Design of C-band 2-stepped Septum Polarizer with dual

matched feed for circular polarization with enhanced gain

Mr. Pramod Kumar*1

, Poonam Bajpai 2

, Dr.A.K.Shrivastav3

, Krishna K. Yadav4 1. Deptt. of ECE, IPEC, GHAZIABAD 2. IGIT, Kashmiri Gate, New Delhi,

3. SAMEER-CEM,Chennai , 4. GCET,Gr. Noida E-mail: 1. er.pramodkumar@gmail.com , 2. Poonambajpai1973@gmail.com , 3.

aks@cem.sameer.gov.in , 4. kkyraju@gmail.com Abstract- In this paper a two stepped septum polarizer is presented with matched feed using three pin to improve the gain and band width while reducing polarization loss. This configuration shows about 10-15% bandwidth and it is therefore suitable for high sensitive, large bandwidth applications. Index Terms—Polarizer, Septum, circularly polarized feed horn.

I. INTRODUCTION Low loss dual-polarization performances are highly required in passive devices for low noise radio astronomy receivers at microwave wavelengths. A polarizer is a three port device. By considering the transmitter-mode operation it has two inputs and one output. With reference to the sketch in Fig. 1, reporting in detail the sub-parts forming a corrugated stepped-septum polarizer, the inputs are circular waveguides (named left input and right input) and the output is also circular waveguide (named common output).The use of waveguide polarizer is generally better than optical wire grids especially at very low frequencies when space occupation and weights become predominant factors. In this report, the design and testing of a C-band (6-8 GHz) septum polarizer is described. The interest in the development of such a device comes from up-grading demands in receiving instrumentation of the radio telescopes of Noto and Arecibo. The measurements showing very good performances are accurately described and comparisons with expected data are presented to show the validity of the design approach. This paper presents a high efficiency polarizer intended to illuminate a passive reflector. The design utilizes the dual mode method developed by the late Dick Turrin, W2IMU, by which two guided waves excited with different modes cancel each other at the circular periphery of the feed in order to provide an excellent radiation pattern. Our polarizer consist 2- stepped septum in circular waveguide, which provides

excellent circularity and isolation between the right and left circularly polarized coaxial feed points. Complete dimensions are presented for 7.0626 GHz, which gives good radiation gain and band width in C-band.

II. DESIGN THEORY

In microwave circuits, where circular polarization is required, septum polarizer constitutes a simple and compact device for converting linear polarization into circular polarization. Some design procedures have been reported in the literatures which are mostly based on experimental analysis. Recently, Albertsen and Skov-Madsen3 analyzed a single and a double notch rectangular septum polarizer using the Weiner-Hopf technique where an infinitely thin bifurcation is assumed and then Galerkin's method is employed. Their results, however, show that the design is useful only for narrowband applications. For broadband applications a stepped septum gives a better performance even than the sloping septum. We have taken reference from the paper [1-3] in which a 5-stepped septum polarizer is presented for L-band application. Proposed model is rescaled for 2-stepped septum for satellite communication from the above paper and then optimized for desired result and also for enhancement of gain by three cylindrical pins inserted after the conical structure as shown in fig-1. These pin height are optimized and simulated for every height and a good response is obtained for 7mm pin height.

978-1-4244-9190-2/11/$26.00 ©2011 IEEE

Fig.1 3-D view of proposed model of

II. DESIGN ANALY

As it is already discussed that septum model is rescaled from 5-septumsimulated for scaled structure but ressatisfactory then dimension of septumposition of feeding is tuned to get theAfter that three cylindrical pin insertedangular distance after the conical structof pin is tuned and at 7mm height 1obtained that was the improvement froin reference paper. Proposed model is with all dimensions in mm. Design desare as follows

Fig-2 . Dimension of septum used in First step

First we design a polarizer with sepwithout conical horn.

f Polarizer

SIS

this 2-stepped m polarizer and sponse was not m is tuned also e desired result. d at 1200 equal ture. The height 10.7 dB gain is om 8.6 dB gain shown in fig-1

scription details

n polarizer

Fig.3 Two stepped modal of polarize

Fig. 4. S11 and S21 plo Radiation pattern is not perfect cirbandwidth is very broad 6.3 to 10only 3dB so we add conical heahorn.

