Claims:1. A low loss harmonic suppression filter for achieving harmonic suppressions, the filter comprising:
a cavity housing (X), wherein the cavity housing forms the combline configuration in slab line fashion for achieving compact size and low loss;
a stepped impedance resonators (A) positioned horizontally inside the cavity housing, wherein the stepped impedance resonators consists of low and high impedance sections (C) for forming step in the resonators for harmonic suppressions;
a short circuit section of the resonator (D) positioned at one end of cavity housing of the filter (base of stepped impedance resonators) is used to suppress the harmonic at 2fo;
a additional grounding walls (E) positioned horizontally to the base of each stepped impedance resonators (A), wherein the additional grounding walls are used to suppress the waveguide modes at least upto 3.5fo;
a tapped line as a input and output co-axial feed (B) coupled with first and last resonators, wherein the optimized location (F) of tapped line feeding is for better matching at the input and output of the filter; and
a tuning rods (G) positioned over the cavity housing which is exactly above each stepped impedance resonators, wherein the tuning rods are used to change the frequency of operation for any other applications (0.8GHz to 1.7GHz).

2. The filter as claimed in claim 1, wherein the short circuited resonators comprising of open circuit at one end.

3. The filter as claimed in claim 1, wherein the short circuited resonators with Low and high impedance section is used for harmonic suppressions at 3fo (thrice the centre frequency of operation).

4. The filter as claimed in claim 1, wherein the total length of the low and high impedance resonator is 32.4mm.

5. The filter as claimed in claim 1, wherein the resonators along with the coupling generates the required band pass characteristics in the filter.

6. The filter as claimed in claim 1, wherein the stepped impedance resonators (C) reduce the phase velocity of the signal and helps in reducing the size of the filter.

7. The filter as claimed in claim 1, wherein the spacing (H) between the stepped impedance resonators (A) determines the bandwidth of the filter.

8. The filter as claimed in claim 1, wherein the length of the stepped impedance resonators L determines the centre frequency of the filter.

9. The filter as claimed in claim 1, wherein the optimized length, width and height of the filter is 9cm, 3.9cm and 2cm respectively to achieve the required rejection and insertion loss.
, Description:FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10, rule 13)
“A Low Loss Harmonic Rejection Filter”
By
Bharat Electronics Limited,
Central Research Laboratory
Jalahalli P.O., Bangalore – 560013

The following specification particularly describes the invention and the manner in which it is to be performed.

