ATMS INDIA

2015

COMPACT DUAL PROXIMITY COUPLER ON A SINGLE COUPLED LINE IN S-BAND FOR SEMI

ACTIVE PHASED ARRAY ANTENNA

Sanjay Choube- Dy General Manager, Balmukund Jha- Dy Manager, Akhil Gupta-Dy Engineer Development & Engineering-Antenna

Bharat Electronics Limited, Bharat Nagar, Ghaziabad (U.P.) 201010 sanjaychoube@bel.co.in

Abstract- The paper presents in-line monitoring of Semi Active Phased Array Antenna Systems using a unique dual coupled line coupler in Strip Line technology in S-Band. In-line monitoring of RF outputs from T/R modules during test/evaluation of Antenna Electronic and calibration at NFTR is required to ease calibration and radiation pattern measurements by examine the health of T/R modules. The coupler is based on proximity coupling on a single coupled line such that both the couplers are parallel on either side of the coupled line. The unique feature of this coupler is availability of 2 coupled ports at different power levels, compactness and light weight. It helps in identifying faulty T/R module whenever measured radiation pattern is not as per requirement. The dual coupler has been successfully verified by simulating using HFSS & IE3D RF simulation tools. The prototype has also been successfully validated and measured

results are in close agreement with simulation results.

I. INTRODUCTION

Phased Array Antenna for radars is State of the Art technology where Antenna Beam is steered electronically at a very high speed. Over the years, Phased Array Antenna technology has been evolved from Passive Phased Array Antenna to Active Phased Array Antenna. An active Phased Array Antenna typically requires as many Transmit/Receive Modules as total number of radiating elements in the Array Antenna. Large number of T/R modules not only increases the cost but also impose critical requirement of thermal management and expensive maintenance. To minimise cost as well as bringing down criticalities in thermal management and maintenance, an approach where less number of T/R modules but with increased power output is in practice and this approach is named as Semi Active Phased Array Antenna. In Semi Active Phased Array Antenna, a group of Radiating Elements are fed by a single T/R module. Typically in a planar Array Antenna entire Linear Antenna Row is fed by a single T/R module. Thus for a Planar Array Antenna of size NxM radiating elements(where N is Radiating elements in the Linear Array and M is the number of Linear Array) only M number of T/R modules will be required Fig-1. Although, Semi Active Phased Array Antenna being less critical in terms of cost, thermal management and maintenance, they have a drawback in terms of not having redundancy whenever any T/R module is not functional in

the Array Antenna. Any failure will drastically affect the radiation pattern. On-line monitoring between Radiating Linear Antenna and T/R module can help in quick diagnosis of faulty T/R module. On-line monitoring is also of great help during calibration of the antenna at NFTR. NxM Planar Array Radiating Aperture

Fig-1: Block Diagram of Semi Active Phased Array Antenna The RF module which is conventionally used for in-line monitoring is a coupled line coupler. Conventionally a directional coupler is a 4-port microwave junction Fig-2. An ideal directional coupler has the property that a wave incident at part 1 couples power into ports 2 and 3 but not into part 4. Similarly, power incident in part 4 couples into ports 2 and 3 but not into part 1. Thus ports 1 and 4 are uncoupled. For wave incident in port 2 or 3, the power is again uncoupled. Typical parameters of a coupler other than coupling loss (C) are Directivity and Insertion loss. These parameters are defined as given under C =10 *log10 (P1/P3)

Fig-2: Context diagram of 4-port coupler

Tx-Power Divider & AGR

T/R module-1

T/R module-M

T/R module-1

Coupler

Coupler

Coupler

ATMS INDIA

2015

Ideally, the power coupled in the backward direction in arm 4 should be zero. The extent to which this is achieved is measured by the directivity D, which is defined as D=10 *log10 (P3/P4)

The directivity is a measure of how well the power can be coupled in the desired direction.

