metallic case for housing the circuit, said case comprising a body and a lid therefore, wherein at least one of said housing and said lid are provided with at least one groove therein in the area of the at least one opening in the ground plate to prevent short-circuiting of the circuit.