A stripline tapped-line filter is disclosed including a first substrate upon which a plurality of N hairpin resonators are disposed alternately on opposite surfaces of the first substrate. Each one of the hairpin resonators is in a parallel coupled relationship with an adjacent hairpin resonator disposed on an opposite surface of the first substrate. The first and last hairpin resonators each have an interconnected member disposed on the substrate for respectively coupling a signal into and out of the plurality of N hairpin resonators. Second and third substrates are included with each being respectively located adjacent to ones of the plurality of N hairpin resonators on opposite surfaces of the first substrate. First and second groundplanes are included with each respectively located adjacent the second and third substrates.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention relates to stripline filters. More specifically, the 
present invention relates to a novel stripline tapped-line hairpin filter. 
2. Background Art 
Microstrip and stripline filters are employed for filtering microwave 
frequency signals or other types of high frequency signals. Microstrip and 
stripline filters are commonly used in high frequency filtering 
applications. One such application is in a radar system wherein received 
signals are filtered, i.e., signals of a particular frequency range pass 
through the filter, for further processing. Striplines have the inherent 
advantage over microstrip in that opposite surfaces of a substrate may 
have circuit elements disposed thereon. The stripline circuit 
element-substrate-circuit element structure is sandwiched between the two 
conductive groundplanes and insulated therefrom by two dielectric 
substrates. Microstrips typically have circuit elements formed on one 
surface of a dielectric substrate and a groundplane formed on the opposite 
surface. 
One type of microwave frequency filter uses a microstrip parallel coupled 
filter. The microstrip parallel coupled filter has the disadvantage that 
input and output end sections are required to couple the signals into and 
out of the filter which is comprised of a plurality of N circuit elements. 
The input and output end sections are respectively parallel coupled to the 
first and last resonators of the N circuit element filter. Thus, 
additional surface area is required to form the additional input and 
output end sections for the N circuit element filter. A further 
disadvantage occurs in the situation where the parallel coupling at the 
end sections becomes very tight and physical realization becomes 
impractical. 
Other types of microstrip filters include the tapped-line interdigital and 
combline filters. These type of microstrip filters have the advantage over 
the parallel coupled filters by virtue of the tapped-line feature. The 
tapped-line feature allows the first and last resonators to also serve as 
the input and output sections. This provides savings in space and an 
improvement in filter bandwidth. For example, a 20 to 30 percent bandwidth 
may be achieved using the tapped-line interdigital filter. However, 
physical limitations exist due to the coupling spacing requirements 
between adjacent microstrip filter elements, thus limiting further 
expansion of the filter bandwidth. 
Another type of microstrip filter is the parallel coupled hairpin filter. 
The parallel coupled hairpin filter uses a plurality of N hairpin shaped 
resonators disposed on a surface of the substrate with alternating 
orientation. The parallel coupled hairpin filter requires input and output 
end sections which provide parallel coupling of the signal in and out of 
the filter. However, in certain situations the parallel coupling between 
the end sections and the first and last resonators may become very tight 
and physical realization may not be practical. Therefore, this type of 
filter is limited in bandwidth due to the tight coupling at the end 
sections. In addition, extra space is required for the end sections on the 
surface of the substrate. 
Another type of microstrip filter is the tapped-line hairpin filter. The 
microstrip tapped-line hairpin filter eliminates the need for end sections 
to couple signals into and out of the parallel coupled hairpin resonators. 
This allows an increase of the bandwidth in the range of 30 to 40 percent. 
A design using the tapped-line hairpin filter is described in an article 
entitled "Microstrip Tapped-Line Filter Design" by Joseph S. Wong, IEEE 
Transactions On Microwave Theory and Techniques, Volume MTT-27, No. 1, 
January 1979. 
In many applications the microstrip filter provides sufficient bandwidth. 
However, in some applications a greater bandwidth is required, such as in 
excess of 40 percent. Microstrip filters of this type will not permit 
bandwidths higher than 40 percent due to the physical limitations, i.e., 
required spacing between adjacent filter elements. Thus, the spacing 
requirement between adjacent microstrip filter elements limits the overall 
frequency bandwidth of the filter. 
The microstrip approach limits the bandwidth due to the adjacent 
construction of the filter elements, on a single surface of the substrate. 
