Dielectric filters and duplexers incorporating same

Coaxial resonators serving as input and output stages are arranged on a dielectric substrate having input and output coupling strip lines and a grounding electrode formed in the same plane as the strip lines and surrounding the strip lines. The substrate is shaped substantially in conformity with the bottom contour of the resonators as arranged on the substrate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to dielectric filters for use in mobile 
communication systems for the microwave band, and duplexers having such 
filters incorporated therein for use in radio devices. 
2. Description of the Related 
Conventional dielectric filters wherein coaxial resonators are used 
include, for example, the one disclosed in Examined Japanese Utility Model 
Publication No. 44566/1987 (FIG. 28). The disclosed dielectric filter 
comprises a plurality of quarter-wavelength coaxial resonators 1 each 
including a dielectric member 12 which has a through hole 4. The outer 
peripheral surface of the dielectric member and the inner peripheral 
surface thereof defining the through hole are covered with an electrically 
conductive material to provide an outer conductor 5 and an inner conductor 
6, respectively. The dielectric member has an open end face 1a where the 
outer conductor 5 and the inner conductor 6 each have an open end, and a 
short-circuit end face 1b where the other ends of the conductors are 
short-circuited. 
A connecting member comprising a dielectric bush 14 and a connecting bar 15 
is fitted in the through hole 4 of each coaxial resonator 1, and the other 
end of the bar 15 is joined to a coupling electrode 13 on a substrate 16, 
whereby the dielectric filter is capacitance-coupled to an external 
circuit. 
It has been required in recent years that mobile communication devices be 
made smaller in size and lightweight. To comply with this requirement, the 
dielectric filter, one of the components of these devices, also need to be 
made more compact. 
In providing compact dielectric filters, the ratio of the diameter of the 
inner conductor 6 to the outer conductor 5 must be 3.6 in order to obtain 
a high Qu value (no-loaded Q factor). If the diameter of the outer 
conductor 5 is up to 4 mm, the diameter of the inner conductor 6 is up to 
1.1 mm, whereas extreme difficulties are encountered in the prior art in 
inserting the connecting members 14, 15 into the through hole 4 of the 
coaxial resonator for connection to the external circuit 13. Thus, making 
the dielectric filter compact becomes difficult. 
In mobile communication devices, on the other hand, signals of different 
frequencies are separated according to the frequency or are combined 
together using duplexers. Such duplexers comprise a transmitting 
dielectric filter and a receiving dielectric filter which are different in 
center frequency. With the trend of mobile communication toward higher 
frequencies, the difference between the receiving band and the 
transmitting band in the center frequency becomes smaller, making it 
difficult for these dielectric filters to attain the desired attenuation 
outside the pass band. Accordingly, the characteristics of the dielectric 
filters for use in the duplexer must involve a local minimum of 
attenuation. 
The present applicant filed a patent application with the Patent Office of 
Japan for a dielectric filter which is free of the above problem and which 
has the construction shown in FIG. 29 (Japanese Patent Application No. 
46796/1991). A U.S. patent has been granted for the filter as U.S. Pat. 
No. 5,144,269. 
This dielectric filter comprises a plurality of coaxial resonators 1 
arranged side by side and each having a dielectric member 12 formed with a 
through hole 4. The outer and inner peripheral surfaces of the dielectric 
member 12 are covered with a conductive material to provide an outer 
conductor 5 and an inner conductor 6, respectively. The resonator has a 
short-circuit end face and an open end face, in the vicinity of which the 
outer conductor 6 is partially removed, along with a portion of the 
dielectric member when so desired, to form a recess 17. A dielectric 
substrate 19 provided with an external connection electrode 18 is attached 
to the recessed portion 17. 
At least three coaxial resonators are used in the filter to provide a local 
minimum of attenuation. More specifically, a capacitance is formed between 
the external connection electrode 18 and a capacitance-forming electrode 
provided on the dielectric substrate 19 to obtain frequency 
characteristics involving a local minimum in the attenuation region. 
With the filter described above, however, the resonator needs machining for 
forming the recessed portion 17 for attaching the dielectric substrate 
thereto and therefore can not be made compact without limitations. The 
characteristics of the filter also have the problem that sufficient 
suppression is not available outside the pass band. 
The dielectric filter has another problem in that the coaxial resonator can 
not be reduced in its overall length because the substrate 19 needs to be 
partly projected from the dielectric member 12 for coupling to an external 
circuit. 
