Dipoles in front of a Reflector Plane

Description of Antenna

The diagram below shows the applicable dimensions for two or more dipoles in front of a reflector plane, namely the dipole spacing (d) and reflector spacing (s) as well as the reflector dimensions. The reflector is assumed to be infinitely thin and a perfect conductor. For a single dipole, reflector spacing and dimensions are the only relevant parameters. For the purpose of creating the following design curves, the reflector dimensions are kept constant at a width and height of 0.444 lambda.

Design Curves

The following two design curves illustrate the gain/bandwith trade-off characteristic of 2, 3 and 4 dipoles at a distance 0.125? in front of a reflector plane with a length and breadth of 0.444?. The gain and bandwidth are related to the spacing between the dipoles.

Figure 1: Gain vs Spacing between Dipoles	Figure 2: Impedance Bandwidth vs Spacing betwenn Dipoles

In case of a single dipole in front of a reflector, the gain and bandwidth can be related to the spacing between the dipole and a reflector plate with a width and height of 0.444?. The relationships can be seen in figure 3 and figure 4.

Figure 3: Gain vs Spacing from Reflector Plate	Figure 4: Impedance Bandwidth vs Spacing from Reflector Plate

Example

An antenna consisting of dipoles in front of a reflector plane is required that fits the following specifications:

* A gain of at least 9.5 dBi
* An Impedance bandwidth of at least 10%
* The space between the dipoles and must be greater than 0.04 lambda
* Operating frequency of 450 - 900Mhz

A single dipole will satisfy the required bandwidth at the specified distance (figure 4), but the maximum gain (for a single dipole) at 0.04 lambda is only 8.75dBi, which is insufficient. More than one dipole is therefore needed. Two dipoles spaced 0.025 lambda apart will satisfy both the gain (9.85dBi - figure 1) and bandwidth (13.1% - figure 2) requirements. The spacing between the two dipoles is not critical however, and can vary between 0.025 lambda and 0.05 lambda while still satisfying the given specifications.

The antenna thus has the following characteristics:

* a reflector plane with width and height equal to 0.444 lambda (1m)
* two dipoles, spaced between 0.025 lambda (56mm) and 0.05 lambda (112mm) apart from each other and 0.125 lambda (281mm) from the reflector plane
* a minimum gain of 9.85dBi
* a minimum bandwidth of 13.1%

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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 Dipoles in front of a Reflector Plane" prepared by Bily Purushothaman for the School of Electrical and Information Engineering, University of the Witwatersrand, August 2003.