Dielectric filter

A dielectric filter is provided which permits one to obtain desired external coupling easily without lowering Qo of resonators. The filter comprises a dielectric block having an open end surface and a shorted end surface and provided with resonator holes. Excitation holes are formed in the block outside the resonator holes, respectively. Input/output electrodes are formed on the open end-surface. The electrodes are electrically connected with conductors formed inside the excitation holes but isolated from an outer conductor. The conductors inside the excitation holes are electrically connected with the outer conductor on the shorted end surface. The excitation holes are electromagnetically coupled to the respective adjacent resonator holes, thus providing external coupling.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a dielectric filter for use in a mobile 
communication device such as a cellular telephone or other portable 
telephone. 
2. Description of the Related Art 
The structure of a prior art dielectric filter comprising a dielectric 
block is shown in FIG. 14. In the following figures, the shaded portions 
indicate visible portions of the dielectric material of the dielectric 
block. On these visible portions, no conductor is formed. 
As shown in FIG. 14, this dielectric filter has, for example, two resonator 
holes 2 extending between a pair of opposite end surfaces of the 
dielectric filter, indicated by reference numeral 1. Inner conductors 3 
are formed on the inner surfaces of the resonator holes 2. An outer 
conductor 4 is formed on the outer surface of the block 1. A pair of 
input/output electrodes 7 are formed at desired locations on the outer 
surface of the dielectric block. No inner conductors 3 are not formed at 
portions (hereinafter referred to as nonconductive portions) close to one 
end surface 1a (hereinafter referred to as the open end surface) of the 
openings of the resonator holes 2. These nonconductive portions are 
isolated from the outer conductor 4. At the opposite surface 1b 
(hereinafter referred to as the shorted end surface), the nonconductive 
portions are electrically connected or shorted to the outer conductor 4. 
This dielectric filter consists of two resonators stages each of which is 
formed in one of the resonator holes 2. These resonators are 
interconnected in a so-called comb-line connection (coupling) by stray 
capacitance created in the nonconductive portions. 
In this structure, an external coupling capacitance Ce is produced between 
each input/output electrode 7 and the corresponding inner conductor 3, as 
shown in FIG. 14. This external coupling capacitance Ce provides external 
coupling. 
When an antenna filter is constructed by using two such dielectric filters, 
a phase-adjusting circuit is inserted between one end of each filter, and 
an antenna terminal acting as the common input/output to and from both 
filters, so that the phase of reflected waves in the passband of one 
filter will cause the opposite filter to appear as an open circuit. A 
lumped constant device such as a capacitive device or an inductive device 
or a distributed constant line such as a cable or stripline is used as the 
phrase-adjusting circuit. 
In the above-described prior art filter which makes use of the external 
coupling capacitance Ce to obtain external coupling, if a wide passband or 
strong external coupling is needed, the area of the input/output 
electrodes may be increased. Alternatively, the resonator holes may be 
positioned in eccentric positions to shorten the distance between each 
input/output electrode and the corresponding inner conductor. In this way, 
adequate external coupling is derived. 
However, the foregoing technique requires use of input/output electrodes 
having a different shape or different dimensions, whenever a desired 
external coupling is to be obtained. This makes it difficult to 
standardize the input/output electrodes. 
Furthermore, when the area of the input/output electrodes is increased or 
the resonator holes are positioned in eccentric locations, the unloaded Q 
(or, Qo) of each resonator drops. In addition, an increase in the area of 
the input/output electrodes reduces the effective dielectric constant, 
thus increasing the resonator's electrical length. 
Moreover, when an antenna filter or the like is made, using the prior art 
dielectric filters as described above, phase-adjusting components such as 
capacitors, coils, or striplines are required in addition to the 
dielectric filters. Additionally, an operation for mounting and soldering 
them to a substrate or for forming them on a substrate is required. 
Consequently, it is difficult to miniaturize the antenna filters. Hence, 
the cost of the components or fabrication cost is increased. 
In particular, in the prior art dielectric filter, once the degree of 
external coupling at the input and output portions is determined, its 
phase is also determined. This makes it impossible to set external 
coupling and phase independently In consequence, it is difficult to obtain 
a desired external coupling and a desired phase simultaneously. Where a 
desired phase is associated with a connection to another filter or 
external circuit, it is necessary to add a separate part for adjusting the 
phase. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention is intended to solve the foregoing 
problems with the prior art techniques. It is an object of the invention 
to provide a dielectric filter permitting one to obtain appropriate 
external coupling easily without modifying the shape or dimensions of the 
input/output electrodes and without decreasing the Qo of the resonators. 
It is another object of the invention to provide a dielectric filter which 
permits one to set the phase at the input and output portions at a desired 
value without adding phase-adjusting parts, whereby the filter is made up 
of fewer components and can be made cheaper and smaller than heretofore. 
