[0001] The invention relates to a dielectric resonator filter having, a particular, but
not necessarily an exclusive, application in a dual mode filter for communications
satellite payloads.
[0002] With communications satellite payloads, the main problem encountered in realising
practical dielectric resonator filters is in supporting the dielectric resonator in
a spatial central position within the cavity of the filter.
[0003] Ideally, the dielectric resonator should hang in free space but, in practice, it
is necessary to provide a support structure for the resonator within the cavity. The
support structures of known dielectric resonator filters degrade the unloaded electrical
quality factor of the resonator. This is due to additional losses induced in the fabric
of the support structure.
[0004] The resonator support structures used in dielectric resonator filters for communications
satellite payloads must be sufficiently rugged and stable to withstand both the high
levels of vibration experienced by space hardware during the launch phase of a mission
and also the long term effects of repeated thermal cycling experienced over the duration
of the mission.
[0005] In addition, the perturbation of the resonators electrical performance must be minimised
and, in particular, all additional electrical losses, given rise to by the resonator
support structure, must be minimised in order to achieve the extremely high levels
of unloaded Q required by narrow band filters.
[0006] It is an object of the present invention to overcome the foregoing problems by providing
a dielectric resonator filter having a support structure for the dielectric resonator
which has a high mechanical ruggedness, minimises thermally induced stress and has
low electrical loss.
[0007] The thermally induced stress is minimised by closely matching the thermal expansion
coefficients of all of the materials from which the filter and support structure are
fabricated.
[0008] The low electrical loss is due to low loss tangents, low dielectric filling factors
and the positioning of the support member within the cavity of the resonator so as
to avoid areas of high electric field concentration.
[0009] The invention provides a dielectric resonator filter including at least one microwave
resonator having a cylindrical conductive cavity symmetrically disposed about a longitudinal
axis and a cylindrical dielectric resonator element supported within the cavity characterised
in that the cavity (1) has an internally projecting flange (3), in that the support
for the resonator element (4) comprises a unitary dielectric support member (5) having
a coefficient of thermal expansion to match that of the resonator element (4) and
in that the dielectric resonator element (4) is secured to the said internally projecting
flange (3) and adapted to support the resonator element (4), at the peripheral surface
thereof, in a spacial central position within the cavity (1) whereby the longitudinal
axes of the cavity (1) and the resonator element (4) are coaxial.
[0010] The said internally projecting flange is preferably formed as an integral part of
the cavity and consists of four equi-spaced lugs to which the support member is secured,
the lugs being located at the low azimuthal field positions of the HEH
11δ mode of the dielectric resonator.
[0011] The support member can be releasably secured to the equi-spaced lugs by either silver
plated bolts, or silver soldering.
[0012] The support member can be secured to the resonator element either by ceramic bonding,
or by low loss adhesive.
[0013] According to one aspect of the present invention the support member is in the form
of a ceramic disc having a central aperture within which the dielectric resonator
is securely located at the periphery thereof, the shape of the aperture being such
that the resonator element is secured to the support member at its low azimuthal field
positions of the HEH
11δ mode. In a preferred arrangement the thickness of the ceramic disc is less than one
third of the length of the cylindrical dielectric resonator element.
[0014] The support member is preferably of a ceramic material, for example, a glass ceramic
material, having high stability and ruggedness, and low permitivity and loss tangent,
and be such that its electrical, mechanical and thermal expansion properties can be
optimised during fabrication.
[0015] According to another aspect of the present invention the coefficient of thermal expansion
of the conductive cavity is matched to that of the ceramic support member.
[0016] According to a further aspect of the present invention, the surface of the ceramic
support member at or near the periphery thereof is metallised, and the metallised
surface is secured to the internally projecting flange of the cavity. The securing
of the support member to the internally projecting flange can be effected by either
brasing or silver soldering, the joint formed thereby being of high ruggedness and
low loss.
[0017] According to a further feature of the present invention, the cylindrical conductive
cavity can be fabricated from either silver plated beryllium metal or silver plated
titanium, the dielectric resonator element having a coefficient of thermal expansion
to match that of the beryllium or the titanium, as the case may be.
