78

CHAPTER-5 CORRUGATED MATCHED FEED

The idea of corrugated horn was conceived by two independent groups. In USA,

A. F. Kay proposed a corrugated horn with pattern symmetry and negligible sidelobes

over a wide bandwidth [25]. At the same time, Minnot and Thomas [71] of CSIRO,

Australia reported the radiation properties of corrugated waveguide feed. Since then

the corrugated horns are in use as primary feeds for reflector antennas. In comparison

to smooth walled conical horns, the corrugated horns possess two important properties,

i.e., axial beam symmetry and low cross-polarization over a wide bandwidth [25].

These properties ensure high antenna gain, low spillover and minimum contribution

from the sidelobes.

In order to produce symmetric E-plane and H-plane patterns with a very low

cross-polarization, it is necessary that the horn should produce linear aperture electric

fields [25]. However, it is known that a pure TE or TM mode cannot produce linear

electric fields and hence the radiation patterns of smooth-walled conical horns

(supporting TE and/or TM modes) are not symmetric. As reported in the published

literature, the hybrid mode can only produce the required linear electric fields. The

hybrid mode is basically a mixture of TE and TM modes, e.g., fundamental HE11 mode

is a mixture of TE11 and TM11 modes. Such hybrid mode(s) can be generated by a

horn, whose inner surface is corrugated. In fact, the corrugations on the walls of the

horn modify the electric and the magnetic fields such that the horn produces symmetric

co-polar patterns with less cross-polar radiation.

Although the fundamental HE11 mode guided corrugated horn is the best feed

option for a symmetrical reflector, however, it is not suitable as a feed for the offset

79

parabolic reflector antenna. This is because the conventional corrugated feed cannot

suppress the cross-polarization introduced by the offset geometry of the reflector.

Thus, a pure HE11 mode guided cylindrical corrugated feed is not suitable for an offset

parabolic reflector antenna.

As suggested in [21], if a corrugated matched feed can be designed, whose

aperture fields are a conjugate match to the focal region fields of the reflector, it is

possible to suppress the unwanted cross-polarization of the offset reflector. In a

corrugated structure, such a matched feed can be configured by adding an additional

hybrid mode (HE21 mode) to the fundamental HE11 mode.

To the best of authors knowledge, a corrugated matched feed has not been

reported in the published literature. This has motivated the author to design a dual-

mode corrugated matched feed. This chapter presents three different designs of a

corrugated matched feed. The necessary field expressions for the corrugated matched

feed are summarized in section 5.1. Following this, the numerical results obtained

through various simulations are discussed. For a selected feed design, the measured

return-loss characteristics and the primary radiation patterns are also presented.

Finally, the effectiveness of the prototype matched feed with the offset reflector has

been verified through experimental results.

5.1 FIELD EXPRESSIONS FOR THE CORRUGATED MATCHED FEED

In case of a corrugated cylindrical wave-guide structure, the matched feed is a

dual-mode feed with two modes, i.e., HE11 and HE21. The fundamental HE11 mode

ensures that the feed itself will not radiate high cross-polarization, while a small

component of HE21 mode compensates the asymmetric cross-polarization added by the

80

offset geometry. For the satisfactory operation of the matched feed, it is necessary that

the HE21 mode should maintain -90 phase relationship with the HE11 mode. The field

expressions of the dual-mode corrugated matched feed can be written as,

(5.1)

(5.2)

where, is the arbitrary constant defining the relative power in HE21 mode with

respect to the fundamental HE11 mode. Using the general expressions for and

from [72], the expressions for , and can be obtained as,

(5.3)

(5.4)

(5.5)

(5.6)

In (5.3) to (5.6),

= aperture radius of the horn (in meter),

R=distance from the aperture centre to the observation point (in meter),

= wavelength of operation (in meter),

= free-space propagation constant,

= normalized phase-change coefficients of HE11 mode,

= normalized phase-change coefficients of HE21 mode,

81

= normalized hybrid factor (referred as and for the HE11 and HE21

modes, respectively),

= free-space wave admittance, and

(5.7)

(5.8)

(5.9)

= the first root of the Bessel function for HE11 mode (=2.405),

= the first root of the Bessel function for HE21 mode (=3.8317),

= Bessel function of the first kind and order m,

The actual values of have been obtained by solving the characteristic

equation under a balanced hybrid condition [72].