The above objects may be achieved by a first feature of the invention which 
lies in a dielectric filter comprising a dielectric block having two 
opposite end surfaces and an outer surface; resonator holes formed in the 
dielectric block between said end surfaces and acting as input/output 
stages; inner conductors formed on inner surfaces of the resonator holes, 
respectively; and an outer conductor formed on the outer surface of the 
dielectric block. This dielectric filter is characterized in that 
excitation holes are formed in the block adjacently to the resonator holes 
and have inner conductors formed inside the excitation holes, and that the 
excitation holes are electromagnetically coupled to the resonator holes 
acting as the input/output stages, respectively, thereby providing 
external coupling. 
With the first feature described above, the excitation holes are 
electromagnetically coupled to their respective resonator holes, whereby 
the filter provides external coupling. The degree of the external coupling 
is adjusted or set by varying the diameters or positions of the excitation 
holes. 
A second feature of the invention lies in a dielectric filter comprising a 
dielectric block having two opposite end surfaces and an outer surface, 
resonator holes formed in the dielectric block between said end surfaces 
and acting as input/output stages; inner conductors formed on inner 
surfaces of the resonator holes, respectively; and an outer conductor 
formed on the outer surface of the dielectric block. This dielectric 
filter is characterized in that excitation holes are formed in the block 
adjacently to the resonator holes acting as the input/output stages and 
have inner conductors formed inside the excitation holes, and that the 
positions, shapes, or sizes of the excitation holes have been so set that 
desired external coupling and phase are obtained. 
With the second feature described above, the excitation holes are 
electromagnetically coupled to their respective resonator holes, whereby 
the filter provides external coupling. Desired external coupling and phase 
can be established by varying the positions, the shapes, or the sizes of 
the excitation holes. 
A third feature of the invention lies in a dielectric filter comprising: a 
dielectric block having two opposite end surfaces and an outer surface; 
resonator holes formed in the dielectric block between said end surfaces; 
inner conductors formed on inner surfaces of the resonator holes, 
respectively; and an outer conductor formed on the outer surface of the 
dielectric block. This dielectric filter is characterized in that 
excitation holes are formed in the block adjacently to the resonator holes 
and have inner conductors formed inside the excitation holes, and that 
external coupling-adjusting holes are formed in the block close to the 
excitation holes, respectively, acting as input/output stages and have 
inner conductors formed on inner surfaces of the external 
coupling-adjusting holes, respectively. 
With the third feature described above, desired external coupling is 
provided by varying the positions, the shapes, or the sizes of the 
external coupling-adjusting holes. That is, the external coupling can be 
established with a greater degree of freedom because the external 
coupling-adjusting holes are provided. Where resonator holes are formed on 
opposite sides of each excitation hole, the coupling between two resonator 
holes on opposite sides of at least one excitation hole can be suppressed. 
A fourth feature of the invention is based on any one of the first through 
third features described above and characterized in that input/output 
electrodes are formed on one end surface of the dielectric block or extend 
from this end surface to one side surface of the dielectric block, are 
electrically connected with the inner conductors: formed inside 
the-excitation holes, and are not connected to the outer conductor. 
The fourth feature described above yields the above-described advantages. 
In addition, the filter can be connected with an external circuit, or a 
packaging substrate, through the input/output electrodes electrically 
connected with the conductors formed inside the excitation holes. These 
input/output electrodes are not intended to provide external coupling. 
Rather, the shapes and the dimensions of these electrodes can be set at 
will. That is, the shapes and the dimensions can be set in such a way that 
the characteristics such as Qo are not deteriorated. When the input/output 
electrodes are designed to extend from one end surface to one side 
surface, the end surface and/or the side surface can be used as a mounting 
surface. That is, the dielectric filter can be placed either horizontally 
or vertically. 
A fifth feature of the invention is based on any one of the first through 
third features described above and characterized in that the dielectric 
block has regions in which said excitation holes are formed, and that the 
regions have been partially removed so that one end surface of the 
dielectric block has steps. 
A sixth feature of the invention is based on any one of the first through 
third features described above and characterized in that the inner 
conductors formed inside the excitation holes or the inner conductors 
formed inside the external coupling-adjusting holes have been partially 
removed to adjust external coupling and phase. Thus, the external coupling 
and phase can be adjusted. 
With the fifth feature described above, the dielectric block has been 
partially removed, so that the length of the excitation holes is adjusted. 
The degree of external coupling can be varied by varying the length of the 
excitation holes, as well as the diameter or the positions of the 
excitation holes. Therefore, the external coupling can be adjusted or 
established with a greater degree of freedom Hence, more appropriate 
external coupling can be obtained. 
A seventh feature of the invention is based on any one of the first through 
third features described above and characterized in that there are further 
provided input/output terminals which are inserted in the excitation holes 
and electrically connected with the inner conductors formed inside the 
excitation holes. 
With the seventh feature described above, the filter can be connected with 
an external circuit, or a packaging substrate, via the input/output 
terminals electrically connected with the conductors formed inside the 
excitation holes. That is, the filter can be mounted on a terminal 
insertion type packaging substrate. The dielectric filter can be placed 
either horizontally or vertically by bending the input/output terminals. 