As the bandwidth increases the impedance between adjacent filter elements 
correspondingly increase, i.e., coupling becomes tighter. Since the 
tightest coupling occurs at the input and output adjacent resonators, once 
the coupling is too tight in these areas, the filter is no longer 
realizable. 
It is, therefore, an object of the present invention to provide a wideband 
microwave filter. 
It is another object of the present invention to provide a stripline 
bandpass filter for microwave applications having a wide bandwidth 
capability. 
It is yet another object of the present invention to provide a stripline 
bandpass filter using hairpin resonators spaced alternately on opposite 
surfaces of a dielectric substrate wherein the first and last hairpin 
resonators are respectively tapped for signal input and output. 
SUMMARY OF THE INVENTION 
The present invention provides a stripline tapped-line hairpin filter 
including a first substrate; a plurality of N hairpin resonators disposed 
alternately on opposite surfaces of the first substrate, ones of said 
plurality of N hairpin resonators located on each of the opposite surfaces 
of the first substrate being in a spaced parallel relationship with 
respect to another such that each one of the plurality of N hairpin 
resonator is in a parallel coupled relationship with an adjacent one of 
said plurality of N hairpin resonators disposed on an opposite surface of 
the first substrate, and the first and last ones of the plurality of N 
hairpin resonators having an interconnected tapping member disposed on the 
substrate for respectively coupling a signal into and out of the plurality 
of N hairpin resonators; second and third substrates each respectively 
located adjacent to ones of said plurality of N hairpin resonators on 
opposite surfaces of the first substrate; and first and second 
groundplanes each respectively located adjacent the second and third 
substrates. 
The present invention provides for a stripline tapped-line hairpin filter 
wherein the hairpin resonators are located alternately on opposite 
surfaces of a substrate. The first and last hairpin resonators are tapped 
to permit signal input and output of the filter. The hairpin resonator 
carrying substrate is sandwiched between two groundplanes insulated 
therefrom by a pair of dielectric substrates. The stripline tapped-line 
hairpin filter allows for a bandpass filter which has a greater band-width 
than previous designs. The alternately formed hairpin resonators may be 
spaced in overlapping fashion to obtain coupling that was physically 
impossible in microstrip application where the reasonators were located on 
a single surface. The tapped input and output hairpin resonators permit 
direct input and output coupling to the filter without the limitations of 
additional parallel coupled end sections and the physical spacing 
limitations associated therewith.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention comprises a novel stripline tapped-line hairpin 
filter. Referring to FIG. 1, there is shown a stripline tapped-line 
hairpin filter 10. Filter 10 includes a dielectric substrate 12 upon which 
a plurality of N hairpin resonators, 14-18, are disposed thereupon in 
alternating sequence on opposite surfaces of substrate 12. Each hairpin 
resonator is comprised of a conductive material. Although only five 
hairpin resonators are illustrated in FIG. 1, the disclosure of the 
present invention is not limited to five hairpin resonators. 
Substrate 12, with hairpin resonators 14-18 disposed thereon, is sandwiched 
between a pair of dielectric substrates 20 and 24. It is preferred that 
substrates 20 and 24 be coextensive with and parallel to substrate 12. In 
addition, it is preferred that substrates 20 and 24 have the same 
dielectric constant as substrate 12. 
Substrates 20 and 24 have respectively disposed upon a surface thereof, 
electrically conductive groundplanes 22 and 26. Groundplane 22 is disposed 
on a surface of substrate 20 which is opposite a surface of substrate 20 
facing hairpin resonators 14, 16, and 18. Groundplane 26 is similarly 
disposed on a surface of substrate 24 which is opposite the surface facing 
hairpin resonator 15 and 17. 
Respectively disposed adjacent to groundplanes 22 and 26 are electrically 
conductive plates 28 and 30. Conductive plates 28 and 30 are used in 
holding the stripline structure together. Conductive plates 28 and 30 are 
substantially coextensive with and parallel to groundplanes 22 and 26. 
