Dielectric Resonator Apparatus Including Means for Adjusting the Degree of Coupling

This invention relates to dielectric resonators and, more particularly, to a dielectric resonator apparatus having a plurality of dielectric coaxial resonators unistructurally formed inside a single dielectric block.

In electronic circuits containing resonance circuit sections such as high-frequency band filters (e.g., band pass filters and band rejection filters) or oscillators, it is common to make use of resonance parts using dielectric members (hereinafter referred to as dielectric resonator apparatus) in the resonance circuit section in order to achieve improvements in circuit characteristics and reduction in size. With prior art dielectric resonator apparatus, desired frequency characteristics were obtained by aligning a plurality of dielectric resonators inside a single case and connecting these dielectric resonators by means of external connector elements. Dielectric resonator apparatus thus structured are disadvantageous in that the number of components is large and that they tend to be heavy. For this reason, demands for smaller and lighter dielectric resonator apparatus have been growing. In applications to mobile communication systems such as portable telephones and car telephones, it is particularly important to provide smaller and lighter dielectric resonator apparatus.

The present applicant has earlier proposed a single-body type dielectric resonator apparatus which can be made smaller and lighter than prior art dielectric resonator apparatus (Japanese Patent Application 4-9207). As a background to the present invention, this dielectric resonator apparatus earlier considered but not published, will be described next in some detail with reference to FIGS. 17-22.

FIG. 17 is an external diagonal view of this earlier considered dielectric resonator apparatus when it is about to be completed, having a dielectric block 1 approximately in the shape of a parallelepiped with two throughholes 5a and 5b penetrating therethrough from one end surface 1a to the opposite end surface 1b. An outer conductor 4 is formed nearly all over the outer surfaces of the dielectric block 1, except there are two signal input and output electrodes 9a and 9b also formed on the outer surfaces of the dielectric block 1 but in an electrically insulated condition from the outer conductor 4. The outer conductor 4 is grounded when the dielectric resonator apparatus is mounted on a circuit board (not shown), and the signal input and output electrodes 9a and 9b are connected to signal input and output terminals on this circuit board. As shown in FIG. 18, which is a sectional view of the dielectric block 1 of FIG. 17 across a plane longitudinally cutting across either of the throughholes 5a or 5b, an inner conductor 3 is formed all over the inner surfaces of the throughholes 5a and 5b.

When the dielectric resonator apparatus is in the condition shown in FIGS. 17 and 18, a portion of the inner conductor 3 is removed from the interior of the throughholes 5a and 5b as shown in FIGS. 19 and 20. As shown more clearly in FIG. 19, the portion of the inner conductor 3 which is removed is that portion with a specified longitudinal length t from one end surface 1a of the block 1 inward, thereby leaving an uncovered part .alpha. inside each of the throughholes 5a and 5b. This means that the portion of the outer conductor 4 which is on the (first) end surface 1a is in an electrically "open" (or not contacting) relationship with the inner conductor 3 inside each of the throughholes 5a and 5b, while another portion of the outer conductor 4 on the other (or second) end surface 1b of the dielectric block 1 is electrically connected to the inner conductor 3 inside each of the throughholes 5a and 5b. For this reason, the first and second end surfaces 1a and 1b of the dielectric block 1 may be referred to as the open end surface and the short-circuited end surface, respectively.

FIG. 21 is an exploded view of the dielectric resonator apparatus of FIG. 20 placed upside down and cut horizontally by a plane passing through the central axes of the two throughholes 5a and 5b. In FIG. 21, symbols C.sub.s each indicate the capacitance between a tip part of the inner conductor 3 and the outer conductor 4 formed on the open end surface 1a (hereinafter referred to as the front end capacitance), and symbol C.sub.e indicates the capacitance between each inner conductor 3 and the nearer one thereto of the signal input or output electrode 9a or 9b (hereinafter referred to as the external capacitance). The front end capacitance C.sub.s can be adjusted by varying the length t to thereby control the resonance frequencies of the dielectric resonators as well as the coupling therebetween.