SUMMARY OF THE INVENTION 
The present invention provides a dielectric filter of reduced size which 
comprises a substrate having approximately the same shape and area as the 
bottom contour of an arrangement of coaxial resonators and wherein the 
arrangement of resonators is mounted on the substrate in register 
therewith without permitting the substrate to project beyond the 
resonators. 
An object of the present invention is to provide a dielectric filter which 
is reduced in size and which has excellent characteristics involving a 
local minimum of attenuation to attain the desired attenuation outside the 
pass band. 
Another object of the invention is to provide a duplexer comprising such 
dielectric filters as transmitting and receiving filters. 
The present invention provides a dielectric filter which is characterized 
in that the filter comprises a dielectric substrate having input and 
output coupling strip lines on a surface thereof, and a plurality of 
coaxial resonators, each of the resonators comprising a dielectric member 
having a through hole, an outer peripheral surface and an inner peripheral 
surface, the outer and inner peripheral surfaces being covered with an 
electrically conductive material to provide an outer conductor and an 
inner conductor, respectively, the outer conductor being partially 
removed, the plurality of coaxial resonators including an input resonator 
and an output resonator arranged on the dielectric substrate with open end 
faces thereof oriented in directions opposite to each other, the 
dielectric substrate being shaped in conformity with the bottom contour of 
the plurality of coaxial resonators as arranged on the substrate. 
The present invention also provides a duplexer which comprises a receiving 
filter and a transmitting filter each comprising three coaxial resonators 
serving respectively as an input stage, an output stage and an 
intermediate stage, each of the resonators having the above construction, 
and a dielectric substrate provided with input and output coupling strip 
lines for transmitting therethrough inputs and outputs of the coaxial 
resonators and with a receiving matching circuit and a transmitting 
matching circuit for connecting the receiving filter and the transmitting 
filter to one antenna.

DETAILED DESCRIPTION OF THE INVENTION 
First Embodiment 
FIGS. 1 to 7 show a dielectric filter of a first embodiment of the present 
embodiment, which comprises two coaxial resonators 1, 2 and a dielectric 
substrate 3. 
Each of the two coaxial resonators 1, 2 is a quarter-wavelength coaxial 
resonator, which comprises, as shown in FIG. 2, a dielectric member 12 in 
the form of a prism and having a through hole 4. The outer and inner 
peripheral surfaces of the dielectric member 12 are covered with silver or 
like electrically conductive material to provide an outer conductor 5 and 
an inner conductor 6, respectively. One end face of the resonator is 
covered with the conductive material to provide a short-circuit end face 
1b where the outer and inner conductors 5, 6 are short-circuited. The 
dielectric member is left exposed at the other end face of the resonator 
to provide an open end face 1a. The two resonators 1, 2 are arranged 
side-by-side and joined together by soldering, with their open end faces 
oriented in directions opposite each other. The outer conductor is removed 
from the bottom of the assembly to form a bottom portion 7. 
Each of the coaxial resonators 1, 2 is adapted for use with frequencies in 
the range of several hundreds of MHz to 3 GHz. The resonator has a square 
cross section measuring slightly less than 2.0 mm in each side, is 0.7 mm 
in the diameter of the through hole 4 and has a length which is 4.2 mm 
when it is used for a frequency of 1.9 GHz. 
The two coaxial resonators 1, 2 are formed in the center of their adjacent 
surfaces with interstage coupling windows 8, 8', respectively, by removing 
the outer conductors 5 perpendicular to the direction of the through holes 
4. As shown in FIG. 3, the windows 8 and 8' have widths W and W', 
respectively, which are different from each other. Accordingly, even if 
the two resonators 1, 2 are joined together by soldering as slightly 
displaced from each other, the assembly has a definite effective 
interstage coupling width, and the degree of coupling will not vary from 
product to product. 
The coaxial resonators 1, 2 are placed on the dielectric substrate 3 with 
the bottom portion 7 down and with their open end faces 1a, 2a oriented in 
directions opposite to each other. 
FIG. 4 shows the dielectric substrate 3. FIG. 4, (A) shows the front 
surface of the substrate, FIG. 4, (B) the rear surface thereof, and FIG. 
4, (C) and (D) side faces thereof. 