[0018] According to a further feature of the present invention, the cylindrical conductive
cavity is of a nickel/iron alloy, the coefficient of thermal expansion of the cavity
being matched to that of the dielectric resonator by varying the nickel content of
the alloy.
[0019] The foregoing and other features according to the present invention will be better
understood from the following description with reference to the accompanying drawings,
in which:-
Figure 1 illustrates, in a front view, a dielectric resonator cavity for a dielectric
resonator filter according to the present invention,
Figure 2 illustrates, in a front view and side elevation, a ceramic support member
for supporting the dielectric resonator in the cavity illustrated in Figure 1 of the
drawings, and
Figure 3 illustrates, in a pictorial view, the dielectric resonator cavity illustrated
in Figure 1 of the drawings.
[0020] The dielectric resonator according to the present invention includes at least one
dielectric resonator cavity which is constructed, in a preferred arrangement, in the
manner illustrated in Figures 1 and 3 of the accompanying drawings.
[0021] As illustrated in Figures 2 and 3, the resonant cavity 1 is formed in a housing member
2 having an internally projecting flange formed by four equi-spaced lugs 3. The cylindrical
cavity 1 is symmetrically disposed about the longitudinal axis of the housing member
2. In a preferred arrangement, the internally projecting flange is formed integrally
with the housing member 2.
[0022] A cylindrical dielectric resonator 4 is centrally located within the cylindrical
cavity 1 by means of a ceramic support member 5. The longitudinal axes of the cavity
1 and resonator 4 are coaxial.
[0023] In a preferred arrangement, the resonator 4 is located at the longitudinal centre
of the cavity 1 by the support member 5, i.e. the longitudinal centres of the resonator
4 and cavity 1 are coincident.
[0024] The housing member 2 is fabricated from a metal having a coefficient of thermal expansion
matched to that of the resonator 4 and support member 5. Examples of metals from which
the housing member 2 can be fabricated are titanium, beryllium and a nickel/iron alloy.
In particular, silver plated beryllium metal or silver plated titanium. The coefficient
of thermal expansion of the nickel/iron alloy can be varied in a controllable manner
by varying the nickel content of the alloy. Thus, the coefficient of thermal expansion
of a nickel/iron alloy housing member 2 can be readily matched to that of the ceramic
resonator element 4. The choice of material for the housing member 2 is dependent
on the ceramic material used for resonator 4 and support member 5.
[0025] The silver plated beryllium which has a coefficient of thermal expansion that is
very closely matched to the typical coefficients of thermal expansion for microwave
dielectric resonator ceramics, is particularly suitable for dielectric resonator filters
adapted for operation at lower frequencies i.e. of the order of 4GHz or less, where
the diameter of the dielectric resonator filter begins to exceed 30mm.
[0026] Similarly, silver plated titanium is, for the foregoing reason, also an ideal material
for the housing member 2, but the higher density of titanium in comparison to beryllium
makes it more suitable for dielectric resonator filters operating at frequencies of
the order of 12GHz.
[0027] As illustrated in a front view and side elevation in Figure 2 of the drawings, the
ceramic support member is in the form of a single planar disc having a central aperture
6 formed therein within which the dielectric resonator 4 is centrally located, as
is illustrated in Figures 1 and 3.
[0028] As can be seen from the drawings, the equatorial planes of the dielectric resonator
4 and the ceramic support member 5 are coincident.
[0029] The shape of the aperture 6 in the support member 5 which is the characteristic clover
leaf shape, is such that the dielectric resonator 4 is, as is best illustrated in
Figure 1 of the drawings, secured at four equi-spaced points on the periphery of the
cylindrical resonator. In operation of the cavity, these four points are the low azimuthal
field positions of the HEH
11δ mode of the dielectric resonator 4.
[0030] Thus, the clover leaf shape of the aperture 6 results in the removal of those sections
of the support member 5 that are in the high field locations of the HEH
11δ mode as viewed, as in Figure 1 of the drawings, from the end face of the resonator.
This maximises field confinement and minimises dielectric losses.
