Helix Antenna Antenna and Microwave Laboratory In the name of God Babol Noshirvani University of Technology, Iran Helix Antenna Import video, voice + dipole 60cm

1. Applications: 2.4 GHz WiFi End fire Helix Antenna End fire helical satellite communications antenna, Scott Air Force base, Illinois, USA. A common form of normal-mode helical antenna is the Rubber Ducky antenna used in portable radios.

2. General Description: Design Parameters: (Antenna Theory, C.A. Balanis, Ch.10, Page 566) f0: center frequency a: wire radius N: number of turns S: Spacing between each turn D: Diameter of one turn Dg: Diameter of ground plane C=πD : circumference of one turn α=Arctan(S/C) Pitch angle L0=sqrt(S^2+C^2) Length of the wire between each turn L=NL0: Total Length of Helix

1) Pattern 3. Electromagnetic Properties: B. end-fire mode (L > λ0) A. normal mode (L << λ0)

2) Polarization Helix antenna can provide Circular polarization both in normal mode and end-fire mode Circular Polarizations are divided to Right-Hand Circular Polarization (RHCP) and Left-Hand Circular Polarization (LHCP). IEEE definition of polarization: the circular polarization of an antenna is defined in the way that the waves propagates away from the antenna.

Let`s mathematically Solve the Problem 2) Polarization ? Suppose two helix antennas, one as the Tx, and the other one as the Rx. On what condition, the transmitted signal is received without any loss. Tx Antenna: RHCP Rx Antenna RHCP or LHCP ? Let`s mathematically Solve the Problem In order to have zero polarization loss factor, the two antennas must be exactly same as each other, so their polarization are same as well: RHCP vs RHCP, LHCP vs LHCP

3) Input Impedance The Zin (input impedance) of a helix radiating in end-fire mode is nearly resistive (Xin≈0) with values between 100 – 200 ohms. Zin= Rin + jXin Z0

4. Design procedure for optimum end-fire mode: Optimum means: Circular Polarization, Pencil Beam Pattern, 14 dBi Gain, Bandwidth 1.6:1, input impedance 140 ohm selecting center frequency f0: 1) N>3 2) 3/4 < C/λ0 < 4/3 (C/λ0 ≈ 1) 3) S ≈ λ0/4 4) 12 < α <14 (α≈13) 5) ground plane Dg >= λ0/2 6) fed by offset coaxial cable

5. Feeding Method: Outer conductor of coaxial cable sticks to the ground plane Inner conductor of coaxial cable extends to form the helical structure Offset fed outer conductor Center fed inner conductor

6. Match the antenna to 50/75 ohm line: Since, the practical transmission lines have characteristic impedance of about 50/75 ohms, in order to provide a better match, the input impedance of the helix must be reduced to near that value. Zin= 100-200 ohms Z0= 50/75 ohms

Kraus method for matching the Helix Antenna: [12] J.D. Kraus, “ A 50-ohm Input Impedance for Helical Beam Antennas,” IEEE Trans. Antennas Propagat., Nov. 1977 The wire of the first 1/4 turn should be flat in the form of strip, and the transition into a helix should be very gradual. Matching design parameters: w, h, εr, Z0(line) height the first 1/4 turn

Now, Let`s get familiar with a brand new software : Antenna Magus

Go to CST and simulate a example And complete the work sheet

Worksheet Please complete the work sheet to your example: Frequency band : ……… The figure of antenna in CST : ……… The of size antenna in λ and in cm/mm : ……… S11 : ……… (set Z0=75 ohm) Zin (Re and Im) in CST : ……… Surface Current (phase=0 deg) : ……… linear 3D Power Pattern in CST : ……… 2D Power Pattern for theory & CST : ……… HPBW for theory & CST : ………