5.2 NUMERICAL RESULTS

Prior to the actual fabrication of the corrugated matched feed, its effectiveness

with the offset parabolic reflector has been verified through a series of computer

simulations. For all the simulations, the offset geometry described in the beginning of

section 3.2 has been taken into consideration. First, the performance of the individual

matched feed has been simulated and then the same feed has been used to simulate the

overall performance of the reflector system. All the simulated results are presented in

the subsequent sub-sections.

5.2.1 Simulated Far-field (Primary) Radiation Patterns of the Corrugated Matched Feed

Using the field expressions derived in section 5.1, a MATLAB based computer

program was developed to estimate the primary radiation patterns of the corrugated

82

matched feed. The results obtained through the MATLAB program were compared to

that of the commercially available GRASP-8W software.

(a)

(b)

Fig. 5.1 Simulated radiation patterns of the corrugated matched feed (a) For

Phi=0 (b) For Phi=90

-90 -60 -30 0 30 60 90

Theta (Degree)

-40

-30

-20

-10

0

Rel

ati

ve

Po

wer

Lev

el (

dB

)

Co Pol. (Computed Results)

Co Pol. (GRASP-8W Results)

-90 -60 -30 0 30 60 90

Theta (Degree)

-40

-30

-20

-10

0

Rel

ati

ve

Po

wer

Lev

el (

dB

)

Co Pol. (Computed Results)

Co Pol. (GRASP-8W Results)

Cross Pol. (Computed Results)

Cross Pol. (GRASP-8W Results)

83

As shown in Fig. 5.1, there is an excellent correlation between the MATLAB results

and the GRASP-8W results. It is worthwhile to note that the high cross-polarization

observed in Fig. 5.1(b) is due to the presence of the additional HE21 mode. Ultimately

this cross-polarization will cancel out the cross-polarization introduced by the offset

reflector geometry. Further, a slight beam-squint noticed in the co-pol pattern (Fig.

5.1(a)) may have resulted because of the asymmetric HE21 mode.

5.2.2 Variation in Peak Cross-Polarization as a Function of Relative Power in HE21 Mode

After obtaining the satisfactory performance of the feed, the next important task

was to decide the numerical value of the constant . To accomplish this, the peak

cross-polarization in the secondary radiation pattern was computed for different values

of the constant . As evident from Fig. 5.2, only one value of gives the least peak

cross-polarization in the reflector far-field pattern. This unique value has been fixed up

for all subsequent simulations.

Fig. 5.2 Variation in peak cross-polarization as a function of relative power in

HE21 mode

0.0 0.2 0.4 0.6 0.8 1.0

Relative Power in HE21 Mode

-60

-50

-40

-30

-20

Cro

ss P

ola

riza

tio

n (

dB

)

84

5.2.3 Offset Angles (0) Versus Relative Power in HE21 Mode

As reported by Chu and Turrin [8], in an offset parabolic reflector antenna, cross-

polarization highly depends on the offset angle ( . The higher offset angle makes the

reflector geometry more asymmetric, which ultimately results into high cross-polar

radiation. In order to compensate this high cross-polarization, using the corrugated

matched feed, a relatively higher amount of modal power will be needed in the HE21

mode. In other words, as the value of the offset angle increases, the modal power in the

HE21 mode should also increase. This explanation is in line with those discussed in

section 3.2.3 and 4.2.3 for rectangular and cylindrical matched feed, respectively. The

same has been verified for the corrugated matched feed and the results are presented in

Fig. 5.3.