Field of the Invention
The present invention mainly relates to a low loss and compact band pass filter and more particularly to harmonic spurious signal suppression filter for suppressing harmonically related spurious signals.
Background of the invention
Research demand exists for the wireless communication products in the recent years. Every communication transmitter and receiver while generating the desired signals also generates undesired harmonics and spurious signals. The spurious emissions are well known in the art which is a radio frequency not deliberately created or transmitted, especially in a device which normally does create other frequencies. The harmonic or other signal outside a transmitter's assigned channel would be considered a spurious emission/signal. These spurious signals produce electromagnetic interferences to nearby systems. To circumvent this problem, filters are used with harmonic suppression characteristics. Traditionally, parallel coupled line filters have been used in many communication devices to achieve narrow fractional bandwidth due to their relatively weak coupling. This type of filter has desirable advantages such as low-cost fabrication and easy integration. However, parallel coupled line filters have problems such as longer size and the presence of harmonics. This second and third harmonics of the filter will degrade the performance of system components such as mixers, amplifiers and oscillators in any wireless communication product.
Harmonics are generated in conventional parallel coupled line filters by the large difference between the even and odd-mode effective dielectric constants of the micro strip coupled lines. But in practical, the velocities become unequal. Equalization of phase velocities at 2fo (twice the centre frequency of operation) helps in cancellation of even and odd mode response at 2fo. There are several ways reported in literature to suppress the harmonic frequencies in the filter. Researchers have added reactive components along with the filter to reject the harmonic frequencies at 2fo. In addition, the Lumped elements have been loaded along with resonators loads for the purpose of harmonic suppression. Defected ground structures and dielectric overlays are also used to achieve harmonic rejection.
Further, techniques use either stepped impedance resonator, wiggly coupled line resonator or mostly relies on choosing the proper dimensions for coupled lines and air filled cavity depths for PCB to achieve equalization of phase velocities at 2fo. In addition, spurious suppression in micro strip line filter is achieved using floating conductors in ground. Using periodical non-uniform micro strip coupled lines, spurious rejection has been achieved.
For example, document US 5893026 describes “Low pass filter for suppressing harmonic of radio transmitter” describes about a low pass filter capable of passing a radio paging signals of good quality through suppressing harmonic in a radio transmitter. A filter for suppressing second and third harmonic or more of the radio paging signal is connected between the first filter and an antenna. This harmonic suppression filter consists of multistage capacitors and parallel inductances. But this filter allows the low frequency contents below the cut off frequency.
Further, document US 8354898 B2 describes “Harmonic suppression resonator, harmonic propagation blocking filter, and radar apparatus” explains about a harmonic suppression resonator comprising of a plurality of waveguide resonators respectively including resonant regions in each of which a fundamental wave resonates in TE mode, the harmonic suppression resonator having adjoining waveguide resonators therein coupled with each other via a coupling window. This filter offers harmonic rejection, but it occupies more space.
Further, document US 7098759 B2 describes “Harmonic spurious signal suppression filter” provides a harmonic suppression filter where input and output sections of the filter are connected through inter digital circuit and meandered micro-strip lines. In this invention, the size of the filter is largely reduced by the use of the filter that is fabricated on the printed circuit board of the wireless network product, such that the micro strip input end, a first, second, third and fourth micro strip line and a micro strip output end is disposed on the printed circuit board. The filter thus uses only a limited space on the printed circuit board, which can largely suppress the high frequency harmonic spurious signal generated. This filter can be useful for low to medium power handling applications.
Furthermore, document US 5929721 A describes “Ceramic filter with integrated harmonic response suppression using orthogonally oriented low-pass filter ” has a ceramic monolithic block filter having a predetermined pass band defined by tuned resonators located between an input and an output and at least one of a harmonic trap filter, a low pass filter having an inductive and a capacitive component. This is achievable with a design which incorporates an integrated harmonic response suppression filter directly in or on the dielectric ceramic monolithic block. But the cascade of filters may offer higher insertion loss.
Also, in document US 3496497 A describes “High-power harmonic suppression filters” comprises a coaxial transmission line along which high-frequency electromagnetic energy may be transmitted, a waveguide surrounding part of the length of the coaxial line and formed as a septate coaxial waveguide or lunar line having as one member thereof the outer member of the coaxial line, coupling means between the coaxial line and waveguide, and energy absorption means in the waveguide multiplexer a pulse generating circuit and a buffer circuit. The buffer circuit is coupled to the pulse generator and includes a switch circuit and a common mode buffer.
All of the above mentioned prior art have their own limitations. In view of the above, there is a need in the art for harmonic spurious signal suppression filter to solve the above mentioned limitations.