Referring to FIG. 2, there is shown the stripline tapped-line hairpin 
filter of FIG. 1 in a side plan view with the component parts mounted 
together. Hairpin resonators 14, 16, 18, are disposed on an upper surface 
of substrate 12 while hairpin resonators 15 and 17 are disposed on a lower 
surface of substrate 12. Thus, the hairpin resonators 14-18 are 
alternately disposed on opposite surfaces of substrate 12. The dimension B 
is indicated as being the thickness of filter 10 between groundplanes 22 
and 26. The dimension B used in Filter 10 is the same dimensional 
groundplane-to-groundplane thickness used in a test structure wherein the 
coupling coefficient versus spacing is experimentally determined. 
As illustrated in FIG. 2, the stripline components are mounted adjacent 
each other between conductive plates 28 and 30. The filter components may 
be clamped together by screws (not shown) or bonded together by means well 
known in the art. 
FIG. 3 illustrates a top plan view of dielectric substrate 12 having 
disposed upon a top surface thereof, hairpin resonators 14, 16, and 18. 
Disposed upon a bottom surface of substrate 12 are hairpin resonators 15 
and 17. Hairpin resonators 14, 16, and 18 are disposed upon the top 
surface of substrate 12 such that there exists an opposite orientation 
from hairpin resonators 15 and 17 located on the bottom surface of 
substrate 12. Hairpin resonators 14, 16 and 18 are arranged in a parallel 
spaced relationship with respect to each other. Hairpin resonators 15 and 
17 are also arranged in a parallel spaced relationship with respect to 
each other. 
Each hairpin resonator formed on substrate 12 is comprised of a pair of 
spaced parallel conductive members interconnected at one end by a third 
member perpendicular to the parallel spaced members. For example, hairpin 
resonator 16 is comprised of parallel spaced members 32 and 34. Parallel 
members 32 and 34 are interconnected at one end by perpendicular member 
36. The ends of parallel members 32 and 34 are therefore shorted by 
perpendicular member 36. The ends of parallel members 32 and 34 opposite 
perpendicular member 36 are therefore open circuited. 
The first and last hairpin resonators on substrate 12 are hairpin 
resonators 14 and 18. Hairpin resonators 14 and 18 are tapped to permit 
signal input and output for the filter. Hairpin resonator 14 includes a 
conductive tapping member 38 disposed on substrate 12 which 
perpendicularly intersects an outermost parallel member of the paired 
parallel members of hairpin resonator 14, with respect to the adjacent 
hairpin resonator 15. Conductive member 38 extends outwardly from the 
outermost one of the paired parallel members of hairpin resonator 14 along 
substrate 12 in a direction away from hairpin resonator 15 and toward one 
end of substrate 12. Hairpin resonator 18 includes a conductive tapping 
member 40 disposed upon substrate 12 which perpendicularly intersects with 
an outermost parallel member of the paired parallel members of hairpin 
resonator 18, with respect to the adjacent hairpin resonator 17. 
Conductive member 40 extends outwardly from the outermost one of the 
paired parallel members of hairpin resonator 18 along substrate 12 in a 
direction away from hairpin resonator 17 towards the other end of 
substrate 12. 
Hairpin resonators 14, 15, 16, 17, and 18 are illustrated in FIG. 3 as 
having parallel members, which at the end opposite the perpendicular 
member have squared off corners. In addition, hairpin resonators 14, 15, 
16, 17, and 18 have squared off exterior corners, at the end where the 
parallel and perpendicular members intersect. 
FIG. 4 illustrates a sectional view taken across line 4--4 of substrate 12 
of FIG. 3. In the preferred embodiment of the hairpin resonators, all of 
the parallel, perpendicular, and tapping members are fixed at an equal 
stripwidth W. Thus, the stripwidth of parallel members 32 and 34, 
perpendicular member 36, and tapping members 38 and 40 are equal. Since 
adjacent hairpin resonators are formed alternately on opposite surfaces of 
substrate 12 they may be spaced apart or overlap in the plane parallel to 
the surface of substrate 12. For example, hairpin resonators 14 and 15 are 
separated in the plane parallel to the surface of substrate 12 i.e., 
lateral direction, by a gap space defined as S.sub.12. Hairpin resonators 
15 and 16 are separated in the lateral direction by space S.sub.23. 
Hairpin resonators 16 and 17 are separated in the lateral direction by 
space S.sub.34 while hairpin resonators 17 and 18 are separated in the 
lateral direction by space S.sub.45. The thickness of substrate 12 is 
defined by the thickness H. 