Fig. 5.3 Offset angles (0) versus relative power in HE21 mode

20 25 30 35 40 45 50

Offset Angle (Degree)

0.20

0.28

0.36

0.44

0.52

0.60

Rel

ati

ve

Po

wer

in

HE

21

Mo

de

85

5.2.4 Simulated Far-field (Secondary) Radiation Patterns of the Offset Reflector Antenna Fed by a Linearly Polarized Corrugated Matched

Feed

This sub-section presents the far-field radiation patterns of the offset parabolic

reflector antenna illuminated by two different corrugated feeds. Initially, the

conventional corrugated horn (which supports only the fundamental HE11 mode) has

been used as a primary feed. Very high cross-polarization was noticed in this case (see

Fig. 5.4). Later on, for the same reflector, a corrugated matched feed was used as a

primary feed. The results are plotted in Fig. 5.5. Comparison of Fig. 5.4 and Fig. 5.5

shows a substantial cross-polarization reduction in case of a matched feed illuminated

reflector.

Fig. 5.4 Simulated secondary radiation patterns of the offset reflector illuminated

by a linearly polarized HE11 mode guided conventional corrugated horn

-8 -6 -4 -2 0 2 4 6 8

Theta (Degree)

-60

-50

-40

-30

-20

-10

0

Rel

ati

ve

Po

wer

Lev

el (

dB

)

Co Pol. (Phi=0)

Co Pol. (Phi=90)

Cross Pol. (Phi=90)

86

Fig. 5.5 Simulated secondary radiation patterns of the offset reflector illuminated

by a linearly polarized corrugated matched feed

5.2.5 Simulated Far-field (Secondary) Radiation Patterns of the Offset Reflector Antenna Fed by a Circularly Polarized Corrugated

Matched Feed

Structural asymmetry of the offset parabolic reflector results in beam squinting

especially when circular polarization is employed. In this reference, it has been shown

that the use of either rectangular matched feed or cylindrical matched feed as the

primary feed, successfully removes the beam squinting effects. On the similar ground,

the dual-mode corrugated matched feed was also tested with the offset reflector

antenna.

Fig. 5.6 shows the far-field radiation patterns of the offset parabolic reflector

antenna illuminated by a circularly polarized conventional corrugated horn (HE11 mode

guided), while Fig. 5.7 represents the far-field patterns of the same reflector when

illuminated by a circularly polarized corrugated matched feed. It is observed that the

-8 -6 -4 -2 0 2 4 6 8

Theta (Degree)

-60

-50

-40

-30

-20

-10

0

Rel

ati

ve

Po

wer

Lev

el (

dB

)

Co Pol. (Phi=0)

Co Pol. (Phi=90)

Cross Pol. (Phi=90)

87

patterns are squint-free in Fig. 5.7 as compared to those of Fig. 5.6. Thus, it can be

said that the use of a circularly polarized corrugated matched feed, in conjunction with

the offset reflector, effectively removes the beam squinting.

Fig. 5.6 Simulated secondary radiation patterns of the offset reflector illuminated

by a circularly polarized HE11 mode guided conventional corrugated horn

Fig. 5.7 Simulated secondary radiation patterns of the offset reflector illuminated

by a circularly polarized corrugated matched feed

-6 -4 -2 0 2 4 6

Theta (Degree)

-50

-40

-30

-20

-10

0

Rel

ati

ve

Po

wer

Lev

el (

dB

)

Left Circular

Right Circular

-6 -4 -2 0 2 4 6

Theta (Degree)

-50

-40

-30

-20

-10

0

Rel

ati

ve

Po

wer

Lev

el (

dB

)

Left Circular

Right Circular

88

5.3 CORRUGATED MATCHED FEED DESIGN

After obtaining satisfactory numerical results, the next stage was to design the

actual model of the corrugated matched feed. Due to the successive slots and teeth to

form the series of corrugations on the walls of the horn, the modeling of such feed was

very challenging even with the HFSS. The process of corrugation modeling, using

HFSS software, is briefly outlined in the next paragraph.

