Filter for very short electromagnetic waves

A filter for very short electromagnetic waves is disclosed in which a plurality of resonators are formed in a row with each resonator containing two filter circuits. The resonators are operated in a dual mode and the filter circuits are arranged in an electrical sequential manner. First and last filter circuits in a direction of the transmitted energy are provided with input and output lines, respectively. At least one additional coupling is provided between at least two filter circuits which do not directly follow one another in electrical sequential fashion. At least one of the resonators provides two filter circuits which are not adjacent with respect to one another in electrical sequential fashion and are coupled via an additional coupling.

BACKGROUND OF THE INVENTION 
The invention relates to a filter for very short electromagnetic waves, 
consisting of a plurality of resonators forming filter circuits and which 
are coupled to one another, are operated in the dual-mode, and whose first 
and last filter circuits in the direction of the transmitted energy are 
provided with connection lines for the supply and discharge of the 
electromagnetic energy. An additional coupling is provided between at 
least two filter circuits which do not directly follow one another in 
electrical mode of operation and the individual resonators are arranged in 
the form of rows next to one another. 
Filters in micro-wave technology are, as is known, constructed from a 
plurality of micro-wave resonators which are coupled to one another, the 
coupling of which can take place either capacitively or inductively. The 
resonators themselves can consist, for example, of so-called coaxial line 
resonators or wave guide resonators. 
In contrast to filters constructed with concentrated or lumped circuit 
elements, as a result of the geometrically predetermined configuration of 
the resonators it is not possible for every circuit which can be 
constructed in the concentrated technique to be readily transferred to the 
micro-wave frequency region. This difficulty occurs, in particular, when 
it is necessary to produce attenuation poles in the attenuation 
characteristic of the filter and/or a transit time leveling in the pass 
band of the filter by means of additional couplings of filter circuits. 
This difficulty is eliminated by means of the arrangement, described in 
the German OS 1 942 867, of resonators in adjacent rows with additional 
over-couplings in the common partition wall of two resonators arranged in 
different rows. 
The possibility is also known of constructing micro-wave filters with 
cavity resonators which are simultaneously operated in more than one mode 
("micro-wave filters employing a single cavity excited in more than one 
mode", "Journal of Applied Physics", Vol. 22, No. 8, August 1951 by 
Wei-Guan Lin; "A Four Cavity Elliptic Waveguide Filter", "IEEE 
Transactions on Microwave Theory and Techniques", Vol. -MTT. 18, No. 12, 
December 1970 by Williams, A. E.). Here preferably two identical but 
orthogonal loads are employed in H.sub.101 - or H.sub.111 - resonators and 
are coupled to one another by means of a coupling screw arranged at 
45.degree. to the direction of the E-vectors (dual mode). In this way two 
electric oscillating circuits of a filter can be constructed in a 
technically effective fashion in one single cavity resonator. On account 
of the reduction in weight and volume of up to 50%, an important field of 
application consists in satellite technology, particularly since high 
electrical requirements are made on the filters employed therein which 
become manifest in a relatively large number of electric oscillating 
circuits. 
As these filters also require attenuation poles and/or a leveling of the 
transit time in the pass band, it is obviously desirable to find suitable 
filter circuits in the dual mode technique for this purpose. In this 
connection a proposed construction is known ("Nonminimum-Phase 
Optimum-Amplitude Band pass Waveguide Filters", "IEEE Transactions on 
Microwave Theory and Techniques", Vol. MTT-22, No. 4, April 1974 by Atia, 
A. E. and Williams, A. E.), which, however, is restricted to filter 
circuits which are symmetrical both with respect to structure and with 
respect to element values. Furthermore they exhibit additional couplings 
which frequently overlap, and cannot be preselected with respect to number 
and geometric position within the filter arrangement. Also, the number of 
electric oscillating circuits of the filter circuits must amount to a 
multiple of 4 so that this proposal frequently cannot be practically 
realized. 
