Hybrid T-shaped Monopole / Dielectric Resonator Antenna as EMI sensor

Saswati Ghosh

Professor, Department of ECE, Durgapur Institute of Advanced Technology & Management, Rajbandh, West Bengal, India

(Former Scientist, Kalpana Chawla Space Technology Cell, Indian Institute of Technology, Kharagpur, West Bengal), India E-mail:saswatikgp@gmail.com

Abstract-The increased use of high frequency electronic equipment in modern day technologies causes electromagnetic interference with each other. To avoid this, the detection of unwanted electromagnetic radiations is required in industry where electromagnetic interference and compatibility (EMI/EMC) problems are quite an important issue. For EMI measurement, it is required to place a sensor to receive the radiation from the equipment. In this paper, a dielectric loaded T-shaped ultra wideband monopole antenna is designed and used as sensors for measuring electromagnetic interference. The performance of a sensor depends on its antenna factor, which is the ratio of the incident electric field on the antenna surface to the received voltage at the load end. Here the antenna factor of the same antenna is presented for the desired and cross polarization of the incident field. The results for antenna factor show an ultra wide bandwidth with the cross polarization isolation better than 86 dB / m for the T-monopole / DRA as EMI sensor. The simulated return loss characteristics show well agreement with the measured data of a prototype antenna.

I. INTRODUCTION

The electromagnetic interference (EMI) has become a crucial issue in the performance of modern high frequency electronic equipment. To avoid the electromagnetic interference and related hazards, stringent electromagnetic compatibility (EMC) testing of the electronic devices is necessary. Accurate measurements in any field require accurately calibrated equipment. When measuring radiated signals, the front end of the measurement system is the sensor which is actually an antenna in receiving mode. While using an antenna as a sensor, especially in the industrial environment where different types of equipment are operating simultaneously, usually very large frequency coverage is necessary for measurements made according the standards. The hybrid monopole / dielectric resonator antenna (DRA) has found wide application as ultra wideband antenna [1 − 2]. The loaded wire antennas (e.g., inverted L, T, I and C antennas) are used to reduce the height of the unloaded (e.g. simple monopole and dipole) antenna and at the same time to achieve broadband performance [3 − 4]. In this paper, a T-shaped monopole along with a DRA is proposed as EMI sensor. The design guidelines for the DRA are followed from the literature [2]. The load arm lengths of the T-shaped monopole are chosen for various main-arm lengths to obtain the resonant effect in the transmitting mode. For the

simulation of the return loss the electromagnetic software WIPL-D Pro v5.1 is used [5]. To validate the simulation, a prototype antenna is fabricated and experimental results for return loss are compared with the simulated results. The performance of this hybrid T monopole antenna as EMI sensor is studied in terms of the antenna factor. The extra loading of the T-shaped monopole / hybrid antenna is likely to introduce the reception of cross-polarized component of incident electric field that may degrade the performance of the sensor. In this paper the antenna factor of the antenna is studied for the desired as well as cross polarized incident electric field.

II. ANTENNA CONFIGURATION

The hybrid antenna consists of a thin T-shaped monopole and an annular DRA, both sharing the same axial reference and mounted on a ground plane (Fig. 1). The T-shaped monopole is designed to have a resonance at the lower end of the frequency band, while the DRA is designed to have a resonance near the upper end of the desired spectrum range. The combined effect of the T antenna and DRA produces the intermediate resonance and maintains the 10dB return loss over a wide frequency range.

Fig. 1 Cross section of a T-shaped monopole / DRA.

