Note: Descriptions are shown in the official language in which they were submitted.
CA 02375879 2001-12-17
Dielectric Resonator Configuration for Microwave-Multipole
Bandpass Filters
Mixed electromagnetic waves (HEM waves) exhibit a degeneration with respect to
the axis
of rotation (z-axis), i.e. two waves having identical natural resonance
frequency, in a
cylindrical dielectric resonator which is situated in a metal screen cavity.
The HEM wave
with the lowest resonance frequency is the HEM,IS wave. With aid of a coupling
screw
which is inserted into the screen, it is possible to couple the energy between
the two HEM,Is
waves and to cause the energy in the resonator to be split into a pair of
orthogonal waves. A
coupling of this type makes it possible to realize a two-pole filter with only
one resonator. In
conventional resonator configurations in which, for example, a cylindrical
dielectric
resonator is placed on a base plate, the HEM,IS wave exhibits a low unloaded Q-
factor (Qo
factor). Moreover, it is difficult to attain a sufficiently strong coupling
between the HEM"s
wave and the coaxial cable that leads to a measuring device.
A dielectric resonator having dielectric spacers is known finm WO-A1-9912225,
said
spacers spatially separating the dielectric resonator from the end plates of a
screen housing.
A resonance Q-factor can be improved thereby that the spacers are dielectric.
Further steps
for improving the resonsance quality or further features which are causally
responsible for
2 0 improving a resonance factor cannot be found in the aforementioned prior
art.
Therefore, it is the object of the present invention to create an arrangement
of a dielectric
resonator with which it is possible to attain a high unloaded Q-factor for a
HEM"s wave and,
at the same time, attain a decreased loss effect of the end plates of a screen
housing.
This object is solved with the characterizing features of claim 1.
With this resonator configuration, a very high unloaded Q-factor (Qo factor),
a strong
modulation of the HEM"s wave, a strong coupling between the two HEM,IS waves
and a
strong coupling between the HEM,IS wave and a coaxial cable or an antenna can
be attained
for the HEM"s wave. A negative and a positive cross coupling can also be
attained
therewith between the non-adjacent waves both along the direction of the z-
axis and in
direction of the xy-plane. With these cross couplings, it is possible to
obtain N = 2k (k = 2,
3, 4,...)-pole bandpass filters having quasi-elliptical properties.
CA 02375879 2001-12-17
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The construction and the functioning of the resonator configuration according
to the
invention will be described in greater detail in the following with reference
to an example of
an embodiment and the drawings, showing:
Fig. 1 a schematic side view of a resonator configuration according to the
invention,
wherein the dielectric resonator is separated from its two end plates by two
dielectric spacers;
Fig. 2a the electric field distribution in the meridian plane cp = 0°
for the HEM"s
wave;
Fig. 2b the magnetic field distribution in the meridian plane cp = 90°
for the HEMI,s
wave;
Fig. 3 a schematic view of the coupling of an HEM"s wave with a coaxial cable
and
a waveguide;
Fig. 4a a schematic side view of a quasi-elliptical four-pole filter with two
dielectric
2 0 resonators which are arranged along the main axis (z-axis);
Fig. 4b a top view onto the common plate with the aperture slits that are used
for the
coupling;
2 5 Fig. 4c a top view onto the upper resonator, wherein a dual-mode
modulating device,
a dual-mode coupler and an input antenna are inserted from the upper plate;
Fig. 4d a top view onto the lower resonator, wherein the dual-mode modulating
device, the dual-mode coupler and an output antenna are inserted from the
3 0 lower plate;
Fig. 5 a schematic top view onto a quasi-elliptical four-pole filter with two
dielectric
resonators which are arranged in the xy-plane;
CA 02375879 2001-12-17
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Fig. 6 a schematic top view onto a quasi-elliptical eight-pole filter with
four
dielectric resonators which are arranged in the xy-plane;
Fig. 7 a schematic side view of a dielectric resonator structure, wherein the
head of
a modulating element is situated within an aperture that has been drilled into
the dielectric body; and
Fig. 8 a schematic side view of a dielectric resonator structure having an
aperture
which was drilled by the spacers through the resonator in the central part
about their z-axis.
Fig. 1 shows a side view of an example of an embodiment of the resonator
configuration
according to the invention. A dielectric resonator 3 is thereby separated from
the upper end
plate 4 and the lower end plate 6 of a metal screen housing by two dielectric
spacers 1 and 2.
The dielectric resonator 3 has a 4k-times (k = 1, 2,..) rotational symmetry
about its z-axis,
for example, with horizontal cross sections of a circle, of a square and the
like. Some
modifications can be made to the resonator, for example, an aperture can be
drilled into the
resonator body. This is taken into consideration by the designation of a quasi
4k-times
2 0 rotational symmetry.
The shape of the spacers 1 and 2, i.e. the cross section in the xy-plane, is
flexible. However,
it is advantageous to select the same shape for the spacer as for the
dielectric resonator. The
simplest is to make the resonator and the spacers from a piece of a dielectric
material. The
2 5 shape of the housing wall 6 is also flexible. Usually, round walls or
square walls or a
combination of a round wall and a square wall can be used.
Fig. 2a shows the electric field distribution in the meridian plane cp =
0° for the HEM, Is wave.
It can be seen in this field distribution that the strongest electric field,
which is indicated by
3 0 thick arrows, is located in the free space between the dielectric
resonator and the upper and
lower end plates. This produces a sufficiently strong coupling and modulation
by using
electric sensors.
