A Novel Design of Single-Wall Cavity Backed Helical Antenna for ReConfigurable Gain and Bandwidth

A Novel Design of Single-Wall Cavity Backed Helical Antenna for ReConfigurable Gain and Bandwidth

1 Rahul Yadav, 2 Vinitkumar Jayprakash Dongre

Dept. of Electronics and Telecommunication, Thakur College of Eng. Technology, Mumbai University, India

Abstract

A single-wall cavity backed helical antenna has been designed to achieve re-configurability on gain and bandwidth. The helix is loaded inside the single-wall cavity in such a way that it can be rotated manually and thus provides variable resonance between C-Ku bands. The antenna has peak gain of 9.2 dB and highest measured bandwidth of 5.3 GHz. Computer Simulation Tool is used for the designing.

1. Introduction

   The helical antenna was first introduced by Kraus in 1946 [1] and in the past five decades it has gone through various design modifications with aim of improving basic antenna parameters. Since the rigorous analysis of helix is extremely complicated, therefore radiation properties of the helix such as gain, far-field pattern and bandwidth needs to be investigated. Also now a days, re-configurability of the antenna is playing important role in communication as it allows to use the single designed antenna in various frequency bands and thereby supporting variety of applications.

   The helical antenna is one of the selective antenna which is used for military applications and therefore a demand arises to employ a design with conformal shape, reconfigurable characteristics and robustness which can significantly help in communication at various bands as and when application demands. So to incorporate all such features, a partial cavity has been designed in the shape of Single-wall which back a 1½ turn helical antenna. It is important to find the best suited orientation of helix inside the cavity which results in desirable

   resonating bands. This is done by rotating the helical antenna with an offset angle of 45o degree in

   anticlockwise direction inside the cavity. At these various angles, the antenna response is extracted using Computer Simulation Tool.

2. Antenna Design

   The design begins with the formation of Single-wall shaped partial cavity as shown in fig. 1. The cavity has a height of 22mm, width of 24mm and thickness of 2mm. Only Single-wall has been designed due to the fact that with minimum number of enclosing walls also its possible to confine the electric field vector, which thereby helps to improve the resonance and gain of the antenna.

   1. Perspective view (b) Front view

      (c) Helix Design (d) Implemented helix

      Fig. 1 Design of Single-wall cavity backed 1½ turn helical antenna

      To achieve more variations in the resonant bands, 1½ turns for the helix is chosen with the fact that it will provide odd symmetry with respect to the cavity walls in various helix rotation as shown in fig.2.

      (a) 0o (b) 45o (c) 90o (d) 135o

      Form 315o and 45o, the effect of wall is only partially dominant which results in the bandwidth of 1.64 GHz and 0.39 GHz respectively. Now there is variation in the bandwidth of the antenna due to asymmetry in the design which is the main aspect, reflection from the single wall and different spacing between helix and wall at various helical rotations. Initially when the helical antenna is at 0o position, the distance between the wall and the antenna is closer and then later the distance increase eventually from 0o to 180o as shown in fig.3. The bandwidth at 0o position is 2.26 GHz, which is the highest bandwidth for this design. Then the bandwidth is in decreasing manner, at 315o position the bandwidth is lowest i.e. 0.39 GHz. Only at 0o position the dual band is observed. The other bands are shown in table 1.

      Table 1 Bandwidth analysis of single wall partial cavity

      (e) 180o	(f) 225o	(g) 270o	(h) 315o	Sr.	Helix	No. of	Frequency	Bandwidth(GHz)
      				No	Position	Bands	Bands

      Fig. 2 Top view of helix rotation inside Pi-wall partial cavity

      (GHz)

      1 0o Dual 7.45-8.71

      & 13.26-

      14.26

      2.26

3. Results and Discussion

   2 45o Single 9.30-10.94 1.64

   This section illustrated the analysis of helical antenna orientations and response at various rotation angles. All the potential parameters are extracted using time domain solve of CST Microwave Studio. The analysis begins with the extraction of reflection coefficient plot

   3 90o Single 13.02-

   13.93

   4 135o Single 13.38-

   14.36

   5 180o Single 13.80-

   14.78

   0.85

   0.98

   0.98

   as shown in fig. 3

   Fig. 3 Comparative plot of reflection coefficient at various helix rotation angles

   It is observed that at 0o position, dual bands are resonated due to the effect of reflection from the wall.