For preparing the corrugations of desired dimensions, the successive slots and

teethes were modeled using cylinders of different radii. All such cylinders were then

united as shown in Fig. 5.8(a). The complete model of the corrugated structure is

shown in Fig. 5.8(b).

(a) (b)

Fig. 5.8 (a) Process of uniting the individual cylinders to form corrugations

(b) Model of the corrugated structure

Three different designs of the corrugated matched feed have been prepared and

are discussed in the subsequent sub-sections. The feeds have been designed for an

operating frequency of 6.6 GHz.

89

5.3.1 Design-I

The geometry of the proposed dual-mode corrugated matched feed [73] is shown

in Fig. 5.9. Using the HFSS software, the feed dimensions were optimized to obtain

the desired modes at the horn aperture.

(a) (b)

Fig. 5.9 HFSS simulated design of the dual-mode corrugated matched feed

(Design-I) (a) Front View (b) Side View

(D1=36 mm, D2=76 mm, D3=98 mm, L1=10 mm, L2=50 mm, L3=10 mm, L4=70 mm)

The input waveguide of the feed was excited by a pure TE11 mode. The

diameter of the input waveguide ( ) was selected such that it supports the smooth

propagation of the TE11 mode. Next, the asymmetric TE21 mode was generated using

the three cylindrical pins of equal dimensions. The height of the pin decides the modal

amplitude of the TE21 mode. The diameter D2 was chosen to cut off all higher order

modes than the TE21 mode. Then, the TE11 and TE21 modes were allowed to pass

through the corrugated structure of the horn. In the corrugated section, the new

boundary conditions convert the TE11 mode in to the HE11 mode and the TE21 mode in

to the HE21 mode. The corrugation pitch and the pitch-to-width ratio were decided

90

based on the guidelines provided in a standard corrugated horn design primer [74]. The

depth of the slots was kept same for all the corrugations. The required phase

relationship amongst the two modes (HE11 and HE21) was established by carefully

selecting the lengths of the various parts of the horn.

After finishing the modeling of the feed, its return-loss and the radiation

performance were simulated in HFSS. The HFSS generated feed radiation patterns

(primary patterns) were given as input to the GRASP-8W software to further simulate

the overall performance of the offset parabolic reflector. The results are summarized in

Table 5.1.

Table 5.1 Simulated performance of the corrugated matched feed (Design-I) #Feed return-loss

##Peak

cross-polarization

(with conventional

corrugated horn)

##Peak

cross-

polarization

(with corrugated

matched feed)

Cross-

polarization

improvement

-9 dB -24.8 dB -42.6 dB 17.8 dB # Shows only the minimum value of the return-loss over a frequency band of 6 to 7 GHz

## Shows the reflector peak cross-polarization in the asymmetric plane

5.3.2 Design-II

Fig. 5.10 shows another interesting geometry of the dual-mode corrugated

matched feed [75] prepared using HFSS. The horn dimensions have been finalized

after a set of parametric studies. The horn was excited by a pure TE11 mode. The

diameter of the input waveguide ( was selected by satisfying a condition,

. Where, is the cut-off wave number for the TE11 mode (= 1.84118), and

is the operating wavelength. The next step is to generate a non-unity azimuthally-

dependant TE21 mode. As reported in [26], by inserting a metallic pin (post) or a

septum into the waveguide, the TE21 mode can be excited. However, it was observed

that the use of pins degrades the return-loss performance of the feed [73]. Therefore,

91

three arc shaped septums were used to excite the TE21 mode. The modal amplitude of

the TE21 mode could be controlled by varying the dimensions of the septums. The TE11

and the TE21 modes were allowed to pass through the corrugated structure. The mode

converter in a corrugated section imposes new boundary conditions and converts the

TE11 mode in the HE11 mode and the TE21 mode in the HE21 mode. The most

commonly used variable-depth-slot (slot depth decreases from to type of

mode converter [74] scheme was chosen. The pitch dimension was chosen to be

approximately The required amounts of modal amplitudes and the phase

relationship (-90) amongst the two modes (HE11 and HE21) were established by

adjusting the horn length and the septum dimensions. Extensive computer simulations

were carried out to decide the proportion of HE21 mode and finally the value which

gave the least peak cross-polarization in the secondary radiation pattern was selected.