As shown in FIG. 22, which is a circuit diagram of an equivalent circuit of the dielectric resonator apparatus shown in FIGS. 20 and 21, this dielectric resonator apparatus may be regarded as being a structure with dielectric coaxial resonators R1 and R2 respectively formed around the throughholes 5a and 5b and connected in a so-called comb-line manner. The front end capacitance C.sub.s is deemed to be inserted between each of the dielectric coaxial resonators R1 and R2 and the grounded outer conductor 4, and the external capacitance C.sub.e is deemed to be inserted between one of the dielectric coaxial resonators R1 and the nearer one of the signal input or output electrodes 9a and between the other of the dielectric coaxial resonators R2 and the other of the signal input or output electrodes 9b.

Dielectric resonator apparatus which are structured as shown in FIG. 20 with a plurality of dielectric coaxial resonators combined together inside a single dielectric block, are advantageous in that they are generally smaller and lighter than prior art apparatus with individually discrete dielectric coaxial resonators combined together.

The pass band of a dielectric resonator apparatus shown in FIGS. 20 and 21 is determined mainly by the magnitude of the front end capacitance C.sub.s and the pitch S between the two throughholes 5a and 5b. In order to vary the pass band, therefore, one has only to change the magnitude of the front end capacitance C.sub.s and/or the pitch S. It is not possible, however, to change the magnitude of the front end capacitance C.sub.s beyond a certain limit because of physical restrictions. If the pitch S is changed, on the other hand, the external dimensions of the dielectric block 1 are affected, and the size of the dielectric blocks 1 cannot easily be standardized. In other words, dielectric blocks of different sizes will have to be prepared for different frequency characteristics, and this will adversely affect the production cost. If the pitch S is reduced in order to obtain broad band characteristics, the distance between the signal input or output electrode 9a and the throughhole 5a and that between the other signal input or output electrode 9b and the other throughhole 5b become smaller accordingly. As a result, unwanted parasitic capacitance C.sub.x (as shown in FIGS. 21 and 22) becomes too large and the symmetry of the waveforms between the dielectric coaxial resonance elements R1 and R2 becomes poor, adversely affecting the attenuation characteristic of the dielectric resonator apparatus as a whole. If the pitch S is increased in order to obtain narrow-range characteristics, on the other hand, the size of the dielectric block 1 increases, and this defeats the purpose of providing more compact resonator apparatus.

It is therefore an object of this invention to provide small and light dielectric resonator apparatus with good performance characteristics.

A dielectric resonator apparatus with a plurality of unistructurally formed dielectric coaxial resonators according to a first embodiment of the invention may be characterized as comprising not only a dielectric block with a plurality of throughholes formed therethrough and an outer conductor almost entirely covering the outer surfaces of this block, but also inner conductors formed on the inner surfaces of the throughholes so as to leave portions of the inner surfaces of the throughholes uncovered such that an end portion of each inner conductor is maintained in an electrically non-contacting relationship with the outer conductor, and means disposed between the throughholes for adjusting the degree of coupling between dielectric coaxial resonators. With a dielectric resonator apparatus thus structured, its characteristics can be adjusted by varying the degree of coupling between the dielectric coaxial resonators. The coupling-adjusting means may comprise a groove formed on the dielectric block such that the degree of coupling between the dielectric coaxial resonators can be controlled by varying the depth, width, length and/or position of the groove. If the coupling-adjusting means comprises a hole formed through the dielectric block and a conductive film formed on its inner surface and connected to the outer conductor, the degree of coupling can be controlled by varying the diameter, length and/or position of the hole. If the coupling-adjusting means comprises a slit formed on the dielectric block and having its surface covered with the outer conductor, the degree of coupling can be controlled by varying the width, length and/or position of the slit. If the coupling-adjusting means comprises an electrically conductive plate embedded in the dielectric block and electrically connected to the outer conductor, the degree of coupling can be controlled by varying the thickness, area and/or position of the conductive plate.

