2011 Cross Strait Quad-Regional Radio Science and Wireless Technology Conference

A Novel2-18GHz TEM Double-Ridged Horn Antenna for Wideband Applications

Hana Amjadi* and Farzad Tavakkol Hamedani**

*,** Electrical and Computer Engineering Department, Semnan University Semnan, Iran

*hana.amjadi@gmail.com

** farzadtavakkol@gmail.com

Abstract: In this paper we will present the new design of TEM

double-ridged horn antenna, resulting in a better VSWR and improved gain of antenna. A cavity back and a new technique for tapering the flared section of the TEM horn antenna are introduced to improve the return loss and matching of the impedance, respectively. By tapering the ridges of antenna both

laterally and longitudinally it is possible to extend the operating frequency band while decreasing the size of antenna. The proposed antenna is simulated with two commercially available packages, namely Ansoft HFSS and CST microwave studio.

Stimulation results for the VSWR, radiation patterns, and gain of the designed TEM horn antenna over the frequency band 2-18 GHz are presented.

Keywords: TEM hom antenna, wideband application, double-ridged, radiation pattern, cavity back.

1. Introduction

The utilizing of wideband antennas continues to increase. These antennas are commonly employed in wide variety of applications, including electromagnetic compatibility (EMC) testing, radar, detection systems, and broad band communication systems.

Recently, transverse electromagnetic (TEM) hom antenna has been used very successfully as ultra wideband antennas for pulse radiation and ground penetrating radar (GPR) applications [1] - [3].

The TEM hom antenna is an end-fire, travelling-wave structure. These antennas have the advantages of wideband, relatively easy construction, no dispersion, high gain and directivity performance. However it has the disadvantage of a large size. Several methods have been proposed to improve the performance of the antenna and reduce its size. Kanda suggested the idea of loading the TEM hom antenna with resistors to enlarge the bandwidth [4]. Also in [3], Shalger introduced a TEM hom antenna with resistive sheet to reduce the distortion. However, loading the TEM hom with resistive material reduces its efficiency. An exponentially tapered TEM hom with microstrip balun is designed in [5] to increase the bandwith of the TEM hom antenna; however we can't use this antenna in ultra wide band range.

It has been proved that the practical bandwidth of hom antennas can be increased greatly by adding metallic

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ridges to the waveguide and flared sections [6], [7]. Several methods have been used for tapering the flared section of the hom antenna.

In this contribution, we introduce a new method for tapering the ridges of the double-ridged hom antenna that increases antenna performance. A TEM double ridged transition including a 50n coaxial feed input and a cavity back for 2-18 GHz frequency band is also presented.

2. Description of the Antenna Structure

The configuration of the suggested TEM hom antenna is illustrated in Fig. 1. Total length of the antenna is 75 mm with an aperture size of 50x50 mm2. To achieve a single mode operation in the waveguide transition over the 2-18 GHz an aperture size of 14x20 mm2 is required for the waveguide. These parameters are optimized using HFSS optimization to satisfy the required characteristics. The material for the conducting plates is aluminium with 1 mm thickness.

The antenna consists of flared section and TEM double-ridged transition including a coaxial excitation. The transition section is divided into two parts, a TEM double-ridged waveguide and a cavity back located at the back of the waveguide. The design process of each part is investigated completely in the next sections.

TEM Double-Ridged

Coaxial Cable

Fig.l: Configuration of the proposed double-ridged hom antenna.

July 26-30, 2011

2.1 Design of the Ridges in Flared Section

Tapering of the two identical ridges is the most significant part in the double-ridged hom antenna design. It is desired that the ridge height and width taper must be such that the associated impedance taper is a smooth transition from the ridge (500) impedance to the impedance of free space (3770) [5].

In order to increase the impedance matching between the double-ridged waveguide and the free space, ridges are tapered both laterally and longitudinally, it means that height and width of ridges varies in flared part of hom antenna. Tapering of the ridges in longitudinal plate (y axis) is based on exponential function and in lateral plate (x axis), ridges are tapered linearly. The curvature of the ridges along the longitudinal direction is determined by a modified exponential function as Equation (1):

Z(y) = 0.02y +Zo ek.y, (O� Y � I) (1)

in which y is the distance from the double-ridged waveguide aperture and l is the axial length (with l = 60mm) of the antenna opening. The k is calculated as follow [8]:

(2)

wherein Zo and Zz is the characteristic impedance at the waveguide and impedance of the hom at the aperture, respectively. The following method is introduced to determine the dimensions of the ridges:

The axial length of the hom opening (I) is divided into eight sections. Now we have 8 smaller double-ridged waveguides. Then, the height of each double-ridged waveguides should be optimized by Ansoft HFSS in such a way that the corresponding characteristic impedance be equal to Eq. (1). After obtaining the height of each part we connect them together. It can be seen that at first, the height of ridge increases and then decreases (Fig.2). Characteristic impedance of each section and detail design dimensions of taped part are tabulated in Table I.

1= 60mm

Fig . 2: The designed antenna made from eight smaller waveguides

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TABLE I: DETAIL CHARACTERISTICS OF TAPERED PART

Distance Width of

Waveguide Characteristic between the impedance the waveguide number

(0) ridges aperture (mm) (mm)

0 50.00 1.00 6.00

1 69.77 2.52 11.45

2 97.38 5.00 17.82

3 135.90 8.56 23.54

4 165.98 12.48 28.20

5 202.73 19.42 33.36

6 247.61 26.25 38.98

7 302.42 36.05 44.60

8 376.50 50.00 50.00

2.2 Design of the Feed Section TEM double-ridged transition is designed to match the

impedance of the hom section to the coaxial line. The configuration of the feed section is shown in Fig. 3. The antenna is excited by a SMA coaxial cable. As shown, the shield of the coaxial probe is connected to the lower ridge and its inner conductor is connected to the upper ridge.