Furthermore, the location at which the connection with the packaging 
substrate is made can be set at will by varying the length of the 
input/output terminals. In this case, it is not necessary to form the 
input/output electrodes. Qo can be improved further. 
An eighth feature of the invention is based on any one of the first through 
third features described above and characterized in that there is further 
provided a metallic casing mounted on the dielectric block so as to cover 
at least a part of the block. 
With the eighth feature described above, leakage of electro-magnetic field 
from the openings of the resonator holes can be reduced by the presence of 
the metallic casing. 
Other objects and features of the invention will appear in the course of 
the description of embodiments thereof, which follows .

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Several preferred embodiments of the invention are hereinafter described 
with reference to the accompanying drawings, in which like components are 
indicated by like reference numerals. The structure of a dielectric filter 
that is a first example of the present invention is shown in FIG. 1. 
As shown in FIG. 1, this dielectric filter comprises a dielectric block 1 
taking the form of a substantially rectangular parallelepiped. Two 
resonator holes 2 and a pair of excitation holes 5 are formed in the block 
1. The resonator holes 2 extend between two opposite end surfaces of the 
block. Inner conductors 3 are formed on the inner surfaces of the 
resonator holes 2, respectively. An outer conductor 4 is formed 
substantially over the whole surface of the dielectric block 1. The 
excitation holes 5 are formed outside the resonator holes 2, respectively. 
A pair of input/output electrodes 7 extend from the open end surface 1a to 
one side surface 1c (the top surface in the figure). The electrodes 7 are 
electrically connected with inner conductors 3a in the excitation holes 5 
but disconnected from the outer conductor 4, That is, the inner conductors 
3a in the excitation holes 5 are disconnected from the outer conductor 4 
at the open end surface 1a and electrically connected with the outer 
conductor 4 at the shorted end surface 1b. 
Nonconductive portions are formed in the inner conductors 3 inside the 
resonator holes 2 near the open end surface 1a. At the shorted end surface 
1b, the inner conductor 3 are electrically connected or shorted to the 
outer conductor 4. Resonators formed by the resonator holes 2, 
respectively, are connected to each other in so-called comb-line 
connection by stray capacitance created in the nonconductive portions. 
In this structure, the excitation holes 5 and their respective adjacent 
resonator holes 2 are electromagnetically coupled together. This 
electromagnetic coupling provides external coupling of the input/output 
portions of the dielectric filter. The input/output electrodes 7 are 
formed simply to make a connection with an external circuit. 
The degree of the external coupling can be adjusted or established by 
varying the distance between the conductor 3a inside each excitation hole 
5 and the conductor 3 inside the adjacent resonator hole 2, which is 
accomplished by varying the inside diameter or the position of the 
excitation hole 5. That is, if the inside diameter of each excitation hole 
5 is increased, or if it is brought closer to the resonator hole 2, then 
the distance between the adjacent inner conductors is reduced. This 
provides stronger external coupling. 
In this structure, the external coupling is determined neither by the shape 
nor by the dimensions of the input/output electrodes 7. Therefore, the 
strength of the external coupling can be changed independently of the 
shape and the dimensions of the input/output electrodes 7. Hence, the 
input/output electrodes 7 can be standardized. This permits 
standardization of patterns on packaging substrates. As a results costs of 
mounting can be curtailed. 
Furthermore, the area of the input/output electrodes can be reduced and so 
the drop in Qo which would normally be caused by large input/output 
electrodes does not take place. Additionally, an increase in resonator 
electrical length which would normally be caused by a decrease in 
effective dielectric constant is prevented. Moreover, it is not necessary 
to place the resonator holes 2 in greatly eccentrically shifted positions. 
Consequently, the drop of Qo which would normally be caused by eccentric 
positioning of the resonator holes 2 can be suppressed. Hence, a 
small-sized dielectric filter which has high Qo, produces only a small 
amount of insertion loss, and provides desired external coupling can be 
obtained. 
Since the input/output electrodes 7 are so formed as to extend from the 
open end surface 1a to one side surface 1c, either the open end surface 1a 
or the side surface 1c can be mounted on a packaging substrate. That is, 
the dielectric filter of the present example can be placed either 
horizontally or vertically on the packaging substrate. 
The structure of a dielectric filter according to a second example of the 
invention is shown in FIG. 2. As shown in FIG. 2, this dielectric filter 
is similar to the dielectric filter already described in connection with 
FIG. 1 except that a pair of input/output electrodes 7 extend from the 
shorted end surface 1b to one side surface 1c (the top surface in the 
figure) and are electrically connected with the inner conductors 3a in the 
excitation holes 5, respectively, but are not connected to the outer 
conductor 4. That is, the conductors 3a inside the excitation holes 5 are 
electrically connected with the outer conductor 4 at the open end surface 
1a but isolated from the outer conductor 4 at the shorted end surface 1b. 