A possibility of improving the realizability of filter circuits which are 
asymmetrical, particularly with respect to element values and which can be 
operated in the dual mode has been disclosed by the German OS 2 511 800 
and consists in arranging the resonators in adjacent rows and providing a 
different number of resonators in the rows. 
The coupling of filter circuits in spatially different resonators is 
subject to the condition that the relevant two filter circuits should be 
spatially orientated in like manner, so that e.g. their E vectors run 
parallel with one another. This condition restricts the number of 
theoretically conceivable couplings and thus the spectrum of possible 
realizations or permits realizations only without the use of additional 
couplings, which in themselves are desirable, or only with production 
technology disadvantages. 
SUMMARY OF THE INVENTION 
An object of the invention is to overcome the above-mentioned difficulties 
in a simple fashion, and, in particular, to provide practical filter 
circuits with resonators operated in the dual mode and additional 
couplings which, in the previously known above-mentioned arrangements, 
either are not possible or are possible only with considerable 
disadvantages. 
According to the invention, a filter for very short electromagnetic waves 
is provided consisting of a plurality of resonators with filter circuits 
and which are coupled to one another, are operated in the dual mode, and 
whose first and last filter circuits in the direction of the transmitted 
energy are provided with connection lines for the supply and discharge of 
the electromagnetic energy. An additional coupling is provided between at 
least two filter circuits which do not directly follow one another in the 
electrical mode of operation and the individual resonators are arranged 
next to one another in the form of rows. In accordance with the invention, 
two filter circuits which are not adjacent in counting mode and which are 
coupled via an additional coupling are provided by means of at least one 
resonator. 
The invention is based upon the recognition that in the previously known 
arrangements, the source of the circuitry limitations is that two filter 
circuits which are consecutive in counting mode are always assigned to the 
same dual mode resonator. 
A particular advantage of the invention is that the limitations concerning 
the circuit structure which exist in the known prior art are avoided so 
that consequently the number of filter circuits which can be constructed 
in the dual mode is considerably increased. 
A production technology advantage in comparison to known filters is that 
the filter structures which, in accordance with the prior art must be 
constructed with two or more adjacent resonator rows, are constructed in 
accordance with the invention in one row and the individual resonators can 
thus be assembled in a simple manner by means of flange connections.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is an equivalent circuit of a six circuit Cauer bandpass filter with 
concentrated or lumped circuit elements. This is a four-pole circuit, in 
the shunt arms of which are arranged the parallel oscillating circuits S1 
to S6, and which are coupled via the coupling inductances 1/2, 2/3, 3/4, 
4/5 and 5/6 arranged in the series arms. By way of additional coupling, a 
coupling inductance 1/6 is introduced between the parallel oscillating 
circuits S1 and S6, and a coupling capacitance 2/5 is introduced between 
the parallel oscillating circuits S2 and S5. 
It has hitherto been impossible to construct a band-pass filter of this 
kind by means of dual-mode resonators arranged next to one another in a 
row, since, for example, in an experiment on a construction employing 
H.sub.101 resonators, the E vectors of the filter circuits S1, S6 and S2, 
S5 are arranged orthogonally to one another and thus cannot be coupled via 
coupling slots in a resonator wall. 
For this reason previously only "pseudo-Cauer filters" of the n=6 order 
have been known, for example having been disclosed in the article 
"Prototype characteristics of a class of dual-mode filters" by R. D. 
Wanselow, published in IEEE Transactions on Microwave Theory and 
Techniques of August 1975. These filters only possess one attenuation pole 
above and below the pass band and thus are not optimal. 
In the exemplary embodiment illustrated in FIG. 2, a filter arrangement in 
accordance with the invention is illustrated which consists of three 
cavity resonators 1 to 3, and whose physical equivalent circuit diagram is 
the circuit in FIG. 1. Here the resonators are arranged in one row in such 
manner that resonators arranged next to one another in each case possess a 
common partition wall. The coupling elements which serve to couple the 
resonators operated in the dual-mode are designed in known manner, for 
example in accordance with the arrangement in FIG. 2 of German Os 2 511 
800, as slot openings and as shown in the instant case at 20 and 21 in 
FIG. 2. 
