Tapered Helix Antenna

Description of Antenna

A schematic illustrating the relevant attributes of a "space-tapered" helix is shown below:

A space-tapered helix is defined as a helix with turn spacing that varies logarithmically with the length of the helix. The taper ratio is given by the start turn spacing (Sstart) divided by the stop turn spacing (Sstop).

Design Curves

The following design curves make it possible to obtain the physical parameters (axial length, start pitch) of the space-tapered helix with respect to gain and beamwidth. The curves are generated for a helix of 1 lambda circumference, with an optimal taper ratio of 5:1.

Figure 1 shows the optimum gain of the antenna as a function of axial length.
	Figure 1: Gain vs Axial Length
If a specific axial ratio is required, the axial length may be determined using figure 2.
	Figure 2: Axial Ratio vs Axial Length
The optimal pitch for a given axial length may be obtained from figure 3. This pitch is a weighted average(oC), and the start pitch (ostart) may be obtained using equation 1. ostart = (2.4 x taper ratio x oC) / (taper ratio + 1).............1
	Figure 3: Optimum Pitch vs Axial Length
Once the optimal start pitch is known, the antenna's beamwidth can be determined using figure 4.
	Figure 4: Gain Beamwidth vs Axial Length

Example

A space-tapered helix is required that meets the following specifications:

* axial length of 30cm
* operating frequency of 5.15GHz ~ 5.85GHz
* minimum axial ratio 0f 0.9
* maximum possible gain.

At the centre frequency of 5.5 GHz, a physical axial length of 30 cm corresponds to an electrical axial length of 5.5 lambda (in free space) and from figure 1 the gain for an axial length of 5.5 lambda is approximately 17 dBi. Figure 2 shows the axial ratio to be 0.91, which is within the required parameters. The optimum (weighted average) pitch is found from figure 3, and is 9.75 degrees. The optimum start pitch is 19.5 degrees from equation 1, and if more beamwidth is required (at a sacrifice in gain) this may be increased.

The antenna therefore has the following characteristics:

* axial length of 30cm
* operating frequency of 5.15GHz ~ 5.85GHz
* axial ratio 0f 0.91
* maximum possible gain of 17 dBi
* start pitch of 19.5 degrees.

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Reference:
Fourth Year Design Report, "Investigate the Design of a Tapered Helix Antenna" prepared by David Starfield for the School of Electrical and Information Engineering, University of the Witwatersrand, August 2003.