[0031] In practice, the support member 5 is of a thickness less than the length of the cylindrical
resonator 4 and is suitably secured to the four points on the periphery of the resonator
4 within the length of the resonator, i.e. it must not extend beyond either of the
end faces of the resonator 4. In a preferred arrangement, the thickness of the support
member 5 is less than one third of the length of the cylindrical dielectric resonator
4 in order to minimise the amount of support dielectric within the cavity and to thereby
minimise the "filling factor".
[0032] The support member 5 is of a ceramic material having high stability and ruggedness,
and low permitivity and loss tangent. Furthermore, the ceramic material must be such
that the electrical, mechanical and thermal expansion properties of the support member
5 can be optimised during fabrication. In particular, the ability to optimise the
coefficient of thermal expansion to match that of the dielectric resonator 4 is of
particular importance because of the need to minimise thermally induced stresses.
[0033] If the thermally induced stresses are not minimised, then this would compromise survivability
of the resonator over extended lifetimes of typically ten years.
[0034] The use of glass ceramic materials, such as cordierite, enstatite, or forsterite,
for the support member 5 are preferred. With such materials, the support member 5
can be made from a single sheet of glass ceramic which is laser cut to the shape illustrated
in Figure 2 of the drawings.
[0035] Glass ceramics also have high Weibel moduli so that support strength is predictable
and hence design margins are smaller and reliability is more predictable.
[0036] The bonding mechanism for securing the support member 5 to the lugs 3 of the internally
projecting flange and to the periphery of the dielectric resonator 4 can be any suitable
arrangement which provides highly rugged and low loss joints and which, for some application,
allows removal of the support member 5 from the cavity to be effected, i.e. the support
member 5 is releasably secured to the lugs 3.
[0037] As stated above, the material of the support member 5 is engineered to have a coefficient
of thermal expansion matched to that of the dielectric resonator 4. The coefficient
of thermal expansion of glass ceramic materials is variable and is a function of the
ceramic used, the quantities and types of additives and the processing parameters.
[0038] As illustrated in the accompanying drawings, a surface of the support member 5 at
or near the periphery thereof lies against the inside face of the flange. This surface
is metallised and secured to each of the lugs 3 either by brasing or soldering, for
example, silver soldering. The joints formed between the support member 5 and the
lugs 3 of the internally projecting flange are highly rugged, require no adhesives
and are, therefore, low loss. In the case of soldering the support member could, if
desired, be readily removed from the cavity by reheating of the solder joint.
[0039] The metallisation of the support member 5 can be effected by the application of metal
ink to the required areas of the support member 5 and co-firing the support member
4 and metal ink at a high temperature, for example, in the range 400°C to 800°C.
[0040] As is illustrated in Figures 1 and 3 of the drawings, the support member 5 can be
releasably secured to the four lugs 3 to form low loss joints by the use of silver
plated bolts and washers of a low loss material. With this arrangement, a screw threaded
hole 7 is provided in each of the lugs 3 and the support member 5 is, as illustrated
in Figure 2, provided with four holes 8 corresponding to the screw-threaded holes
7. The support member 5 is secured to the lugs 3 by silver plated bolts with a washer
of low loss material positioned between the head of the bolt and the support member
5.
[0041] This arrangement for securing the support member 5 in position is ideally suited
for dielectric resonator filters operating at frequencies of the order of 4GHz.
[0042] The securing of the support member 5 to the periphery of the dielectric resonator
4 can be effected by the application of suitable low loss adhesives but for those
structures in which the coefficients of thermal expansion for the support member 5
and dielectric resonator 4 are matched to within 1.0ppm/°C, ceramic bonding can be
used. This involves the positioning of a piece of green state ceramic between the
periphery of the resonator 4 and each of the four forks of the shaped aperture 6 in
the support member 5. The assembly is then fired in a known manner to produce a continuous
ceramic bond between the support member 5 and resonator 4 at each of the contact points.
These bonds are both very strong and very low loss due to the absence of adhesives.
