Schematic representation of the antenna:
The antenna has circular polarization, but can work with both horizontally and vertically polarized waves. In this case, the antenna gain will be 3 dB less than the calculated one.
The calculation is based on two ANSYS HFSS antenna models.
- The first model for VHF and UHF frequencies up to 1000 MHz. In this case, a relatively thin wire is wound around a PVC tube to maintain the mechanical strength of the antenna.
- For microwave frequencies above 1000 MHz, a model with a relatively thick wire mounted on dielectric stands is used, as in the drawing below.
INPUT DATA:
Javascript source code:
© 2014- 3G-Aerial
Limitations and features of antenna calculations
The Wi-Fi helical antenna model was verified and optimized in the ANSYS HFSS electromagnetic simulator. The characteristics of the antenna are shown below. Based on the analysis of the antenna characteristics, it can be concluded that the helical antenna radiates a wave not with an ideal circular polarization, but with an elliptical one. In addition, it is clearly seen that the antenna can work well with a 75-ohm feeder.
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| Input impedance | VSWR | Radiation pattern |
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| Gain | Axial Ratio | Front/back ratio |
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The antenna is characterized by several basic “mechanical” parameters on which its “electrical” properties depend:
- The perimeter of one turn, on which, by-turn, the winding diameter depends
- Winding pitch
- Wire diameter
The helix is a slower wave structure with surface waves. In practice, it was determined that there is a preferable perimeter of one turn C ≈ λ and a winding pitch of about s ≈ 0.24λ, at which the radiation pattern is most optimal. If the values of C and s deviate significantly from the optimal ones, unwanted wave modes begin to propagate along the wire and the radiation pattern falls apart. Nevertheless, the antenna is quite insensitive to small deviations in size from the optimal ones, which determines its broadband and good repeatability. The greater the number of turns, the greater the gain of the antenna and the better its polarization properties, i.e. less Polarization Axial Ratio. When the number of turns is less than three, it can no longer be considered that the antenna emits a circularly polarized wave. The input impedance of the antenna significantly depends on the diameter of the wire, so it is advisable to use a wire with a diameter as close as possible to the calculated one. To match with the feeder, an impedance transformer is used in the form of a tapered line in the shape of a triangle made of copper foil. In a number of designs, a quarter-wave transformer can be found as a matching element. A tapered line with variable characteristic impedance was chosen as more broadband so as not to limit the bandwidth of the antenna system in whole. The author of this matching scheme is Australian radio amateur Jason Hecker.
You are operating with excessive precision in this case. Firstly, the exact integer number of turns is not necessary. Secondly, the length of 2.5 mm, even for frequencies of 5.8 GHz, is less than 5% of the wavelength, so the Helix antenna parameters will not change to unacceptable values if the triangle is recessed into the beginning of the helix. Moreover, you won't even catch these changes.