The substrate 3 has approximately the same shape and area as the contour of 
the bottom portion 7 of assembly of the two resonators 1, 2 as arranged 
side-by-side as shown in FIG. 1. Input and output coupling strip lines 9, 
10 are formed on the front and rear surfaces of the dielectric substrate 
3, whereby the resonators 1, 2 are connected to an external circuit for 
input-output coupling. Indicated at 11 is a grounding electrode extending 
along the side end faces of the substrate 3 to electrically connect the 
front surface to the rear surface, and is provided around the input 
coupling strip line 9 and the output coupling strip line 10 to prevent 
electrical interference between the strip lines 9, 10. 
As shown in FIG. 1, the resonators 1, 2 have their open end faces 1a, 2a 
positioned on the input and output coupling strip lines 9, 10, 
respectively, and are affixed to the dielectric substrate 3 with an epoxy 
or like adhesive. The outer conductors 5 of the resonators 1, 2 are 
thereafter soldered to the grounding electrode 11 on the substrate 3. 
Thus, the dielectric filter is completed. 
The dielectric filter thus constructed is shown in FIG. 5 as an equivalent 
circuit. Capacitances C1, C2 are formed between the inner conductors 6 of 
the coaxial resonators 1, 2 and the respective input and output coupling 
strip lines 9, 10 for capacitance coupling. A capacitance C3 is formed by 
the inner conductors 6, 6 of the resonators 1, 2 owing to the presence of 
the interstage coupling windows 8, 8' for capacitance coupling, whereby a 
filter is provided. The filter characteristics of this embodiment are 
shown in FIG. 6. 
The dielectric substrate 3 has approximately the same area and contour as 
the bottom portion of assembly of the two coaxial resonators 1, 2 and does 
not have the portion 15 greatly projecting outward unlike the conventional 
filter (FIG. 28). When incorporating the present filter, communication 
devices can therefore be made smaller in size. 
FIG. 7 shows the higher-order pass band characteristics of the present 
embodiment. The solid line represents the dielectric filter of the 
invention, and the broken line the prior-art device (FIG. 28). According 
to the present invention, coaxial resonators serving as an input stage and 
an output stage are arranged on a dielectric substrate with their open end 
faces spaced apart as oriented in opposite directions. This eliminates 
matching at 3 fo and 5 fo owing to variations in higher-order frequency, 
affording improved ability to suppresss waves other than the dominant 
wave. 
Second Embodiment 
FIGS. 8 to 12 show a second embodiment of the invention which also 
comprises the coaxial resonators 1, 2 shown in FIG. 2. 
The dielectric substrate 3 to be used in the present embodiment is shown in 
FIG. 8. The front surface of the substrate for placing the resonators 1, 2 
thereon is shown in FIG. 8, (A), the rear surface thereof in FIG. 8, (B) 
and side faces thereof in FIG. 8, (C) and (D). The dielectric substrate 3 
has approximately the shape and area as the contour of the bottom portion 
of arrangement of the two coaxial resonators 1, 2. Indicated at 9, 10 are 
input and output coupling strip lines, through which the resonators 1, 2 
are connected to an external circuit for input-output coupling. Indicated 
at 11 is a grounding electrode provided on the resonator bearing surface 
of the substrate formed with the strip lines 9, 10 so as to surround these 
strip lines 9, 10. The electrode 11 is electrically connected to a 
grounding electrode formed approximately over the entire rear surface of 
the substrate, on the front and rear surfaces by means of through holes 23 
and through holes 24. The electrode 11 also serves to obviate electrical 
interference between the input coupling strip line 9 and the output 
coupling strip line 10. Each of the input and output coupling strip lines 
9, 10 is electrically connected to a corresponding one on the rear surface 
by a through hole 25. 
FIG. 9 shows the front surface of dielectric substrates 3, 3 in the course 
of preparation. These substrates 3, 3 are formed by printing input and 
output strip lines 9, 10 and grounding electrode 11 on each of opposite 
surfaces of a substrate blank 26 of dielectric material, printing a 
grounding electrode on the rear surface of the blank 26, forming through 
holes 23 and 24 for connection and through holes 25 for inputting and 
outputting in the blank 26, and cutting the blank along the broken lines 
shown in the drawing. 
