Post Coupled

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Post Coupled
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Design, Development and Fabrication of Post-coupled Band pass Waveguide Filter @ 11.2GHz for Radiometer
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Microwave methods, both passive and active, play an important role amongst modern diagnostics of fusion plasma research. Microwave Reflectometer is used to measure plasma density profile and its fluctuations. Electron Cyclotron Emission (ECE) waves which has a frequency range in the microwave as well as millimeter range is used to measure plasma temperature and its fluctuation. ECE frequency depends on toroidal magnetic field used for plasma confinement. Toroidal magnetic field has radial profile. This radial profile gives good radial correlation between cyclotron frequency and spatial location of ECE.

The power level in cyclotron radiation is very low (nW - &umW). A heterodyne radiometer can be used to measure such a low power level. In the Radiometer IF section, the combination of power divider and filter bank is used to resolve the spatial location of the cyclotron radiation. The existing E-band radiometer has 1-14GHz IF frequency. For this frequency range, only 1-10 GHz filter bank is available. Next filter frequency required is 11.2GHz.
With the above requirements, the authors designed, developed and fabricated a post-coupled Band pass waveguide filter @ 11.2GHz. The dimensions of the filter were decided using certain calculation and criteria. After carrying out various simulations using ANSOFT-HFSS, the filter dimensions were optimized for required electrical parameters. Although the attenuation is higher in the post-coupled design in comparison to an iris-coupled construction, the new design outperforms the latter with regards to ease of fabrication and thus time and cost constraints
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Design, Development and Fabrication of Post- coupled Band pass
Waveguide Filter @ 11.2GHz for Radiometer
RAVISH R. SHAH, AMIT PATEL, VED VYAS DWIVEDI, EC Department, Charotar
Institute of Technology, Changa, Gujarat, India.
Microwave Diagnostics, Institute for Plasma Research, Gandhinagar 382 428, Gujarat,
Microwave methods, both passive and active, p lay an important role amongst modern d iagnostics of fusio n
plasma research. Microwave Reflectometer is used to measure p lasma density profile and its fluctuations.
Electron Cyclotron Emission (ECE) waves which has a frequency ran ge in the micr owave as well as
millimeter range is used to measure plasma temperature and its fluctuation. ECE frequency depends on
toroidal magnetic field used for plasma con finement. Toroidal magnetic field has radial profile. This rad ial
profile gives good radial correlation between cyclotron frequenc y and spatial location of ECE.
The pow er lev el in cyclotron radiation is very lo w (nW - W). A h eterodyne radiometer can be used to
measure such a low power level. In the Radio meter IF section, the combination o f power divider and filter
bank is used to resolve the spatial location of the cyclotr on r adiation. T he existing E -band radiometer has 1-
14GHz I F frequency. For this frequenc y ra nge, onl y 1 -10 GHz filter bank is available. Next filter frequency
With the above requirements, the authors designed, developed and fabricated a post-coupled Band pass
waveguide filter @ 11.2GHz. The dimensions of the filter were decided using certain ca lculation and criter ia.
After carrying out various simulations using ANSOFT-HFSS, the filter dimensions were optimized for
required electrical parameters. Although the attenuation is higher in the post-coupled design in comparison to
an iris-coupled construction, the ne w design outperforms the latter with regards to ease of fabrication and thus
time and cost constraints, while achieving the required electrical p erformance.
Key words : Plasma, Electron Cyclotron Emission, Band Pass filter, Radiometer, Tokamak
The Electron Cyclotron Emission (ECE) is used as plasma diagnostic method on a tokamak
fusion researc h device[1]. In the tokamak devices, whic h confine plasmas of sufficient
density and temperature, the ECE offers a way to measure the electron temperature. The
ECE radiate power in the range of nW -µW with microwave as well as millim eter range of
frequency. A Radiometer (60 – 90 GHz) is proposed to measure ECE radiation coming
from Superconducting Steady-state Tokamak-1 (SST-1)[2].