FIG. 5 illustrates an alternate embodiment of the hairpin resonators formed 
upon substrate 12a. Hairpin resonators 14a, 15a, 16a, 17a, and 18a are 
formed as previously described on alternate surfaces of substrate 12a. 
Hairpin resonators 14a and 18a have respectively formed therewith upon 
substrate 12a tapping member 38a and 40a. Each of the hairpin resonators 
in FIG. 5 comprise parallel spaced conductive members having a 
perpendicular member intersecting the parallel members at one end thereof. 
Hairpin resonator 16a is comprised of parallel members 32a and 34a. 
Parallel members 32a and 34a are interconnected at one end by 
perpendicular member 36a. At the end opposite of perpendicular member 36a, 
the edges of parallel members 32a and 34a are rounded in the plane 
parallel to substrate 12a. At the end of parallel members 32a and 34a 
where perpendicular member 36a intersects thereat the corners exterior to 
parallel members 32a and 34a are also rounded in the plane parallel to the 
surface of substrate 12a. 
The present invention takes advantage of parallel coupling between adjacent 
hairpin resonators located on opposite surfaces of a substrate. To design 
a hairpin resonator filter according to the present invention, the singly 
loaded Q (Q.sub.s) of the first and last hairpin resonators produced by 
tapping and the coupling coefficient (K) must be determined. The article 
entitled "Microstrip Tapped-Line Filter Design" previously described, 
discusses a technique for experimentally determining the coupling 
coefficients of a pair of hairpin resonators. 
Using the experimental procedure to determine the coupling coefficients, a 
plurality of hairpin resonators are disposed alternately on opposite 
surfaces of a selected dielectric substrate material having a fixed 
reference thickness (H.sub.1). The hairpin resonators are disposed such 
that the spacing between hairpin resonators varies in a lateral direction, 
with reference to the surfaces of the substrate, from overlapping to wide 
spacing between adjacent hairpin resonators. The dielectric material 
selected for the substrate has a fixed dielectric constant (E.sub.R). Each 
hairpin resonator has an equal reference stripwidth (W.sub.1) when 
disposed upon the surface of the substrate. The hairpin resonator carrying 
substrate is then disposed between a pair of substrates having a fixed 
thickness and being of the same dielectric material and permittivity as is 
the hairpin resonator carrying substrate. Groundplanes are disposed upon 
parallel surfaces of the substrate materials exterior to the stack of 
substrates, which sandwich the hairpin resonator carrying substrate. The 
distance between groundplanes becomes a fixed reference thickness 
(B.sub.1). 
Upon constructing the test structure as just described, the coupling 
coefficient (K) of each spaced pair of adjacent hairpin resonators may be 
determined. 
A frequency generator is capacitively coupled to a first hairpin resonator 
at the open-circuited end. The second or adjacent hairpin resonator 
located on the opposite surface of the substrate has the parallel members 
connected at the open-circuited end to provide an RF short. A detector 
circuit is used to detect the response of the first hairpin resonator with 
the second resonator RF shorted. The detected response reveals frequency 
f.sub.0 which corresponds to a single-tuned circuit resonant frequency. 
The connection at the open-circuited ends of the second adjacent hairpin 
resonator is now removed to create a double-tuned circuit. A detector 
circuit is again used to detect the response of the pair of hairpin 
resonators. The detected response reveals frequencies f.sub.1 and f.sub.2. 
The coupling coefficient (K) of the pair of hairpin resonators is 
represented by the following relationship: 
EQU K=(f.sub.2 -f.sub.1)/f.sub.0 [ 1] 
where f.sub.2 is the double-tuned circuit higher resonant frequency, 
f.sub.1 is the double-tuned circuit lower resonant frequency, and f.sub.0 
is the single-tuned circuit resonant frequency. 
This process is repeated for various resonator-pair spacings. A curve is 
thus generated wherein the coupling coefficient is plotted as a function 
of the spacing between adjacent resonators located on opposite surfaces of 
the substrate for a fixed H and W/B ratio. A representative curve is 
illustrated in FIG. 6. The negative spacing in the spacing axis indicates 
spatial overlap of adjacent hairpin resonators located on opposite 
surfaces of the substrate. The curve of FIG. 6 is then used in the design 
of a hairpin filter having substrates with identical thicknesses and 
dielectric constant along with identical dimensions of W and B as of the 
test structure. 