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III. ANTENNA FACTOR

The most common performance descriptor of a sensor is the antenna factor. For EMI measurement, the sensor / antenna is illuminated by the incident plane wave. A receiver such as a spectrum analyzer is attached to the terminals of this measurement antenna. The voltage measured by this instrument is denoted as Vrec. It is desired to relate this received voltage to the incident electric field. This is done with the antenna factor, which is given as follows

( )(1)

Re ( )incIncident Electric Field E

Antenna factorceivedVoltage Vrec

=

10( )

20log (2)Re ( )

incdB

Incident Electric Field EAntenna factor

ceived Voltage Vrec⎛ ⎞

= ⎜ ⎟⎝ ⎠

The ratio of the incident electric field on the surface of the sensor to the received voltage at the antenna terminal when terminated with 50 ohms load is known as the antenna factor [6]. Here it should be noted that the impedance of the receiver is taken as 50 Ω and the polarization of the antenna is parallel to the incident electric field. The Thevenin’s equivalent circuit diagram of an EMI sensor is shown in Fig. 2. The receiving antenna is replaced by an equivalent open circuit voltage Voc at the two terminals of the antenna and its impedance. Generally the receiver (e.g. spectrum analyzer) impedance is considered as 50ohm.

Fig. 2. Equivalent circuit diagram of an EMI sensor. Due to the presence of the top loading, the antenna (Fig.1) is likely to suffer from cross polarization pick-up. Here the cross polarization characteristic is studied in terms of the antenna factor for the desired and cross-polarized electric field.

IV. SIMULATION AND MEASURED RESULTS

To design the T - monopole / DRA with suitable broadband characteristics, it is required to study the return loss characteristics of the antenna. For the simulation of the antenna in transmitting mode using WIPL-D, the antenna is considered to be connected with a delta-gap source of 1Volt. The simulation result for the return loss versus frequency for a DRA loaded monopole obtained using WIPL-D (Fig. 3) shows the same nature as the experimental data available in literature [2]. The dimensions of the antennas studied in this section are presented in Table 1. Fig. 4 presents the return loss versus frequency plot of the T-shaped monopole / DRA

compared to a simple monopole. Fig. 5 shows the return loss versus frequency plot for different main – arm and load – arm lengths. It is noticed from the figure that the desired ultra wideband performance is achieved by adjusting the main – arm and load – arm lengths keeping the dielectric parameters and dimensions unchanged. To verify the results experimentally, a prototype antenna is fabricated (Fig. 6). A circular finite ground plane of radius 50 mm is used for the measurement. The annular DRA is machined from a cylindrical rod of dielectric constant plastic material of EMERSON & CUMING MICROWAVE PRODUCTS (type: Eccostock HI-K; relative permittivity εr

= 10 ± 3 %). A comparison between the simulated and measured return loss using HP 8757 C network analyzer is presented in Fig. 7. The dimensions of the prototype antenna are presented in Table 1. The design parameters of the prototype are as follows: L = 12 mm; 2L1 = 6 mm; s = 0.84 mm; a = 5 mm; b = 1.49 mm; r = 0.65 mm; h = 6 mm ; εr = 10.

-30

-25

-20

-15

-10

-5

4 5 6 7 8 9 10 11 12 13Frequency (GHz)

S11

in d

B

simulatedmeasured [2]

Fig. 3 Return loss versus frequency for DRA-loaded monopole antenna.

-35

-30

-25

-20

-15

-10

-5

0

4 5 6 7 8 9 10 11 12 13 14 15 16Frequency (GHz)

S11

in d

B

T -monopoleT-monopole / DRAmonopole

Fig. 4 Plot of return loss versus frequency of T-shaped monopole / DRA. When the same antenna is studied as EMI sensor, the antenna connected with a 50 Ω load at the gap, is considered to be illuminated by a plane wave with z-directed incident electric field of magnitude 1 volt / m. This incident electric field induces a current distribution on the antenna surface which produces a voltage drop across the load. The plots of the antenna factor versus frequency for the desired and cross polarization of incident electric field are presented in Fig. 8 -

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9. The dimensions of the hybrid antenna are presented in Table 1.

-35

-30

-25

-20

-15

-10

-5

4 5 6 7 8 9 10 11 12 13 14 15 16Frequency (GHz)

S11

in d

B

L=11 mmL=12 mmL=13 mmL=14 mm

Fig. 5 Plot of return loss versus frequency of T-shaped monopole / DRA with varying main arm and load arm length.

Fig. 6(a) Prototype hybrid antenna – top view.

Fig. 6(b) Prototype hybrid antenna – side view.

Fig. 7 Return loss versus frequency plot of T-shaped monopole / DRA prototype antenna with dimensions given in Table 1.