FIG. 10 shows an electrically conductive cover 27, which is prepared by 
blanking out a piece of the illustrated configuration from a conductive 
member and thereafter bending the piece along the broken lines shown. The 
conductive cover 27 has a plurality of connecting end portions 28 
resembling comb teeth and formed at each of its opposite ends by blanking, 
and an interstage coupling degree adjusting window 29 and soldering 
windows 30 formed approximately at the center of the cover by punching. 
The pitch P of the connecting end portions 28 is equal to the pitch of the 
through holes 23 in the dielectric substrate. The end portions 28 have a 
width W" which is equal to or slightly smaller than the diameter of the 
through holes 23. Suitable for the material of the conductive member is 
copper or like material having high conductivity, while in view of the 
coefficient of expansion and strength, it is suitable to use a copper 
alloy. The material is most suitably a copper alloy comprising up to 0.2 
wt. % of Fe, up to 0.1 wt. % of P, up to 1.0 wt. % of Sn and the balance 
Cu. 
The second embodiment of the invention is assembled in the manner to be 
described below with reference to FIG. 11. The coaxial resonators 1, 2 are 
arranged with their open end faces 1a, 2a oriented in directions opposite 
to each other and with these faces 1a, 2a lapping over the respective 
input and output coupling strip lines 9, 10. The bottom portion 7 of the 
arrangement is affixed to the dielectric substrate 3 with an epoxy resin 
or like adhesive, with a cream solder applied to the outer conductors 5 of 
the two resonators 1, 2. 
The conductive cover 27 is then placed over the resonators 1, 2. At this 
time, the connecting end portions 28 fit into the through holes 23 in the 
substrate 3 and are temporarily fixed in position. The resulting assembly 
is then heated in a reflow oven and soldered with the cream solder 
applied. This procedure electrically connects the outer conductors 5 of 
the resonators 1, 2 to the cover 27 and the end portions 28 of the cover 
27 to the through holes 23 to connect the outer conductors 5 of the 
resonators 1, 2 to the grounding electrodes 11, 11 of the substrate 3. 
The interstage coupling window 8 is thereafter trimmed with a diamond bar 
or the like through the coupling degree adjusting window 29 formed in the 
cover 27 to thereby adjust the pass band characteristics. Finally, a seal 
member 31 is affixed to the cover 27 to assure the filter of reliability. 
If a material having an aluminum or like metal layer formed by vacuum 
evaporation is used as the seal member, a leakage magnetic field will 
penetrate through the seal member to entail an increased energy loss, 
which results in problems such as an increased insertion loss and 
variations in the center frequency. It is therefore desired to use resin, 
paper or like insulating material for the seal member 31. 
With the dielectric filter of the present embodiment, the outer conductors 
of the coaxial resonators serving as an input stage and an output stage 
are electrically connected to the grounding electrode via the comb-toothed 
connecting end portions of the cover and through holes in the dielectric 
substrate, so that the resonators can be connected to the substrate with 
good stability, assuring a filter of high performance free of variations 
in its characteristics and impairment of the characteristics due to 
variations in the grounding condition. 
Third and Fourth Embodiments 
FIGS. 13 and 14 show other dielectric filters of the invention, i.e., third 
and fourth embodiments, respectively. More specifically, each of these 
drawings shows the dielectric substrate 3 to be used in the embodiment. 
Throughout FIGS. 8, 13 and 14, like parts are designated by like reference 
numerals and will not be described again. In FIGS. 13 and 14, (A) shows 
the front surface for placing coaxial resonators 1, 2 on, (B) shows the 
rear surface, and (C) and (D) show side faces. 
With the third embodiment shown in FIG. 13, a grounding electrode 11 
provided so as to surround the input and output coupling strip lines 9, 10 
is electrically connected to a grounding electrode 11 formed approximately 
over the entire rear surface of the dielectric substrate 3 by means of a 
plurality of through holes 23 formed in each of right and left opposite 
side faces of the substrate 3 where the strip lines 9, 10 are not formed. 
With the fourth embodiment shown in FIG. 14, a grounding electrode 11 
provided around the input and output coupling strip lines 9, 10 is 
electrically connected to a grounding electrode 11 formed approximately 
over the entire rear surface of the dielectric substrate 3 by means of 
through holes 23 formed in the right and left opposite side faces of the 
substrate 3 having neither of the strip lines 9, 10 and also by means of a 
through hole 36 formed in one of the front and rear side faces having the 
input and output coupling strip lines 9, 10, respectively. 