In this dielectric filter, the input/output electrodes 7 are formed on the 
side of the shorted end surface 1b in an opposite relation to the 
structure of the first example previously described in conjunction with 
FIG. 1. 
In this structure, the shorted end surface 1b is affected to a greater 
extent by a magnetic field than the open end surface 1a. Therefore, this 
second example can provide stronger external coupling, or stronger 
electromagnetic coupling, than the first example. Also in this example, 
the degree of external coupling can be adjusted or set by varying the 
diameter or positions of the excitation holes 5 without changing the 
positions or dimensions of the input/output electrodes or the positions of 
the resonator holes 2. This makes it easy to standardize the input/output 
electrodes 7. Also, the Qo is prevented from dropping. 
In the above-described examples, the inner conductors 3a in the excitation 
holes 5 are electrically connected with the outer conductor 4 at one end 
of each excitation hole 5. This structure can provide stronger external 
coupling, or stronger electromagnetic coupling, wherein a structure that 
the excitation holes 5 not electrically connected to the outer conductor 
4. 
In the above-described examples, the input/output electrodes 7 extend from 
one end surface of the dielectric block 1 to an adjacent side surface. The 
invention is not limited to this structure. As shown in FIG. 3, the 
electrodes 7 may be formed only on one end surface. As shown in FIG. 4, 
the electrodes may extend from the top side surface to the bottom side 
surface across one end surface. As shown in FIG. 5, each electrode 7 may 
extend from one end surface to two adjacent side surfaces which are 
perpendicular to each other. In the dielectric filter shown in FIG. 4, any 
one of the three surfaces on which the input/output electrodes 7 are 
formed may be used as a mounting surface and attached to a mounting 
substrate. 
In the examples in FIGS. 1-4 described above, the excitation holes 5 are 
formed substantially along the center plane passing through the center of 
the dielectric block 1 in the direction of the thickness. As shown in FIG. 
5, the excitation holes 5 may be shifted from the center plane toward the 
top or bottom side of the dielectric block 1. No restrictions are imposed 
on the vertical positions of the excitation holes 5 in the dielectric 
block 1. 
The structure of a dielectric filter (antenna duplexer) according to a 
third example of the invention is shown in FIG. 6. As shown in FIG. 6, 
five resonator holes 2a, 2b, 2c, 2d, and 2e extend between a pair of end 
surfaces of a dielectric block 1 An excitation hole 5a is located toward 
the outside the resonator hole 2a. Another excitation hole 5b is formed 
between the resonator holes 2b and 2c. A further excitation hole 5c is 
located toward the outside from the resonator hole 2e. Inner conductors 3 
are formed on the inner surfaces of the resonator holes 2a-2e and inner 
conductors 3a-3c are formed on the inner surfaces of the excitation holes 
5a, 5b, and 5c, respectively An outer conductor 4 is formed substantially 
over the whole outer surface of the dielectric block 1. Three input/output 
electrodes 7a, 7b, and 7c extend from the open end surface 1a to one side 
surface 1c and are electrically connected with the inner conductors 3 in 
the excitation holes 5a-5c but not connected to the outer conductor 4. 
The inner conductors 3a, 3b and 3c in the recitation holes 5a, 5b, and 5c 
are electrically connected with the outer conductor 4 at the shorted end 
surface 1b. The inner conductors 3 in the resonator holes 2a-2e are 
disconnected from the outer conductor 4 by nonconductive portions at the 
open end surface 1a. The inner conductors 3 are electrically connected 
with the outer conductor 4 at the shorted end surface 1b. 
In this structure, two resonators formed by the resonator holes 2a and 2b 
cooperate to form a transmission filter or reception filter. Three 
resonators formed by the resonator holes 2c, 2d, and 2e constitute a 
reception filter or transmission filter. 
The excitation holes 5a and 5c are electromagnetically coupled to the 
resonator holes 2a and 2e, respectively. The excitation hole 5b is 
electromagnetically coupled to the adjacent resonators 2b and 2c. These 
electromagnetic couplings provide external coupling. The input/output 
electrodes 7a, 7b, and 7c are formed simply for external connection with 
an external circuit. The input/output electrode 7b between the resonator 
holes 2b and 2c is an antenna electrode shared by the inputs and outputs 
of the transmission and reception filters. 
Also in this example, external coupling is provided by electromagnetic 
coupling between each excitation hole 5a, 5b, or 5c and the adjacent 
resonator hole 2a, 2b, 2c, or 2e. Therefore, the degree of external 
coupling can be adjusted or set by varying the diameters or positions of 
the excitation holes 5a, 5b, and 5c without changing the positions or 
dimensions of the input/output electrodes 7 or the positions of the 
resonator holes 2a-2e. Consequently, the input/output electrodes 7 can be 
standardized with ease. Also, Qo is prevented from decreasing. 
Moreover, in dielectric filters of the above-described various examples, 
the phase as well as external coupling can be set, by varying the 
positions, the shape, or the inside diameter of the excitation holes. That 
is, the phase can be varied while maintaining the external coupling 
constant. 