Summary of the Invention
An aspect of the present invention is to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below.
Accordingly, in one aspect of the present invention relates to a low loss harmonic suppression filter for achieving harmonic suppressions, the filter comprising: a cavity housing (X), wherein the cavity housing forms the combline configuration in slab line fashion for achieving compact size and low loss, a stepped impedance resonators (A) positioned horizontally inside the cavity housing, wherein the stepped impedance resonators consists of low and high impedance sections (C) for forming step in the resonators for harmonic suppressions, a short circuit section of the resonator (D) positioned at one end of cavity housing of the filter (base of stepped impedance resonators) is used to suppress the harmonic at 2fo, a additional grounding walls (E) positioned horizontally to the base of each stepped impedance resonators (A), wherein the additional grounding walls are used to suppress the waveguide modes at least upto 3.5fo, a tapped line as a input and output co-axial feed (B) connected/coupled with first and last resonators, wherein the optimized location of tapped line feeding (F) is for better matching at the input and output of the filter, and a tuning rods (G) positioned over the cavity housing which is exactly above each stepped impedance resonators, wherein the tuning rods are used to change the frequency of operation for any other applications (0.8GHz to 1.7GHz).
Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention.
Brief description of the drawings
The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which:
Figure 1 illustrates the low loss harmonic suppression filter according to one embodiment of the present invention.
Figure 2 shows the Stepped Impedance Resonators according to one embodiment of the present invention.
Figure 3 shows the single section of Stepped Impedance Resonator according to one embodiment of the present invention.
Figure 4 illustrates the realized harmonic suppression filter according to one embodiment of the present invention.
Figure 5 shows the measured results of harmonic suppression filter according to one embodiment of the present invention.
Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure. Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.
The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces.
By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide.
Figs. 1 through 5, discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way that would limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged communications system. The terms used to describe various embodiments are exemplary. It should be understood that these are provided to merely aid the understanding of the description, and that their use and definitions, in no way limit the scope of the invention. Terms first, second, and the like are used to differentiate between objects having the same terminology and are in no way intended to represent a chronological order, unless where explicitly stated otherwise. A set is defined as a non-empty set including at least one element.
The present invention relates to the design of low loss band pass filter in slab line configuration with harmonic suppressions for wireless communication systems. In this filter, harmonic frequencies are re-located or moved away from its original locations. Harmonic rejection filters are used in wide variety of applications like amplifiers, oscillators and microwave mixers to suppress the unwanted frequency components. The invented filter circuit consists of slab line stepped Impedance resonators for harmonic suppressions and tapped line feeding structure for the purpose of miniaturization. The proposed filter is made up of short circuited resonators and metallic housing. Step (Low and High impedance sections) in the resonators reduces the physical size of the overall resonator into less than quarter wave at the centre frequency of operation. This characteristics moves the harmonic frequencies beyond or below its fundamental locations (twice or thrice the centre frequency).
The present invention filter circuit uses stepped impedance combline structure and tapped line mechanism. Combline structure rejects the harmonics at 2fo (twice the centre frequency of operation). Further steps in the resonator move the spurious frequency away from 3fo (thrice the centre frequency of operation). An experimental band pass filter designed over 1.435GHz to 1.535GHz is used to illustrate the present innovation. This experimental filter offers stop band region up to 4.9GHz (3.3fo) with rejection better than 40dB. Second and third harmonics (2fo and 3fo) are rejected to the level of more than 80dB and 50 dB respectively.
The present invention provides a band pass filter designed in slab line configuration for suppressing harmonic signals. This harmonic suppression filter uses stepped impedance resonators (SIRs) in combline structure with tapped line feeding to build harmonic suppression characteristics. Filter uses coupled resonators with low and high impedance sections for establishing the band pass characteristics and the harmonic rejections. The second harmonic of the filter is suppressed by resonators short circuited on the wall whereas third harmonic is suppressed by chosen SIR structure. Harmonics are moved away from 3fo by properly choosing the impedance ratio in SIR. Tapped line mechanism is used to achieve impedance matching and to avoid the transformers. This type of filter is well suited for wireless communication system products, where harmonic suppression is essential. Validation of the device is explained through the design of an experimental harmonic rejection filter.
The harmonic spurious signal suppression filter comprising of resonators with stepped impedance lines made in slab line configuration for suppressing harmonically related spurious signals upto three times of the fundamental frequency which are generated from a nonlinear distortion of a power amplifier of the wireless communication systems.
Figure 1 illustrates the low loss harmonic suppression filter according to one embodiment of the present invention.
Figure 2 shows the Stepped Impedance Resonators according to one embodiment of the present invention.
The figure illustrates the low loss harmonic suppression filter. The present invention is a low loss harmonic suppression filter and a method for designing and achieving harmonic suppressions with compact size. Band pass filters with wide stop band are essential component in modern communication systems. Classical-parallel coupled line filters are mostly used due to its simplicity and easy synthesis procedure. However, when the classical filters are realized, it suffers from poor upper stop band performance and have spurious frequencies located at harmonics of the fundamental pass band frequency 'fo'. This behaviour degrades the overall performance of system.
