helical antenna, construction, geometry and design criteria and example

helical antenna, construction and design.

1. Antenna & Propagation ( AWP ) Prof. T. S. Patel

2. Helical Antenna Outline : Introduction 1. Narrow band antenna 2. Wide band antenna 3. Broad band antenna Geometry of helical antenna Mode of operation 1. Normal Mode and 2. Axial Mode Radiation Pattern Design of Helical Antenna Example design of antenna.

3. Helical Antenna A helical antenna is an antenna consisting of a conducting wire wound in the form of a helix. Properties: 1.Broadband antenna 2. Circular polarization Helical antennas are mounted over a ground plane.The feed line is connected between the bottom of the helix and the ground plane. Helical antennas can operate in one of two principal modes: 1) normal mode (2) axial mode .

4. Helical Antenna (Geometry of Helix antenna) D: D iameter of a turn on the helix antenna . C:C ircumference of a turn on the helixantenna (C = πD) S: V ertical separation between turns for helical antenna . α : pitch angle, which controls how far thehelix antenna grows in the "'direction per turn,and is given b y tan α = S / C L: length of one turns N: Number of turns on the helix antenna. H : Total height of helix antenna,(Axial length H=NS)

5. Geometry of Helix antenna The radiation pattern will be maximum in the Z direction (along the helical axis )  . The design of helical antennas is primaril y based on empirical results, and the fundamental e q uations will be presented here. Helix antennas of at least 3 turns will have close to circular polari z ation in the Z direction when the circumference ) is close to a wavelength: 3/4< C/ λ<4/3

6. Geometry of Helix antenna Pitch Angle: α=tan -1 (S/C)= tan -1 (S/πD) Important parameter,which is tangent to helix axix and perpendiclar to helix axix. α =0 ◦ spacing between turns zero helical structure will reduce to loop antenna α =90 ◦ Diameter D to be Zero helical structure will reduce to a linear antenna ◦ < α < 90 ◦ resultas in true helix

7. Normal Mode of Operation In the normal mode or broadside helix,the dimensions of the helix (the diameter and the pitch aresmall compared with the wavelength. The antenna acts similarl y to an electricall y short dipole ormonopole, and the radiation pattern, similar to these antennas is omnidirectional, with maximum radiation atright angles to the helix axis. The radiation is linearl! polari"ed parallel to the helix axis.

8. Axial Mode of Operation In the axial mode or end-fire helix, the dimensions of the helixis e q ual to wavelength. The antenna functions as a directional antenna radiating a beam off the ends of the helix, along the antennas axis. It radiates circularl y polari z ed radio waves.The benefits of this helix antenna axial mode or end-fire helix . it has a wide bandwidth, is easil y constructed, and has real input impedance.

9. Design of Helical Antenna The design of helical antennas is primaril y based on empirical results, and the fundamental e q uations will be presented here. Parameter: 1. Helix antennas of at least N=3 turns will have close to circular polarization 2. C ircumference C is close to a wavelength: 3/4<C/ λ/4/3 3. P itch angles : 12 ◦ < α < 14 ◦

10. Design of Helical Antenna HPBW: The Half Power Beamwidth for helical antennas can be approximated (in degrees by:   FNBW: The First Null Beamwidth for helical antennas can be approximated (in degrees by:  

11. Design of Helical Antenna Directivity: The gain of the helix antenna can be approximated by: =15(C/ λ) 2 N(S/λ)   Gain: The gain of the helix antenna can be approximated by: G =15(C/ λ) 2 N(S/λ) (lossless helix)  

12. Design of Helical Antenna Axial Ratio: =1 for finite ratio N-very large The axial ratio for helix antennas decreases as the number of loops N is added,   input Impedence: the input impedance is primaril! real and can be approximated in Ohms by: R in = 140(C/ λ) Ω(ohm)

13. Design of Helical Antenna Normalized far field pattern: where, ) C= λ, α=14 ◦  

14. Example: A 16 turn helical beam antenna has circumference of λ and turns spacing of λ/4 find: 1. HPBW 2. Axial ratio 3. Power pattern Ans: N=16 C= λ S= λ/4 HPBW=26 ◦ AR=1.03 ≈1 G=60(17.78 dB)

15. Example: Calculate the directivity HPBW and terminating resistance for helix operating in axial mode Input: N=9 f=2 GHz C=0.8 λ S=0.2 λ Ans: D = 17.28 HPBW=48.45 ◦ Terminating Resistance R L =R in =140(C/ λ)=112 Ω

16. Example: A helix operated at 2 GHz has dimensions S=5 cm, D=10 cm, N=20, find out 3 dB beamwidth and directivity. Input: f= 2 GHz S=5 cm D=10 cm N=20 Ans: λ =c/f=15 cm C= πD=31.42 cm HPBW=9.61 ◦ D = 438.8 (26.42 dB)

17. It is used for transmission and reception of VHF signals through ionosphere • It is used for satellite and radiometry applications. Advantages: It is simple in design. it uses circular polarised pattern, It can be used for broadband applications due to wider bandwidth. It can be used at HF/VHF frequencies for transmission and reception. It offers higher directivity. It is very robust in construction. Disadvantages It is large in size. This requires more space for installation. For higher number of turns its efficiency decreases. The maximum efficiency of about 80% can be achieved with the use of 3 to 4 turns. It is higher in cost.