Fig. 5.10 HFSS simulated design of the dual-mode corrugated matched feed

(Design-II) (a) Front View (b) Side View

Table 5.2 Simulated performance of the corrugated matched feed (Design-II) #Feed return-loss

##Peak

cross-polarization

(with conventional

corrugated horn)

##Peak

cross-polarization

(with corrugated

matched feed)

Cross-

polarization

improvement

-28 dB -24.8 dB -41.1 dB 16.3 dB # Shows only the minimum value of the return-loss over a frequency band of 6 to 7 GHz

## Shows the reflector peak cross-polarization in the asymmetric plane

92

The designed feed was then tested for the return-loss performance. The return-

loss of the feed was found to be better than 28 dB over the specified frequency band.

The primary radiation patterns were also found satisfactory. Later on, the feed was

used to illuminate the offset parabolic reflector antenna and the far-field radiation

patterns were estimated using the GRASP- 8W software. The important results are

summarized in Table 5.2.

5.3.3 Design-III

The geometry of the prototype horn is shown in Fig. 5.11. The dimensions of the

dual-mode corrugated matched feed [76] were finalized after extensive simulations

performed using HFSS software. The computer simulations took a few weeks time to

arrive at an optimum horn design. The horn was excited by a pure TE11 mode and the

input diameter of the feed was chosen such that it allows the TE11 mode to propagate.

Following this, a mode converter was introduced to convert the TE11 mode into a

hybrid HE11 mode. In the mode converter section, the depth of corrugations varies

gradually from an initial depth of near the throat of the horn to approximately

at the fourth corrugation. Except the fifth corrugation, all the remaining corrugations

(sixth onwards) maintain a constant depth of . It was observed that this type of

variable-depth-slot mode converter offers better return-loss performance as compared

to the constant-depth-slot (fixed at ) mode converter as employed in Design-I. In

the present prototype design, approximately 6.5 corrugations have been accommodated

per wavelength. The pitch length and the pitch-to-width ratio were decided based on

the guidelines provided in a standard corrugated horn design primer [74].

The required HE21 mode was generated, by inserting three arc shaped

symmetrical septums. In Fig. 5.11(a), the fifth corrugation represents the septum. It is

93

to be noted that the amplitude of the HE21 mode largely depends on the dimensions of

the septums. Since, the improper septum dimensions may degrade the overall

performance of the reflector, utmost care was taken while deciding the dimensions of

the septums. The required phase relationship (-90) amongst the two hybrid modes was

established by properly adjusting the length L3.

Accurate performance prediction of such a complicated design was very

challenging. Appropriate setting of the convergence limit with the suitable simulation

parameters became necessary for satisfactory simulations. It is important to note that, a

high-speed computer with minimum 4 GB RAM is essential to accomplish the

computations for such type of complex feed structures. Also, for such geometries,

relatively more computation time is needed for convergence of the results. The major

results are highlighted in Table 5.3.

Fig. 5.11 HFSS simulated design of the dual-mode corrugated matched feed

(Design-III) (a) Front View (b) Side View

(D1=34 mm, D2=50 mm, D3=68 mm, D4=102 mm L1=30 mm, L2=15 mm, L3=98 mm)

Table 5.3 Simulated performance of the corrugated matched feed (Design-III) #Feed return-loss

##Peak

cross-polarization

(with conventional

corrugated horn)

##Peak

cross-

polarization

(with corrugated

matched feed)

Cross-

polarization

improvement

-20 dB -24.8 dB -40.9 dB 16.1 dB #

Shows only the minimum value of the return-loss over a frequency band of 6 to 7 GHz ## Shows the reflector peak cross-polarization in the asymmetric plane

94

5.4 MEASURED RESULTS

After analyzing the simulated results of all the three corrugated matched feeds

described in the previous section, it was decided to fabricate one prototype feed.

