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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.

HITESH B PANDYA

Microwave Diagnostics, Institute for Plasma Research, Gandhinagar 382 428, Gujarat,

India.

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 - W). 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, while achieving the required electrical performance.

Key words : Plasma, Electron Cyclotron Emission, Band Pass filter, Radiometer, Tokamak

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INTRODUCTION:

The Electron Cyclotron Emission (ECE) is used as plasma diagnostic method on a tokamak

fusion research device[1]. In the tokamak devices, which 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 millimeter 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 frequency is decided by

the available bandpass filter in the IF section of the radiometer. A filter frequency of

11.2GHz was required.

The Specifications of the filter to be designed include a Center Frequency fc=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 filter are decided using certain calculation

and criteria. ANSOFT-HFSS helped in optimization of various specifications, and

determination of filter dimensions and resulting S-parameters.

THEORY AND DESIGN

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)

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(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 WR-90 waveguide (X-band) and cut it to a length based on

equation (1).

(1) (f/c)2 = (l/2d)

2 + (n/2a)

2 + (m/2b)

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Where f stands for frequency of the wave, C is velocity of light in vacuum (= 3 x 1011

mm/sec), m and n are the number of electromagnetic cycles transmitting through

waveguide generally even (here, l=8), mode index in E- plane and mode index in H-plane

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respectively. The quantities a and b are width (= 22.86mm) and height (= 10.16mm) of

waveguide. The length of the waveguide, which satisfied the two conditions above, came

out to be 132.23mm.

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 methods of using posts and screws. A post or

screw, which only partially 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 parameters for the posts, the ratio 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.

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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 λ0 is c/ fc (= 26.79mm) for fc equal to 11.2GHz. λc is equal to 2a(= 45.72mm).

These values of λ0 and λc give a value of 33.05 mm for λg . Varying the length between

the two posts either resulted in dual bands or very low return loss, with the 2nd

post length

slightly varied to be compliant with the given specifications.

SIMULATION:

ANSOFT-HFSS was used in the optimization of various specifications, determining the

filter dimensions and obtaining S-parameters. Figure 3 shows the finalized design and the

simulated results. Two posts of diameter 3.05mm and 3.3mm were kept at 76.2mm and

107.81mm respectively from the input end of waveguide. The simulated result gave a

bandpass of 200MHz with a peak at 11.3GHz and return loss of -15dB.

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TESTING:

The filter was fabricated with the dimensions motioned in table 1. The picture of the filter

is shown in figure 4. The filter tested by using spectrum analyzer and X-Band voltage

control oscillator. The final testing was done with a Network analyzer. The test results are

shown in figure 5.

Table 1.

Length of

the

waveguide

Width of

the

waveguide

Height of

the

waveguide

Diameter

of post 1

Diameter

of post 2

Distance

of 1st

post

from

source

Distance

of 2nd

post

from

source

132.23mm 22.86mm 10.16mm 3.3mm 3.05mm 76.2mm 107.81mm

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Using the Spectrum Analyzer:

The Spectrum Analyzer shows the power at discrete frequency points. By changing the

bias voltage of the X-band 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 considered as the response. Hence final testing

was done on network analyzer.

Testing of Filter using Network Analyzer:

For accurate results, testing was performed with a network analyzer. 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.

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Table 2

Power in put

direction

Center

frequency fc

GHz

Insertion Loss

dB

Return Loss

DB

Bandwidth

GHz

At 1st post port 11.19 -1.28 -26.8 < 0.1

At 2nd

post port 11.19 -0.45 -14.41

CONCLUSION

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

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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

References

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

pp 169-176, 2002.

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-

522

4 N. Marcuvitz, Waveguide Handbook, Electromagnetic waves series, IEE, vol 21

Peter Peregrinus, London, 1986,

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Figure captions

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