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Патент USA US3035244

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May 15, -1962
3,035,236
H. J. RIBLET
MICROWAVE FILTERS
Filed Aug. l5, 1958
€92595|5
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FIG.3
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FIG.
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T
JNVENTOR.
HENRY J. RIBLET
ATTORNEY
L4
United States Patent O
er
3,035,236
CC
Patented May 15, 1962
1
2
3,035,236
FIG. 5 is a lengthwise sectional view through a wave
guide filter having the selectivity of the filter of FIG. 3
and capable of handling high power levels, but more com
MICROWAVE FILTERS
Henry J. Riblet, 35 Edmunds Road, Wellesley, Mass.
Filed Aug. 15, 1958, Ser. No. 755,304
8 Claims. (Cl. S33-73)
The present invention relates in general to direct
coupled or quarter-wave coupled microwave filters and
more particularly concerns novel high-Q narrow-band
microwave filters capable of transmitting selected spectral
components of incident microwave energy at very high
power levels while minimizing the response of the filtering
pactly arranged.
’
With reference now to the drawing and more particu
larly FIG. l thereof, there is illustrated a lengthwise sec
tional view through a filter section constructed according
to the invention. Since the arrangement of irises within
a rectangular waveguide is well-known, sectional views
10 are employed to best illustrate the principles of Áthe in
sections to frequencies outside the narrow band.
lt is well known in the microwave filter art that many
vention by showing the spacing between adjacent refiect
ing elements within the waveguide. Thus, the waveguide
section 11 may typically be a rectangular waveguide
dimensioned to support the propagation of microwave
of the techniques developed in connection with the syn 15 energy of the frequencies selectively transmitted by the
I’thesis of lumped parameter linear passive networks may
filter. Each end of the section 11 includes a reñecting
be adapted for synthesizing microwave filters formed of a
element such as inductive irises 112 and »13. It is im
cascade of large, generally lossless, reflecting elements
portant to note that the separation between irises 12 and
regularly spaced in a uniform waveguide. General syn
13 is an integral multiple of substantially one half the
thesis procedures for “quarter-wave coupled” and “direct 20 guide wavelength of microwave energy at the center fre
coupled” filters are set forth in Microwave Transmission
quency of the uîlter formed by section 11 cascaded with
Circuits, vol. 9 of the M.I.T. Radiation Laboratory Series,
other sections. While 4the other sections may be any in
pp. 661-706. For both types of filters, synthesis pro
tegral multiple of half this guide wavelength, section 11
e
cedures are described based on the use of a ladder net
work prototype having a prescribed insertion loss func
tion. A closely related synthesis procedure for the design
of direct coupled filters is also disclosed in a paper by
S. B. Cohn entitled Direct-Coupled Resonator Filters in
is two or more times the half guide wavelength.
The irises 12 and 13 are characterized by coefficients
r2 and r1, respectively, which relate current and voltage
on the load and source side of each iris. These relations
are set forth in detail in the discussion below concern
the “Proceedings of the I.R.E.,” vol. 45, pp. 187-196,
ing the mode of operation.
for February 1957.
30
A view of iris 12 is better seen in FIG. 2 which shows
Generally, prior high-Q Ifilters have consisted of a num
a sectional View of section 11 through section 2_2 of
ber of reflecting elements spaced within the waveguide
FIG. l. From FIGS. 1 and 2, it is seen that the sec
by one-fourth or one-half the guide wavelength of micro
tion 11 includes top and bottom walls 14 and 15 and
wave energy at the filter center frequency. While the
side walls `16 and 17. The particular dimensions of
desired filter selectivity is readily obtained with such 35 the irises are determined to provide the desired filtering l
structures, at high power levels the potentials developed
characteristics by techniques described in the publica
in the waveguide portions between the reflecting elements
tions cited above or any other suitable method.
may exceed the breakdown potential and arcing may
The following analysis will be helpful in understanding
occur.
how separating the reflecting elements by an integral
Accordingly, the present invention contemplates and 40 multiple of half guide wavelengths, where the integral
has as a primary object materially increasing the power
multiple is at least two, preserves .the selectivity charac
handling capabilities of high-Q waveguide filters while still
teristics while increasing the -power handling capabilities.
providing the desired degree of selectivity.