Toroidal magnetic field is used to confine the fusion plasma in the Tokomak. This magnetic
field has a radial profile that gives good radial correlation between cyclotron frequency and
spatial location of the cyclotron radiation. The measured cyclotron freque ncy is decided by
the available bandpass filter in the IF section of the radiometer. A filter frequency of
The Specifications of the filter to be designed include a Center Frequency f c =11.2GHz (X-
band), Max. Bandwidth = ±100MHz, Insertion Loss LA < -1.5dB and Return Loss LB > -
15dB in the pass band. The dimensions of the fi lter are decided usin g certain calculation
and criteria. ANSOFT-HFSS helped in optimization of various specifications, and
determination of filter dimensions and resulting S-parameters.
The first and foremost criterion is selection of the length of the X-band waveguide. For
deciding the length of the waveguide, two conditions were taken into consideration:
(1) The electric field must be highest at both the end of the waveguide (Figure 1)
(2 ) To achieve for optimum coupling, the electric field must be highest at the center point
of the waveguide, where the middle post is kept.
The authors modeled a piece of W R-90 waveguide (X-band) and cut it to a length based on
(1) (f/c) 2 = (l/2d) 2 + (n/2a) 2 + (m/2b) 2
Where f stands for frequency of the wave, C is velocity of lig ht in vacuum (= 3 x 10 11
mm/sec), m and n a re the number of electromagnetic cycles transmitting through
waveguide generally ev en (here, l=8), mode index in E- plane and mode index in H-plane
respectively. The quantities a and b are width (= 22.86mm) and hei ght (= 10.16mm) of
waveguide. The length of the waveguide, which satisfied the two conditions above, came
Now dimensions of the posts were decided based on its behavior[3]. Posts and Screws
made from conductive material can be used for impedance-changing devices in
waveguides. Figure 2 illustrates two basic metho ds of using posts and screws. A post or
screw, which only partiall y penetrates into the waveguide, acts as a shunt capacitive
reactance. When the post or screw extends completely through the waveguide, making
contact with the top and bottom walls, it acts as an inductive reactance. Note that when
screws are used the amount of reactance can be varied.
While deciding on various para meters for the posts, the ra tio of diameter of post to the
width of the waveguide, d/a<0.25, should be satisfied[4], above which dispersion increases.
Moreover, 2a>> >>2a/3 should be followed for the perfect transmission of modes.
The authors tried out various diameters of the posts. While increasing the diameter of the
posts, the center frequency shifts increased, the bandwidth became narrower and the losses
increased (high insertion loss and low return loss). Placing the resonators at one-quarter
guide wavelength intervals resulted in strong interaction among the fringing fields in the
vicinity of the coupled posts. So, instead of one very high peak of attenuation, there were
several relatively low peaks. To avoid this interaction between the fringing fields at the
various resonator posts, the resonators are spaced 3 g/4 apart from each other, with λg
equaling the guide wavelength, shown in equation 2.
(2) g = 0 /[1 – ( 0 / c ) 2 ] 0.5
Where λ 0 is c/ f c (= 26.79mm) for f c equal to 11.2GHz. λ c is equal to 2a(= 45.72mm).
These values of λ 0 and λ c give a value of 33.05 mm for λ g . Varying the length between
the two posts either resulted in dual bands or very low return loss, with the 2 nd post length
slightly varied to be compliant with the given specifications.
ANSOFT-HFSS was used in the optimization o f various specifications, determining the
filter dimensions and obtaining S-parameters. Figure 3 shows the finaliz ed design and the
simulated results. Two posts of diameter 3.05mm and 3.3mm were kept at 76.2mm and
107.81mm respectively fr om the input end of waveguide. The simulated result gave a
bandpass of 200MHz with a peak at 11.3GHz and return loss of -15dB.
The filter was fabricated with the dimensions motioned in table 1. The picture of the filter
is shown in figure 4. T he filter tested by using spectrum analyzer and X-Band voltage
control oscillator. The final testing w as done with a Network analyzer. The test results are
The Spectrum Analyzer shows the power at discrete frequency points. By changing the
bias voltage of the X-b and source, the power at discrete frequencies can be obtained.