The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings:

FIG. 1A is a diagonal external view of a dielectric resonator apparatus to a first embodiment of the invention, and FIG. 1B is its sectional view taken along the line 1B--1B in FIG. 1A;

FIG. 2 is an equivalent circuit diagram of the embodiment shown in FIGS. 1A and 1B;

FIG. 3A is a diagonal external view of a dielectric resonator apparatus according to a second embodiment of the invention, and FIG. 3B is its sectional view taken along the line 3B--3B in FIG. 3A;

FIG. 4A is a diagonal external view of a dielectric resonator apparatus according to a third embodiment of the invention, and FIG. 4B is its sectional view taken along the line 4B--4B in FIG. 4A;

FIG. 5A is a diagonal external view of a dielectric resonator apparatus according to a fourth embodiment of the invention, and FIG. 5B is its sectional view taken along the line 5B--5B in FIG. 5A;

FIG. 6A is a front view of a dielectric resonator apparatus according to a fifth embodiment of the invention, and FIG. 6B is its sectional view taken along the line 6B--6B in FIG. 6A;

FIG. 7A is a front view of a dielectric resonator apparatus according to a sixth embodiment of the invention, and FIG. 7B is its sectional view taken along the line 7B--7B in FIG. 7A;

FIG. 8A is a front view of a dielectric resonator apparatus according to a seventh embodiment of the invention, and FIG. 8B is its sectional view taken along the line 8B--8B in FIG. 8A;

FIG. 9A is a front view of a dielectric resonator apparatus according to an eighth embodiment of the invention, and FIG. 9B is its sectional view taken along the line 9B--9B in FIG. 9A;

FIG. 10A is a front view of a dielectric resonator apparatus according to a ninth embodiment of the invention, and FIG. 10B is its sectional view taken along the line 10B--10B in FIG. 8A;

FIG. 11A is a front view of a dielectric resonator apparatus according to a tenth embodiment of the invention, FIG. 11B is its vertical sectional view taken along the line 11B--11B in FIG. 11A, and FIG. 11C is its horizontal sectional view taken along the line 11C--11C in FIG. 11A;

FIGS. 12A, 12B, 12C and 12D are sectional views of dielectric resonator apparatus according to variations on the tenth embodiment of the invention shown in FIGS. 11A, 11B and 11C;

FIG. 13A is a front view of a dielectric resonator apparatus according to an eleventh embodiment of the invention, and FIG. 13B is its sectional view taken along the line 13B--13B in FIG. 13A;

FIG. 14 is a diagonal perspective view of a dielectric resonator apparatus according to a twelfth embodiment of the invention;

FIG. 15 is a diagonal perspective view of a dielectric resonator apparatus according to a thirteenth embodiment of the invention;

FIG. 16 is a diagonal perspective view of a dielectric resonator apparatus according to a fourteenth embodiment of the invention;

FIG. 17 is a diagonal perspective view of an earlier considered dielectric resonator apparatus before its production is completed;

FIG. 18 is a sectional view of the dielectric resonator apparatus of FIG. 17 before its production is completed;

FIG. 19 is a sectional view of the dielectric resonator apparatus of FIGS. 17 and 18 after its completion;

FIG. 20 is a diagonal perspective view of the dielectric resonator apparatus of FIG. 19 after its completion;

FIG. 21 is an exploded diagonal perspective view of the dielectric resonator apparatus of FIGS. 19 and 20; and

FIG. 22 is an equivalent circuit diagram of the dielectric resonator apparatus of FIGS. 19, 20 and 21.

Throughout the figures, components which are substantially the same are indicated by the same numerals.