38

40

42

44

46

48

50

4 6 8 10 12 14 16Frequency in GHz

Ant

enna

Fac

tor (

dB m

-1)

design#1design#2design#3design#4monopole/DRA

Fig. 8 Plot of Antenna factor versus frequency of T-shaped monopole / DRA with varying main arm and load arm length for the desired polarization.

130

140

150

160

170

180

190

4 6 8 10 12 14 16Frequency in GHz

Ant

enna

Fac

tor (

dB m

-1)

design#1design#2design#3design#4monopole/DRA

Fig. 9 Plot of Antenna factor versus frequency of T-shaped monopole / DRA with varying main arm and load arm length for the cross polarization.

TABLE I ANTENNA PARAMETERS OF DIFFERENT ANTENNAS

Antenna Parameter

(mm) (refer Fig.1)

Shown in

Fig. 4 Fig. 5, 8 – 9

T-monopole / DRA

Design #1

Design#2

Design#3

Design#4

L 12 11 12 13 14 2L1 6 8 6 4 2

s 0.84 0.84 0.84 0.84 0.84

a 5 5 5 5 5 b 1.49 1.49 1.49 1.49 1.49

r 0.65 0.65 0.65 0.65 0.65

h 6 6 6 6 6

V. DISCUSSIONS

The plot of return loss versus frequency (Fig. 4) for the DRA-loaded T-shaped monopole antenna shows a wider bandwidth (10 dB bandwidth of 110%) compared to a monopole or T-shaped monopole and also to a DRA-loaded monopole. From Fig. 5 it is noticed that by suitably adjusting the load-arm and main-arm length the desired bandwidth can be achieved. However, the height of the dielectric cylinder is kept

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constant. While using the T-monopole/DRA as EMI sensor, it is noticed that the antenna factor of the antenna remains almost constant (varies from 38 dB / m to 46 dB / m) within the frequency range 4 - 15 GHz (Fig. 8). Fig. 9 shows that the cross polarization isolation is better than 86 dB / m for the T-monopole / DRA. It should be noted that for a good sensor the cross polarization pick up of the antenna is expected to be minimum. Hence the greater the value of cross polarization isolation, the better is the performance of the antenna as EMI sensor. The antenna factor of the antenna also depends on the direction of the incident wave. The antenna factor variation with the incident direction will follow the radiation pattern form.

VI. CONCLUSION

This paper presents the initial investigation on the wideband performance of a hybrid antenna consisting of a thin T-monopole and an annular DRA as EMI sensor. It can be concluded from this work that the use of T-monopole instead of a simple monopole has increased the operating bandwidth (both in transmitting and receiving mode) appreciably without making any major compromise in the performance in terms of cross polarization pick up.

REFERENCES

[1] Marc Lapierre, Yahia M.M. Antar, A. Ittipiboon, A. Petosa, “Ultra Wideband Monopole/Dielectric Resonator Antenna,” IEEE Microwave and Wireless Components Letters, Vol. 15, No. 1, January 2005.

[2] Debatosh Guha, Yahia M.M. Antar, A. Ittipiboon, A. Petosa, David Lee, “Improved Design Guidelines for the Ultra Wideband Monopole-Dielectric Resonator Antenna,” IEEE Antennas and Wireless Propagation Letters, Vol. 5, 2006, pp. 373-376.

[3] T. L. Simpson, “The theory of top-loaded antennas: Integral equations for the currents,” IEEE Trans. Antennas and Propagation, vol. AP-19, 186–190, March 1971.

[4] S. Ghosh, A. Chakrabarty and S. Sanyal, “Loaded Wire Antenna as EMI Sensor”, Progress in Electromagnetics Research, PIER 54, page 19-36, 2005.

[5] B. M. Kolundzija, J. S. Ognjanovic, T.K. Sarkar, WIPL-D software: Electromagnetic Modeling of Composite Metallic and Dielectric Structures –Software and User’s Manual, Artech House, pp.1-332.

[6] C. R. Paul, Introduction to Electromagnetic Compatibility, New York: John Wiley & Sons Inc., 1992, pp. 1-236.

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