In order to achieve low VSWR, the cavity back length, the initial distance between ridges in the rectangular waveguide and probe spacing from the ridged edge should be optimized. From the optimization process it was found that the probe spacing from the ridged edge affects the gain of the antenna and shaping of the main lobe at high frequencies. A lot of efforts have been made to identify the appropriate position of the coaxial cable.

It is very common to use a cavity back to obtain a much lower return loss in the coaxial to double-ridged waveguide transitions. It was found that the VSWR of the antenna is critically dependent on the shape and dimensions of the cavity back. Hence, we consider a polygonal shaped cavity.

6mm

2.5 mm 6.5 mm � • •• • Imm

i

4.12 mm

Fig. 3: Configuration of the feed section

3. Simulation Results

In this section simulation results of the introduced antenna are presented. To confirm the results, the corresponding antenna characteristics are verified by Ansoft HFSS which is based on the finite element method and CST microwave studio which is based on the finite integral technique. Both show very close results confirming that the simulated results are reasonably accurate. Fig. 4 shows the VSWR of the proposed antenna. As shown, the TEM hom antenna has the frequency band of 2 to 18 GHz for VSWR< 2.0.

2,---,---,---,----,---,---,----,---,

1.6

1.4

I HFSS � / _-+--1----+--+ - - - CST " I I I I I I I

1.21---+---+ -0·,;-1--;1--+----+- IiI------+------I

1L--�-�--�--L-�--� ___ � 2 4 6 8 10 12 14

Frequency (GHz) 16

Fig. 4: VSWR of the designed antenna vs. frequency

18

Fig. 5 shows the simulated E and H-plan radiation patterns at 2, lO and 18 GHz .From this figure it is seen that, the back lobe and side lobe levels (SLL) are quite low. Furthermore as the frequency grows, the pattern becomes more direction. The radiation patterns of Fig. 5 are obtained through HFSS.

10

5

iii � 0

E Q) :t: CIS -5 11. c: 0 � :c -10 CIS

�

_20 L--L----�--�----�----L----L----��

-150 -100 -50 0 50

Angel (Oeg) (a)

100 150

343

20,-�---,,---,-----,---,----,----,__,

10

iii 0 � E � -10 CIS

11. c: o

:;::; .!!! '0 r:. -30

-40

\ , , '\ , , , ,I , ,I ,

',' � , , I

I _50L-

-L----L---�----�----L----L----L-�

III � c:

� CIS 11. c: o :;::; .!!! '0 CIS a::

-150 -100 -50 0 50 100 150

Angel (Oeg) (b)

_30L--L----L---�----�----L----L----L-� -150 -100 -50 0 50 100 150

Angel (Oeg) (c)

E-plane H-plane

Fig. 5: E-plane and H-plane radiation pattern of the antenna at: (a) 20Hz (b) 10 OHz, (cl 180Hz

14 16 Frequency(GHz)

Fig .6: Total gain versus frequency for the suggested TEM double­ridged antenna

18

Fig. 6 presents the total gain of the designed TEM hom [8]

antenna over the frequency range of 2 to 18 G Hz, and Fig. 7 shows the total gain of the TEM double-ridged hom antenna that has the ridges tapering only longitudinally in the flared part. As shown in these figures, the gain of the proposed antenna increases as frequency increases with a maximum value more than 16 dB at 18GHz while the gain for usual TEM double-ridged antenna has a peak value of 13 dB.

14,---.----.----,----.---.----.----.---,

12

- 10 m � c: 'M (!) 8

6

4 6 8 10 12 14 16 18 Frequency(GHz)

Fig .7: Total gain ofTEM double-ridged hom antenna with fixed-width ridges

4. Conclusion

In this paper, a new TEM double-ridged hom antenna

has been proposed for the 2-18G Hz frequency band. We

have shown that the new design for tapering the flared

part of TEM double-ridged hom antenna leads to a

significant size reduction of the device. Furthermore,

Stimulation results show that the designed antenna

provides good VSWR (less than 2), satisfactory far-field

radiation characteristics and high gain over the practical

bandwidth. This antenna that covers the 2-18 G Hz can be

useful for EMC testing and the other broadband

applications.

References

[I] K. Chang and S. Yang, "Design of a wideband TEM hom antenna" IEEE Trans. AP-S., vol. 1, 229-232, June 2003

[2] A. S. Turk, "Ultra wideband TEM hom design for ground penetrating impulse radar system," Microwave and Optical

Technology Letters., vol. 41, No.5, 333-336, June 2004 [3] K. L. Shalger, G. S. Smith, and 1. G. Maloney, ''IEM hom

antenna for pulse radiation: An improved design," Microwave and Optical Technology letters., vol. 12, no 2, pp 86-90, June 1996

[4] M. Kanda, "The effects of resistive loading of "TEM" horns," IEEE Transaction on Electromagnetic Compatibility., vol. 24, no 2, pp 246-255, May 1982

[5] K.H. Chung, S.H. Pyun, S.Y. Chung and 1.H. Choi, "The design of a wideband TEM hom antenna with a microstrip-type balun" , IEEE Antennas and Propagation Society International Symposium, pp. 1899-1902, June 2004

[6] K. L. Walton and V. C. Sundberg, "Broadband ridged hom design," Microwave J., vol. 4, pp 96-101, Apr. 1964.

[7] S. B. Cohn, "Properties of ridged waveguide," Proc. IRE., vol. 35, pp 778 - 783, Aug. 1947.

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J. L. Kerr, "Short axial length broad-band horns," lEE Trans Antennas Propagate., vol. AP-21, no. 5, pp. 710---714, Sep. 1973.