Each of these dielectric substrates thus obtained is used to assemble a 
dielectric filter like the second embodiment shown in FIG. 11. 
FIG. 15 shows an equivalent circuit diagram of the dielectric filters thus 
constructed. Indicated at C4 and C5 are coupling capacitances formed 
between the inner conductors of the coaxial resonators 1, 2 and the 
respective input and output coupling strip lines 9, 10 on the dielectric 
substrate 3. Indicated at C6 is a coupling capacitance between the coaxial 
resonators provided by the interstage coupling window 8 formed in the 
resonators 1, 2, and at C7 is a coupling capacitance formed between the 
outer conductors 5 of the resonators 1, 2 and the grounding electrode 11 
on the rear surface of the substrate 3. The value of C7 is dependent on 
the strength of electrical coupling between the outer conductors 5 of the 
resonators 1, 2 and the rear grounding electrode 11 on the substrate 3, 
i.e., the presence or absence of the through holes 23 and 36, whereby the 
characteristics of the dielectric filter is made to have a local minimum 
of attenuation. 
FIG. 16 shows the filter characteristics of dielectric filters of the 
present invention. With reference to the drawing, A respresents the case 
wherein the dielectric substrate 3 of the third embodiment FIG. 13) is 
used, the substrate 3 having the through holes 23 only in the right and 
left opposite side faces thereof. B represents the case wherein the 
dielectric substrate 3 of the fourth embodiment (FIG. 14) is used, the 
substrate having, in addition to the through holes 23, the through hole 36 
formed in one of the front and rear side faces having the strip lines 9, 
10 respectively. C represents the characteristics of the first embodiment 
shown in FIG. 1 and the second embodiment shown in FIG. 12. Reliable 
electrical coupling is achieved between the outer conductors 5 of the 
coaxial resonators 1, 2 and the rear grounding electrode 11 on the 
substrate. 
As represented by A and B in FIG. 16, the position of local minimum in the 
attenuation region is controllable according to the strength of electrical 
coupling between the resonator outer conductors 5 and the rear grounding 
electrode 11 on the dielectric substrate 3, i.e., according to presence or 
absence of the through hole 36 in one of the front and rear side faces 
having the strip lines 9, 10. 
Fifth Embodiment 
FIG. 17 shows a fifth embodiment of the present invention which comprises 
three coaxial resonators, i.e., coaxial resonoators 1, 2 of input and 
output stages, and a coaxial resonators 37 of intermediate stage 
interposed between the resonators 1, 2. The resonators 1, 2 and 37 are 
arranged on a dielectric substrate 3. As shown in FIG. 18, the substrate 3 
has a grounding electrode or pattern 11 surrounding input and output 
coupling strip lines 9, 10 and electrically connected at side faces of the 
substrate to a grounding pattern 11 formed substantially over the entire 
rear surface of the substrate. Alternatively, these grounding patterns 11, 
11 may be connected together by means of through holes as in the second 
embodiment shown in FIG. 8. The input and output coupling strip lines 9, 
10 extend from one side of the dielectric substrate 3 toward one 
direction. Indicated at 42 is a resonator length correcting strip line for 
making the length of the resonator 37 of intermediate stage to that of the 
resonators 1, 2 of input and output stages. The coaxial resonators 1, 37 
and 2 are arranged as oriented alternately in opposite directions. The 
resonators are first fixed onto the substrate 3 with an adhesive, and the 
outer conductors are thereafter soldered to the grounding electrode 11 on 
the substrate 3 for electrical connection. 
FIG. 19 is an equivalent circuit diagram of the fifth embodiment. The inner 
conductors 6 of the resonators 1, 2 are capacitor-coupled to the 
respective input and output coupling strip lines 9, 10 by capacitances C8, 
C9, and the resonators 1, 37 and 2 are coupled to one another by 
capacitances C10 and C11 provided by interstage coupling windows 8. 
whereby a filter is constructed. FIG. 20 showing the filter 
characteristics of this embodiment reveals that the filter is more 
excellent in suppression in a low frequency range than those having two 
resonators. 