Experiments were conducted on the relations among the positions of the 
excitation holes, the shape, the external coupling, and the phase. The 
experiments and results are now described. FIGS. 7(a)-7(d) are schematic 
cross sections of dielectric filters, taken close to the location of one 
excitation hole. These figures illustrate a method of establishing the 
self-capacitance C11 of the excitation hole 5 formed between the conductor 
inside the excitation hole 5 and the outer conductor and the mutual 
capacitance C12 created between the excitation hole 5 and the conductor 
inside the resonator hole 2. 
In FIG. 7(a), the excitation hole 5 is shifted toward either the upper or 
lower side of the dielectric block. In this illustrated example, the hole 
is shifted toward the lower side, to increase the self-capacitance C11 and 
to reduce the mutual capacitance C12. In FIGS. 7(b) and 7(c), the 
excitation hole 5 assumes substantially an elliptical shape. The 
self-capacitance C11 and the mutual capacitance C12 can be set to various 
values by varying the longitudinal direction of the excitation hole 5. In 
FIG. 7(d), the inside diameter of the excitation hole 5 is increased to 
increase both self-capacitance C11 and mutual capacitance C12. In this 
way, the self-capacitance C11 and mutual capacitance C12 can be changed by 
varying the position, shape, or size of the excitation hole. 
The relations of these capacitances C11 and C12 to the external coupling 
and to the phase of the dielectric filter according to the second example 
are shown in FIG. 8. FIG. 8 shows results of measured reflection phases 
about this filter having a center frequency of 836.5 MHz in the passband 
of the opposite filter, the passband lying in the frequency range of 869 
to 894 MHz. In FIG. 8, the relation between the self-capacitance C11 of 
the excitation hole and the mutual capacitance C12 obtained where the 
external coupling is constant is indicated by triangles. Under this 
condition, the relation between the self-capacitance C.sub.11 and the 
reflection phase at 869 MHz is indicated by white circles. The relation 
between the self-capacitance C.sub.11 and the reflection phase at 894 MHz 
is indicated by black circles. 
As shown in FIG. 8, the external coupling can be maintained constant by 
varying the position, the shape, or other factor of the excitation hole so 
as to vary the self-capacitance C11 and the mutual capacitance C12. That 
is, the reflection phase can be reduced while maintaining the external 
coupling constant, by reducing both self-capacitance C11 and mutual 
capacitance C12. In other words, the reflection phase can be made to 
approach the open state. 
Therefore, where an antenna filter is built using such dielectric filters, 
if the positions, the shapes, or the sizes of the excitation holes in one 
filter corresponding to an antenna end are varied, then the reflection 
phase in the passband of the opposite filter can be made to assume an open 
state. Consequently, an antenna filter can be easily built without adding 
separate phase-adjusting components such as capacitive devices, inductive 
devices, or striplines. In particular, an antenna filter can be 
constructed simply by using two such dielectric filters or by using one 
such dielectric filter together with the prior art dielectric filter shown 
in FIG. 14 and then directly interconnecting respective input or output 
electrodes of the two filters. 
It is to be understood that application of the invention is not limited to 
antenna filters. Where a connection with an external circuit is made and 
it is necessary to vary the phase at the input/output portion, appropriate 
matching to the external circuit can be obtained similarly without adding 
separate phase-adjusting components. 
Each excitation hole can be shaped into any desired form. For example, the 
cross-sectional shape of the hole can be an ellipse, rectangle, triangle, 
or any other form. In the above examples, the dielectric filter is 
composed of two stages of resonators. The filter may also consist of only 
one stage of resonator. Furthermore, the filter may be made up of three or 
more stages of resonators. 
The structure of a dielectric filter (antenna duplexer) according to a 
fourth example of the invention is shown in FIGS. 9(a) and 9(b). FIG. 9(a) 
is a perspective view of the dielectric filter (antenna duplexer) as 
viewed from the side of the open end surface. The bottom surface 1c 
forming a mounting surface is shown to be located at the top. FIG. 9(b) is 
a plan view of the shorted end surface. The bottom surface 1c forming a 
mounting surface is shown to be located at the bottom. 
As shown in FIGS. 9(a) and 9(b), the dielectric filter (antenna duplexer) 
of the present example comprises a dielectric block 1 substantially in the 
form of a rectangular parallelepiped. This block has a pair of opposite 
end surfaces 1a and 1b (See FIG. 9a). Seven resonator holes 2a, 2b, 2c, 
2d, 2e, 2f, 2g extend between these end surfaces 1a and 1b. An excitation 
hole 5a and an external coupling-adjusting hole 6a are formed between the 
resonator holes 2a and 2b. An excitation hole 5b and an external 
coupling-adjusting hole 6b are formed between the resonator holes 2c and 
2d. An excitation hole 5c and an external coupling-adjusting hole 6c are 
formed between the resonator holes 2f and 2g. Conductors 3 are formed on 
the inner surfaces of the resonator holes 2a-2g, conductors 3a are formed 
on the inner surfaces of the excitation holes 5a, and conductors 3b are 
formed on the inner surfaces of the external coupling-adjusting holes 
6a-6c. An outer conductor 4 is formed substantially over the whole outer 
surface of the dielectric block 1. 
