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Design of Ka-Band Front End of Fully Polarized Radiometer

Ya-Fen Ge, Qing Li, Kai Zhou, Hong-Da Lu, Yong Liu, Li-Ming Si, and Xin Lv

Beijing Key Laboratory of Millimeter Wave and Terahertz Technology,

Department of Electronic Engineering, School of Information and Electronics,

Beijing Institute of Technology, Beijing 100081, Peoples Republic of China

A Ka-band front end of fully polarized microwave radiometer is designed, which consists of ortho-mode transducer (OMT) andmultimode horn (MMH). The front end is analyzed using the finite element method. The simulation results show it has a goodperformance. The two ports isolation of the front end is larger than 53dB, the gain of the front end is larger than 22.4dBi, the sidelobelevel is less than -24.9dB and the VSWR in the bandwidth is lower than 1.2.

Index Termsfully polarized microwave radiometer, ortho-mode transducer (OMT), multimode horn (MMH).

I. INTRODUCTION

Fully polarized microwave radiometer plays an importantrole in remotely sensing sea surface wind vector from space, it

gets all the polarization information of the target by measuring

the Stokes vector of the target, realizing the completely use of

electromagnetic wave frequency, phase, amplitude and polari-

zation in microwave remote sensing[1-3]. And it can provide

crucial valuable information for short-term weather forecast,

meteorological and oceanographic studies. However, perform-

ance of fully polarized microwave radiometer system mostly

depends on the performance of the front end, therefore, an

excel-lent front end is quite important [4-7].

In this paper, we took the method of combination of ortho-

mode transducer (OMT) and multimode horn (MMH), and a

front end of fully polarized microwave radiometer with excel-

lent electrical performance, compact and reasonable structure

is designed. The OMT has three ports, one is square, the othe-

rs is rectangle. We took the design ideas of square waveguide

staircase impedance matching and waveguide aperture coupl-

ed, and its structure is compact, simple, stability and facilitate

processing [8]. But most of all, its electrical performance is

excellent, it has high isolation, low insertion loss and VSWR

characterristics. Multimode conical horn also has outstanding

electrical performance, its main advantage contain high gain,

very symmetrical direction pattern, low sidelobe. The front

end has a range of applications in engineering.

The purpose of this work is to present an analysis of theOMT and the MMH, this analysis can give useful direction in

designing a front end of fully polarized microwave radiometer.

In Section II, the mechanism of operation of the OMT is

briefly discussed. In Section III, the mechanism of operation

of the MMH is briefly discussed. In Section IV, an analysis of

8mm OMT and MMH is presented, simulation results of both

are also given.

II. OMT DESIGN

OMT can be expressed as a three-port network on the phys-

ical structure (but it is a four-port network electrically), and it

consists of a square waveguide (1 in Fig.1), a side waveguide

(2 in Fig.1), a ladder match blocks (3 in Fig.1)[8]. Port A

transmits orthogonal TE10 mode and TE01 mode, ports B andC transmit orthogonal TE10 fundamental mode. The block

size of the ladder impedance matching, the position and size

of the coupling aperture are key parameters in designing such

a structure of OMT.

Fig.1 the structure of OMT

A. Square Waveguide Design

The relationship between the cut-off wavelength c of themain modes TE10, TE01 in the square waveguide and the

square waveguide edge length a can be described as

c=2a (1)

In order to suppress the generation of high-order mode in

waveguide, based on engineering experience, when working

bandwidth is determined, the ratio of the low frequency fL inthe working band to the cut-off frequency fcof the square wa-

veguide is a constant k (k generally value 1.1-1.5 )[6]. Further

expression is given as

fL=kfc (2)

Combine equations (1) and (2), we can calculate the edge

length of the square waveguide.

B. Impedance Matching Design

As shown in Fig.1, in order to achieve impedance matching

between the square waveguide and the rectangular waveguide

B, we use the ladder impedance transformation structure [7-8].

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Now well explain how to design it. Its principle structure is

shown in Fig.2

Fig.2 principle structure of impedance matching

Now consider the principle structure as shown in figure.2,

Z0-Z4is the equivalent impedance of each order respectively,h1, h2, h3 is the step height, and t is the ladder length. The

equivalent resistance of the rectangular waveguide is given by

(3)

Chebyshev ladder impedance transformer design principles

are not mentioned here, we only give its conclusions. The

relationship between impedance and ladder height is

(4)

And the length of each stage is

(5)

The theoretical size of the ladder impedance matching devi-

ce may be determined by the three formulas.

C. Coupling Aperture Design

The microwave field at the coupling aperture of the side wa-

ll of the square waveguide is very complex and require quite

difficult theoretical calculations only could get an approximate

value. In order to facilitate the application of engineering

practice, the size of the hole is generally determined by the

empirical formulas [3]. As we all know, the waveguide

wavelen-gth of rectangular waveguide is expressed as

(6)

The relationship between the length of the coupling aperture

and the wavelength of the waveguide at the center frequency

is given by

d= (0.3~0.4) g0 (7)And the relationship between the width of the coupling ap-

erture and the wavelength of the waveguide at the center freq-

uency is

w= (0.1~0.2) g0 (8)

The theoretical size of the ladder impedance matching devi-

ce may be determined by the three formulas.