II. THEORY OF EDGE COUPLED

STRIPLINE COUPLER

Fig-3: Edge Coupled Strip line Coupler

In the Fig-3 for Spacing (s), width (w), Ground plane spacing (b) and for voltage coupling coefficient Cv, the

normalised width and normalised spacing can be

calculated using the following set of equations[3].

= , β=

∆ =

+ k q=

f0=

f0 0.8825424)

III. RF MONITORING APPLICATION

Now a days, directional coupler is designed as dual directional coupler. Dual directional couplers are ideal for sampling and monitoring both forward and return RF powers at the same time. Additionally this can be utilized for ensuring the gain flatness of the amplifier. Various waveguide and coaxial directional couplers have been utilized for the purpose of RF monitoring but off late the directional couplers on planar transmission lines (strip line in this case) have gained popularity because of their reduced size. A dual coupler with two coupling values of 30 dB/ 50 dB in S Band in strip line with minimum loss is developed in-house. For Transmit,-50 dB coupling value is preferred and for Receive Power -30 dB can be utilized. As -50 dB coupling is a very low value, normal practice is to use attenuation pad of 20 dB in tapping sample of high power which is avoided in this design.

IV. CONFIGURATION

This dual coupler is a 6-port module (Fig-5) and can be configured in both the directions as per the coupling requirement.

Configuration 1:

Coupling

PORT

1 2 3 4 5 6

50dB I/P Through Coupled Isolated Terminated Terminated

30dB I/P Through Terminated Terminated Coupled Isolated

Configuration 2:

Coupl

ing PORT

1 2 3 4 5 6

50dB Through I/P Isolated Coupled Terminated Terminated

30dB Through I/P Terminated Terminated Isolated Coupled

ATMS INDIA

2015

V. DESIGN & SIMULATION

This coupler is a 6-Port directional coupler in strip Line Technology. Fig-4 depicts HFSS model of the dual coupler for simulation of all critical parameters.

Fig-4: HFSS model of 6-Port Dual Coupler

Simulation Results:

3.10 3.20 3.30 3.40 3.50Freq [GHz]

-40.00

-30.00

-20.00

-10.00

0.00

dB

(S(1

,1))

HFSSDesign1Return Loss at Port1 ANSOFT

Curve Info

dB(S(1,1))Setup1 : Sw eep

3.10 3.20 3.30 3.40 3.50Freq [GHz]

-60.00

-50.00

-40.00

-30.00

-20.00

-10.00

0.00

dB

(S(1

,3))

HFSSDesign1Coupling (-50dB) ANSOFT

Curve Info

dB(S(1,3))Setup1 : Sw eep

3.10 3.20 3.30 3.40 3.50Freq [GHz]

-50.00

-40.00

-30.00

-20.00

-10.00

0.00

dB(S

(1,5

))

HFSSDesign1Coupling (-30dB) ANSOFT

Curve Info

dB(S(1,5))Setup1 : Sw eep

3.10 3.20 3.30 3.40 3.50Freq [GHz]

-80.00

-70.00

-60.00

-50.00

-40.00

-30.00

-20.00

-10.00

0.00

Iso

latio

n

HFSSDesign1Isolation for -50dB ANSOFT

Curve Info

dB(S(1,4))Setup1 : Sw eep

3.10 3.20 3.30 3.40 3.50

Freq [GHz]

-60.00

-50.00

-40.00

-30.00

-20.00

-10.00

0.00

dB

(S(1

,6))

HFSSDesign1Isolation for -30dB ANSOFT

Curve Info

dB(S(1,6))Setup1 : Sw eep

VI. VALIADATION & MEASUREMENTS

The coupler has been successfully validated by realising a Prototype. As shown in Fig-5:

Fig-5: Prototype of 6-Port Dual Coupler Measured Vs Simulation Results were found to

be same in both the configuration:

Test results for configuration 1:

Return loss

ATMS INDIA

2015

Coupling (for 30 dB)

Coupling (for 50 dB)

Isolation (for 30 dB)

Isolation (for 50 dB)

Tabulated Results of Configuration 1 :

Freq

(GHz)