The invention reports yet another technique for suppressing the spurious response at 2fo and 3fo. The design of band pass filter for harmonic suppressions with low loss and compact size according to the present invention may be better understood with reference to the drawings. This invention claims a low loss harmonic suppression filter and as used herein and harmonic suppressions are upto 3.3fo.
The electrical specifications of the filter are listed in Table I. As shown in Fig. 1, the main elements of the filter are tapped input and output feed lines, Impedance resonators with steps, coupling mechanism, short circuit at one end of the resonators and Tuning rods. The filter is designed with the width of 39mm and height of 20mm.
This present invention provides a method for achieving harmonic suppressions in band pass filter with low loss and miniaturized size and a method for suppressing waveguide modes in filter’s electrical characteristics. This harmonic suppression filter comprises: Cavity housing (X), Stepped Impedance Resonators (A), tapped line (B) Low and High Impedance sections in SIR (C), Short Circuit Section of the Resonator (D), Additional Grounding walls (E), Tapping Location (F), and Tuning rods (G). When a signal is fed into the filter through tapped lines B, coupling occurs between the stepped impedance resonators as shown in Figures 1-2.
The low loss harmonic suppression filter for achieving harmonic suppressions, the filter comprising: a cavity housing (X), wherein the cavity housing forms the combline configuration in slab line fashion for achieving compact size and low loss, a stepped impedance resonators (A) positioned horizontally inside the cavity housing, wherein the stepped impedance resonators consists of low and high impedance sections (C) for forming step in the resonators for harmonic suppressions, a short circuit section of the resonator (D) positioned at one end of cavity housing of the filter (base of stepped impedance resonators) is used to suppress the harmonic at 2fo, a additional grounding walls (E) positioned horizontally to the base of each stepped impedance resonators, wherein the additional grounding walls are used to suppress the waveguide modes at least upto 3.5fo, a tapped line as a input and output co-axial feed (B) coupled/connected with first and last resonators, wherein the optimized location (F) of tapped line feeding is for better matching at the input and output of the filter and a tuning rods (G) positioned over the cavity housing which is exactly above each stepped impedance resonators, wherein the tuning rods are used to change the frequency of operation for any other applications (0.8GHz to 1.7GHz).
The key elements of the filter are listed below.
Cavity housing (X): Cavity housing forms the combline configuration in slab line fashion for achieving compact size and low loss.
Stepped Impedance Resonators (A): This consists of low and high impedance sections for forming step in the resonators for harmonic suppressions.
Short Circuit Section of the Resonator (D): This is used to suppress the harmonic at 2fo.
Additional Grounding walls (E): These are used to suppress the waveguide modes at least upto 3.5fo.
Tapping Location (F): Tapping location plays a prime role for achieving impedance matching in the filter
Tuning rods (G): These will be used to change the frequency of operation for any other applications.
Figure 3 shows the single section of Stepped Impedance Resonator according to one embodiment of the present invention.
The figure shows the single section of Stepped Impedance Resonator. Resonators along with the coupling generate the required band pass characteristics in the filter. The tapped line B and tapping location F determines the matching at the input and output. Stepped impedance resonators C reduce the phase velocity of the signal and this helps in reducing the size of the filter. The short circuited section D is a prime factor for the harmonic suppression at 2fo. Stepped impedance resonator is a prime factor for the harmonic suppression at 3fo. The harmonic can be moved away from 3fo based on the impedance ratio of the SIR. Presently, the impedance ratio (High impedance/Low impedance) is 0.52 to move the harmonics to 3.3fo. The spacing between the resonators H determines the bandwidth of the filter. The length of the stepped impedance resonators L determines the centre frequency of the filter. The total length of the low and high impedance resonator is 32.4mm. Additional grounding walls E are used in the filter as shown in Fig. 1 for waveguide mode suppression and the tuning rods shown G in Fig. 5 are useful for changing the frequency of operation for any other applications. The filter is designed with the specifications listed in Table 1 as per the following steps.
• Calculation of coupling coefficients using filter’s prototype values, center frequency and bandwidth.
• Determining the spacing between the Resonators for the required coupling co-efficient
• Finding the Tapping Location for better impedance matching
• Finding the required resonator lengths using the impedance steps and center frequency of operations
Figure 4 illustrates the realized harmonic suppression filter according to one embodiment of the present invention. The figure 4 illustrates the assembled harmonic suppression filter. The assembled harmonic suppression filter comprises a tuning rod over the harmonic suppression filter which is used for changing the frequency of operation for any other applications.
Figure 5 shows the measured results of harmonic suppression filter according to one embodiment of the present invention.
The figure shows the measured results of harmonic suppression filter obtained from the HFSS simulator. The measured bandwidth of the filter is from 1435MHz to 1535MHz. Table II lists the measured pass band electrical performance of the filter. The maximum insertion loss of the filter over the band is 0.6dB. Harmonic rejection characteristics of the filter are listed in Table III. This harmonic suppression filter offers stop band region up to 4.9GHz (3.3fo) with rejection better than 40dB.Second and third harmonics (2fo and 3fo) are rejected to the level of more than 80dB and 50 dB respectively. The filter has been tested for 100W power handling and is working for the required specifications. The minimum gap between the lines in the structure of the filter is 3.3mm. Electromagnetic analysis was carried out to identify the maximum field strength at the spacing for the power levels of 100W (required) and 500W. Even for 500W, the filed strength doesn’t exceed the breakdown filed strength of air. The optimized width and height of the invented filter is 3.9cm and 2cm respectively to achieve the required rejection and insertion loss. Overall the filter is compact and measures the size of 9cm× 3.9cm×2cm.
Table I: Filter specifications
Parameters Values
Frequency of Operation 1.435GHz to 1.535GHz
Insertion loss Less than 0.5dB
Return loss Better than 15dB
Harmonic suppression at
2fo Better than 70dB
and 3fo Better than 50dB
Table II. Measured pass band performance
Frequency Insertion Loss Return loss
1435MHz 0.6dB 20dB
1485MHz 0.5dB 18dB
1535MHz 0.6dB 20dB