Three input/output electrodes 7a, 7b, and 7c extend from the shorted end 
surface 1b to one side surface, or the bottom surface. The input/output 
electrodes 7a, 7b, and 7c are electrically connected with the conductors 
3a (See FIG. 9a) inside the excitation holes 5a-5c but isolated from the 
outer conductor 4. That is, the conductors 3a inside the excitation holes 
5a-5c are electrically connected with the outer conductor 4 at the open 
end surface 1a and disconnected from the outer conductor 4 (See FIG. 9a) 
at the shorted end surface 1b. The conductors 3 (See FIG. 9a) inside the 
resonator holes 2a-2e are isolated from the outer conductor 4 by 
nonconductive portions formed in the inner conductors close to the open 
end surface 1a and are electrically connected with the outer conductor 4 
at the shorted end surface 1b. 
The external coupling-adjusting holes 6a, 6b, and 6c are formed close to 
the excitation holes 5a, 5b, and 5c, respectively. The array of the 
adjusting holes 6a-6c is parallel to the array of the excitation holes 
5a-5c. The conductors 3b (See FIG. 9a) formed inside the external 
coupling-adjusting holes 6a, 6b, and 6c are electrically connected with 
the outer conductor 4 at the open end surface 1a, as well as at the 
shorted end surface 1b. That is, the conductors 3b inside the adjusting 
holes 6a-6c act as grounding conductors similarly to the outer conductor 
4. 
In this structure, the excitation hole 5a is electromagnetically coupled to 
the adjacent resonator holes 2a and 2b. The excitation hole 5b is 
electromagnetically coupled to the adjacent resonator holes 2c and 2d. The 
excitation hole 5c is electromagnetically coupled to the adjacent 
resonator holes 2f and 2g. External coupling is provided by these 
electromagnetic couplings. The filter is connected with an external 
circuit via the input/output electrodes 7a, 7b, and 7c which are 
electrically connected with the conductors 3a inside the excitation holes 
5a-5c. The input/output electrode 7b is an antenna electrode acting as one 
input/output of a transmission filter and also as one input/output of a 
reception filter. 
In the antenna filter of the present example, the self-capacitance of each 
excitation hole can be increased and reduced by varying the location, 
shape, or inside diameter of the external coupling-adjusting hole formed 
close to the excitation hole. Therefore, the external coupling can be 
modified, and external coupling can be established more appropriately That 
is, the external coupling can be established with a greater degree of 
freedom by adding the external coupling-adjusting holes. 
The self-capacitance of each excitation hole is the capacitance created 
between the conductor inside the excitation hole and the grounding 
conductor, or the outer conductor plus the conductor inside the external 
coupling-adjusting hole. The self-capacitance of each excitation hole can 
be increased by providing the external coupling-adjusting hole. By 
reducing the distance between the excitation hole and the external 
coupling-adjusting hole, the self-capacitance of the excitation hole can 
be increased, and the external coupling can be weakened. Conversely, by 
increasing the distance between the excitation hole and the external 
coupling-adjusting hole, the self-capacitance of the excitation hole can 
be reduced and the external coupling can be intensified. 
Since the external coupling can be weakened by providing the external 
coupling-adjusting holes in this way, the distance between each excitation 
hole and the adjacent resonator hole can be reduced. Hence, the size of 
the filter can be reduced. That is, in the present example, the distance 
between the resonator holes 2a and 2b, the distance between the resonator 
holes 2c and 2d, and the distance between the resonator holes 2f and 2g 
can be reduced. 
Furthermore, the coupling between two resonator holes between which one 
excitation hole and one external coupling-adjusting hole are located can 
be suppressed by the external coupling-adjusting hole. In the present 
example, direct coupling between the resonator holes 2a and 2b, direct 
coupling between the resonator holes 2c and 2d, and direct coupling 
between the resonator holes 2f and 2g can be suppressed by the external 
coupling-adjusting holes 6a, 6b, and 6c, respectively. Specifically, 
direct coupling of the trap formed by the resonator hole 2a can be reduced 
greatly. Also, direct coupling of the filter formed by the resonator holes 
2b, 2c, the filter formed by the resonator holes 2d, 2e, 2f, and the trap 
formed by the resonator hole 2g can be reduced greatly. In consequence, 
the characteristics of the filters and traps can be adjusted readily. As a 
result, good characteristics can be obtained. 
Once a filter is constructed, the self-capacitance or other factor of each 
excitation hole can be varied by grinding parts of the conductors either 
in the excitation holes or in the external coupling-adjusting holes with a 
grinding tool or grindstone. In this manner, the external coupling and 
phase can be adjusted. Therefore, the characteristics can be improved. 