It is convenient to designate current and voltage rela
Still another object of the invention is to provide high-Q
tions in the vicinity of the reflecting elements at a fixed
waveguide filters in -accordance with the preceding ob 45 time with the subscripts L and S designating the load
ject of the minimum size required to selectively transmit
and source side of the elements, respectively. Assum
a predetermined segment of the frequency spectrum of
ing that the power flows from Ileft to right in the struc
input microwave energy at a given power level.
ture of FIG. 1 4and labeling parameters in the Vicinity
According to the invention, the power handling capa
of irises 13 and 12 with subscripts 1 and 2 respectively,
bilities are increased by spacing the reflecting elements
the following equations relate the currents, i, to the
an integral number of half wavelengths apart, at least
two adjacent reflecting elements being no less than a
volt-ages, v.
wavelength apart. Space is conserved by arranging the
spacing between adjacent elements so that the maximum
potential developed in each cavity formed between ad 55
jacent refiecting elements is just under the breakdown
potential of the cavity in View of the power level and
bandwidth of incident microwave energy.
Other features, objects and advantages of the invention
will be better understood from the following specification 60
when read in connection with the accompanying drawing
in which:
FIG. 1 is a lengthwise sectional view through a Wave
guide filter section constructed according to Vthe invention;
FIG. 2 is a view along section 2_2 of FIG. l to better 65
illustrate a typical reflecting element;
FIG. 3 is a lengthwise sectional View through a repre
sentative embodiment of a waveguide filter constructed
according to the invention;
T2
where
_- i à@
p-J S n M
and r is a positive real factor corresponding to the cou
pling reactance X-2 referred to in the portion of vol. 9
FIG. 4 is a graphical representation of the square of 70 of the M.I.T. Radiation Laboratory Series cited above.
the maximum voltage as a function of frequency in the
L/Àgo is substantially lone-half at »the center frequency
different sections of the filter of FIG. 3 ; and
of the ñlter including the section 11 when K=1, L being
3,035,236
the distance between reflecting elements 12 and 13. The
the r’s are determined by a suitable synthesis method.
'When these coefiicients are determined, irises or other
guide wavelength `at center frequency is designated Ago.
Solution of the above equations may be effected by
suitable reflecting elements are designed by well-'known
matrix manipulation to express vgS/i-ZS as a function of
VIL/im. This relationship establishes the frequency char
acteristics of `filter section »11 and is found by performing
the following multiplication of matrices.
Wl tml Wl
j/t/Eo
p1
¿NEG
4
ing lhigh power filters. First, the number and value of
analytical Iand/or experimental techniques to provide
corresponding values for the VSWR at the plane in the
waveguide where each is located. r.The square of the
maximum voltage in the cavities between reflecting ele
ments spaced by a half wavelength may then be deter
10 mined by analytical, graphical or experimental means.
From a `knowledge of the prescribed power handling
capabilities of the filter, it may be determined in which
It is convenient to designate these three matrices -as
R2, W21 and R1, respectively, corresponding to the char
cavities the maximum voltage squared should be re
duced in order to avoid breakdown. The elements sep
tion coupling reflecting element 12 yto reflecting element 15 -arated by these cavities are then moved one or more
acteristics of refiecting element 12, the waveguide sec
13 and reflecting element 13, respectively.
additional half wavelengths apart. '_The increased sep
The fre
quency-sensitive characteristics of section 11 are then
-aration between only some elements will not appreciably
fully characterized by
affect the frequency response characteristics of preceding
sections.
[Ri'Wzi’Rzl
This will be better understood from the fol-
lowing considerations.
This matrix product is
The relation between the current on the source side
of iris 13 in section 11 to develop a given voltage on the
load side is
25
It is desired to increase the power handling capabilities
of ñlter section 11 without altering the value of this
Thus, the current on the source side of iris 13 has been
reduced by a factor of vk. Itis well known in the micro
wave art that a current maximum occurs at an inductive
iris and that a voltage maximum occurs substantially a
quarter wavelength away from the current maximum and
30
will be seen that this is accomplished by separating the
product matrix.
From the analysis which follows, it
reflecting elements by k half guide wavelengths where
k is an integer greater than one.
As a practical matter, small values of p are of interest.
Since
QTL
p__-j sin Äg
is proportional to the value of the current maximum.
Therefore, the maximum voltage in the cavity is reduced
by the same factor \/k. Since the peak power is pro
portional to the square of the maximum voltage, it fol
lows that the power transmitted through the section may
be increased by a factor k in a k half wavelength section
over that which may be transmitted through a section
only a half wavelength long before the breakdown po
tential of the cavity is exceeded.