   1. 225o Single Notch Notch

   2. 270o Single Notch Notch

      8 315o Single 6.16-6.55 0.39

      Now looking at the gain analysis which is shown in fig.5, it is found that there is variation in the gain since only single-wall is present providing an odd symmetry around the helical antenna and parameters like spacing between the walls, turn visibility are also changing with significant offset.

      The turn visibility is defined here as the number of turns visible from the wall perspective view. This helps to provide an information can be used to set desired gain and bandwidth by adjusting this parameter. At 315o position, more turns are visible from the single wall perspective and this reflects the radiation from the helical antenna. Beyond 180o position the number of

      turn in from of the single wall is only one, this result in Table 2 Gain analysis of Single wall partial cavity

      reduced gain from 180o to 270o. The perspective view

      of turn visibility of the helical antenna with respect to the wall is shown in fig.4.

      (a) 0o (b) 45o (c) 90o (d) 135o

      (e) 180o (f) 225o (g) 270o (h) 315o

      Fig. 4 Turn visibility for various positions of single-wall with 1½ turns helical antenna rotation

      Fig. 5 Simulated gain plot for single-wall configuration at various helix rotation angles

      A peak gain of 9.25 dB and minimum gain of 2.8 dB is observed for 135o and 45o respectively. Table.2 shows the values of peak gain at the respective helix rotation.

      Sr. No Helix Position Turns Visibility Peak Gain (dB)

      1 0o 1.5 7.4

      2	45o	2	7.8
      3	90o	2	8.5
      4	135o	2	9.2
      5	180o	1.5	8.6
      6	225o	1	8.2
      7	270o	1	8
      8	315o	1	7.5

      The antenna has been tested at 135o position. Fig.6 shows the measure results for 135o position of helix with respect to the wall. It is observed that, the measured bandwidth for 0o helix rotation is 5.3 GHz whereas the simulated bandwidth is found to be 0.98 GHz only. This increase in the measured bandwidth is due to the lofting of helix base and feed pin joint in practical implementation whereas in simulatin, the joining point between feed pin and helix base is abrupt, which can be clearly seen in fig.1c-1d. So a peculiar observation has been noted down that, lofting between helix base and feed pin helps to improve the geometry continuity and thereby improves the current distribution along the antenna.

      Fig. 6 Measure results of single wall cavity backed helix at 135o position

      The radiation pattern of electric field vector for 0o and 135o position is shown in fig.7

      1. 0o Position

      2. 135o Position

   Fig. 7 E-field polar plot for Single-wall cavity at phi = 0

   Moreover, the electric field intensity of the antenna system is also investigated to obtain the maximum power handling capacity which is given as [2]

   Fig. 8 Near E-field distribution

   So, here electric field of 21249 v/m is observed in the near field zone of the single-wall cavity loaded helical antenna and thus maximum power handling capacity is found to be 5.53 MW assuming that breakdown threshold for helical antenna which is made of copper as 50 MV/m.

4. Conclusion

   The designed Single-wall partial cavity backed 1½ turn helical antenna meets the requirement of re- configurable gain and bandwidth, conformability and robustness making it suitable to use for various microwave bands. It is found that by varying the spacing between helical antenna and the walls, an application dependent response can be achieved. Also it observed that the practical result is having a remarkable improvement over simulated results.

5. References

1. J. D. Kraus and R. J. Marhefka, Antenna: For All Applications, 3rd ed. New York: McGraw-Hill, 2002.

2. Xiang-Qiang Li, Qing-Xiang Liu, Xiao-Jiang Wu, Liu Zhao, Jian-Qiong Zhang, Zheng-Quan Zhang, A GW Level High-Power Radial Line Helical Array Antenna, IEEE Trans. Antennas Propag., vol.56, pp. 2943 2948, 2008.

Power handeling capacity ( Breakdown Threshold )2

Electric field int ensity

(1)

The electric field distribution is shown in fig.8.