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Tapered Helix
Antenna
Description
of Antenna
The diagram below illustrates
the configuration of a step-tapered helix. This is a helical
antenna with two main sections of different diameters, joined
by tapered sections.
Design Curves
The design curves for this
antenna provide gain and bandwidth as a function of tip radius
to base radius ratio (Tr:Br), and take the following assumptions
into account:
* Only axial length's greater
than 3.5 lambda are considered
* BL:TL is set at 8:1
* The tip pitch is set at 5 degrees to maximise gain and ensure
stable bandwidth
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Figure
1: Gain vs Tip to Base Radius Ratio (Axial length i.t.o.lambda)
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Figure
2: Percentage Bandwidth vs Tip to Base Radius Ratio
(Axial length i.t.o.lambda)
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Example
A helical antenna with the
following characteristics is required:
* Operating frequency of 5.5GHz
* Axial length fixed at 235mm
* Minimum Bandwidth of 60%
The physical length of 235mm
corresponds to an electrical length of 4.3 lambda at 5.5GHz.
In order to obtain a bandwidth of 60% a tip to base ratio
of 2.2 is required (figure 2). Using figure 1 the gain for
the antenna can be determined as 13dBi.
The antenna's physical characteristics
can therefore be summarised as follows:
* BL:TL of 8:1
* Tip pitch of 5 degrees
* TR : BR of 2.2:1
* axial length of 235mm
With the resultant electrical
characteristics being:
* Gain of 13dBi
* Bandwidth of 60%
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Note: these design curves provide only a rough
estimate of a particular antenna's dimensions and performance.
In order to fully specify the antenna it should be simulated,
using the values obtained from the curves if so desired.
Reference:
Fourth Year Design Report, "Investigate the Design of
a Tapered Helix Antenna" prepared by Benjamin Klein for
the School of Electrical and Information Engineering, University
of the Witwatersrand, August 2003.
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