Also, the percentage of defective products can be reduced. In this case, 
the dielectric substance can be ground together with the inner conductors. 
In the above examples, one external coupling-adjusting hole is formed 
corresponding to each one excitation hole. The present invention is not 
limited to this structure. A plurality of external coupling-adjusting 
holes may be formed corresponding to each one excitation hole. The 
external coupling-adjusting holes may be shaped into any arbitrary form, 
which can be an ellipse, rectangle, triangle, or rhomboid. 
In the above examples, two filters and two traps are formed in one 
dielectric block. In this way, the dielectric filter or antenna resonator 
has a complicated structure. It is to be noted that the present invention 
is not restricted to this structure. The present invention is also 
applicable to a dielectric filter comprising a dielectric block 1 in which 
one filter is formed, as shown in FIG. 10. 
In the dielectric filter shown in FIG. 10, the dielectric block 1 is 
provided with two resonator holes 2. Excitation holes 5 and external 
coupling-adjusting holes 6 are formed outside their respective resonator 
holes 2. Also in this dielectric filter, the degree of external coupling 
can be varied by varying the position, shape, or inside diameter of each 
external coupling-adjusting hole. Furthermore, the external coupling and 
phase can be adjusted by grinding parts of conductors formed inside the 
excitation holes and inside the external coupling-adjusting holes. Also, 
the number of resonator holes formed in the dielectric block can be unity. 
In the above examples, every excitation hole has at least one corresponding 
external coupling-adjusting hole or holes. The invention is not limited to 
this structure. Each external coupling-adjusting hole may be formed 
corresponding to more than one excitation hole. 
The structure of a dielectric filter according to a fifth example of the 
invention is shown in FIG. 11. As shown in FIG. 11, this dielectric filter 
comprises a dielectric block 1 having an open end surface 1a and one side 
surface 1c. This block has recessed portions 11 in which excitation holes 
5 are formed on the side of the open end surface 1a. Thus, the open end 
surface 1a has a stepped shape. Each input/output electrode 7 extends from 
the corresponding recessed surface 11 to the side surface 1c. The 
excitation holes 5 extend from the recessed surfaces 11. The electrodes 7 
are electrically connected with conductors 3a formed inside the excitation 
holes 5, respectively, and not connected to an outer conductor 4. This 
dielectric filter is similar in structure to the dielectric filter already 
described in connection with FIG. 1 except for these points and so those 
components which have been already described are not described here. 
In this structure, the degree of coupling due to the electromagnetic 
coupling of each excitation hole 5 to the adjacent resonator hole 2 can be 
adjusted and set by varying the length of the excitation hole 5. That is, 
the degree of external coupling can be changed by varying the length of 
the excitation holes 5, as well as the diameter of the holes 5 and the 
positions of the holes 5. Hence, the external coupling can be adjusted and 
set with greater degree of freedom. As a result, more appropriate external 
coupling can be derived. 
In this example, steps are formed on the side of the open side surface la. 
The invention is not restricted to this structure. The steps may also be 
formed on the side of the shorted end surface 1b. Furthermore, steps may 
be formed on both end surfaces 1a and 1b. The other examples above of a 
dielectric filter or antenna filter may also be modified to have these 
recessed surfaces 11. 
The structure of a dielectric filter according to a sixth example of the 
invention is shown in FIG. 12. As shown in FIG. 12, this dielectric filter 
has an open end surface 1a on which input/output electrodes 7 are formed. 
The filter is provided with excitation holes 5, and conductors 3a are 
formed inside the holes 5, respectively Input/output terminals 20 which 
are electrically connected with the conductors 3a inside the holes 5 are 
brought out from the open end surface 1a. Each input/output terminal 20 is 
a rod-like member made of a metal. These terminals 20 are inserted into 
the excitation holes 5, respectively, and soldered to the conductors 3a, 
respectively, inside the excitation holes 5 or to the input/output 
electrodes 7, respectively, when the terminals 20 are mounted. This 
dielectric filter is similar in structure to the dielectric filter 
previously described in conjunction with FIG. 1 except for these points. 
That is, this dielectric filter is similar to the dielectric filter shown 
in FIG. 1 except that the input/output terminals 20 are connected. 
Where connection with an external circuit is made through the input/output 
terminals 20 as in this example, it is not always necessary to form the 
input/output electrodes 7. Where the input/output electrodes 7 are not 
formed, those portions of the outer conductor 4 which are on the end 
surface located on the side of the input/output terminals 20 or those 
portions of the conductors 3a inside the excitation holes 5 which are 
close to the end surface are partially removed to disconnect the 
input/output terminals 20 from the outer conductor 4. 
This structure can be mounted on a mounting substrate of the terminal 
insertion type. The dielectric filter can be placed either horizontally or 
vertically by bending the input/output terminals 20. Furthermore, the 
locations at which the filter is connected with the packaging substrate 
can be set at will by varying the length of the input/output terminals 20. 
