Dipoles in Front of a Reflector Plane


Description of Antenna:

The layout of different numbers of dipoles in front of a reflector plane are shown below in the diagram.

The reflector-dipole spacing (d) is the same for each dipole in the case of multiple dipoles. The parameter s' is applicable only in the four dipole case, and represents the lateral spacing between two pairs of dipoles which are spaced at the normal dipole-dipole spacing s.

Design Curves

The gain and bandwidth characteristics of dipoles in front of a reflector plane are illustrated below in figure 1 and figure 2 as a function of reflector-dipole spacing. These curves were generated using the following optimum dimensions for the reflector, with dipole-dipole spacing (s and s') as shown:

No. Dipoles Present
Reflector Width (lambda)
Reflector Height (lambda)
Dipole-Dipole Spacing (s, s')
1
1.3
1.16
-
2
1.3
1.16
0.48lambda
3
1.4
1.26
0.6lambda
4
1.7
1.53
0.48lambda, 0.12lambda

 
Figure 1: Gain vs Reflector-Dipole Spacing (d)
Figure 2: Bandwidth vs Reflector-Dipole Spacing (d)

Alternatively, the gain and bandwidth characteristics can be related to the dipole-dipole spacing (figure 3 and figure 4), with reflector dimensions and s' as stated in the table above, and reflector-dipole spacing set at 0.16lambda. These curves are only useful for the two and four dipole cases.

Figure 3: Gain vs Dipole-Dipole Spacing (s)

Figure 4: Bandwidth vs Dipole-Dipole Spacing (s)

Example

An antenna with the following specifications is required:

* minimum gain of 12 dBi
* minimum bandwidth of 30%
* centre operating frequency of 675Mhz

From figure 1, both three and four dipoles are able satisfy the required gain. However, three dipoles do not produce sufficient bandwidth and four dipoles are therefore used, with a reflector dipole spacing of 0.2 lambda producing a bandwidth of 40% (from figure 2).

The antenna therefore has the following characteristics:

* Gain of 12dBi
* Bandwidth of 40%
* Dipole-Dipole Spacing of s = 0.48 lambda (213mm) and s' = 0.12 lambda(53mm)
* Reflector-Dipole Spacing of 0.2 lambda (89mm)
* Reflector of 1.7 lambda(756mm) wide and 1.53 lambda (680mm) high.


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.

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Reference:
Fourth Year Design Report, "Investigate the Design of Dipoles in front of a Reflector Plane" prepared by Deo Dusabe for the School of Electrical and Information Engineering, University of the Witwatersrand, August 2003.