Cavity Backed Dipole Antenna

Description of Antenna

Below is a diagram of the cavity backed dipole antenna:

The dipole is centred in terms of cavity length (i.e. in the y-z plane) and situated a quarter of a wavelength away from the back of the cavity.

The cavity-backed dipole antenna is primarily used in communications applications (VHF, UHF and microwave frequency bands). They are useful in some satellite applications due to folowing advantageous characteristics:

* High-gain element
* Low side and back lobes
* Lightweight antenna element
* Ease of design for high power applications
* Simple to match
* Compactness and simple manufacture

Design Curves

The design curves are developed for varying cavity length (of between 0.65 lambda and 1 lambda) and illustrate the gain and bandwidth characteristics of the antenna. The cavity depth was found to not have a large effect on the antenna's performance, but

Figure 1 shows the gain as a function of cavity length, for different cavity depths. The gain generally increases with cavity length (while cavity length is less than 1 lambda)

Figure 1: Gain vs Cavity Length for varying Cavity Depth	Figure 2: Bandwidth vs Cavity Length for varying Cavity Depth

Example

A dipole antenna is required for an aviation communications application that meets the following specifications:

* operating frequency of 400MHz (therefore lambda = 0.75m)
* minimum gain of 8dbi
* maximum possible bandwidth

In order to optimise the performance of the antenna, a reflective cavity is to be used. Due to physical constraints the dimensions of the cavity must be as follows:

* 0.3m deep (cavity depth), with other dimensions (i.e. cavity length) as small as possible.

0.3m corresponds to 0.4 lambda, and from the curves above the minimum cavity length required for a gain of 8dBi is approximately 0.8 lambda (0.6m). The bandwidth can therefore be determined as being 40% from figure 2.

The antenna therefore has the following characteristics:

* cavity depth of 0.3m (0.4 lambda)
* cavity length of 0.6m (0.8 lambda)
* gain of 8 dBi
* bandwidth of 40%

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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 Cavity Backed Dipole Antenna" prepared by Keapoletswa Dithejane for the School of Electrical and Information Engineering, University of the Witwatersrand, August 2003.