A comb-line filter incorporating a plurality of bandpass resonators set off center between ground planes is provided with input and output coupling means comprised of dual notching resonators.

FIELD OF THE INVENTION 
The present invention relates to comb-line filters provided with notch 
resonators in the input and output coupling means. 
BACKGROUND OF THE INVENTION 
It has long been a practice to utilize a notch resonator in the input and 
output coupling of a comb-line filter. Such prior art devices rely 
generally on short resonators with excessive capacitance to the ground 
planes and one another. 
Coupling between the resonators of such a device is achieved between 
resonators, which are less than a quarter-wave length long at resonance, 
by electromagnetic fringing fields. The electrical properties are provided 
by structure which includes rod diameters and spacing and lumped 
capacitances that prevent the resonators from being a full quarter-wave 
length long at resonance. If the resonators were a quarter-wave length, 
the structure would have no bandpass because without reactive loading at 
the ends of the resonator elements, the electromagnetic coupling effects 
cancel. 
Because of the variety of coupling sets involved in the fringing fields, 
equating the performance of comb-line filters to mathematical models is so 
unwieldy that creating a structure having a predetermined response cannot 
be accomplished by a simple analysis of the structure. 
OBJECTIVES OF THE INVENTION 
It is a primary objective of the present invention to provide a comb-line 
filter with dual notch resonator inputs and outputs arranged such that the 
individual notch resonators will not couple to one another. 
Another objective of the present invention is to provide a comb-line filter 
which may be set to a plurality of individual notch frequencies to enhance 
the roll off above and below the pass band. 
Another objective of the present invention is to provide a comb-line filter 
with integral notch filters that is more cost effective than combining a 
plurality of external notch filters. 
Another objective of the present invention is to provide a comb-line filter 
which is easy to tune and match impedances in the bandpass. 
A further objective of the invention is to provide a comb-line filter with 
a plurality of bandpass resonators. 
A still further objective of the invention is to provide a comb-line filter 
wherein bandpass resonators are physically staggered about the center line 
to reduce mutual coupling between bandpass resonators without lengthening 
the comb-line chassis. 
SUMMARY OF THE INVENTION 
A comb-line filter is provided with two notch resonator at each end to 
enhance the selectivity of the filter. The dual notching resonators are 
mutually coupled to the first bandpass resonator at each of the comb-line 
and the two end bandpass resonators are coupled via a plurality of 
bandpass resonators offset from the center running the length of the 
ground planes. A shield is positioned between each pair of notching 
resonators to prevent cross coupling therebetween. The shield is on 
alignment with the adjacent bandpass resonator so that mutual coupling 
between the bandpass resonator and each of the notch resonators is 
achieved.

DETAILED DESCRIPTION OF THE INVENTION 
The best mode for carrying out the invention comprises an elongated casing 
of electrically conductive material as illustrated by FIG. 1 with a 
plurality of resonator rods, 21-26, in a comb-line configuration, creating 
a coupling network between pairs of notch resonators. In the preferred 
embodiment the rods are beams having a circular cross section but the 
beams may have any desired or convenient cross section such as elliptical, 
rectangular or square for example. The casing is comprised of conductive 
ground planes 11 and 12 which are joined by conductive end plates 13 and 
14. Conductive side walls, 15 and 16 complete the structure. Electrical 
signals are introduced and extracted from the comb-line filter by coaxial 
connectors 17 and 18 which function as input and output means via coupling 
loops such as 19 of FIG. 1 which is immediately adjacent to a bandpass 
resonator. 
The comb-line bandpass resonators, 21 through 26, and notch resonators, 31 
through 34, of FIGS. 2 and 3 are rods which include a tuning means 27 for 
adjusting the lump capacitance of the resonator and the electrical length 
of the rod between the conductive ground planes 11 and 12. The dual notch 
resonators at each end of the structure are comprised of a pair of rods 
similar to the bandpass resonators but due to location are coupled to the 
input and output bandpass resonators and function as notch filters. They 
are separated by resonator shields 35 and 36 which are electrically 
connected to adjacent end covers 13 and 14 and between the conductive 
ground planes 11 and 12 leaving unconnected edges 37 and 38 which are 
adjacent to the respective input or output bandpass filter resonator rod 
21 or 26. 
The bandpass resonators 21 through 26 form a comb-line filter with bandpass 
resonators 21 and 26 serving as input and output filter means 
electromagnetically coupled to the inductive loops 19 of their respective 
coaxial connectors 17 and 18. The bandpass resonators 22 through 25 are 
staggered on either side of the center line 28 as best seen in FIG. 3. 
They couple the input and output resonators 21 and 26 together at the 
passband frequency. They are staggered on opposite sides of the line 28 to 
shorten the physical length of the filter assembly while maintaining the 
preferred distance between resonator rods. In the preferred embodiment 
line 28 is a center line bisecting the filter cavity but it may be 
off-center or skewed, the controlling concept for this imaginary line is 
that it is a straight line passing through the unconnected edge 37 of 
notch resonator shield 35, its adjacent input/output bandpass resonator 
rod 21, the input/output bandpass resonator rod 26 and the unconnected 
edge 38 of its adjacent notch resonator shield 36. 
The bandpass resonators are mounted to the narrow wall, 12, and the spacing 
between the mounting holes is graduated larger to smaller from center 
resonators 23 and 24 to the input/output resonators 21 and 26 at the 
opposite ends of the comb-line filter as best seen in FIG. 3. In this 
embodiment, it ranges from 0.178 wavelength, or 4.58 inches, line 30, to 
0.142 wavelength, or 3.65 inches, lines 29, computed for air dielectric, 
which is the filter medium here. Further, by way of example, the spacing 
between resonators 22 and 23 and between resonators 24 and 25 may be 0.168 
wavelength, or 4.34 inches. This spacing is a key factor in setting the 
bandpass width and flatness, and will change with any particular filter 
design, including the number and diameter of resonators. The exemplary 
embodiment, resonator diameter is 0.0146 wavelength in air, or about 
0.375." 
