Note: Descriptions are shown in the official language in which they were submitted.
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TEMPERATURE STABLE FOLDED WAVEGUIDE
FILTER OF REDUCED LENGTH
TECHNICAL FIELD
The present invention relates to a temperature-
stable folded waveguide bandpass filter of reduced
length and a method of constructing same.
BACKGROUND ART
Bandpass filters are widely used in radio communi-
cation systems. At microwave frequencies, electrical
signals are often guided by transmission lines in the
form of rectangular waveguides to minimize losses of
the signals. Waveguide filters may be implemented
using shunt inductive iris along the waveguide
structure forming resonating cavities coupling to one
another. Such filter design is well described in the
book, "Microwave Filters, Impedance-Matching Networks,
and Coupling Structures" by G. Matthaie, L. Yong and
E.M.T. Jones at pages 450 to 459.
One problem with waveguide filters, specially at
lower frequencies, is the size or more specifically the
length of the filter, which is a limiting factor when
it comes to integrate them in today's compact radio
systems. For example a six-cavity filter at 5 GHz is
about 12-inches long. This length can be reduced by
one-half when superposing every other adjacent cavity
to implement a folded structure. These "folded" struc-
tures can be machined out of a block of brass or
copper, but in some cases, to achieve the required
temperature stability, a more stable material has to be
used. Machining the filter from a block of INVAR is
very difficult and expensive.
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SUMMARY OF INVENTION
It is a feature of the present invention to imple-
ment a folded filter using standard copper or copper-
clad INVAR waveguide sections, in order to achieve size
(length) reduction as well as temperature stability.
A further feature of the present invention is to
provide a temperature-stable folded waveguide bandpass
filter which is of reduced length while maintaining the
performance characteristics of a straight folded
waveguide filter which is substantially longer.
Another feature of the present invention is to
provide a temperature-stable folded waveguide bandpass
filter of reduced length which is easier to construct
and more economical than known folded waveguide
bandpass filters having a serpentine arrangement of
resonating cavities machined from metal blocks.
Another feature of the present invention is to
provide a temperature-stable folded waveguide bandpass
filter of reduced length which is lighter than similar
folded waveguide bandpass filters which are machined
from metal blocks.
According to the above features, from a broad
aspect, the present invention provides a temperature-
stable folded waveguide bandpass filter of reduced
length and comprised of two straight waveguide sections
of rectangular cross-section. At least one of the
sections has opposed parallel sidewalls and opposed
parallel broadwalls. The other of the section has
opposed parallel sidewalls and at least one outer
broadwall. One end of each section is an open end and
the opposite end is closed by an end wall. Connecting
means is provided adjacent the open end. Each of the
waveguide sections has transfer slits formed in their
respective sidewalls and an outer broadwall at
predetermined locations to receive therein shunt
inductive iris plates and cavity wall plates to form
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resonating cavities in both the waveguide sections.
The waveguide sections are interconnected superposed
along an inner coupling broadwall of at least one of
the sections thereof with the open end of each wave-
guide section disposed at opposed ends. The inner
coupling broadwall of at least one of the waveguide
sections has inductive coupling holes. Means is
provided to tune the frequency of the resonating
cavities.
According to another broad aspect, the present
invention provides a temperature-stable folded
waveguide bandpass filter of reduced length. The
folded waveguide bandpass filter comprises two straight
waveguide sections of rectangular cross-section each
defined by opposed parallel side walls and opposed
parallel broadwalls. One end of each of these sections
is an open end and the opposite end is closed by an end
wall. Connecting means is provided adjacent the open
end. Each waveguide section has transverse slits
formed in its sidewalls and broadwalls at predetermined
locations to receive therein shunt inductive iris
plates and cavity wall plates to form resonating
cavities in both the waveguide sections. The waveguide
sections are interconnected superposed along a coupling
broadwall thereof with the open end of each waveguide
section disposed at opposed ends. The coupling
broadwall of each waveguide section has inductive
coupling holes. The coupling holes in the coupling
broadwall of each waveguide section is juxtaposed when
the waveguide sections are interconnected. Means is
provided to tune the frequency of the resonating
cavities.
According to a still further broad aspect of the
present invention there is provided a method of
constructing a temperature-stable folded waveguide
filter of reduced length while maintaining the
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performance of a straight waveguide filter which is
substantially longer. The method comprises the steps
of providing two straight waveguide sections of
rectangular cross-section and at least one having
opposed parallel sidewalls and opposed parallel
exterior and interior broadwalls, and the other having
opposed parallel sidewalls and at least said exterior
broadwall. One end of each section is an open end and
the opposite end is closed by an end wall. Connecting
means is provided adjacent the open end. Transverse
slits are formed in the respective sidewalls and the
exterior broadwalls of the waveguide sections at
predetermined locations. Shunt inductive iris plates
and cavity wall plates are secured in the slits to form
resonating cavities in both the waveguide sections.