Although, the overall performance of Design-II was slightly better than that of Design-

III, considering the fabrication difficulties with Design-II, it was decided to select

Design-III for fabrication. The horn was fabricated in three parts. Part-1 of the horn

comprises of the input waveguide section and the first four slots. From the fabrication

point of view, this was the most difficult part. The septum (fifth slot) was fabricated as

part-2. In order to test the performance of the horn with septums of different

dimensions, three such pieces of different arc size were fabricated. The third part of the

horn includes all the slots (i.e., slot 6 onwards) subsequent to the septum. During the

fabrication process, utmost care was taken to maintain the consistency of all

dimensions. Some of the adverse effects due to changes in the horn dimensions are

listed in Table 5.4.

Table 5.4 Effects of changes in the horn dimensions

Type of change Effects

Change in slot depth Influence the cross-polar performance of the feed itself

Change in horn length Affects the phase relationship between the HE11 and

HE21 mode

Change in septum diameter Affects the modal amplitude of the HE21 mode

After fabrication of all the three parts, their dimensions were physically verified

with high precision measuring instruments. Then, each part was tightened to the

successive part such that there is no air-gap between any two parts. An air-gap may

lead to a discontinuity in the current flow, which alters the fields in the horn and

ultimately result in poor performance of the horn [25]. After integrating all the three

95

parts, the horn was connected to the rectangular-to-circular transition. The photograph

of the entire horn, with the transition, is shown in Fig. 5.12.

Fig. 5.12 The photograph of the proposed dual-mode corrugated matched feed

(Design-III) (a) Front View (b) Side View

The next important stage was to test the performance of the feed as an individual

element and then with the offset parabolic reflector. All measurements were carried out

at SAC, ISRO, Ahmedabad. The results of three crucial measurements are presented in

the following sub-sections.

5.4.1 Feed Return-Loss Measurement

Firstly, the return-loss of the proposed corrugated matched feed was measured

over a frequency band of 6.0 to 7.0 GHz. The results are plotted in Fig. 5.13. It is

important to note that the horn return-loss is satisfactory over a wide frequency range,

even though the septums are present in the horn.

5.4.2 Far-field (Primary) Radiation Patterns of the Corrugated Matched Feed

The radiation characteristics of the proposed dual-mode corrugated feed was

measured and compared with the theoretical predictions. Since the feed under testing

was relatively small in wavelength, the anechoic chamber was preferred for radiation

96

pattern measurements. The photograph of the feed under test at the anechoic chamber

is shown in Fig. 5.14.

Fig. 5.13 The measured return-loss characteristic of the corrugated matched feed

(Design-III)

Fig. 5.14 The corrugated matched feed under test at the anechoic chamber (SAC,

ISRO, Ahmedabad, India)

6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 7.0

Frequency (GHz)

-50

-45

-40

-35

-30

-25

-20

-15

-10R

etu

rn L

oss

(d

B)

97

Fig. 5.15 shows the measured and the simulated far-field radiation patterns of the

corrugated matched feed.

(a)

(b)

Fig. 5.15 The normalized simulated and measured radiation patterns of the HFSS

designed corrugated matched feed (Design-III) (a) For Phi=0 (b) For

Phi=90

-90 -60 -30 0 30 60 90

Theta (Degree)

-40

-30

-20

-10

0

Rel

ati

ve

Po

wer

Lev

el (

dB

)

Co Pol. Simulated (Phi=0)

Co Pol. Measured (Phi=0)

-90 -60 -30 0 30 60 90

Theta (Degree)

-40

-30

-20

-10

0

Rel

ati

ve

Po

wer

Lev

el (

dB

)

Co Pol. Simulated (Phi=90)

Cx. Pol. Simulated (Phi=90)

Co Pol. Measured (Phi=90)

Cx. Pol. Measured (Phi=90)

98

For both and plane, the measured and the computed patterns

were found in almost close agreement. However, in Fig. 5.15(b), slight mis-match in

the cross-polar patterns is visible. One of the reasons behind this mis-match could be

the measurement set-up, which generally has 1 dB error at the level of 20 dB.