Additionally, the input/output electrodes 7 can be made smaller. 
Alternatively, the characteristics such as Qo can be improved further 
without the need to form the input/output electrodes 7. 
In the above examples, in addition to the example of FIG. 12, input/output 
terminals 20 like those shown in FIG. 12 can be inserted into the 
excitation holes 5, respectively, from the end surface on which the 
input/output electrodes 7 are formed, and then the terminals 20 are 
connected. Moreover, restrictions are imposed neither on the shape of the 
input/output terminals 20 nor on the manner in which the terminals 20 are 
connected with the conductors 3 inside the excitation holes 5 For 
instance, each input/output terminal can be fabricated by rolling a sheet 
metal plate into a tube and pressing it against the conductors 3a inside 
the excitation holes 5 for connection. 
The structure of a dielectric filter according to a seventh example of the 
invention is shown in FIG. 13. As shown in FIG. 13, this dielectric filter 
has an open end surface la into which input/output terminals 20 are 
inserted. A metallic casing 30 is mounted on the dielectric block 1 so as 
to cover the open end surface 1a. The metallic casing 30 is soldered to 
the outer conductor 4, thus constructing the dielectric filter. Parts of 
the metallic casing 30 have apertures to permit the input/output terminals 
20 to be brought out and to prevent the casing 30 from touching the 
input/output electrodes 7. This dielectric filter is similar in structure 
to the filter shown in FIG. 12 except for these points. That is, this 
example of dielectric filter is similar to the sixth example of dielectric 
filter shown in FIG. 12 except that the metallic casing 30 is mounted on 
it. A substrate may be inserted between the open end surface 1a and the 
metallic casing 30. 
When this dielectric filter is mounted on a packaging substrate (not 
shown), input/output terminals 20 and protruding portions 30a of the 
metallic casing 30 are inserted into the packaging substrate. In this 
structure, the open end surface 1a is covered with the metallic casing 30 
and so leakage of electro-magnetic field through the opening of each 
resonator hole 2 can be reduced. This metallic casing 30 can also be 
mounted to other examples of dielectric filter. 
In the above examples described thus far, coupling between adjacent 
resonators is provided by stray capacitance created in nonconductive 
portions in the inner conductors. The invention is not limited to this 
structure. Coupling holes or other coupling means may also be used to 
couple together the adjacent resonators. Furthermore, the manner in which 
the conductors inside the resonator holes are disconnected from the outer 
conductor at the open end surface is not limited to the method of the 
illustrated examples. 
As described thus far, in a dielectric filter according to the present 
invention, the input/output portions are provided with excitation holes. 
External coupling is provided by electromagnetic coupling of each 
excitation hole to the adjacent resonator hole. The best external coupling 
can be obtained by appropriately establishing the inside diameter, 
positions, or the length of the excitation holes so as to adjust or 
establish the degree of external coupling. Furthermore, it is not 
necessary to make the resonator holes in eccentric positions in order to 
adjust the external coupling. Hence, the Qo is prevented from decreasing 
In another dielectric filter according to the invention, external 
coupling-adjusting holes are formed close to external coupling excitation 
holes. Desired external coupling and phase can be obtained by 
appropriately establishing the positions, shape, and dimensions of the 
external coupling-adjusting holes. In consequence, the external coupling 
and phase can be established with greater degree of freedom. The external 
coupling can be weakened by forming the external coupling-adjusting holes. 
Therefore, the distance between each excitation hole and the adjacent 
resonator hole can be reduced. This enables miniaturization of the filter. 
Furthermore, the coupling between two resonator holes which are adjacent 
to each other via an excitation hole can be suppressed by the external 
coupling-adjusting holes. Therefore, even where a plurality of filters are 
formed in one dielectric block, interference between the filters can be 
prevented. The characteristics of the filters can be adjusted easily. 
Hence, good characteristics can be obtained. After a filter has been 
constructed, external coupling and phase can be adjusted by grinding parts 
of conductors or dielectric substances inside excitation holes. Therefore, 
the characteristics can be improved. Also, the defective percentage can be 
reduced greatly. Hence, the fabrication cost can be reduced. Moreover, the 
input/output electrodes can be made smaller than previously. The resonator 
length can be shortened without deteriorating Qo. 
If the filter is connected with an external circuit by the use of 
input/output terminals, it is not necessary to form input/output 
electrodes. Furthermore, Qo is prevented from droppings The filter can be 
mounted on a mounting substrate of the terminal insertion types In 
addition, leakage of electro-magnetic field can be reduced by mounting a 
metallic casing. 
Thus, according to the present invention, a small-sized dielectric filter 
which can be easily mounted on a substrate can be variously mounted, has 
high Qo, and has optimum external coupling and phase can be obtained. 
Although several preferred embodiments and features of the invention have 
been disclosed herein, the claimed invention is not limited to those 
embodiments, but rather should be considered to include all modifications, 
variations and equivalents thereto that may occur to those having the 
ordinary level of skill in the pertinent art.