Using these values a graph can be plotted, but this would not be an accurate graph as it is
not continuous and therefore cannot be consid ered as the response. Hence final t esting
Testing of Filter using Network Analyzer :
For accurate results, testing was performed with a network anal yzer. A network analyzer
is comprised of a built-in source and receiver , therefore it is quite easy to view the whole
response of this X-band band pass waveguide filter at a glance. The achieved parameters
are presented in table 2. Figure 5 shows the insertion loss and return loss of the filter.
The design of the post-coupled band pass filter is such that the mechanical fabrication is
feasible for in-house facility in mechanical workshop. The designed band pass filter gave
better results than the required specifications. In the existing filter, applying layer of gold
or silver, which has higher conductivity than copper, can reduce the losses. Moreover,
such post-coupled filters result in higher attenuation compared to iris-coupled filters and
hence certain modifications like dumbbell shaped posts or flat posts can attempted for
superior results. The filter is a part of Radiometer on Tokamak at IPR to study the radial
position behavior of cyclotron emission and in turn helping in plasma diagnostic
1 H. B. Pandya and K. K. Jain: “ Electron Cyclotron Emission Measurement Diagnostics
for SST-1 ” Proc. 12 Joint Workshop on ECE and ECRH, Aix- en -Provence, France
2 Bora Dhiraj, “SST and ADITYA Tokamak Research in India” Brazilian Journal of
Physics, vol 32 , no. 1, pp 193 – 216, 2002
3 G. L. Matthaei, Leo Young, and E.M.T. Jones, Microwave Filters, Impedance
Matching Networks, and Coupling Structures., McGraw-Hill, New York, 1964 pp 521-
4 N. Marcuvitz, Waveguide Handbook, Elec tromagnetic waves series, IEE, vol 21
Figure 1 Electric Field propagation in a waveguide
Figure 2 capacitive reactance and inductive reactance
Figure 3 ANSOFT simulated S-parameters of the filter
Figure 4 Picture of the Band pass filter
Figure 5 Frequency response of the band pass filter
... Low-pass and bandpass filters are generally used to suppress higher order harmonics or spurious signal in microwave and millimeter wave system [1, 2]. The realization of these components should be such that they provide high performance with compact size and low cost. ...
... The realization of these components should be such that they provide high performance with compact size and low cost. Numbers of techniques are adopted to design filters such as microstrip-line, waveguide [1] , substrate integrated waveguide [3,4], photonic band gap structure, defected ground structure, ground plane aperture, low temperature co-fire ceramic technology. The function of low-pass filter is to pass low frequency up to certain frequency called as a cutoff frequency. ...
... Volumetric source nature and high radiation efficiency of lowprofile Dielectric Resonator Antenna (DRA) makes it suitable for wideband applications, especially in Microwave and Millimeter wave range where a Patch antenna shows more losses [1] [2][3][4]. Satellite communication and Radar applications prefer circularly polarized antenna (CP) over linearly polarization as it provides flexibility to maintain the exactly same polarization for transmitting and receiving antenna as needed in linearly polarization. Generally circularly polarized nature can be obtained with single or multiple feeds for a limited band of frequencies. ...
This article represents microstrip low-pass filter using defected ground structure (DGS) for the wireless communication application. DGS is used as the basic building block for the realization of a low-pass filter. A low-Pass filter with a 23 dB cutoff frequency of 3.5 GHz has been designed and simulated using high frequency structure simulator. Tuning of frequency is carried out by changing the dimension of DGS. An equivalent LC circuit of proposed structure is derived and simulated it with advanced design system software. Proposed low-pass filter has been fabricated and provides better performance with compact size; it can be used in wireless communication application such as PCS-1900, UMTS, Bluetooth, WLAN, Wi-max, IMS, and RFID.
... Nowadays, multipleband bandpass resonator design is one of the most critical and interesting componenet in RF (radio frequency) and microwave Communication due to rapid technological changes and requirements of users [1] . A wide number of different topologies like planar and nonplanar allow obtaining a specific response to a wide range of applications. ...