Inductive coupling holes are formed in the interior
broadwall of at least the waveguide section having
opposed broadwalls at predetermined positions. The
waveguide sections are interconnected by superimposing
the waveguide sections in a predetermined manner with
the outermost of the exterior broadwalls and connecting
them together with the open end of each section at an
opposed end.
BRIEF DESCRIPTION OF DRAWINGS
A preferred embodiment of the invention will now
be described with reference to the accompanying
drawings, in which:
FIG. 1 is a perspective view of a standard wave-
guide bandpass filter equipped with inductive iris
plates to form serially disposed resonating cavities;
FIG. 2 is an exploded perspective view of the
filter of the present invention showing the position of
the iris plates and the division walls as well as the
coupling holes in the waveguide section;
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FIG. 3 is a perspective view of the six-cavity
folded waveguide filter of Figure 2 with the two super-
posed waveguide sections, and showing the tuning screws
relative to the position of the irises to control the
frequency and coupling factors; and
FIG. 4 is a graph showing a typical six-cavity
folded filter response.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to Figure 1, there is shown generally at
10 a straight standard waveguide bandpass filter of the
prior art. It comprises a waveguide tube 11 having
slits 12 formed in a sidewall 13 and broadwall 14
thereof. Inductive iris plates 15 of various lengths
are positioned within the slits and secured therein.
The filter is open at opposed ends and provided with an
input securing flange 16 and an output securing flange
17 to interconnect the waveguide to associated
electronic radio communication hardware (not shown). A
disadvantage of such bandpass filters is that they
require a long space for their installation, particu-
larly at lower microwave frequencies, and this is
sometimes undesirable.
Referring now to Figures 2 and 3 there is shown
the construction of the temperature-stable folded
waveguide bandpass filter 20 of the present invention.
The folded waveguide bandpass filter is comprised of
two straight waveguide sections 21 and 22. Each
section is of a rectangular cross-section, and is
defined by opposed parallel sidewalls 21' and 22',
respectively, and opposed parallel broadwalls 21" and
22", respectively. One end of each of the waveguide
sections is an open end 23 and 24, respectively, and
the opposite end is provided with an end wall 25 and
26, respectively. A connecting flange 27 and 27' is
also provided about the open ends 23 and 24, respec-
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tively, to interconnect the folded waveguide. When the
waveguide sections are interconnected as shown in
Figure 3, one of their broadwalls is an outer broadwall
and the superposed broadwalls are the inner broadwalls.
As clearly shown in these Figures, each of the
waveguide sections 21 and 22 have transverse slits 28
and 29 formed transversely in one of their sidewalls
21' and 22' and one of their broadwalls 21" and 22".
Transverse slit 29 extends across the broadwall while
transverse slit 28 extends only a predetermined
distance across the broadwall, and this is to receive
shunt inductive iris plates 30 in the slits 28 and
cavity wall plates 31 within the slits 29, the latter
to segment the inner space of the waveguide sections,
and to form resonating cavities 32 in both the
waveguide sections.
One of the broadwalls 21" and 22" of each wave-
guide section 21 and 22 are provided with coupling
holes 33 which are positioned at predetermined
locations with respect to the cavities 32. These holes
are elongated rectangular holes having rounded ends and
are of a predetermined size to provide the proper
coupling of the resonating cavities. These holes 33
are formed in a coupling broadwall of each of the
waveguide sections.
The shunt inductive iris plates 30 and the cavity
wall plates 31 are soldered or brazed in position along
the slits 28 and 29 respectively. This soldering or
brazing is effected with care, and the surface of the
broadwall and side wall, where the slits are
positioned, is polished so that this interconnection is
flush with at least the coupling broadwall which is
intended to be interconnected.
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As shown in Figure 3 the coupling broadwalls 21"
and 22" having the coupling holes therein are
positioned juxtaposed with one another with the
coupling holes in alignment. This can be done by
suitable aligning pins (not shown) to provide substan-
tially perfect alignment of the holes 33. The open
ends 23 and 24 of the waveguides are positioned at
opposed ends of the juxtaposed sections, and these
sections are soldered or brazed with one another to
form the folded waveguide 20 of the present invention.