As shown in FIG. 1A, a dielectric resonator apparatus according to a first embodiment of the invention comprises a dielectric block 1 which is approximately a parallelepiped in shape and has a plurality of throughholes (only a mutually adjacent pair of throughholes 5a and 5b being shown) formed therethrough, connecting its open end surface 1a with its short-circuited end surface 1b. The outer surfaces of the dielectric block 1 is nearly completely covered by an outer conductor 4. Also formed on the outer surfaces of the dielectric block 1 is a pair of signal input or output electrodes 9a and 9b which are in electrically insulated relationship with the outer conductor 4. As shown in FIG. 1B, an inner conductor 3 is formed on the inner surfaces of the throughholes 5a and 5b, leaving uncovered annular areas .beta. which, unlike the uncovered part .alpha. shown in FIGS. 19 and 21, are not directly adjacent to the opening of the throughhole 5a or 5b to the open end surface 1a but are somewhat recessed therefrom. In other words, portions of the inner conductor 3 are left inside the throughholes 5a and 5b as front end conductors 30 connected to the part of the outer conductor 4 on the open end surface 1a. This is substantially equivalent to having parts of the outer conductor 4 folded into the throughholes 5a and 5b, and this structure is advantageous in that the leak of electromagnetic fields from the interiors of the throughholes 5a and 5b can be thereby prevented nearly completely. As indicated also in FIG. 1(B), front end capacitance C.sub.s appears between the inner conductors 3 and the front end conductors 30. Generation of external capacitance C.sub.e between the inner conductors 3 and the signal input or output electrodes 9a and 9b is indicated in the circuit diagram of its equivalent circuit shown in FIG. 2.

The dielectric resonator apparatus shown in FIGS. 1A and lB is further characterized as having grooves 11a and 11b formed on the dielectric block 1, serving as coupling adjusting means. One of the grooves 11a is shown as being formed on one of the main outer surfaces of the dielectric block 1 and between the two throughholes 5a and 5b. The other groove 11b is formed on the other main outer surface of the dielectric block 1 and also between the two throughholes 5a and 5b. Both grooves 11a and 11b extend parallel to the throughholes 5a and 5b, starting at the open end surface 1a and ending at a position approximately half way between the open end surface 1a and the short-circuited end surface 1b. The inner surfaces of the grooves 11a and 11b are both covered by the outer conductor 4. Although FIGS. 1A and 1B show the grooves 11a and 11b as having a semi-circular cross-sectional shape, the cross-sections of the grooves 11a and 11b may be U-shaped, rectangular or triangular.

The dielectric resonator apparatus of FIG. 1 is structured, as shown in FIG. 2 which is its equivalent circuit diagram, as a so-called comb-line coupling of the dielectric coaxial resonators R1 and R2 respectively formed around the throughholes 5a and 5b. Because of the grooves 11a and 11b provided between the two dielectric coaxial resonators R1 and R2 affect the impedance in the longitudinal direction, each of the dielectric coaxial resonators R1 and R2 may be considered as each having a first resonance section r1 near which the grooves 11a and 11b are provided and a second resonance section r2 which is farther away from the grooves 11a and 11b (as also shown schematically in FIG. 1B). If the values of characteristic impedance of the first resonance section r1 in even and odd modes are respectively Ze.sub.1 and Zo.sub.1, and if those of the second resonance section r2 in even and odd modes are respectively Ze.sub.2 and Zo.sub.2, Ze.sub.1 /Ze.sub.2 is smaller than Zo.sub.1 /Zo.sub.2 for the dielectric resonator apparatus shown in FIG. 1. As a result, inductive coupling becomes stronger and the pass band becomes broader. Since the degree of inductive coupling between the dielectric coaxial resonators R1 and R2 varies as the lengths, widths, positions and cross-sectional shapes of the grooves 11a and 11b are changed, the pass band characteristics can be adjusted without changing the pitch between the throughholes 5a and 5b, that is, without changing the external dimensions of the dielectric block 1. In other words, the dielectric block 1 can be standardized and made compact. Moreover, since the grooves 11a and 11b provide air-filled gaps between the dielectric coaxial resonators R1 and R2, the parasitic capacitance C.sub.e as shown in FIG. 22 can be reduced. Accordingly, the asymmetry in the attenuation characteristics between the dielectric coaxial resonators R1 and R2 can be improved and, as a further result, the attenuation characteristics of the dielectric resonator apparatus as a whole can be improved. FIGS. 3A and 3B show a dielectric resonator apparatus according to a second embodiment of the invention characterized as having a bottomed hole 12 (that is, not a throughhole) formed between the throughholes 5a and 5b inside the dielectric block 1, instead of the grooves 11a and 11b of FIGS. 1A and 1B, serving as coupling adjusting means. This bottomed hole 12 extends parallel to the throughholes 5a and 5b, starting from the open end surface 1a of the dielectric block 1 and ending at the bottom which is approximately half way between the open end surface 1a and the short-circuited end surface 1b. A conductive film connected to the outer conductor 4 is formed on the inner surface of the bottomed hole 12. In other aspects, the embodiment shown in FIGS. 3A and 3B is the same as that shown in FIGS. 1A and 1B, and corresponding parts are indicated by the same numerals. The cross-sectional shape of the bottomed hole 12 is not limited to be circular but may be different, such as polygonal or elliptical.