Using the experimentally determined coupling coefficients a five pole 
Chebyshev filter may be designed and constructed. In this case the chosen 
filter ripple characteristic is equal to 0.001 dB and the passband is 
equal to 0.167f.sub.0 (f.sub.0 being the resonant frequency of the 
filter). The substrate material used for the filter is a double side one 
ounce copper clad fiberglass material which has a dielectric constant, 
E.sub.R, of 2.45. The hairpin resonator stripwidth is W.sub.1, and the 
thickness between groundplanes, B.sub.1, is 0.130 inches. The normalized 
coupling coefficient (k) and the normalized q for the five pole Chebyshev 
filter are obtained from the publication Reference Data For Radio 
Engineers, Sixth Edition, International Telephone and Telegraph Corp., 
Howard W. Sams Co. Inc., wherein: 
EQU q.sub.2 =q.sub.3 =q.sub.4 =.infin. 
EQU q.sub.1 =q.sub.5 =0.822 
EQU k.sub.12 =k.sub.45 =0.845 
EQU k.sub.23 =k.sub.34 =0.545 
The symbol q.sub.1 designates the normalized q for the first hairpin 
resonator, q.sub.2 designates the normalized q for the second hairpin 
resonator, and so on for q.sub.3 through q.sub.5. The symbol k.sub.12 
designates the normalized coupling coefficient between the first and 
second hairpin resonators, k.sub.23 designates normalized the coupling 
coefficient between the second and third hairpin resonators, and so forth. 
The normalized coupling coefficients (k) to the actual coupling 
coefficients (K) is related by the following expression: 
##EQU1## 
where BW.sub.3 dB is the filter 3 dB bandwidth and f.sub.0 is the filter 
center frequency. The 3 dB bandwidth for the five pole Chebyshev filter is 
found from the referenced publication Reference Data For Radio Engineers 
as follows: 
##EQU2## 
where A=Bandwidth Ratio between ripple level and 3 dB points. 
Therefore by determining the filter 3 dB bandwidth the coupling 
coefficients (K) may be calculated. The coupling coefficient for the first 
and second hairpin resonators, respectively hairpin resonators 14 and 15 
of FIG. 1, are as follows: 
##EQU3## 
Similarly, the coupling coefficients may be calculated for the remaining 
hairpin resonators. Therefore, for hairpin resonators 15 and 16 the 
coupling coefficient K.sub.23 =0.116 and for hairpin resonator 16 and 17 
the coupling coefficient K.sub.34 =0.116. The coupling coefficient of 
hairpin resonators 17 and 18 is K.sub.45 =0.179. 
Summarizing the above, 
EQU K.sub.12 =K.sub.45 =0.179 and 
EQU K.sub.23 =K.sub.34 =0.116. 
Having determined the coupling coefficients of the hairpin resonators, the 
spacing (S) between the hairpin resonators can be found from the curve 
illustrated in FIG. 6. In FIG. 4 the spacing between hairpin resonators 14 
and 15 is designated as spacing, S.sub.12. The spacing between hairpin 
resonators 15 and 16 is designated as spacing, S.sub.23. The spacing 
between hairpin resonators 16 and 17 is designated as spacing, S.sub.34 ; 
while the spacing between hairpin resonators 17 and 18 is designated as 
spacing, S.sub.45. 
Since coupling coefficients K.sub.12 =K.sub.45, then from FIG. 6 spacings 
S.sub.12 =S.sub.45 wherein spacing S.sub.12 and S.sub.45 are determined to 
be 0.002 inches. Accordingly, since coupling coefficients K.sub.23 
=K.sub.34, then from FIG. 6 spacings S.sub.23 =S.sub.34 wherein spacing 
S.sub.23 and S.sub.34 are determined to be 0.017 inches. 
Summarizing the above, 
EQU S.sub.12 =S.sub.45 =0.002 inches 
EQU S.sub.23 =S.sub.34 =0.017 inches 
The hairpin resonator spacings discussed above are not difficult to realize 
since all the hairpin resonators are etched alternately on opposite 
surfaces of substrate 12. In the present embodiment of the invention it is 
preferred that substrate 12 have a thickness (H.sub.1) of 0.010 inches. 
