layer will be studied in a subsequent paper. This paper has only studied the simpler lossless second layer, so that the ranges of application formula (14) can be checked for extensive variations of dielectric constants of both the first and second layer of the substrate. As seen from the above results, the ranges are indeed large.The capacitance formula of one layer substrate of Eq. (5) is simple as opposed to the "a little more complicated" formula of two layers of Eq. (14) in this paper. This is expected when there is one extra layer. However, since both are derived from the same synthetic asymptote technique (as pointed out at the end of subsection 3.3), the formula of two layers readily reduces to that of one layer of Eq. (5) , the impedance bandwidth of a patch antenna can be enhanced to over 30%. It is low in cost and simple in structure. However, this technique has the disadvantage of high cross-polarization, which is contributed by the portion of the probe normal to the ground plane. Using the phase-cancellation technique can effectively suppress the cross-polarized radiation of a single element antenna [3,4] or an array antenna [5][6][7]. However, the power divider increases the complexity of the antenna, which increases the antenna's fabrication cost. Recently, a suspended probe-fed plate antenna [8] and a meandering probe-fed patch antenna [9] were suggested for suppressing the cross-polarization level and increasing the impedance bandwidth of a probe-fed patch antennas. Even though the suspended probe and the meandering-shaped probe are easy to design and fabricate; one end is connected to the patch and the other is connected to a SMA connector. The fabricating process could be simpler if the connection point between the feeding probe and the patch was removed. Based on the concept of a meandering probe and an L-shaped probe, a modified L-shaped probe feed for the patch antenna is proposed in this paper. The antenna has characteristics of wide impedance bandwidth and low cross-polarization level. The experimental results show good agreement with the numerical simulations.
ANTENNA STRUCTUREThe geometry of the modified L-shaped probe fed patch antenna is shown in Figure 1. For operation at 1.72 GHz as the center frequency, a 1-mm-thick rectangular aluminum patch, which has a width W ϭ 70 mm (0.4 0 ), a length L ϭ 60 mm (0.344 0 ), is selected. The patch locates at a height of H ϭ 17.5 mm (0.1 0 ) and is supported by plastic posts. The modified L-shaped probe is a combination of three horizontal parts and three vertical parts. It is connected to the inner conductor of a 50⍀ SMA launcher. The three vertical parts are related to the two gaps, g 1 and g 2 , which are both equal to 1.5 mm (0.0086 0 ); therefore, the lengths of the
RESULTS AND DISCUSSIONAll results shown in this paper were obtained using Zeland IE3D version 9.35. The measured and simulated SWR and gain of the proposed antenna have been obtained and are shown in Figure 2. It shows that the simulated impedance bandwidth of the antenna is 21% ...
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