Design of Helical Antenna with Balun at 2.2 GHz

Design of Helical Antenna with Balun at 2.2 GHz

Ayisha B S

Indian Institute of Space science and Technology Trivandrum, India

Abstract: The design of a helical antenna having following specification are presented:

Operating frequency: 2.2 GHz Gain: >5dBi

Polarization: Circular Design platform is HFSS.

Keywords: Helix, Balun, Coax, Axial-Ratio Bandwidth, Polarization

1. Introduction

   Helical antenna is a broad band radiator fed using a coaxial cable. It is basically a conducting wire wound in the form of a screw thread forming a helix. Helical antenna is usually designed in the presence of a ground plane of minimum diameter of 3/4. Two modes of operation is possible, axial and normal mode.in normal mode the field radiated by the antenna is maximum in the plane normal to the helical axis and in axial mode the maximum will be along the axis of the helix. Axial mode is more practical as it can achieve circular polarization over a wider band width. The terminal impedance of a standard helix will be around 100-200 ohms and the impedance of a standard feed line is 50 ohms. To get impedance matching between these two, a balun is designed.

2. HELICAL ANTENNA DESIGN The antenna is designed in axial mode.

   Fig 1: Helical antenna in axial mode

   To excite this mode, the diameter D and the spacing S must be large fractions of wavelength. To achieve circular polarization primarily in the major lobe the circumference C of the helix must be in the range 3/4<C/<4/3 and spacing S approximately = /4.

   The pitch angle is usually 12° 14°. The ground plane diameter is at a minimum of /2 and is fed by a coaxial line. Radius of helix = 0.01989m

   Radius of wire = 0.001m Pitch = 0.03123m Number of turns =4

3. BALUN DESIGN

   The calculated resistance R of this helix is approximately. R~140(C/) ~140 ohms

   The characteristic impedance of a standard coaxial line is 50 ohms. To provide a better match between these two, there are many ways. One common method is to reduce the input impedance of the helix by properly designing the first ¼th turn of the helix and bring it down to 50 ohms. For this the wire of first ¼ turn should be flat in the form of a strip and the transition to helix must be gradual. Design and construction of such a structure can turn out to be tedious and it need not be accurate.

   A new alternative is to design a balun, which can act as an impedance converter. A thin metal strip of length /4 is implemented as a balun. The balun will be connected from inner conductor of the coax to the tip of the helix.

   Length of the balun = 0.03125 m Width of the balun = 0.001m Thickness of the balun =0.00025m

   Fig2: balun between helix and coax

4. COAXIAL FEED DESIGN Coax is designed at 2.2GHz for 50 ohms. Dielectric constant = 4.7

   Inner diameter = 0.001m Outer diameter=0.006 m.

   Fig3: Complete structure

   5a. S11

5. SIMULATION RESULTS

   Fig4:S11

   5b. Gain

   5c.Polarization

   Fig4: gain

   Gain obtained is around 9.7 dBi

   Fig5: polarization

   Antenna is right circularly polarized

6. REFERENCES

1. Constantine A Balanis, Antenna theory analysis and design 3rd edition

2. John D Kraus , Antennas for all applications.

3. S. Abdullah, S. I. Syed Hassan, Design Small Size of High Frequency (HF) HelicalAntenna2009 5th International Colloquium on Signal Processing & Its Applications (CSPA)

4. Mike B. Young, Peter Norgard, Randy D. Curry, Senior Member, IEEE University of Missouri-Columbia

   ,Design, Simulation, Construction, and Characterization of a Wideband 900MHz-2.25GHz Helical Antenna.

5. Samihah Abdullah,Dayang Suhaida Awang Damit,Belinda Chong Chiew Meng, Rohaiza Baharuddin 2.46 GHz Reduce Size Helical Antenna Simulation, 2011 IEEE International Conference on Control System, Computing and Engineering.