With reference to FIG. 3, there is shown a lengthwise
40
sectional view of a ñlter having four cavities 31-34
separating iìve inductive irises 41-45 by one and one
the guide half wavelength at center frequency; for small
half guide wavelengths at the filter center frequency.
values rof p, the sine may be approximated by the dif
The square of the maximum voltage of each cavity was
ference between an appropriate integral multiple of 11
experimentally determined, the irises being dimensioned
and its argument. Thus,
and L is k times
to provide a filter exhibiting Tchebyshev characteristics
within the pass band. These measurements are graph
ically represented on a common frequency scale about the
filter center frequency in FIG. 4 with the amplitude
In conventional filters, k=l.
It can be shown that increasing the length of section
normalized. The curves are labelled with the number
11 by a factor k correspond-s to substituting kp for p in 50 of the cavity having the plotted maximum Voltage squared.
the matrix representations of characteristics of the filter
Examination of these curves shows that the two inner
section. The Itransfer characteristics of the section are
cavities 32 and 33 have a maximum voltage squared
then represented by the product matrix [R2] [W21]k[R1].
below two over the frequency band from 9450 to 959@
To show an example illustrating the propriety of re
megacycles while the two end cavities 31 and 34 have
placing p by kp when k half wavelength sections are 55 a maximum voltage squared below one over the entire
between adjacent irises, consider `the case where k=-2.
range. lf it is desired to provide a filter operative over
this frequency band of minimum length to transmit a
Then [w21]2 is given by
prescribed power, the inner sections should be twice as
long as the end sections.
If inner cavities a guide wavelength long are capable
60
which reduces to
of transmitting the desired power levels, then the end
[il] lì?
cavities need only be a half guide wavelength. A length
wise sectional view of such a filter is shown in FlG. 5
with elements designated by the reference numeral primed
If p is replaced by a factor kp, the magnitude of the 65 of a corresponding element shown in FIG. 3. This em
functional relationships characterized by this matrix re
bodiment of the invention provides the same degree of
mains unchanged if r1 and r2 are each decreased by a
«factor k. The matrix product remains
selectively as the filter of PEG. 3 but minimizes the size
of the filter in view of the power level requirements.
The information in FIG. 4 is also helpful in deter
70 mining the proper lengths of sections when operation
0
over a wider bandwidth is required. For example, con
sider operation over `a bandwidth from 9425 to 9525
megacycles. The maximum voltages squared of the cavi
ties 31-34 are below one, 2.2, 3.7 and 3.3, respectively.
ing the description which follows of a method for mali 76 The ratio of lengths would then be 1:2.5:4:4. ln terms
The preceding discussion should facilitate understand
3,035,236
6
of guide half wavelengths, the integer k for the cavities in
waveguide between adjacent sections coacting with said
order from the source end to the load end would be 2,
sections to provide a filter exhibiting a predetermined selec
tivity characteristic whereby only a narrow band of said
5, 8 and 8. In general, the pattern for selecting the
cavity lengths in the optimum manner involves choos
spectral components is transmitted therethrough, each of
ing the length of the cavity having the highest maximum
said sections having a length substantially equal to an in
voltage squared within the desired band to be the small
est integral multiple of a half guide wavelength which
tegral multiple k of half the guide wavelength at said
center frequency, k being at least two for an intermediate
one of said sections, the reflection coeñicient of those of
said reflecting elements adjacent to the ends of said one
section being reduced by said factor k over the value re
results in transmission of the desired power levels with
out exceeding the breakdown potential of the cavity.
The remaining cavities are then made a lesser or equal
integral number of half guide wavelengths long, the par
ticular integer also being the minimum which results in
quired to provide said predetermined selectivity character
istic with said one section being only half said guide
transmission of the desired power levels without exceed
wavelength long.
ing the breakdown potentials of the respective cavities.
5. A direct-coupled resonant cavity microwave ñlter re
In a representative embodiment of the type illustrated 15 sponsive to microwave energy having spectral components
in FIG. 5, wherein the waveguide section was 2.85 cm.
near a predetermined center microwave frequency com
wide by 1.25 cm. across the narrow dimension, the length
prising, a plurality of cascaded sections of waveguide hav
of inner cavities 32’ and 33’ was 3.75 cm. and that of
ing a uniform cross-section and being dimensioned to nor
end cavities 31’ and 34', 1.875 cm. The width of the
mally propagate said energy, reflecting elements in said
centered conducting plates defining inner inductive irises 20 waveguide between adjacent sections coacting with said
42', 43’ and 44’ was 1.6 cm. and the width of the centered
sections to provide a filter exhibiting a predetermined selec
tivity characteristic whereby only a narrow band of said
conducting plates defining inductive end irises 41’ and
45’ was 0.8 cm.
