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To study the effect of asymmetric slot on the return loss of the antenna, the two slot dimensions, A2 and B2, were varied. Figure 3 shows how return loss changes with variations in these slot dimensions. From Fig. 3(a), it can be seen that as the value of A2 increases, the return loss improves and frequency shifts downward. As A2 increases, the overall slot length increases, resulting in a shift towards lower frequencies. Also, the coupling between the ground and the radiating patch improves with increasing value of A2. This results in better impedance matching and improvement in return loss.

However, after a particular value of A2, the return loss deteriorates with increasing value of A2. So the value of A2 must be optimized; the optimum value was found to be 9.10 mm. From Fig. 3(b), it can be seen that B2 has a similar effect on return loss as A2. The optimized value of B2 was found to be 7.50 mm.

Return loss

As opposed to the asymmetrical slot shown in Fig. 1, if the slot is extended on both sides, it again becomes symmetrical. The effects of this extended symmetric slot on antenna return loss were studied, and found not ideally suited for UWB operation, even when the slot dimensions are optimized. These return-loss characteristics were studies by considering surface-current distribution for the antenna. High current distribution was found at 4.3 GHz, due to the star-shaped patch, with a resonance at 6.5 GHz due to second harmonics of 3.2-GHz fundamental signals.

By considering current distribution, the first resonance can be approximated as:

f1 = c/[SL(εeff)0.5]   (1)

where:

f1 = the lowest resonance frequency (in Hz);

SL = the slot length (in m and equal to 0.065 m);

c = the speed of light (in m/s); and

εeff is the effective relative permittivity which can be approximated by (εr + 1)/2.

The second resonance frequency can be approximated by considering the height of the patch

to λ/4, and it is given by Eq. 2:

f2 = c/[4L(εeff)0.5]   (2)

where:

L = the height of the star-shaped patch.

Unit-cell dimensions

Overall screeen dimensions

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