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.

These wire elements have loads inserted at different points so that their electrical length can be controlled at different frequencies of operation. Using this technique an antenna with a broadband characteristic can be obtained.

Design Curves

The design curves presented here enable the optimisation (in terms of gain and VSWR) of a broadband (in this case, specifically 50-500MHz) skeletal antenna in terms of cone angle, element radius, feed gap and number of elements. The physical parameters of the specific antenna that these curves relate to, unless otherwise specified, are as follows:

* Element Length - 0.75m
* Number of Elements - 6
* Frequency of Operation - 50-500MHz
* Half Angle - 25 degrees
* Feed Gap - 20mm
* Feed Radius - 60mm
* Element Radius - 10mm

The optimum loads used were a pure resistive load of 75 ohm at 500mm from the feed and a pure inductive impedance of 800H at 625mm from the feed.

The following two graphs (figure 1 and figure 2) show the average gain and VSWR as a function of frequency for different cone angles:

Figure 1: Gain vs Frequency for varying Cone Angle	Figure 2: VSWR vs Frequency for varying Cone Angle

The gain-bandwidth trade-off is clearly evident here, with gain being slightly higher and less broadband at smaller cone angles.

Figure 3 shows the VSWR as a function of frequency for different values of radius for the wire element. The main observation here is that an element radius of 5mm is more desirable for frequencies greater than 80 MHz, while for frequencies less than this a larger element radius would be preferable.
	Figure 3: VSWR vs Frequency for varying Element Radius
Figure 4 shows how the VSWR varies for different feed gaps (i.e. the gap between the feed disk and the ground plane).
	Figure 4: VSWR vs Frequency for varying Feed Gap
Figure 5 and figure 6 show the gain and VSWR characteristics with different numbers of elements in the antenna. An increased number of elements results in slightly higher gain, but does not influence VSWR significantly in the upper 80% of the band.
	Figure 5: Gain vs Frequency for varying Number of Elements
	Figure 6: VSWR vs Frequency for varying Number of Elements

Example

A skeletal antenna that operates across the frequency band 50-500MHz that meets the following additional requirements is to be designed:

* minimum gain of 3dBi across the band
* maximum VSWR of 5:1 across at least 80% of the band

The minimum cone angle that satisfies the required gain is 35% (from figure 1), angles greater than this may be used if increased gain is desired, and any number of elements greater than 6 will satisfy the required gain according to figure 5.

The required VSWR is easily obtained as long as both the element radii and feed gap are greater than 20mm.

The antenna thus has the following dimensions:

* Element Length - 0.75m
* Number of Elements - 6
* Frequency of Operation - 50-500MHz
* Cone Angle - 35 degrees or greater
* Feed Gap - 20mm or greater
* Feed Radius - 60mm
* Element Radius - 20mm or greater

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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 Loaded Broadband Skeletal Antenna" prepared by Andrew Philips for the School of Electrical and Information Engineering, University of the Witwatersrand, August 2003.