The bottomed hole 12 of the embodiment in FIGS. 3A and 3B, like the grooves 11a and 11b shown in FIGS. 1A and 1B, serves to make the inductive coupling stronger between the two dielectric coaxial resonators R1 and R2, that is, to broaden the pass band of the dielectric resonator apparatus of FIGS. 3A and 3B. Since the degree of this inductive coupling varies according to the length, diameter, position, etc. of the bottomed hole 12, it is possible to adjust the band pass characteristics without changing the external dimensions of the dielectric block 1.

FIGS. 4A and 4B show a dielectric resonator apparatus according to a third embodiment of the invention characterized as having a slit 13 formed between the throughholes 5a and 5b in the dielectric block 1, serving as coupling adjusting means. This slit 13 extends parallel to the throughholes 5a and 5b, starting from the open end surface 1a of the dielectric block 1 and ending at a position approximately half way between the open end surface 1a and the short-circuited end surface 1b. The inner surface of the slit 13 is covered by the outer conductor 4. In other aspects, the embodiment shown in FIGS. 4A and 4B is the same as that shown in FIGS. 1A and 1B, and corresponding parts are indicated by the same numerals.

The slit 13 of the embodiment in FIGS. 4A and 4B serves to make the inductive coupling stronger between the two dielectric coaxial resonators R1 and R2, that is, to broaden the pass band of the dielectric resonator apparatus of FIGS. 4A and 4B. Since the degree of this inductive coupling varies according to the length, width, position, etc. of the slit 13, it is possible to adjust the band pass characteristics without changing the external dimensions of the dielectric block 1.

FIGS. 5A and 5B show a dielectric resonator apparatus according to a fourth embodiment of the invention characterized as having a conductive plate 14 buried between the throughholes 5a and 5b in the dielectric block 1, serving as coupling adjusting means. This conductive plate 14 extends parallel to the throughholes 5a and 5b, starting from the open end surface 1a of the dielectric block 1 and ending at a position approximately half way between the open end surface 1a and the short-circuited end surface 1b. Such a dielectric resonator apparatus can be obtained, for example, by inserting the conductive plate 14 between two divided dielectric blocks before they are sintered, compressing these dielectric blocks together to make a single body and sintering this unified block and the conductive plate 14 together at the same time. In other aspects, the embodiment shown in FIGS. 5A and 5B is the same as that shown in FIGS. 1A and 1B, and corresponding parts are indicated by the same numerals.

The conductive plate 14 the embodiment in FIGS. 5A and 5B serves to make the inductive coupling stronger between the two dielectric coaxial resonators R1 and R2, that is, to broaden the pass band of the dielectric resonator apparatus of FIGS. 5A and 5B. Since the degree of this inductive coupling varies according to the area, thickness, position, etc. of the conductive plate 14, it is possible to adjust the band pass characteristics without changing the external dimensions of the dielectric block 1.