Sixth Embodiment 
FIG. 21 shows a sixth embodiment of the invention wherein three coaxial 
resonators are used. This embodiment differs from the fifth embodiment in 
that a dielectric substrate 3 has arranged thereon a coaxial resonator 1 
of input stage and a coaxial resonator 37 of intermediate stage which are 
oriented in the same direction, and a coaxial resonator 2 of output stage 
which is oriented in a direction different from the above direction. The 
substrate 3 is provided with input and output coupling strip lines 9, 10 
which are opposed to each other as seen in FIG. 22. 
FIG. 23 is an equivalent circuit diagram of the sixth embodiment. The inner 
conductors 6 of the coaxial resonators 1, 2 are capacitor-coupled to the 
respective input and output coupling strip lines 9, 10 by capacitances C8, 
C9, and the coaxial resonator 37 of intermediate stage is 
capacitor-coupled to the resonator 2 of output stage by a capacitance C11 
provided by an interstage coupling window 8. On the other hand, since the 
resonators 1, 37 of input and intermediate stages have the same 
orientation, the magnetic field distributions concerned are in phase with 
the result that magnetic field coupling predominates to couple the 
resonators 1, 37 by an inductance L1. FIG. 24 showing the filter 
characteristics of this embodiment reveals that the filter is more 
excellent in suppression in a high frequency range than those having two 
resonators. 
Seventh Embodiment 
FIG. 25 shows a seventh embodiment of the present invention, i.e., a 
duplexer which comprises the dielectric filter of fifth embodiment of FIG. 
17 as a transmitting (Tx) filter 46 and the dielectric filter of sixth 
embodiment of FIG. 21 as a receiving (Rx) filter 47. With this embodiment, 
a matching circuit 49 for connecting the transmitting (Tx) filter 46 and 
the receiving (Rx) filter 47 to a single antenna is formed by strip lines 
on a dielectric substrate 3, which has mounted thereon the coaxial 
resonators 1, 37 and 2 of input, intermediate and output stages to provide 
the duplexer. 
FIG. 26 shows the front surface of the substrate 3, which is formed with a 
pattern 51 for the transmitting (Tx) filter 46, pattern 51 for the 
receiving (Rx) filter 47, transmitting matching circuit 52 and receiving 
matching circuit 53. Each of the patterns 50, 51 for the respective 
filters is substantially the same as those of the fourth and fifth 
embodiments shown in FIGS. 18 and 22, and comprises input and output 
coupling strip lines 9, 10, a grounding electrode 11 formed around these 
lines and a resonator length correcting strip line 42. The grounding 
electrode 11 is electrically connected to a grounding electrode formed 
approximately over the entire rear surface of the substrate 3 by means of 
through holes 54. The transmitting matching circuit 52 and the receiving 
matching circuit 53 respectively comprise pattern capacitors 52a, 53a as 
capacitance means, and line inductors 52b, 53b as inductance devices. The 
line inductor 52b is connected to the grounding electrode on the rear 
surface via a through hole 52c. 
FIG. 27 is a schematic equivalent circuit of the duplexer. Indicated at 57 
and 55 are a receiver and a transmitter, respectively, and at 56 is the 
antenna. 
With the duplexer of the present embodiment, the pattern 50 for the 
transmitting (Tx) filter 46, the pattern 51 for the receiving (Rx) filter 
47 and the matching circuits 52, 53 can be formed in the single dielectric 
substrate 3. 
As described above, the dielectric filter of the present invention 
comprises coaxial resonators which are arranged with their open end faces 
spaced apart and oriented alternately in opposite directions, and are 
mounted on a dielectric substrate shaped approximately in conformity with 
the shape of the bottom of the arrangement. This serves to provide more 
compact products. The characteristic impedance of the resonator differs 
between the open-end side and the short-circuit side to produce a change 
in higher-order resonance component to give improved higher-order pass 
band characteristics. Since the grounding electrode is formed so as to 
surround the input and output stip lines, the dielectric filter realized 
has outstanding filter characteristics free from interference between the 
input and the output. 
The duplexer of the present invention comprises dielectric filters having 
the foregoing construction and serving as transmitting and receiving 
filters, and these filters can be provided on a single dielectric 
substrate along with matching circuits. The duplexer is therefore 
simplified in construction and easy to manufacture. 
The embodiments described above are intended to illustrate the present 
invention and should not be construed as limiting the present invention 
defined in the appended claims or reducing the scope thereof. The devices 
of the present invention are not limited to the foregoing embodiments in 
construction but can of course be modified variously within the technical 
scope defined in the claims.