Second Step Now we add a conical horn fo

also insert three cylindrical pinangular distance around at concircular horn junction which impedance matching. . All Dimensbelow.

ptum but

er without conical horn.

ot in dB.

rcular. Its operational 0 GHz but its gain is ad with long mouth

or enhance gain and n separated at 1200 nical structure and

provide excellent sional are mentioned

Fig.5 (a) Probe Detail

Fig.5. (b) 2- stepped Septum Polarizer with

Material used for proposed polarizer and

Component is copper coated wOverall length of polarizer is excitation, two coax feed of 50Ω eafor right hand circular polarizationfor left hand circular polarization (frequency is 7.0626 GHz at which dB and isolation between both porshown in fig-6. From fig-7 it caS22 and S12 = S21 also isolation ioperating band 6.63 GHz to 7.4band width is about 1GHZ i.efrequency. In fig-8 (a) 3D view with structure due to excitation of in fig-8 (b) polarization due to excshown. It can be seen that maximuoperating frequency 7.0626GHz alit is clear that polarization due tsame. In fig-9 polar plot of gfrequency 7.0626 GHz at phi = 90 due to excitation of port 1 and portthat both plot are same as shown in

0 and main lobe direction is 13 widB) 38.10 . In fig-10 (a) and (b) Cais shown at phi = 900 and theta ran1800 for operating frequency 7.0field radiation is observed at differplotted a graph Gain vs. Frequenfig-8. It can be seen from graph different value at different frequenat 7.15 GHz i.e. 11.36 dB.

Fig-6 (a). Return L

h all dimensions

all its

with silver inside.

31.5 cm. For the

ach are used. One

n (RHCP) and other

(LHCP). Resonating

return loss is -39.68

rts is about -19 dB as

an be seen that S11 =

s below 15 dB in

47 GHz and -10dB

e. 14% of operating

of polarization gain

Port 1 is shown and

itation of port 2 is

um gain is 10.7dB at

lso from both figure

o both excitation is

gain is shown for

degree individually

t 2. It can be seen

n fig-9 (a) and (b)

ith angular width (3

artesian plot far field

nge is from -1800 to

0626 GHz. Also far

rent frequencies and

ncy as shown in the

that gain is having

ncy and is maximum

Loss

Fig-6 (b) Isolation between port 1 and port 2 Fig-7. Comparison of S11 with S22 and S12 with S21

Fig-8 (a) 3D far field pattern with structure due to excitation of port 1

Fig-8 (b) 3D far field pattern due to excitation of port 2

Fig-9 (a) polar plot of gain due to excitation of port 1

Fig-9 (b) polar plot of gain due to excita Fig-10 (a) Cartesian plot of far field due

port1

Fig-10 (b) Cartesian plot of far field due port2

ation of port 2

e to excitation of

to excitation of

Fig-11. Plot of Gain vs Fr

IV. CONCLUSION A two septum polarizer with dual simulated. After that both feed is cylindrical pin of 7 mm height asimulated due to which gain is obto 10.7 dB with 97.51 % total raband width is 14% of operating fre

REFERENCES [1] Marc Franco, N2UO, Com

Mode Circularly Powww.on7un.net/files/n2uo_Ced%20Dual%20Mode%20C%20Feedh.pdf

[2] Pavel Hazdra, RastislavMazanek, “optimization offeed for 1.296 GHz. Eme FEE, Dept. of Electromagne166 27, Prague, Czech Repu

[3] N. Ekanayake, “analysis polarizer”, electronics lettersVol. 21 No. 24

[4] G.Balodis,O.ceriņš,“PolariImprovement for Feed Hornwww.etf.rtu.lv/Latvieshu%2ms.../C14_Balodis_2.pdf.

equency

N

feed is designed and matched by using 3

and then again it is btained from 8.6 dB diation efficiency & equency 7.026 GHz.

S

mputer Optimized Dual olarized Feedhorn. Computer%20Optimiz

Circularly%20Polarized

v Galuscak, Milos f the septum polarizer

station.”, ctu Prague, etic Field, Technicka 2, ublic

of stepped septum s 21st November 1985

ization Parameters n”,; 20lapa/jaunumi/jaunu