The E vectors assigned to the individual modes are orthogonal within a 
resonator and in FIG. 2 are provided with the references E1 to E6 in 
accordance with the associated parallel oscillating circuits S1 to S6 in 
FIG. 1. Each of the resonators is furthermore provided with a coupling 
screw K16, K25 and K34 arranged at an angle of 45.degree. between the E 
vectors corresponding to the modes, in order to set the coupling between 
the orthogonal modes operated therein. In the prior art, coupling screws 
of this kind served merely to produce the coupling inductance between two 
adjacent parallel oscillating circuits of the equivalent circuit diagram, 
which thus followed one another in counting mode. In accordance with the 
invention, the exemplary embodiment illustrated in FIG. 2 contains at 
least one resonator, however, and in a special situation the two 
resonators 1 and 2, which are assigned the filter circuits S1, S5 and S2, 
S5, which do not follow one another in counting mode, and therefore the 
coupling screws also produce additional coupling reactances between 
non-adjacent filter circuits. 
In detail, in the exemplary embodiment, the resonator 1 is assigned the 
filter circuits S1 and S6 and the corresponding E vectors E1 and E6, the 
coupling of which is effected via the coupling screw K16 which has an 
inductive action. In this way the resonator 1 simultaneously contains the 
input-coupling terminal I and the output-coupling terminal O of the 
filter. The resonator 2 which adjoins the resonator 1 by a common 
partition wall is assigned the filter circuits S2 and S5 and the 
corresponding, orthogonal E vectors E2 and E5. The coupling of these 
filter circuits is effected via the coupling screw K25 which is arranged 
to be offset relative to the coupling screw K16 by a resonator edge, and 
thus produces a capacitive coupling. 
The coupling of resonator 1 to resonator 2, and of their assigned modes 
with the corresponding, in each case parallel E vectors E1, E2 and E6 is 
effected via coupling slots which are arranged in the common partition 
wall of these resonators in each case at right angles to the E vectors E1, 
E2 and at right angles to E6, E5, and which are shown at 20 and 21 in FIG. 
2. 
The resonator 3 which adjoins the resonator 2 is assigned the filter 
circuits S3 and S4, represented by the corresponding orthogonal E vectors 
E3 and E4, the inductive coupling of which is effected via the coupling 
screw K34. The coupling of resonator 2 to resonator 3 is effected via 
further coupling slots which are arranged in the common partition wall of 
these resonators at right angles to the relevant E vectors E5, E4 and E2, 
E3 and which have not been represented in the Figure. 
In order to indicate the division of the oscillating circuits between the 
individual resonators in the exemplary embodiment, the circuit illustrated 
in FIG. 1 is provided with broken coordination lines which run between two 
oscillating circuits constructed in a resonator and are provided with the 
references assigned to the relevant resonator. 
An exemplary embodiment of the invention for a six-circuit Cauer filter 
consists in a further equivalent circuit which is suitable for 
construction in dual-mode technology and which is represented in FIG. 3. 
This again is a four-pole circuit with parallel oscillating circuits S1 to 
S6 in the shunt arms which are coupled via coupling inductances 1/2 to 
5/6. In place of the additional coupling 2/5 in accordance with FIG. 1, 
however, a capacitive, additional coupling 1/4 has been introduced which 
runs between the oscillating circuits S1 and S4. 
The individual oscillating circuits have again been divided between the 
resonators in the exemplary embodiment in FIG. 4 in accordance with the 
coordination lines of FIG. 3. In accordance with the invention, the filter 
illustrated in FIG. 4 contains a resonator, here the resonator 2, which is 
assigned the filter circuits S1 and S4 which do not follow one another in 
counting mode and are represented by the E vectors E1 and E4, whereas the 
resonator 1 contains the filter circuits S2 and S3 and the resonator 3 
contains the filter circuits S5 and S6. 
