[ieee 2006 7th international symposium on antennas, propagation & em theory - guilin, china...

A Modified Helix antenna with Gain Improvement and Size Reduction

Ying Liu* An Tao-bu, Shu Xi-gongNational Laboratory ofAntennas and Microwave Technology, Xi'an 710071, Shaanxi China

*Email:liuyinggmail.xidian.edu.cn

Abstract

A new modified helix antenna is introduced whichcombines the helix and monopole together and formsimproved current distribution. The character makes itmore convenient for gain improvement and sizereduction. The antenna is designed and simulated withAnsoft HFSS which is based on Finite Element Method(FEM). According to the rule, some models arefabricated for use on two-way radio. The measuredresults are presented and prove the theory efficient.

1. Introduction

Helix antenna [1] is commonly used in mobile phonecommunication as external antenna. Antenna is nowrequired to be with smaller size [2] and higher gain. Amodified helix antenna is introduced which combines thehelix and monopole together. The helix can be viewed asthe load of monopole and changes its current distribution,which leads to improved gain and reduced size. AnsoftHFSS is used to analyze the antenna. The actual antennais fabricated used on two way radio. For comparison,helix antenna with the same length and monopoleantenna with longer length are also fabricated. Themeasured results show that the modified helix antennaworks well in size reduction and gain improvement.

I

Figure 2. The pattern of helix antenna

According to the rules, three antennas are fabricated.Antenna 1# is modified helix antenna with length 9cm.

Antenna 2# is helix antenna with the same length.Antenna 3# is monopole antenna 16cm long. Threeantennas are all detuned to work on two way radio withfrequency 400-420MHz and measured in far filedmeasurement system as shown in figure 3.

Anechoic chamber[

Referenceantenna

A ltennaItuderIl test

2. Results

Ansoft HFSS is used to compute the 3D pattern ofantennas. For comparison, modified helix antenna andhelix antenna are all computed as shown in Figurel andFigure2. It can be seen that the field distribution of twoantennas are different as they have different currentdistribution now.

Figure 1. The pattern of modified helix antenna

Figure 3. The measurement setup

In the same measurement system, three antennas aremeasured on the same two-way radio. The measurementresults comparison between antenna 1# and antenna 2#are as shown in figure 4. The top figure is VSWR andthe bottom is gain level. The measured results of antenna1 are indicated with triangle markers. It can be seen thatthe gain of modified helix antenna is 2.6dB higher thanthat of helix antenna with the same length. Thecomparison between antenna 1# and 3# is shown infigure 5. Also the curves with triangle markers are

measured results of antenna 1#. The gain of antenna 1#is 0.5-0.6dB higher than antenna 3#, while antenna 3# is7cm longer than 1#.

AcknowledgmentsThanks to the direction of Professor Xiao Liangyong andmeasurement setup of Xi'an Haitian AntennaTechnologies Co., Ltd.

References

[1] Elliott, Robert S., Antenna theory and design,Englewood Cliffs, N.J.: Prentice-Hall, 1981.

[2] R.M. Barts and W.L. Stutzman, A reduced sizehelical antenna, Antennas and Propagation SocietyInternational Symposium, IEEE, 1997 Digest, Vol.3,p.1588 (1997).

Figure 4. The measured VSWR and gain levelcomparison between antenna 1# and 2#

Figure 5. The measured VSWR and gain levelcomparison between antenna 1# and 3#

3. Summary

A modified helix antenna is proposed. The helix worksas load to change the antenna current distribution.Measurement results show that the modified antenna canbe used to improve gain with smaller size. The fabricatedantenna works in frequency band 400-420MHz but therule is applicable to any frequency band and can be usedfor antenna design with reduced size and improved gain.