This filter selectively transmitted 80
peak kilowatts of power, its bandwidth being 60 mega
cycles between the one db points.
lt is apparent that those skilled in the art may now
make numerous departures from the speciñc embodi
ments and techniques described herein without departing
from the inventive concepts. Consequently, the invention
25
spectral components is transmitted therethrough, each of
said sections having a length substantially equal to that
integral multiple k of half the guide Wavelength at said
center frequency resulting in approximately equal values
of the square of maximum voltage being developed in
each section, k being at least two for one of the intermedi
ate sections, the reñection coeñìcient of those of said re
is to be construed as limited only by the spirit and scope 30 tlecting elements adjacent to ends of each section being
reduced by the associated factor k over the value required
What is claimed is:
to provide said predetermined selectivity characteristic
1. A direct-coupled resonant cavity microwave ñlter
with all said sections being only half said guide wave
responsive to microwave energy having spectral com
length long.
ponents near a predetermined center microwave frequency 35
6. A direct-coupled resonant cavity microwave filter for
comprising, a waveguide section of uniform cross-section
selectively transmitting a narrow band of frequencies about
having a cutoff frequency below the frequency of said
a predetermined center frequency at high power levels
spectral components, and a plurality of reñective reactive
comprising, a plurality of cascaded reliecting elements,
of the appended claims.
elements in respective planes within said section, said
waveguide sections of uniform cross-section
planes being spaced by an integral multiple of the guide 40 respective
separating adjacent ones of said reñecting elements and
half wavelength of said energy at said center frequency,
coacting therewith to provide a predetermined frequency
at least two of the intermediate adjacent retiective ele
selectivity, said reñecting elements projecting into the
ments being separated by at least one guide wavelength
waveguide formed by said cascaded sections, each of said
of said energy at said center frequency.
sections being dimensioned to normally propagate micro
2. A direct-coupled resonant cavity microwave filter 45 wave energy within said narrow band of frequencies and
responsive to microwave energy having spectral compo
having a length substantially equal to half the guide wave
nents near a predetermined center microwave frequency
length at said center frequency multiplied by an integer
comprising, a waveguide section of uniform cross-section
k, at least one pair of intermediate adjacent reflecting ele
having a cutoff frequency below the frequency of said
ments being separated by a long section of waveguide hav
spectral components, and a plurality of irises in respec 50 ing a length such that said integer k is greater than one,
tive planes within said section, said planes being spaced
the reflection coeiiicients of said one pair of reñecting
by an integral multiple of the guide half wavelength of
elements being reduced by the multiplicative integer k
said energy at said center frequency, at least two inter
of said long section over the value required to provide
mediate adjacent planes being separated by at least one
said
predetermined frequency selectivity with said long
guide wavelength of said energy at said center frequency. 55 section being only half said guide wavelength long.
3. A direct-coupled resonant cavity microwave ñlter re
7. A microwave filter in accordance with claim 6 where
in each section which would develop a maximum voltage
therein greater than its breakdown potential when trans
prising, a plurality of cascaded sections of waveguide di
mitting said high power levels if only half said guide wave
mensioned to normally propagate said energy, all of said 60 length long is the minimum value of k guide half wave
sponsive to microwave energy having spectral components
near a predetermined center microwave frequency com
sections being uniform in cross-section retiecting elements
in said waveguide between adjacent sections coacting with
said sections to selectively transmit only a narrow band
lengths long required to reduce said maximum voltage to
a value below said breakdown potential when transmitting
said high power levels while said filter exhibits said pre
determined selectivity.
of said spectral components, each of said sections having
a length substantially equal to an integral multiple of half 65
8. A microwave iilter in accordance with claim 7 where
the guide wavelength at said center frequency, at least
in said retiecting elements are inductive irises.
one of the intermediate sections being at least one guide
wavelength long.
References Cited in the ñle of this patent
4. A direct-coupled resonant cavity microwave filter
UNITED STATES PATENTS
responsive to microwave energy having spectral compo- 70
nents near a predetermined center microwave frequency
comprising, a plurality of cascaded sections of waveguide
having a uniform cross-section and being dimensioned to
normally propagate said energy, reñecting elements in said
2,546,742
2,623,120
Gutton et al ___________ __ Mar. 27, 1951
Zobel _______________ __ Dec. 23, 1952
2,859,418
Vogelman ____________ __ Nov. 4, 1958
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