FIG. 7A is a schematical representation of a typical single tapped hairpin 
resonator 42. Hairpin resonator 42 comprises spaced parallel members 44 
and 46 intersected at one end by perpendicular member 48. Hairpin 
resonator 42 also comprises tapping member 50 which perpendicularly 
intersects a parallel member, and as illustrated intersects parallel 
member 44. Hairpin resonator 42 is one-half wavelength (.lambda./2) long, 
measured from the end of parallel member 46 opposite perpendicular member 
48 along parallel member 46 towards perpendicular member 48 (dimension 
L.sub.1) plus from perpendicular member 48 along parallel member 44 to the 
end opposite perpendicular member 48 (dimension L.sub.2). The distance 
L.sub.1 =L.sub.2 and L.sub.1 +L.sub.2 =.lambda./2. Thus, the distance 
L.sub.1 and L.sub.2 are each one-quarter wavelength (.lambda./4). The 
length l is used for calculating the position of tapping member 50 along 
parallel member 44. The length l is a portion of the length L.sub.1, 
measured from the intersection of perpendicular member 48 with parallel 
member 44 along the length of parallel member 44 to the intersection of 
tapping member 50 with parallel member 44 at tap point 52. 
Assuming minimum and negligible coupling between the parallel members 44 
and 46, the equivalent circuit of tapped hairpin resonator 42 is 
illustrated in FIG. 7B. At near resonance the input admittance (Y) at the 
tap point 52 is defined by the following expression. 
##EQU4## 
where Y.sub.0 is the characteristic admittance, Q.sub.S is the singly 
loaded Q, and f is the band edge frequency, provided that 
##EQU5## 
and 
##EQU6## 
where .theta. is the half wave electrical length. 
Therefore, the expression defining the singly loaded Q, (Q.sub.s), is as 
follows: 
##EQU7## 
where 
##EQU8## 
and R is the generator impedance and Z.sub.0 is the filter internal 
impedance. 
Using equation [8] and the following equation [10]: 
##EQU9## 
where Q.sub.s is the singly loaded Q tapped into the hairpin resonator, 
and q is the normalized loaded Q. For the exemplified five pole Chebyshev 
filter where Q.sub.1 =Q.sub.5, the corresponding singly loaded Q (Q.sub.1 
and Q.sub.5) may be determined as follows: 
##EQU10## 
The tap point location is then calculated using equation [8] where R and 
Z.sub.0 are 50 ohms and 1/L is found to be approximately 0.43. Since 
L.sub.1 =L.sub.2 =.lambda./4 at f.sub.0 with the dielectric constant 
E.sub.R =2.45, L may be calculated from the following expression: 
##EQU11## 
where C equals the speed of light (3.times.10.sup.8 meters/second) and 
f.sub.0 is the resonant frequency. The distance l, being the distance from 
the short-circuited ends of the hairpin resonators to the tapping member, 
may then be calculated from equation [9]. 
Referring to FIG. 8, Curves A and B respectively illustrate the filter 
response of a five pole Chebyshev square corner hairpin filter and a round 
corner hairpin filter of the present invention. Although both filters are 
designed at a center frequency f.sub.0 the round corner hairpin filter 
embodiment is centered slightly higher than the square corner hairpin 
filter embodiment. This effect is a result of the round corners physically 
shortening the resonator length. Therefore, the round corner hairpin 
resonators electrically appear shorter with a resulting higher frequency. 
The exemplary embodiment described herein illustrates a five pole filter, 
however, it is possible that an even number pole filter may be used. With 
an even number pole filter, the input and output tapped hairpin resonators 
would be located on opposite surfaces of a substrate. An over-and-under 
stripline connection would be required to couple one of the tapped members 
to a stripline input/output connection. 
The previous description of the preferred embodiments are provided to 
enable any person skilled in the art to make or use the present invention. 
Various modifications to these embodiments will be readily apparent to 
those skilled in the art, and the generic principles defined herein may be 
applied to other embodiments without the use of the inventive faculty. 
Thus, the present invention is not intended to be limited to the 
embodiment shown herein, but is to be accorded the widest scope consistent 
with the principles and novel features disclosed herein.