It is pointed out that one of the waveguide
sections 21 or 22 may not have a couping broadwall so
that when the sections 21 or 22 are connected, a single
coupling broadwall serves as a mutual coupling
broadwall.
The shunt inductive iris plates 30 and cavity wall
plates 31 are preferably, but not exclusively, formed
from a metal identified by the Trademark "INVAR". The
end walls 25 and 26 of the sections 21 and 22 are also
formed by plates of INVAR metal soldered or brazed to
the waveguide tube sections. These INVAR plates
provide for temperature stability and material
compatibility of the waveguide. To reduce losses
caused by solder joints of the INVAR plates forming the
irises, copper or precious metal plating can be used to
cover the inside of the filter. Each cavity and
coupling aperture is adjusted with a silver-plated
stainless steel screw 35 which is positioned in the
broadwalls and the sidewalls of the waveguide sections
to optimize the filter performance.
As shown in Figure 2, the center cavity has two
coupling holes. This dual coupling system permits to
have two identical waveguide sections thus reducing the
number of different parts and simplifying the assembly.
This approach is not possible with a single hole, since
the hole cannot be positioned in the center of the
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cavity due to electrical properties of the coupling
structure. The connecting flanges 27 and 27' are also
constructed of INVAR material to mate to associated
radio interface circuitry.
As can be seen from the six-cavity structure
defined by the two juxtaposed waveguide sections, shown
in Figures 2 and 3, each waveguide section delineates
two cavities in the space defined between the end walls
25 and 26 and the intermediate cavity wall plate 31 and
in the space defined between the intermediate cavity
wall plate and the open ends 23 and 24. The two
cavities are delineated by the position of one of the
iris plates 30. The cavity defined between the iris
plate and the intermediate cavity wall in the space
between the end wall and the intermediate cavity wall
has two coupling holes in their broadwall. Once these
cavities are coupled together and after juxtaposing the
sections, the tuning screws 35 are adjusted to tune the
frequency of the resonating cavities. Accordingly, it
can be seen that the signal will follow a serpentine
path through these juxtaposed waveguide sections to
form a folded waveguide wherein the signal path is
equivalent in length to a much longer straight standard
waveguide bandpass filter, as shown in Figure 1.
The invention also envisages the method of
constructing the temperature-stable folded waveguide
filter of reduced length while maintaining the
performance of the straight waveguide filter shown in
Figure 1. The method comprises providing the two
waveguide sections of a construction described herein
with transverse slits formed therein and inductive iris
plates and cavity wall plates secured in these slits
with the coupling holes aligned and the waveguide
sections interconnected to one another on juxtaposed
coupling broadwalls. As earlier stated, one of the
waveguide sections may be void of a coupling broadwall.
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The filter is implemented following a design
method known in the art, and as described by Matthaie
G., L. Yong, and E.M.T. Jones, in the publication
"Microwave Filters, Impedance Matching Networks, and
Coupling Structures", McGraw-Hill Book Company, New
York, 1964. Reprinted by Artech House, 1980. The
guided wavelength at the center frequency is given by:
8 ~1 - (~ 2n)'
where ~ is the free-space wavelength at the center
frequency fO , a is the wider dimension of the
waveguide. We can calculate the K-inverter parameters
KJ,J+I by the following mathematical analysis:
- Ko~ a
Zo ~ 2 gogl
K~J+1 ~a 1 j = 1,..., n - 1-
ZO 2 ~
_n,n+l = ~ ~a
Zo 2 gngn+l
where gO to gn+l are the lowpass prototype elementvalues, ~a is the number of cavities and
RO) f
~ f is the filter bandwith.
From the K-inverter values we can calculate the
normalized shunt reactance X~zo
~I c Kl_l"~/Zo
o
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The length of each resonator is then given by 1~ where:
(Ago) ~ J 2 (~'f ~ ~'~+')
= tan~l (~)
~ = ~,j = 1,2,...,n
A 4.8 GHz six-cavity folded filter was designed,
built and tested. Its frequency response is shown in
Figure 4. With a bandwidth of 52 MHz, the measured
insertion loss was 0.34 dB at the center frequency. In
temperature variation from 0 to +50~C, we could notice
a frequency shift of about 200 KHz, but no changes in
return loss or insertion loss performance was observed.
The superposed constructed folded filter of this
invention has very competitive performances compared to
a standard waveguide filter with half its total length.
It is within the ambit of the present invention to
cover any obvious modifications of the preferred
embodiment described herein, provided such modifica-
tions fall within the scope of the appended claims.
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