FIGS. 6A and 6B show a dielectric resonator apparatus according to a fifth embodiment of the invention characterized as having grooves 110a and 110b formed on the two mutually opposite major surfaces 1c and 1d of the dielectric block 1, serving as coupling adjusting means. These grooves 110a and 110b, unlike the grooves 11a and 11b of the first embodiment of the invention shown in FIGS. 1A and 1B, are formed adjacent to the short-circuited end surface 1b of the dielectric block 1. In other words, the grooves 110a and 110b both start from the short-circuited end surface 1b and end somewhere about half way between the open end surface 1a and the short-circuited end surface 1b. The inner surfaces of the grooves 110a and 110b are covered with the outer conductor 4. In other aspects, the embodiment shown in FIGS. 6A and 6B is the same as that shown in FIGS. 1A and 1B, and corresponding parts are indicated by the same numerals.

Because the grooves 110a and 110b are on the side of the short-circuited end surface 1b, Ze.sub.1 /Ze.sub.2 is greater than Zo.sub.1 /Zo.sub.2 in the case of the dielectric resonator apparatus shown in FIGS. 6A and 6B, where Ze.sub.1, Ze.sub.2, Zo.sub.1 and Zo.sub.2 are as defined above. As a result, the inductive coupling between the dielectric coaxial resonators R1 and R2 is weakened and the pass band becomes narrower. Since the degree of this inductive coupling varies according to the lengths, widths, depths, cross-sectional shapes, etc. of the grooves 110a and 110b, it is possible to adjust the band pass characteristics without changing the external dimensions of the dielectric block 1.

FIGS. 7A and 7B show a dielectric resonator apparatus according to a sixth embodiment of the invention characterized as having a bottomed hole 120 formed between the throughholes 5a and 5b inside the dielectric block 1, serving as coupling adjusting means. This bottomed hole 120, unlike the bottomed hole 12 shown in FIGS. 3A and 3B, is formed on the side of the short-circuited end surface 1b of the dielectric block 1. A conductive film connected to the outer conductor 4 is formed on the inner surface of the bottomed hole 120. In other aspects, the embodiment shown in FIGS. 7A and 7B is the same as that shown in FIGS. 1A and 1B, and corresponding parts are indicated by the same numerals.

Because the bottomed hole 120 is formed from the short-circuited end surface 1b, it has the same effect as the grooves 110a and 110b shown in FIGS. 6A and 6B, causing the inductive coupling between the dielectric coaxial resonators R1 and R2 to become weaker. Accordingly, the pass band of the dielectric resonator apparatus shown in FIG. 7A and 7B becomes narrower. Since the degree of this inductive coupling varies according to the length, diameter, position, etc. of the bottomed hole 120, it is possible to adjust the band pass characteristics without changing the external dimensions of the dielectric block 1.

FIGS. 8A and 8B show a dielectric resonator apparatus according to a seventh embodiment of the invention characterized as having a step 51 inside each of the throughholes 5a and 5b of the embodiment shown in FIGS. 7A and 7B, the rest being the same as the sixth embodiment. With the steps 51 thus formed, the dielectric coaxial resonators R1 and R2 of the dielectric resonator apparatus shown in FIGS. 8A and 8B are capacitively coupled. Thus, its pass band is already narrow, and since the bottomed hole 120 serves to increase the degree of this capacitive coupling, it is possible to obtain a dielectric resonator part with an extremely narrow pass band.

FIGS. 9A and 9B show a dielectric resonator apparatus according to an eighth embodiment of the invention characterized as having a slit 130 formed between the throughholes 5a and 5b in the dielectric block 1. The slit 130, unlike the slit 13 of the third embodiment of the invention, is formed on the short-circuited end surface 1b of the dielectric block 1. The inner surfaces of the slit 130 are covered by the outer conductor 4. In other aspects, the embodiment shown in FIGS. 9A and 9B is the same as that shown in FIGS. 4A and 4B, and corresponding parts are indicated by the same numerals.