[0043] As illustrated in Figure 1 of the drawings, the dielectric resonator filter includes
a coupling screw 8 which extends into the cavity 1 on a radial plane that is at 45°
to the two orthogonal dual mode electrical field orientations of the cavity 1 i.e.
in alignment with one of the lugs 3. The filter also includes two resonance tuning
screws 9 each one of which extends into the cavity 1 on a radial plane coincident
with a respective one of the two orthogonal mode electrical field orientations.
[0044] The screw-threaded holes that are provided in the housing member 2 for the screws
8 and 9 enable the position of the screws to be adjusted, i.e. the extent to which
the screws 8 and 9 extend into the cavity is adjustable. As, and when, a desired position
is reached the screws 8 and 9 are respectively locked in position by locking nuts
10 and 11.
[0045] A coaxial input connector 12 for the cavity 1 is illustrated in Figures 1 and 3 of
the drawings.
[0046] In practice, the dielectric resonator filter includes a plurality of the dielectric
resonators illustrated in Figures 1 and 3 of drawings connected in cascade with resonant
energy coupling means interposed between each pair of adjacent dielectric resonators.
The housing member 2 is provided with flanges 13 and 14 to facilitate the cascaded
couplings. The flange 13 is provided with a number of through nodes 15 and the flange
14 is provided with a matching number of screw-threaded holes 16. The holes 15 and
16 being in alignment when the flange 13 of one cavity is aligned with the flange
14 of an adjacent cavity. The adjacent cavities are connected together by bolts, each
one of which passes through a respective one of the holes 16 and into the corresponding
screw-threaded hole 15. The resonant energy coupling means referred to above which,
in practice, is in the form of a planar member with a coupling iris formed therein,
for example, a cruciform shaped aperture, would be interposed between the flanges
13 and 14, and provided with a number of through holes in alignment with the holes
15 and 16.
[0047] A dielectric resonator filter including a plurality of cascaded dielectrlc resonators
of the type outlined in the preceding paragraphs with reference to the accompanying
drawings, is ideally suited for use as a multiplexer and/or a demultiplexer.
1. A dielectric resonator filter including at least one microwave resonator having a
cylindrical conductive cavity symmetrically disposed about a longitudinal axis and
a cylindrical dielectric resonator element supported within the cavity characterised
in that the cavity (1) has an internally projecting flange (3), in that the support
for the resonator element (4) comprises a unitary dielectric support member (5) having
a coefficient of thermal expansion to match that of the resonator element (4) and
in that the dielectric resonator element (4) is secured to the said internally projecting
flange (3) and adapted to support the resonator element (4), at the peripheral surface
thereof, in a spacial central position within the cavity (1) whereby the longitudinal
axes of the cavity (1) and the resonator element (4) are coaxial.
2. A dielectric resonator filter as claimed in claim 1 characterised in that the support
member is in the form of a ceramic disc (5) having a central aperture (6) within which
the dielectric resonator element (4) is securely located at the periphery thereof,
the shape of the aperture (6) being such that the resonator element (4) is secured
to the support member (5) at its low azimuthal field positions of the HEH11δ mode.
3. A dielectric resonator filter as claimed in claim 2 characterised in that the thickness
of the ceramic disc (5) is less than one third of the length of the cylindrical dielectric
resonator element (4).
4. A dielectric resonator filter as claimed in any one of the preceding claims characterised
in that the support member (5) is of a ceramic material having high stability and
ruggedness, and low permittivity and loss tangent, and in that the electrical, mechanical
and thermal expansion properties of the support member (5) can be optimised during
fabrication.
5. A dielectric resonator filter as claimed in claim 4 characterised in that the ceramic
support member (5) is fabricated from a glass ceramic material.
6. A dielectric resonator filter as claimed in any one of the preceding claims characterised
in that the coefficient of thermal expansion of the conductive cavity (1) is matched
to that of the dielectric support member (5).
7. A dielectric resonator filter as calimed in any one of the Claims 1 to 5 characterised
in that the coefficient of thermal expansion of the conductive cavity (1) is matched
to that of the dielectric support member (5) and the dielectric resonator element
(4).
