Loaded Broadband Skeletal Antenna

Description of Antenna

A diagram of a skeletal antenna is shown below, which uses six wire elements arranged in a conical shape in order to approximate a perfect cone.

Design Curves

The following design curves illustrate the gain and VSWR characteristics (as a function of load parameters) of a loaded broadband skeletal antenna with operating bandwidth of 40-500MHz. The other properties of the antenna are:

* Height - 2.8m
* Feed Radius - 0.06m
* Feed Height - 0.02m
* Cone half-angle - 25 degrees
* Number of wire elements - 6

The following assumptions were made in the generation of the design curves:

* The antenna lies on an infinite ground plane.
* The size of the load is as large as the segment size.
* The wire elements of the antenna are fed from the same source.
* The effects of the antenna's shielding on the electrical properties (current distribution, impedance and electric field) of the antenna are all neglected.
* Capacitive effect between the bottom of the wire and the ground plane are neglected.
* All measurements are specified in wavelengths of the lowest frequency (20MHz), the evaluating bandwidth is however restricted to 40-500MHz due to the poor performance of the antenna at 20MHz.
* The loads added to the antenna elements are referred to as the "top" and "bottom" loads, referring to their respective positions on the wire element. The position of the loads is measured from the base of the antenna.

Figure 1 shows the VSWR as a function of the position of the bottom load, for different top load positions (in wavelengths).
	Figure 1: Maximum VSWR vs Position of Bottom Load for varying Top Load Position
The maximum gain possible in the band is shown below in figure 2.
	Figure 2: Maximum Gain vs Position of Bottom Load for varying Top Load Position
The following two figures (3 and 4) illustrate the gain and VSWR as a function of the position of the bottom load for varying load resistance (100 ohm, 300 ohm, 500 ohm). The top load is fixed at 0.17 lambda.
	Figure 3: Maximum VSWR vs Position of Bottom Load for varying Load Resistance
	Figure 4: Maximum Gain vs Position of Bottom Load for varying Load Resistance
If the length of the wire elements is a critical design factor, then the following two figures (5 and 6) may be used to determine the performance in terms of gain and VSWR with respect to the length of the elements. The top and bottom load positions are 0.16 lambda and 0.1 lambda respectively.
	Figure 5: Maximum VSWR vs Element Length for varying Load Resistance
	Figure 6: Maximum Gain vs Element Length for varying Load Resistance

Example

An antenna with the minimum VSWR possible is required, with a gain of at least 5dBi, element length is restricted to a maximum of 0.3 lambda.

Since the element length is a critical parameter, figure 5 and 6 are used. The lowest possible VSWR is approximately 3:1 at a length of just greater than 0.1 lambda, with 100 ohm loads. The corresponding gain for this combination of parameters is 6.4dBi, which satisfies the remaining conditions.

The properties can be summarised as follows:

* VSWR of 3:1
* Gain of 6.4dBi
* Element Length of 0.2 lambda
* load spacing of 0.16 lambda and 0.1 lambda for the top and bottom loads respectively.
* Feed Radius - 0.06m
* Feed Height - 0.02m
* Cone half-angle - 25 degrees
* Number of wire elements - 6

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Reference:
Fourth Year Design Report, "Investigate the Design of a Loaded Broadband Skeletal Antenna" prepared by Kam Hay Chan for the School of Electrical and Information Engineering, University of the Witwatersrand, August 2003.