Because the slit 130 is formed on the short-circuited end surface 1b, it has the effect of weakening the inductive coupling between the dielectric coaxial resonators R1 and R2. Accordingly, the pass band of the dielectric resonator apparatus shown in FIG. 9A and 9B becomes narrower. Since the degree of this inductive coupling varies according to the length, width, position, etc. of the slit 130, it is possible to adjust the band pass characteristics without changing the external dimensions of the dielectric block 1.

FIGS. 10A and 10B show a dielectric resonator apparatus according to a ninth embodiment of the invention characterized as having a conductive plate 140 buried between the throughholes 5a and 5b in the dielectric block 1. The conductive plate 140, unlike the conductive plate 14 of the fourth embodiment of the invention, is buried into the short-circuited end surface 1b of the dielectric block 1. In other aspects, the embodiment shown in FIGS. 10A and 10B is the same as that shown in FIGS. 5A and 5B, and corresponding parts are indicated by the same numerals.

Because the conductive plate 140 is buried into the short-circuited end surface 1b, it has the effect of weakening the inductive coupling between the dielectric coaxial resonators R1 and R2. Accordingly, the pass band of the dielectric resonator apparatus shown in FIG. 10A and 10B becomes narrower. Since the degree of this inductive coupling varies according to the area, thickness, position, etc. of this conductive plate 140, it is possible to adjust the band pass characteristics without changing the external dimensions of the dielectric block 1.

FIGS. 11A, 11B and 11C show a dielectric resonator apparatus according to a tenth embodiment of the invention characterized as having another throughhole 15 formed between and parallel to the throughholes 5a and 5b through the dielectric block 1. The inner surface of this additional throughhole 15 is covered with a conductive film connected to the outer conductor 4. In other aspects, the tenth embodiment is the same as the sixth embodiment shown in FIGS. 7A and 7B, and corresponding parts are indicated by the same numerals.

With the throughhole 15 thus formed additionally with its inner surface grounded, the self capacitance C.sub.ii of the dielectric coaxial resonators R1 and R2 increases while their mutual capacitance C.sub.ij decreases, the inductive coupling therebetween thereby becoming weaker. This is an equivalent result obtainable by reducing the pitch between the throughholes 5a and 5b, and the pass band characteristic of the dielectric resonator part becomes narrower. Since the degree to which the inductive coupling is weakened depends on the size and position of the throughhole 15, as well as the distance x (as indicated in FIG. 11B) between the central axis of the additional throughhole 15 and the plane defined by the central axes of the throughholes 5a and 5b. In other words, since the degree of the coupling can be varied by changing the position and size of the throughhole 15, it is possible to adjust the band pass characteristics without changing the external dimensions of the dielectric block 1.

Although FIGS. 11A, 11B and 11C show only one additional throughhole 15 between the pair of throughholes 5a and 5b, two additional throughholes 15a and 15b may be provided as shown in FIG. 12A as a variation of the tenth embodiment of the invention. As further variations, FIGS. 12B and 12C show examples with a rectangularly and diamond shaped cross-section 15c and 15d, respectively, in order to effectively weaken the degree of coupling. In the case of dielectric resonator apparatus with four or more coaxial resonance elements such as shown in FIG. 12D, the positions of the additional throughholes 15e, 15f and 15g may be each varied with respect to the throughholes 5a, 5b, 5c and 5d. In this manner, it is possible to produce compact dielectric resonator parts with coaxial resonators at equal intervals.

