II. BASIC ANTENNA STRUCTURE

In a basic antenna, the monopole and ground conductors are located on the same plane, and the edges of the planar monopole and ground have a parabolic structure [Reference Lee and Park1].

The impedance varies by converting the feeder to a discontinuous CPW structure, instead of a coaxial cable. The step-type discontinuous CPW disturbs the electromagnetic field of a normal CPW and adds a reactance component [Reference Simons10]. First, the transmission line length CL, as shown in Fig. 1, is 0.8 mm, which varies starting from 0.625 mm above the bottom side of the antenna. The part that determines the width of the signal line in the middle of the transmission line is denoted as CW. The impedance of the transmission line is determined by the variation in the length of CW. Table 1 shows the impedance of the calculated transmission line as the value of CW increases. When the CW value increases, the impedance also increases, and the return loss value of the upper frequency band increases.

Fig. 1. Basic structure of the antenna with a notch slot.

Table 1. Impedance of transmission line for various CW.

Figure 2 shows the change in return loss value for the change of the CW when there is no notch. The CL value for the result is 0.8 mm. Overall, it is confirmed that the return loss value has increased aside from the 7–9 GHz section where the return loss value slightly decreased when CL value increased. From the optimization result of this parameter, CW was determined to be 0.8 mm.

Fig. 2. Return-loss simulation results for various CW of the no-notch antenna.

According to the changes in MS, which is the width of the matching step, impedance matching in the high-frequency band was effective. Further, the largest 10-dB band was obtained when MS was 15 mm. In addition, the band-rejection center frequency of the proposed antenna, which is 5.4875 GHz, was the median value of the WLAN band (5.15–5.825 GHz). λ/4 was calculated to be 13.6 mm at 5.49 GHz. The length of the notch slot was determined based on this calculated value. By making the notch slot assume a parabolic edge shape similar to that of the monopole and ground conductors, the band rejection characteristics was obtained. The following is the parabolic equation of a basic antenna.

(1) $$\eqalign{& {Y_A} = {a_A}X_A^2 \cr & {Y_B} = {a_B}X_B^2 + g \cr & {Y_C} = {a_C}X_C^2 + ns} $$

In equation (1), ground edge A is expressed as Y _A , monopole edge B is expressed as Y _B , and notch slot edge C is expressed as Y _C . The coefficient of the quadratic term in the parabolic expression for the notch slot edge is designated as a _C . nl and nw are the length and width, respectively, of the notch slot. nl is a variable that moves the center frequency of the notch slot. a _C and nw affect the size and the return loss value of the notched band. Based on the simulation results using different parameters, the obtained optimal values were as follows: a _C  = 0.5, nl = 17.5 mm, nw = 0.4 mm, and ns = 4.5 mm.

Figure 3 shows the return-loss simulation results of the notch-slot-applied antenna and the no-notch antenna. The 10-dB bandwidth is 2.015–11.185 GHz, and in the middle, the notch band is 5.06–5.795 GHz. The notch band did not completely match the WLAN band, and the error rate was approximately 0.5%.

Fig. 3. Return-loss simulation results of the notch-slot-applied antenna and the no-notch antenna.

III. ANTENNA MINIATURIZATION BY APPLYING SCALE CHANGES

The important requirements for a UWB antenna include miniaturization of the UWB antenna itself. For the antenna miniaturization, a scale change was performed to reduce all components in the same proportion. For the feeder, the scale was maintained to be the same because its impedance was set to 50 Ω. The overall size of the antenna was reduced by 1, 0.9, 0.8, 0.7, 0.6, and 0.5 times. Excluding the feeder and notch slot, the overall size and structure of the antenna were reduced in the same scale compared with the original size. Equation (2) shows the substitution of variables using the scales, instead of variables X and Y in equation (1). Here, the scale was set as n. By assuming that the new variables are X′ and Y′, then X′ = nX, Y′ = nY, and g′ = ng.

(2) $$\eqalign{& Y_A^{{\prime}} = {{{a_A}}\over{ n}}X_A^{{\prime}{2}} \cr & Y_B^{{\prime}} = {{a_B}\over {n}}X_B^{{\prime}2}+{ { {g^{{\prime}}}}}.} $$

As the scale decreases, the coefficients of the quadratic terms in the parabolic expressions for the monopole and ground edges increase at a certain proportion according to the scale, whereas the antenna's overall size, MS, and g decrease in the same proportion. Because the antenna structure was similar to a self-complementary structure, we expected that the dependence on the angle formed by the monopole and ground conductors would be larger than that by the total length.

Figure 4 shows the results obtained through simulation of the return-loss characteristic according to the changes in the antenna scale. As the scale decreases from 1 to 0.5, small changes are observed at the lower frequency where the 10-dB band started, and noticeable differences are observed at the upper frequency part. In the results, the size of the antenna became small, and simultaneously, the overall 10-dB band became wide. In addition, while the notch slot was applied to the antenna at each scale, we confirmed that band rejection occurred well in the WLAN band. The antenna scale with the smallest antenna size, while satisfying the UWB band requirement was 0.6. Here, the 10-dB band was 3.013–12.725 GHz, and the notch band was 5.165–5.865 GHz.

Fig. 4. Simulation results of the return loss according to the antenna scale changes with notch slot.

IV. ANTENNA FABRICATION AND MEASUREMENT

Figure 5 shows an actual photograph of the antennas with a notch slot. For the antenna fabrication, an FR4 dielectric substrate with dielectric constant of 4.7 was used. Figure 6 shows the measurement results of the actually fabricated antenna. Figure 7 shows the return-loss characteristics at the scale of 0.6. The 10-dB band is 3.013–12.515 GHz, and the notch band is 5.235–5.848 GHz. A high-frequency result of 0.085 GHz is observed in the low limit frequency for the notch band. Figures 8 and 9 show the graphs illustrating the frequency radiation patterns in the E- and H-planes, respectively, when the scale is 0.6. All the four frequencies display similar radiation patterns, and these patterns are similar to those of a dipole antenna. In the case of the E-plane, an average gain difference of 5 dB is observed at the frequency of 5.5 GHz due to band rejection compared with that at 3.5 GHz. In the case of the H-plane, a small gain is observed compared with those of the other frequencies and the band-rejection characteristics could be clearly observed.

Fig. 5. Photograph of the miniaturized antenna with notch slot.

Fig. 6. Return loss of the antenna with notch slot at various scales (measured result).

Fig. 7. Return loss at a scale of 0.6.

Fig. 8. Radiation pattern at each frequency (E-plane).

Fig. 9. Radiation pattern at each frequency (H-plane).

Figure 10 shows the graph of the maximum and average gains on the E- and H-planes. Because gain reduction occurs at the frequency of 5.5 GHz, band-rejection characteristic is confirmed in the notched bands. Outside the rejection band, the average gain is practically not attenuated in the range from −2.5 to −5.7 dB on the E-plane and from 2.1 to 9 dB on the H-plane.

Fig. 10. Gain at each frequency.

Table 2 shows comparison of the size of the reference antenna and the proposed antenna. While the dimension of antenna in [Reference Azim, Islam and Mobashsher7] was 20.5 × 29 mm² and in [Reference Mobashsher and Abbosh8] was 14.1 × 25 mm², this paper showed a very small size of 15.6 × 18.6 mm². Thus, it was confirmed that the area became 51% smaller about in [Reference Azim, Islam and Mobashsher7] and 18% about in [Reference Mobashsher and Abbosh8].

Table 2. Comparison of the size of the reference antenna and the proposed antenna.