8. A dielectric resonator filter as claimed in any one of the preceding claims characterised
in that the surface of the dielectric support member (5), at or near the periphery
thereof, is metallised and in that the metallised surface of the support member (5)
is secured to the said internally projecting flange (3).
9. A dielectric resonator filter as claimed in claim 8 characterised in that the support
member (5) is secured to the said internally projecting flange (3) by either brazing
or silver soldering, and in that the joint formed thereby is of high ruggedness and
low loss.
10. A dielectric resonator filter as claimed in any one of the preceding claims characterised
in that the cylindrical conductive cavity (1) is of silver plated beryllium metal,
the dielectric resonator element (4) having a coefficient of thermal expansion to
match that of the beryllium.
11. A dielectric resonator filter as claimed in any one of the claims 1 to 9 characterised
in that the cylindrical conductive cavity (1) is of silver plated titanium, the dielectric
resonator element (4) having a coefficient of thermal expansion to match that of the
titanium.
12. A dielectric resonator filter as claimed in any one claims 1 to 9 characterised in
that the cylindrical conductive cavity (1) is of a nickel/iron alloy, the coefficient
of thermal expansion of the cavity (1) being matched to that of the dielectric resonator
element (4) by varying the nickel content of the alloy.
13. A dielectric resonator filter as claimed in any one of the preceding claims characterised
in that the said internally projecting flange is formed as an integral part of the
cavity (1) and consists of four equi-spaced lugs (3), and in that the lugs (3) are
located at the low azimuthal field positions of the HEH11δ mode of the dielectric resonator element (4).
14. A dielectric resonator filter as claimed in any one of the preceding claims characterised
in that the resonator element (4) is located at the longitudinal centre of the cylindrical
conductive cavity (1).
15. A dielectric resonator filter as claimed in any one of the claims 1 to 6 characterised
in that the said internally projecting flange is formed as an integral part of the
cavity (1) and consists of four equi-spaced lugs (3), in that the lugs (3) are located
at the low azimuthal field positions of the HEH11δ mode of the dielectric resonator element (4) and in that the support member (5) is
releasably secured to the four lugs (3) to form low loss joints between the support
member (5) and the lugs (3).
16. A dielectric resonator filter as claimed in claim 14 characterised in that a screw-threaded
hole (7) is provided in each of the lugs (3), in that the support member (5) has holes
(8) formed therein corresponding to the screw-threaded holes (7) in the lugs (3) and
in that the support member (5) is secured to the lugs (3) by silver plated bolts,
a washer of a low loss material being positioned between the head of each of the bolts
and the support member (5).
17. A dielectric resonator filter as claimed in any one of the claims 2 to 16 characterised
in that the coefficients of thermal expansion of the dielectric resonator element
(4) and the ceramic support member (5) are matched to within 1.0ppm/°C, and in that
the dielectric resonator element is secured to the ceramic support member (5) at four
points, on the periphery thereof, by means of a low loss ceramic bond produced by
a firing process.
18. A dielectric resonator filter as claimed in any one of the preceding claims characterised
in that the filter includes a coupling screw (8) which extends into the cavity (1)
on a radial plane that is at 45° to the two orthogonal dual mode electrical field
orientations of the cavity (1) and at least two resonance tuning screws (9) each one
of which extends into the cavity (1) on a radial plane coincident with a respective
one of the said two orthogonal dual mode electrical field orientations, the extent
to which the coupling (8) and tuning (9) screws extend into the cavity (1) being adjustable.
19. A dielectrical resonator filter as claimed in any one of the preceding claims characterised
in that the filter includes a plurality of cascaded dielectric resonators, and resonant
energy coupling means interposed between each pair of adjacent dielectric resonators.
20. A dielectric resonator filter as claimed in any one of the preceding claims characterised
in that the dielectric support member (5) is of cordierite, or enstatite, or forsterite.
21. A multiplexer/demultiplexer characterised in that it includes a dielectric resonator
filter as claimed in any one of the preceding claims.
22. A communication satellite payload characterised in that it includes a demultiplexer
as claimed in claim 21.