The tenth embodiment described above is an example of a comb-line resonator apparatus, but the present invention can be applied to interdigital resonator apparatus as shown in FIGS. 13A and 13B representing an eleventh embodiment of the invention characterized as having an additional throughhole 15 serving as coupling adjusting means between the throughholes 5a and 5b through the dielectric block 1. Inside one of the throughholes 5a is an uncovered part .beta. near but recessed from one of the end surfaces 1a not covered by the inner conductor 3. Inside the other of the throughholes 5b is similarly an uncovered part .beta. near but recessed from the other of the end surfaces 1b not covered by the inner conductor 3. In other aspects, the eleventh embodiment is the same as the tenth embodiment shown in FIGS. 11A, 11B and 11C, and corresponding parts are indicated by the same numerals.

FIG. 14 shows a dielectric resonator apparatus according to a twelfth embodiment of the invention which is similar to the earlier considered apparatus shown in FIGS. 20 and 21 in that an uncovered part .alpha. is formed in each of the throughholes 5a and 5b at its opening to one of the end surfaces 1a. The signal input or output electrodes 9a and 9b are formed entirely on one of the main surfaces 1c of the dielectric block 1, and a groove 11c is formed between and parallel to the throughholes 5a and 5b on this main surface 1c along its entire length between the open and short-circuited end surfaces 1a and 1b to serve as coupling adjusting means. The inner surface of this groove 11c is covered by the outer conductor 4. In other aspects, the twelfth embodiment is the same as the earlier considered apparatus shown in FIGS. 20 and 21.

Because of the groove 11c between the throughholes 5a and 5b according to the twelfth embodiment of the invention, the coupling between the dielectric coaxial resonators can be weakened and, as a result, a dielectric resonator apparatus with a wide pass band can be obtained. Since the degree of coupling varies according to the length, width, position, cross-sectional shape, etc., it is possible to adjust the band pass characteristics without changing the external dimensions of the dielectric block 1. In other words, dielectric blocks can be standardized and made compact.

Although FIG. 14 shows only one groove formed on only one of the main surfaces of the dielectric block 1, two grooves 11c and 11d may be formed, as shown in FIG. 15, one on each of the two main surfaces 1c and 1d of the dielectric block 1. Moreover, the groove to be formed as coupling adjusting means need not be of the same length as the throughholes 5a and 5b. FIG. 16 shows another variation of the embodiment shown in FIG. 14, with a groove 112 with about one-half length of the throughholes 5a and 5b.

Regarding all of the embodiments described above with reference to FIGS. 1-16, the degree of coupling may be adjusted by partially removing a part of the surface or inner conductive film and/or dielectric from any of the grooves 11a-11d, 110a, 110b and 112, the bottomed hole 12 and 120, the slits 13 and 130, and throughholes 15 and 15a-15g. Since such adjustment can be effected after the assembly process has been completed, the yield of the products can be significantly improved.

In summary, the present invention makes it possible to adjust the degree of coupling between dielectric coaxial resonators through adjusting means provided to the dielectric block such that the characteristics can be adjusted without changing the external dimensions of the dielectric block. As a result, dielectric blocks can be standardized and made compact, and the cost of the final product is reduced accordingly. Additionally, asymmetry of the attenuation characteristics can be corrected, and the characteristics can be improved according to the present invention.

Although the invention was described above with reference to a limited number of examples with reference to drawings and although most of the illustrations showed only two throughholes per resonator apparatus forming coaxial resonators, this is not intended to limit the scope of the invention. Dielectric resonator apparatus according to the present invention may include a larger plural number of throughholes 5a, 5b, 5c, . . . formed parallel to one another through a dielectric block with their internal surfaces covered with inner conductor (with an uncover part left thereon). Such dielectric resonator apparatus with a larger number of coaxial resonators unistructurally formed are not separately illustrated. It is then to be understood that the drawings herein shown with only two throughholes 5a and 5b illustrate only a portion of the entire dielectric resonator apparatus according to the present invention including only a mutually adjacent pair of coaxial resonators.

The description provided above is intended to be interpreted broadly as being merely illustrative. Any modifications and variations thereof which may be obvious to a person skilled in relevant arts are intended to be included within the scope of the invention.