III. MMH DESIGN

Multimode conical horn antenna is a high efficiency antenn-a with widely application, and it overcome many disadvantag-

es of base-mode conical horn, such as unequal of beamwidth,

phase center of E-plane and H-plane doesnt coincide, high

sidelobe of E-plane and so on[9-11]. Otherwise, fabrication is

relatively simple, so it is widely used [12].

Physical mechanism of multimode horn is as follows: As

we all know, the base mode of the circular waveguide is the

TE11 mode, and now the radius of the circular is equal to , and the peak level of the cross- polarization of direc-

tion pattern is very low[13-15]. TE1n mode has contribution

to the pattern of E-plane and H-plane, but TM1n mode only

has contribution to the pattern of E-plane. Because the E-plane

pattern of TE11 narrower than that of H-plane when hornwork in the base mode TE11, the peak level of cross-

polarization must be high, so in general, the feed working in

the base mode is a low efficiency feed. If TM11 mode (or

other higher mode) could be introduced to the base mode, and

properly adjusting the relative phase of high-order mode and

base mode, it is possible to make the E-plane pattern almost

the same with the H-plane pattern. Thereby we can obtain

rotation axis symmetric patterns and achieve the purpose of

equalization of E-plane pattern and H-plane. Especially to be

emphasized here is that the so-called equalization is using

TM1n mode to make E-plane pattern move closer to the H-

plane pattern [12]. This is the basic concept of the multimode

mechanism.

Fig.3 Configuration of a multimode circular horn

Multimode horn is actually the properly combination of a

high-order mode excitation device and phase shift segment. In

design multimode horn as shown in Figure.3, the key is to

adjust the modes ratio and phase shift quantity [9]. The

formulas of modes ratio and phase shift is very complex, dont

having the engineering guiding significance, and what we can

do is to optimize its value only by simulation.

The horn-flare half-angle is given by

(9)

where mc is the maximum quadratic-type phase frontdifference (refer to axis) at the horn aperture (approximately

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the same for all the modes) and is the free-spacewavelength at the center frequency.

IV. FRONT END DESIGN OF 8MM FULL POLARIZED RADIOMETER

A front end of 8mm full polarized radiometer is designedbased on the aforementioned microwave approach. The front

end structure is shown in Fig.4. The structure consists of three

parts: MMH, OMT and Balanced output waveguide. Port 1 is

the horizontally polarized port, port 2 is the vertically polari-

zed port. As we all know, whatever the polarization of the

electromagnetic wave is, it can be regarded as a combination

of the horizontal polarization component and the vertical pola-

rization component. Therefore, by measuring the polarization

components of port 1 and port 2, we can know the polarization

of electromagnetic waves into the multimode horn.

Fig.4 Configuration of a front end of 8mm full polarized radiometer

Primary model is simulated and optimized by HFSS softwa-

re and the simulated results are shown as follows. The simula-

ted VSWR of the two ports are presented in Fig.5, respectively.

Fig.5 Simulated VSWR of the two ports

It can be seen that the VSWR of both ports are lower than

1.2 in the frequency band of 35GHz to 39GHz. In actual

applications, the VSWR can be lower by adjusting the tuning

screws in the balanced output waveguide.

Fig.6 ISO of Port1 and Port2

The ISO (Isolation) of Port1 and Port2 is plotted in Fig.6,

and we can see that the ISO is good than 60dB in the whole

frequency band of 34GHz to 40GHz.

The purpose of the front end we designed is working well at

35GHz to 39GHz. Radiation characteristics of front end of full

polarized radiometer is what we are most concerned about.

The gain patterns of 35GHz, 36GHz, 37GHz, 38GHz, and

39GHz are shown in Fig.7 to Fig.11, successively.

Fig.7 Gain pattern of Phi=00and Phi=900at 35GHz

Fig.8 Gain pattern of Phi=00and Phi=900at 36GHz

Port1

Port2MMH OMT

Balanced output waveguide

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Fig.9 Gain pattern of Phi=00and Phi=900 at 37GHz

Fig.10 Gain pattern of Phi=00and Phi=900at 38GHz

Fig.11 Gain pattern of Phi=00and Phi=900at 39GHz

As can be seen from the above five figures, the rotational

symmetry of each pattern is very good, the gain is larger than

22dB, the sidelobe level is less than-25dB in the frequency

band.

V. CONCLUSION

In this paper, we design a Ka-band front end of fully

polarized microwave radiometer, based on theoretical analysis

and HFSS simulation software. The simulation results show it

has a good performance in the frequency band of 35GHz to

39GHz. The method used in the design of the front end is a

simple, efficient solution with highly engineering guiding

significance.

ACKNOWLEDGMENT

This work is supported by the National High Technology

Research and Development Program of China under Grant

Nos. 2012AA8123012 and 2010CB327505, the Basic

Research Foundation of Beijing Institute of Technology under

Grant No. 20120542015, and the Academy of Satellite

Application under grant No. 2012-1692.

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