CA 02375879 2001-12-17
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Fig. 2b shows the magnetic field distribution in the meridian plane cp =
90° for the HEM"s
wave. The strongest magnetic field is found within the dielectric resonator in
this field
distribution. The magnetic field on the two end plates is relatively weak.
This field
distribution results in a very low loss for the two end plates. This gives the
resonator
housing a very good Q-factor. This is in contrast to a conventional resonator
configuration
in which, for example, the dielectric resonator is arranged directly on the
lower end plate. In
this case, a strong magnetic field is found on the lower end plate which
results therein that
the lower end plate causes a strong loss.
Fig. 3 is a schematic representation of a design according to the invention in
order to couple
the HEM~IS wave of a dielectric resonator 11 with a coaxial cable 12 and a
waveguide 13.
For the coupling with the coaxial cable, the coaxial cable is coupled by the
aperture that
opens on the end plate through a coupling antenna 14 which is connected with
the inner
guide of the coaxial cable. For the coupling with the waveguide, the lower end
plate 1 S is
arranged on the broad side 16 of the waveguide. An aperture 17 is opened in
the centre of
the broad side 16 by the waveguide through the lower end plate 15. A coupling
antenna 18
is used in order to couple the resonator with the waveguide. A dielectric ring
19 is used to
form an insulation between the coupling antenna and the wall of the waveguide.
A dielectric
screw 20 having a low dielectricity constant and a slight loss is connected
with the antenna
2 0 18. It can be adjusted from the underside of the waveguide.
To realize the bandpass filters with quasi-elliptical properties, positive and
negative cross
couplings are required between non-adjacent waves. The cross coupling can be
realized by
the resonator configuration according to the invention both along the
direction of the z-axis
2 5 and in the xy-plane. The negative or positive cross coupling is attained
by the positioning of
the direction of the wave polarization in the same or the opposite direction
during coupling.
This is obtained by a suitable arrangement of input and output antennas and by
dual-mode
couplers. Some examples for creating bandpass filters with quasi-elliptical
response are
noted in the following.
Fig. 4a is a schematic side view of a quasi-elliptical four-pole bandpass
filter which uses two
resonators 21a and 21b which are arranged along the direction of the z-axis.
Figs. 4b and 4c
are top views onto the upper or lower resonators 21a or 21b, respectively. An
input antenna
CA 02375879 2001-12-17
22a and an output antenna 22b are coupled with the upper or lower resonators,
respectively.
Dual-mode couplers 23a and 23b and dual-mode modulating devices 24a and 24b
are
inserted from the upper or lower plates for the upper or lower resonators,
respectively. Fig.
4d is the top view onto the common plate with the slits 25 and 26 provided for
the coupling.
5 Modulating elements 27 and 28 can be used to set the coupling coefficients
kz3 and k,4.
Since the directions of the wave polarization for the wave 1 (M 1 ) and the
wave 4 (M4) are
the same, then k,4 is negative.
Fig. 5 is the schematic top view onto a quasi-elliptical four-pole bandpass
filter having two
resonators 31a and 31b which are arranged in the xy-plane. In this
arrangement, the wave
polarization is arranged in a direction of 45° relative to the middle
line of the common side
wall which makes it possible to attain a cross coupling. The negative cross
coupling k,4 is
attained by the arrangement of the input antenna 32a and the output antenna
32b and the
dual-mode couplers 33a and 33b shown in Fig. 5.
Fig. 6 is the schematic top view onto a quasi-elliptical eight-pole bandpass
filter having four
resonators which are arranged in the xy-plane. Three cross couplings (k36,
k2., and k,8) are
established with this design. In these cross couplings, k36 and k2, are
negative and k,8 is
positive.
In a low-pressure gas, a self maintaining discharge can occur when a high-
frequency electric
field is applied, if the applied field is strong enough that it sends out
secondary electrons and
if the emitted secondary electrons jump back and forth between the emitted
electrodes in
resonance with the applied field. If the dielectric resonator is to be used
for high-capacity
2 5 applications in a vacuum or a low-pressure atmosphere, then self
maintaining discharges of
this type must be avoided. With the resonator configuration according to the
invention, a
self maintaining discharge of this type can be easily avoided. The principle
is shown in Fig.
7. The head of a modulating element or an antenna 42 is placed in an aperture
41 which was
drilled into the dielectric resonator body once the modulation has been
completed. The
3 0 electric field within the dielectric body is much less than that of the
two ends of the dielectric
resonator.
Another important point when using filters, in particular for mufti-channel
filters, is the wave
CA 02375879 2001-12-17
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separation. It is necessary to keep all undesirable resonance frequencies in
all channel filters
outside of the operating frequency band. To attain this, the undesirable
resonsance
frequencies must generally be pushed as far away as possible from the
operating resonance
frequency. This can also be attained with the resonator configuration
according to the
invention.
The adjacent waves of the HEM"s wave in the resonator configuration according
to the
invention are the TEo~s wave on the side of the lower frequency and the HEMIZS
wave on the
side of the higher frequency. It was found that the strengths of the electric
fields for the
TEo,s wave, HEM,IS wave and the HEM,zs wave in the centre of the resonator are
quite
different. The electric field is very weak for the TEo,s wave, not very strong
for the HEM"s
wave and very strong for the HEM,zs wave in the centre of the resonator. Thus,
removing a
cylinder stopper 51 from the centre of the resonator and the spacers, as shown
in Fig. 8,
changes the resonance frequency of these three waves in various ways. The
resonance
frequency of the TEols wave remains almost unchanged. A very slight increase
might occur.
On the other hand, with the HEM,IS wave a moderate increase occurs. And with
the HEM,zs
wave, there is a great increase.