Table III. Harmonic rejection performance
Frequency Rejection
2970MHz Better than 80dB
4455MHz Better than 50dB

The unique features of present invention are:
• Usage of short circuited resonators
• Usage of Low and high impedance resonators
• Optimized filter housing for suppressing the waveguide modes in the filter’s characteristics
• Optimized length and width of impedance steps in the resonators
• Tapped lines for achieving size reduction. This removes the usage of transformers at the input and output of the filter.
• Location of tapped lines for achieving impedance matching
• Placement of resonators
• Additional grounding walls for waveguide mode suppression
• Tuning element for changing the filter’s frequency of operation for any other applications between 0.8GHz to 1.7GHz.
Those skilled in this technology can make various alterations and modifications without departing from the scope and spirit of the invention. Therefore, the scope of the invention shall be defined and protected by the following claims and their equivalents.
FIGS. 1-5 are merely representational and are not drawn to scale. Certain portions thereof may be exaggerated, while others may be minimized. FIGS. 1-5 illustrate various embodiments of the invention that can be understood and appropriately carried out by those of ordinary skill in the art.
In the foregoing detailed description of embodiments of the invention, various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments of the invention require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the detailed description of embodiments of the invention, with each claim standing on its own as a separate embodiment.
It is understood that the above description is intended to be illustrative, and not restrictive. It is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein,” respectively.

We claim:
1. A low loss harmonic suppression filter for achieving harmonic suppressions, the filter comprising:
a cavity housing (X), wherein the cavity housing forms the combline configuration in slab line fashion for achieving compact size and low loss;
a stepped impedance resonators (A) positioned horizontally inside the cavity housing, wherein the stepped impedance resonators consists of low and high impedance sections (C) for forming step in the resonators for harmonic suppressions;
a short circuit section of the resonator (D) positioned at one end of cavity housing of the filter (base of stepped impedance resonators) is used to suppress the harmonic at 2fo;
a additional grounding walls (E) positioned horizontally to the base of each stepped impedance resonators (A), wherein the additional grounding walls are used to suppress the waveguide modes at least upto 3.5fo;
a tapped line as a input and output co-axial feed (B) coupled with first and last resonators, wherein the optimized location (F) of tapped line feeding is for better matching at the input and output of the filter; and
a tuning rods (G) positioned over the cavity housing which is exactly above each stepped impedance resonators, wherein the tuning rods are used to change the frequency of operation for any other applications (0.8GHz to 1.7GHz).

2. The filter as claimed in claim 1, wherein the short circuited resonators comprising of open circuit at one end.

3. The filter as claimed in claim 1, wherein the short circuited resonators with Low and high impedance section is used for harmonic suppressions at 3fo (thrice the centre frequency of operation).

4. The filter as claimed in claim 1, wherein the total length of the low and high impedance resonator is 32.4mm.

5. The filter as claimed in claim 1, wherein the resonators along with the coupling generates the required band pass characteristics in the filter.

6. The filter as claimed in claim 1, wherein the stepped impedance resonators (C) reduce the phase velocity of the signal and helps in reducing the size of the filter.

7. The filter as claimed in claim 1, wherein the spacing (H) between the stepped impedance resonators (A) determines the bandwidth of the filter.

8. The filter as claimed in claim 1, wherein the length of the stepped impedance resonators L determines the centre frequency of the filter.

9. The filter as claimed in claim 1, wherein the optimized length, width and height of the filter is 9cm, 3.9cm and 2cm respectively to achieve the required rejection and insertion loss.

Abstract
The present invention mainly relates to a low loss harmonic suppression filter for achieving harmonic suppressions. In one embodiment, the filter comprising: a cavity housing (X), wherein the cavity housing forms the combline configuration in slab line fashion for achieving compact size and low loss, a stepped impedance resonators (A) positioned horizontally inside the cavity housing, wherein the stepped impedance resonators consists of low and high impedance sections (C) for forming step in the resonators for harmonic suppressions, a short circuit section of the resonator (D) positioned at one end of cavity housing of the filter (base of stepped impedance resonators) is used to suppress the harmonic at 2fo, a additional grounding walls (E) positioned horizontally to the base of each stepped impedance resonators (A), wherein the additional grounding walls are used to suppress the waveguide modes at least upto 3.5fo, a tapped line as a input and output co-axial feed coupled with first and last resonators, wherein the optimized location of tapped line feeding is for better matching at the input and output of the filter, and a tuning rods (G) positioned over the cavity housing which is exactly above each stepped impedance resonators, wherein the tuning rods are used to change the frequency of operation for any other applications (0.8GHz to 1.7GHz).
Figure 1(for publication)