... Design of resonators using nonplanar waveguide structures and the analysis of the relevant scattering phenomena has been attracting considerable attention over the past decades [2]. Moreover, large attractions have been carried out used of conductive and dielectric posts filters in rectangulat waveguide, they constitute a strategy of special interest due to their simplicity and easy manufacturing processes associated with these configurations [1] . Several studies have been carried out in the past to improve the electrical characteristics of inductive and capacitive PCWR. ...
... The traditional resonator [1] consists of the classical doublet topology shown in Fig. 1(a) (Where node indicates source, load and cavity resonators and solid lines indicate flow of the electromagnetic field). This structure is known to have a response with one transmission zero if no coupling between the source and the load is considered [8] [10]. ...
This paper demonstrates transverse non planar resonators consisting of symmetrically placed obstacles (posts which made up of same or different type of materials) coupled into rectangular waveguide. Introducing posts inside a waveguide in proposed structure generates discontinuity inside and becomes cavity resonator. It creates a resonance at X band, Ku band and Ka band Frequencies are reported. Simulation results demonstrate the realization of resonators matched to free-space as well as having a superior insertion loss and better return loss of conductive posts compare to dielectric posts. Obstructing the E - field using conductive posts compare to the H-field using dielectric posts are given better insertion loss and impedance matching characteristics. However, Insertion of amalgamated obstacles into rectangular waveguide generates good transmission and reflection zeros compare to other two topologies at resonant frequencies which are described here. Waveguide resonator having high power handling capacity so it can be utilized for Base transceiver station, satellite communication, sensors as well as the fusion plasma diagnostics system. The structure is simulated with Ansoft’s finite-element High Frequency Structure Simulator tool to demonstrate the characteristics of this resonator using a rectangular waveguide with center frequency of 11.2GHz for X-band. In addition, the equivalent model of resonator is simulated in ADS (Advance designing system). Moreover, offered topologies are sensitive to the parameters deviation (Post coupled waveguide resonator performance depends on diameter of conductive post, elevation of post and detachment between posts).
... Moreover, the selection of b 2 gives the conditional choice between the ohmic loss and the intended mode conversion. Based on the previous observation, 27 it is concluded that the reflection coefficient can be reduced by keeping the dominant mode constant along the propagating direction. Here, the dimension of the rectangular waveguide was chosen as 5.68 × 2.84 mm 2 (WR-22). ...
In this manuscript, a new design approach is proposed for □TE10 (rectangular) to ○TE01 (circular) mode transducer for electron cyclotron resonance heating (ECRH) applications. The design consists of two parts: first part has a rectangular waveguide, which transforms □TE10 to □TE20 mode, and in the second part, conversion of □TE20 into ○TE01 mode takes place. The electric field pattern, mode conversion efficiency or transmission efficiency and the operating principle of the individual parts are discussed, optimized, and analyzed. The novelty of this integrated design lies in the fact that it has merits of sidewall and inline coupling. Simulation study and optimization of the individual sections of the proposed mode converter were carried out using computer simulation technology (CST) microwave studio software (MWS). The simulation result shows that the design gives more than 97% of mode conversion efficiency with −1 dB bandwidth of 14.65% at 42 GHz. Copper material is used for fabrication of the mode transducers and are assembled with precise doweling. The simulated and measured high frequency (HF)‐field pattern of the proposed mode transducer is also highlighted in the manuscript.
... Recently there has been rapid expansion of wireless communication systems that operate in the microwave frequency range. It is attracting considerable interest from the academia and industry [1] [2] [3][4][5]. At higher frequencies, wavelengths involved are very small, manufacturing of microstrips at small wavelength requires extremely tight tolerance and is generally not very efficient. ...
... Most suitable antennas for such kind of applications are from planar structure. However, this type of antennas like microstrip antennas suffers from serious losses particularly at bends and discontinuities by increasing frequency [2] . Although numerous planar antennas have been studied for Ka band communication and radar
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