S11

(dB)

S22

(dB)

S33

(dB)

S44

(dB)

S55

(dB)

S66

(dB)

3.0 -23.9 -24.0 -25.9 -26.6 -22.2 -21.7

3.3 -26.9 -25.5 -31.0 -33.6 -26.0 -24.9

3.6 -29.1 -28.7 -37.1 -37.6 -36.8 -32.8

TABLE-1: Measured Return Loss

Freq (GHz)

S12 S21

Amp (dB) Ph(deg) Amp (dB) Ph(deg)

3.0 -0.20 -29.77 -0.20 -29.77

3.3 -0.21 -104.71 -0.21 -104.71

3.6 -0.24 -179.20 -0.24 -179.20

TABLE-2: Measured InsertionLoss

Freq (GHz) S13(dB) S15(dB)

3.0 -50.44 -32.16

3.3 -49.81 -32.23

3.6 -50.94 -32.54

TABLE-3: Measured Coupling

Freq(GHz) S14(dB) S16(dB)

3.0 -68.86 -49.89

3.3 -67.40 -49.14

3.6 -58.40 -50.05

TABLE-4: Measured Isolation

ATMS INDIA

2015

Summary of results of Configuration 1:

Characteristics For 50 dB For 30 dB

Frequency 3.1-3.5GHz 3.1-3.5GHz

Insertion Loss 0.25 max 0.25 max

Coupling -50±1 dB -30±2 dB

Isolation ≤-65 dB ≤-49 dB

Directivity ≥15 dB ≥19 dB

Return Loss ≤-24 dB ≤-24 dB

Summary of results of Configuration 2:

Characteristics For 50 dB For 30 dB

Frequency 3.1-3.5GHz

3.1-3.5GHz

Insertion Loss 0.25 max 0.25 max

Coupling -50±1 dB -30±2 dB

Isolation ≤-63 dB ≤-49 dB

Directivity ≥13 dB ≥19 dB

Return Loss ≤-24 dB ≤-24 dB

VI. CONCLUSION

An approach for on-line monitoring/measurement of RF power for Semi Active Phased Array Antenna is presented. An-In-House developed6-Port dual coupler with simultaneous two different coupling values up to -50 dB and low insertion loss for on-line monitoring of Tx and Rx Power is also presented. The coupler design is compact and meets the requirement of on-line measurement of Semi Active Phased Array Antenna Systems. On line monitoring saves time and man efforts for evaluating/testing Antenna Arrays and quick diagnosis of faulty T/R modules.

REFERENCES

[1] John D. Kraus, “Antennas” [2] www.orbitfr.com, “Antenna Measurement Theory” [3] Bharti Bhat,S K Kaul “Stripline-Like Transmission Lines for MICs” [4] David M.Pozar“Microwave Engineering” [5] Rogers Corporation“Design Equation for Broadaside and Edge-wise Strip[line [6] S.B. Cohn "Shielded Coupled-Strip Transmission Line" IRE Trans MTT October, 1955 p.29-38

BIO DATA OF AUTHOR(S)

Sanjay Choube is an Hons Graduate in Electronics & Telecommunication Engineering. Initially he served M/s CEL, Sahibabad (U.P) and had significant contribution in Design & Production of PCMs for Passive Phased Arrays. Presently he is working with M/s BEL, Ghaziabad as DGM in D&E-Ant and is involved in antenna designs for various radar projects. Bal Mukund Jha is B.Tech in Electronics and Telecommunication Engineering from J.K.Institute (University of Allahabad) and M.Tech (Microwave) from Delhi University. Presently he is working with M/s BEL, Ghaziabad as Dy MGR in D&E-Ant and is involved in antenna designs for various radar projects.

Akhil Gupta has completed B.Tech in Electronics& Communication engineering from NIT Srinagar in July 2012.He is working as a Deputy Engineer in D&E-Antenna, Bharat Electronics Limited, Ghaziabad. He is involved in various ongoing design activities of antenna for radar projects .