Help me with optimization of quadrifilar helix antenna design
I am a beginner in this field..I am designing quadrifilar helix antenna for 2.25 GHz..i have modelled it in Ansoft HFSS v.11.
when i simulate my design, i am getting gain of +6 dB, S11 of about -7dB but getting very poor axial ratio +30dB:( Can anyone help me in optimizing my design so that I may get good axial ratio, S11 and Gain simultaneously...I have noted that while i change my design to increase Gain , the S11 value becomes bad ..why is it so?
I notice you have not had a reply so far. Maybe you have already resolved this.
Just in case not, forgive that I reply to such an old thread.
I have tried modelling quadrifilar for years, using NEC method of moments and generating helix in little segments, and also with a variety of other softwares as I had opportunity. The standard 180 degree helix pair generates a near spherical radiation pattern, and seen straight from the top, has near perfect axial ratio. Also, because of the way the fields are summed, the circular polarization, while it does depart some from perfect, still delivers as better for the off-axis angle than with other circular polarized arrangements, like "Turnstile", or simple multiturn helix.
Getting a better match than S11 = -7dB is possible, at least with the UHF quadrifilars, using quarter-wave transmission line in the feeds. Opposite helix pairs are fed 180 degree phase shifted, with one pair being made 90 degrees delayed. These feed relationships can be achieved with various hybrid combiners, e.g. the "rat race" or "quarter-wave-hybrid". There are many ways the elements can be matched to the combiners, using lengths of transmission line to make the delay at the same time. I was interested in 1.5-turn helix, fed from the bottom, and with open capacitive top.
One trick is to make all the splitters/combiners in stripline or microstrip, all at the wanted quadrifilar impedance. Then, use only a single quarter-wave coaxial match, of impedance SQRT(50*Zin). I did this by making the coax matching section in the central support, feeding a circular low-loss substrate containing all the splitters, set at the top. All these arrangements are by nature, not wideband.
I would say that at 2.4GHz, the quadrifilar gets very small compared to realistic element structures. It can be done, but using at VHF and UHF is easier to get the good radiation patterns because the element thickness is a smaller fraction of the total size.