Further, the increased cross-polarization in the plane indicates the presence

of an additional HE21 mode with the fundamental HE11 mode. As mentioned earlier,

this high cross-polarization will finally cancel-out the cross-polarization of the

reflector.

5.4.3 Far-field (Secondary) Radiation Patterns of the Offset Reflector Antenna Fed by a Linearly Polarized Corrugated Matched Feed

This sub-section presents the far-field radiation patterns of the offset reflector fed

by a linearly polarized corrugated matched feed. The offset reflector specifications are

the same as those described in section 3.2 (Fig. 3.1). Prior to the actual measurement,

the feed was fixed at the geometrical focus of the reflector and its alignment with the

reflector was ensured. The pictorial view of the entire antenna measurement set-up is

shown in Fig. 5.16. The measurements were conducted at the compact antenna test

range (CCR-75/60), SAC, ISRO, Ahmedabad.

The measured co-polar and cross-polar patterns for the two principle planes, i.e.,

=0 and =90 are shown in Fig. 5.17. As justified in the previous chapters,

asymmetrical = 90) plane, was chosen for estimating the actual improvement in

the cross-polarization. As evident from Fig. 5.17 (b), significant improvement in the

cross-polarization was observed when a dual-mode corrugated horn was in the place of

a conventional corrugated horn.

99

Fig. 5.16 The offset reflector and a corrugated matched feed under test at CATR

(SAC, ISRO, Ahmedabad, India)

(a)

-8 -6 -4 -2 0 2 4 6 8

Theta (Degree)

-50

-40

-30

-20

-10

0

Rel

ati

ve

Po

wer

Lev

el (

dB

)

Co Pol. (Measured)

Cross Pol. (Measured)

100

(b)

Fig. 5.17 Measured secondary radiation patterns of the offset reflector illuminated

by a linearly polarized corrugated matched feed (a) For Phi=0 (b) For

Phi=90

5.5 CONCLUSION

In the present chapter, three different designs of a corrugated matched feed are

discussed. In a cylindrical corrugated structure, higher order HE21 mode has been

added with the fundamental HE11 mode to configure the corrugated matched feed horn.

Through experimental results, it is verified that, the corrugated matched feed

effectively suppress the undesired cross-polarization of an offset parabolic reflector

antenna. The preliminary results are very encouraging and further improvement in the

cross-polar performance is possible by adjusting the septum dimensions.

-8 -6 -4 -2 0 2 4 6 8

Theta (Degree)

-50

-40

-30

-20

-10

0

Rel

ati

ve

Po

wer

Lev

el (

dB

)

Co Pol. (Measured)

Cross Pol. (Measured)

101

PUBLICATIONS RELATED TO THE CHAPTER

[1] S. B. Sharma, Dhaval Pujara, S. B. Chakrabarty, and Ranajit Dey, Cross-

Polarization Cancellation in Offset Parabolic Reflector Antenna using a

Corrugated Matched Feed, IEEE Antennas and Wireless Propagation Letters,

vol. 8, pp. 861-864, 2009.

[2] S. B. Sharma, Dhaval Pujara, and S. B. Chakrabarty, Design and Development

of a Dual-mode Corrugated Horn for an Offset Reflector Antenna, Microwave

and Optical Technology Letters, vol. 52, no.1, pp. 113-116, January 2010.

[3] Dhaval Pujara, S. B. Sharma, S. B. Chakrabarty, Ranajit Dey, and V. K. Singh,

Design of a Novel Corrugated Matched Feed for an Offset Parabolic Reflector

Antenna, IEEE International Symposium on Antennas and Propagation,

Charleston, South Carolina, USA, June 2009.