The effect of substrate properties on the radiation pattern and directivity of the antenna were investigated by varying the substrate thickness and εr.
A. Effect of relative permittivity
The relative permittivity is varied for a fixed substrate thickness of h=1 mm. Fig. 6 shows 2D radiation pattern plots obtained when εr was increased. It can be seen that, at lower εr values the antenna has a narrow main lobe, which translates to good directivity. However, as the value increases, the main lobe broadens, and eventually two lobes were formed when εr = 5. This agrees with the observations pointed out by Katehi and Alexopoulos [22]. The pattern generally deteriorated with increasing εr, with more power being radiated in the backside direction. As higher εr is used, more energy is directed towards the antenna sides due to more modes are generated inside the substrate. The rather severe backside lobes are the manifest of edge-feeding used in this design.
Table 5 tabulates the directivity and gain for different εr. The directivity, and hence gain, decreases with increasing εr. This behaviour could have been due to a decrease of the fringing fields and the patch area which happens when a higher εr is used.
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In the previous section, it can be seen that the bandwidth of the antenna increases with substrate thickness. Its effect on the radiation pattern is shown here. The substrate thickness is varied for εr =3. Fig. 7 shows the 2D plots of the radiation pattern when increasing the thickness of the substrate.
From Fig. 7 it can be seen that the width of the main lobe remained almost the same when the substrate thickness was increased. The substrate has a small effect on the radiation pattern. This is because using thicker substrates do not affectthe patch size so much, but only extends the zone of the fringing fields [22-24].
Table 6 lists the directivity and gain for the various substrates. It can be seen that the directivity and gain of the antenna increased with substrate thickness. This is due to the increase in aperture area and patch size.