The bandpass resonators are 0.208 wavelength long, or 5.375" for this 
embodiment. The comb-line design is centered at approximately 458 Mhz, 
having a wavelength in air of 25.76". 
The notch resonators 31 through 34 are also mounted to the narrow wall, 12, 
and spaced from the input/output resonator at each end of the comb-line by 
0.083 wavelength in air, or 2.13", in all four cases, this spacing may 
also be altered to vary notch resonator coupling to the input/out bandpass 
resonator, and hence produce varying notch attenuation. In actuality, this 
spacing is experimentally adjusted to achieve the desired blending of 
bandpass and notch filter frequency response. The notch resonators are the 
same in diameter and length as the bandpass resonators in this embodiment. 
As can best be seen in FIG. 3, the bandpass resonators 22 through 26 are 
not on the center line, 28, between the input/output resonators 21 and 26. 
They are staggered slightly to obtain the fringe field coupling desired in 
a physically shorter filter length. The spacing produces a nominal 4 Mhz 
wide bandpass at 458 Mhz and the filter response is adjustable over a 
range of about 10%, or plus or minus 23 Mhz. 
The end bandpass resonators 21 and 26 are each coupled to a pair of 
individual notch resonators, bandpass resonator 21 to notch resonators 31 
and 32 and bandpass resonator 26 to notch resonators 33 and 34. A shield, 
35 or 36, is positioned between each pair of notch resonators to prevent 
cross coupling which would result in the loss of the ability to allow the 
individual notch frequencies to be adjusted for the most favorable 
addition to the comb-line bandpass response and to enhance the roll off 
immediately above and below the bandpass. In a preferred embodiment, the 
return loss is about -20 db in the center of the bandpass which equates to 
a 1.22:1 VSWR (voltage standing wave ratio), or an impedance variation of 
from 40.9 Ohms to 61 Ohms, with 50 Ohms producing the desired VSWR of 
1.00:1. The ideal is approached by reducing the loss in the passband by 
adding adjustable capacitive coupling slugs between bandpass resonators. 
The basic element of the present invention is the means to mutually couple 
two notch resonators to the same bandpass filter resonator with the two 
notch resonators tuned to two different frequencies positioned close to 
the edge of the passband of the comb-line filter. This is accomplished by 
the shields, 35 and 36, positioned at each end of the comb-line structure. 
The shields are grounded to their respective end covers 13 or 14 and 
conductive ground planes 11 and 12 as illustrated by FIG. 5. Physically 
each shield is comprised of two conductive plates 41 and 42 joined by an 
electrically conductive member 43 to create a shield structure which 
includes an air gap 45. In a preferred embodiment the air gap is adjacent 
to the end cover. Notch resonators 31 and 32 or 33 and 34 are positioned 
on either side of the shield and set back from the edge of the shield 
facing the bandpass resonator 21 or 27 to minimize the intermingling of 
the notch resonator currents. The detrimental effects of the intermingling 
or cross coupling of the resonator currents is further reduced by proper 
selection of the notch resonator frequencies. The physical proximity of 
the notch resonators 31 and 32 or 33 and 34 to the bandpass resonator 21 
or 27 determines the amount of mutual coupling therebetween and the depth 
and sharpness of the notch. The shields, 35 and 36, between the notch 
resonators are constructed so as to not inhibit the mutual coupling of the 
notch resonators to the adjacent bandpass resonator 21 or 26. The setback 
of the notch resonators is only as far from the adjacent bandpass 
resonator as will result in a minimal degradation of the basic comb-line 
selectivity when the notch is tuned close to the edge of the passband. 
Since the notch resonators are stagger tuned, it is preferable to tune one 
notch resonator close to the passband frequency to increase the notch 
depth and width and then tune the companion notch to be further from the 
passband frequency. This will enhance the resultant selectivity by 
reducing response flyback. 
The width of the shields 35 and 36 is determined by the most favorable 
combined notch resonator coupling to the bandpass filter resonator, and 
the reduction of notch resonator current interaction. The notch resonators 
are effectively in four-sided enclosures with one side removed and 
positioned within the basic comb-line filter structure to obtain the 
desired mutual coupling to the first bandpass resonator at each end of the 
comb-line. 
The response curve of the preferred embodiment is illustrated in FIG. 7 
which depicts how the notch resonator tuning is chosen to prevent the 
resonator currents of the notch resonators, located either side of a 
shield, from intermingling and destroying the independent tuning ability 
of each notch response curve. 
For instance, on the response curve notch 31 is tuned by notch resonator 
31, notch resonator 32 tunes notch 32, notch resonator 33 tunes notch 33 
and notch 34 tunes notch 34. If you compare the physical location of the 
notch resonators 31, 32, 33 and 34 in FIG. 3 you will note that the notch 
frequencies which are adjacent to one another, 31 and 33 and 32 and 34, 
have their resonators located at opposite ends of the comb-line filter. 
The "Q" of the notch resonators is sufficiently high so that physically 
adjacent resonators will have widely separated resonant frequencies and 
currents, allowing tuning of the individual notch resonators for the best 
overall selectivity. The dotted curve 39 illustrates what the selectivity 
of the comb-line filter would be without the notch resonators. 
While preferred embodiments of this invention have been illustrated and 
described, variations and modifications may be apparent to those skilled 
in the art. Therefore, I do not wish to be limited thereto and ask that 
the scope and breadth of this invention be determined from the claims 
which follow rather than the above description.