A.R. Djordjevi ć 1 , D.I. Ol ć an 1 , J.R. Mosig 2 Helical Antennas with Improved Gain 1 School of Electrical Engineering, University of Belgrade, Serbia 2 Ecole Polytechnique Fédérale de Lausanne, Switzerland COST Action IC0603 Workshop Joint session with COST 297 Cyprus, April 2008
Contents � Classical helical antennas � uniformly wound and above infinite ground plane � optimization and new design data � Influence of the reflector � finite ground, cup, cone � optimization and design data � Nonuniformly wound helical antennas � with infinite ground plane � without any reflector � interim results
Helical antennas � Uniformly wound � Axial mode � Discrepancy among design data � Influence of the reflector � not widely recognized � Maximize gain � bandwidth � elipticity � matching
Classical design data 3 / 4 / 4 / 3 < C λ < � Optimal circumference 0 . 005 / 0 . 05 < d λ < � Independent on wire diameter 12 14 ° < α < ° � Optimal pitch angle � Minimal size of square ground plane / 0 . 75 λ c = b (counterbalance, reflector) � Gain (Kraus) 2 C L 10 log 15 = g [ dBi] λ λ
Discrepancies 20 19 18 17 16 g max [dBi] 15 14 13 Formula (Kraus) Experiment (King, Wong) 12 Simulation (Emerson) Design curve (Poynting) 11 NB design WB3 design 10 1 2 3 4 5 6 7 L / λ p
Comments � Formula by Kraus overestimates the gain � Simulations (Emerson, Poynting, our results) for infinite ground plane � Experiment for antenna with a cup (King, Wong) � influence of the cup remained unnoticed � wrong estimation of the antenna center � contradictory data for distance between transmitting antenna and receiving antenna � Data by Poynting for a wide range of pitch angles � Our design: narrowband and broadband � optimized over wide range of parameters
Narrowband and broadband 16 NB WB3 15 14 Gain [dBi] 13 12 11 10 Simulation Measurement 9 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 f [GHz]
Our design data � Infinite ground plane � Narrowband design: smaller pitch angles � Wideband design: larger pitch angles � increased bandwidth at lower frequencies caused by reflections from the ground plane � the ground plane must be sufficiently large (2 λ ) � Optimal pitch angles strongly depend on wire radius 3 16 ° < α < ° � Optimal pitch angles
Our design curves 21 1.1 NB WB1 NB WB1 20 WB2 WB3 WB2 WB3 19 1.0 18 g max [dBi] 17 16 C / λ c 0.9 15 14 0.8 13 12 11 0.7 L / C L / C 1 10 1 10 r / C r / C r / C =0.00015 =0.0015 =0.015 P oynting 1 P oynting 2 16 16 NB WB3 14 14 12 12 10 10 o ] α [ o ] 8 8 α [ 6 6 4 4 2 2 0 0 L / C 1 L / C 10 1 10
Influence of reflector � Reflector has significant influence on radiation pattern and gain 1 . 5 = λ b � Optimal square 0 . 25 = λ h � Optimal cup = 1 λ D � Smallest optimal truncated 2 . 5 = λ D cone 2 0 . 5 = λ h 0 . 75 = λ D 1
Influence of reflector Infinite ground plane Square conductor of side b =0.5 λ 17 Optimal square conductor 16 Optimal cylindrical cup Optimal truncated cone 15 Gain [dBi] 14 13 12 11 10 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 Frequency [GHz]
Optimal helix and cone � Simultaneous optimization of helix and cone � Nelder-Mead simplex algorithm � Very large pitch angles (30° ) � Many local optima � Cone collects radiation from the lowest helix turns and redirects it upwards � Low side lobes � For large heights – helicone antenna (Carver)
Optimal helix and cone 22 21 20 19 18 17 g max [dBi] 16 15 14 13 Helix (Milligan) Hansen-Woodyard 12 NB design WB3 design 11 Cone, h =0.5 λ Cone, h =2 λ 10 9 1 2 3 4 5 L / λ
Optimization of pitch and diameter � Simultaneous optimization of helix pitch and diameter (for constant wire radius) � No reflector � simple launcher (with choke) � results comparable to classical antenna with ground plane � Infinite ground plane � helix radiates backwards � ground plane acts as a reflector � results comparable to optimal cone
Preliminary results 19 18 17 16 15 g max [dBi] 14 13 12 Optimal, no ground Optimal, infinite ground 11 NB design WB3 design Cone, h =0.5 λ Helix (Milligan) 10 9 1 2 3 4 5 L / λ
Example: 5 λ λ λ , classical λ
Example: 5 λ λ λ λ , no reflector
Example: 5 λ λ λ , infinite ground λ
Future work � Further optimization of helix with truncated cone for wide range of wire radii � previous results only for one wire radius � Include taller cones to cover transition to helicone antenna � previous optimization up to h =2 λ � Further optimization of helix pitch and diameter; experimental verification � Compare the influence of the reflector to the influence of wave launchers
References A.R. Djordjevi ć , A.G. Zaji ć , M.M. Ili ć , G.L. Stueber, � “Optimization of helical antennas “, IEEE Antennas and Propagation Magazine, vol. 48, December 2006, pp. 107-115. D.I. Ol ć an, A.G. Zaji ć , M.M. Ili ć , A.R. Djordjevi ć , “On the � optimal dimensions of helical antenna with truncated-cone reflector”, Proc. of EuCAP, ESA SP-626, Nice, November 2006. A.R. Djordjevi ć , A.G. Zaji ć , M.M. Ili ć , “Enhancing the gain of � helical antennas by shaping the ground conductor”, IEEE Antennas and Wireless Propagation Letters, Vol. 5, 2006, pp. 138-140. A.R. Djordjevi ć , D.I. Ol ć an, A.G. Zaji ć , M.M. Ili ć , "Optimization � of helical antennas ", Cost Action IC0603 Workshop, Bonn, October 2007.
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