In the resonator arrangement illustrated in FIG. 4, the electric vectors E1 
and E6 of the filter circuits S1 and S6 run parallel to one another and 
can thus be magnetically coupled by means of a slot in the partition wall 
of the resonators 2 and 3. The capacitive additional coupling of the 
filter circuits S1 and S4 is achieved in that the dual-mode coupling screw 
K14 of the resonator 2 is displaced by 90.degree. relative to the 
like-orientated coupling screws K23 and K65 of the resonators 1 and 3. 
In comparison to the exemplary embodiment in FIG. 2, the arrangement in 
FIG. 4 has the advantage that the input-coupling terminal I and the 
output-coupling terminal O of the filter are not contained in the same 
resonator, but in the resonators 2 and 3. This largely avoids the 
occurrence of undesired additional couplings. 
FIG. 5 shows a measured curve of the operating attenuation variation, which 
complies well with theory, of the exemplary embodiment as shown in FIG. 4. 
This is a band-pass filter which has been derived from the Cauer low-pass 
filter C6/26 dB/25 dB/B by transformation, and is operated with H.sub.101 
dual mode resonators. (Theoretical data: middle frequency f.sub.o =4015 
MHz, pass band width .DELTA.fg=40 MHz, echo attenuation a.sub.e .gtoreq.26 
dB, blocking attenuation a.sub.b .gtoreq.25 dB). The two clearly marked 
attenuation poles in each case beneath and above the pass band can be 
gathered from the measured curve. For the echo attenuation, values of 
a.sub.e .gtoreq.21 dB were measured in the required pass band. 
A realization with dual-mode resonators for the eight-circuit Cauer 
band-pass filter with in each case three attenuation poles below and above 
the pass band has not been previously known. The article "Narrow Band-Pass 
Waveguide Filters" by A. E. Atia and A. E. Williams in "IEEE Transactions 
on Microwave Theory and Techniques", Vol. -MTT 20, No. 4, April 1972 only 
reports on a "Pseudo Cauer Filter" of the n=8 order, with only in each 
case two attenuation poles below and above the pass band. 
FIG. 6 illustrates two equivalent circuits of the Cauer band-pass filter of 
the n=8 order with concentrated elements. The second circuit, provided 
with an inductive additional coupling K16, can be constructed in 
accordance with the invention in the dual-mode technique and is 
illustrated in the exemplary embodiment shown in FIG. 7. 
The filter in FIG. 7, which is constructed from the resonators 1 to 4 
contains the two resonators 2 and 3, which accommodate the filter circuits 
S2, S5 and S1, S6 which do not follow one another in counting mode. The 
sign sequence for the additional couplings which has been used in 
accordance with the associated equivalent circuit diagram can be achieved 
with the locations shown in FIG. 7, of the dual-mode coupling screws K34, 
K25, K16 and K78. 
An advantageous application of the invention consists, for example, also in 
the construction of a fourteen-circuit linear filter, of which the 
equivalent circuit diagram is represented in FIG. 8, and which contains 
two circuit sections in each case double-bridged for transit time leveling 
in the pass band. The additional couplings are entirely inductive and run 
between the circuits S1 and S6 (K16), S2 and S5 (K25), S9 and S14 (K914), 
and S10 and S13 (K1013). 
As can be seen from the resonator arrangement in accordance with the 
invention shown in FIG. 9 and having the equivalent circuit of FIG. 8, the 
resonator arrangement consists of the resonators 1 to 7. Filter circuit 
pairs S1, S6; S2, S5; S9, S14; and S10, S13 comprise filter circuits which 
do not follow one another in counting mode and are in each case combined 
in a resonator, the resonators 3, 2, 5 and 6, respectively. An advantage 
with respect to production technology in comparison to known arrangements 
of this type consists in that the resonators are arranged in one single 
row and thus can be easily assembled by means of flange connections. An 
electrical advantage is achieved in particular by the splitting of a 4 
bridge circuit section into two double bridged circuit sections as this 
results in dimensions for the coupling slots which can be more easily 
achieved and simplifies the filter tuning. 
Although various minor modifications may be suggested by those versed in 
the art, it should be understood that I wish to employ within the scope of 
the patent granted hereon all such modifications as reasonably and 
properly come within the scope of my contribution to the art.