II. ANTENNA STRUCTURE

Configuration of the proposed TSA is shown in Fig. 1. A Rogers RO4003C dielectric was used as the substrate with a relative permittivity of 3.38. The thickness of the substrate and the copper layer were 0.508 and 0.07 mm, respectively. The dimensions of the antenna were set to be 50 × 70 mm² (0.43 × 0.61 λ; where λ is the wavelength of the low-end operating frequency). In the proposed antenna, CPW was used as a feed line with a width of 2 mm, and a slot-line gap width of 0.2 mm, which its characteristic impedance matches with a 50 Ω, coaxial line. Moreover, as shown in Fig. 1, the CPW center line width decreases linearly to 0.4 mm and acts as an impedance transformer for impedance matching between the CPW and the slot-line (82 Ω). The exponential profile curvatures indicated by C ₁, C ₂, and C ₃ in Fig. 1 can be described by:

where A _i , R_i , and B _i are the scaling factor, exponential rate, and offset value, respectively.

Fig. 1.

Geometry of the proposed antenna.

In the conventional microstrip to slot-line transition, two radial stubs, or circles, at opposite sides of the substrate were used for modeling the slot-line termination as an open circuit and the microstrip-line as a short circuit. However, in this paper, similar to [Reference Ho and Hart18, Reference Hettak19], we used only one radial stub at the end of the CPW single arm that acts as a broad-band open circuit. In previous works, this radial stub, or circle hollow, located asymmetrically with respect to the tapered flares, led to instability and deviation of the radiation pattern at different frequencies, and also limited the impedance bandwidth. To solve this problem in our proposed antenna, the radial stub was located symmetrically, which caused the radiation pattern to be symmetrical and stable across the frequency band. Results showed that as the radius of the radial stub became larger, the return loss performance at the low and middle frequencies was better, but the high-end operating frequency decreased. Consequently, there was a trade-off. On the other hand, in the proposed CPW antenna, a cross-section of tapered flares on each side of the slot (shaded A and B areas in Fig. 1) were not symmetrical or exactly the same. Because the electrical length of the two tapered flares was different, the waves propagated through them at different distances. This difference caused CPW-slot-line mode-conversion, and the experience of an unwanted current distribution and disturbance of current around the radial stub (region A in Fig. 2(a)). To overcome this problem, we applied an air-bridge in the vicinity of the radial stub to connect the two tapered flares together (Fig. 1). The air-bridge exhibited a short circuit for the slot-line mode [Reference Ma, Qian and Itoh20], and it united the two parts. This technique omitted the disturbance of current by directing the current toward the tapered flares, as shown in region B in Fig. 2(b).

Fig. 2.

Simulated current distribution of the antenna: (a) Without an air-bridge; (b) With an air-bridge.

Important advantages of the proposed transition were its simple design, low fabrication costs, uni-planar structure, low-wave propagation dispersion up to very high frequencies, and independence of via-holes. Therefore, it was preferable to MMIC technologies. Moreover, microstrip was sensitive to the substrate thickness, while CPW was not. To further improve the lower frequency characteristics, we employed a TSE structure (Fig. 1). This increased the electrical length of the antenna and decreased the low-end operating frequency. The length of the slot was chosen to be approximately 0.15 λ according to [Reference Fei, Jiao, Hu and Zhang11]. Simulation results showed that increasing the number of slots does not have much effect on the performance of the antenna. Therefore, we applied only one TSE in our design.

III. RESULTS AND DISCUSSION

Simulations were performed in the CST Microwave Studio environment. After optimization, the final dimension values are listed in Table 1. Moreover, the exponential curvature parameters are listed in Table 2. To validate the results, a prototype of the proposed antenna was fabricated as shown in Fig. 3. A SubMiniature version A (SMA) connector was used to feed the antenna.

Fig. 3.

Fabricated prototype of the antenna.

Table 1.

Optimal dimension values.

Table 2.

Exponential curvatures parameters.

For further comparison, the radiation efficiency for both designs (with and without the air-bridge) is illustrated in Fig. 4. The plot shows that without an air-bridge the radiation efficiency is lower than 80% across a wide range of frequencies. On the other hand, adding the air-bridge increased the efficiency to more than 96% across almost the entire working frequency band. Instead of using an air-bridge, we also could connect two tapered flares at the other side of the substrate using via-holes. However, this technique increased both the complexity and cost of construction, while at higher frequencies the propagation characteristics of the via-holes have a stronger electromagnetic effect on the performance of the antenna that leads to gain reduction. Therefore, the air-bridge technique was a better choice.

Fig. 4.

Radiation efficiency of the antenna with and without an air-bridge.

The measured and simulated return loss in three situations, either with or without an air-bridge and employing a TSE structure, are shown in Fig. 5. As shown in the figure, the low-end operating band decreased from 4.2 to 3 GHz, due to the presence of the air-bridge, and from 3 to 2.6 GHz as a result of employing the TSE structure. However, the high-end operating band was also reduced a little, though that is not significant (20 GHz). The measured result was in good agreement with the simulated results, but there was some deviation, perhaps due to multiple reflections caused by the SMA connector, the soldering process, or fabrication errors. Extra simulation results showed that the width and height of the air-bridge does not have much influence on the overall performance of the antenna. However, the air-bridge worked better when it was not very close to the surface.

Fig. 5.

Measured and simulated return loss (S ₁₁).

Figure 6 shows the measured and simulated E- and H-plane radiation patterns at different frequencies. The figure reveals that the antenna exhibited good end-fire radiation properties and good stability with frequency variations. However, as expected, directivity increased with each frequency increment. The measurements confirmed the simulations. Since cross-polarization components were very small in some directions, the ratios of cross-polarization to co-polarization in the E- and H-planes for different frequencies are presented in Fig. 7. As shown in Fig. 7, in the propagation direction (z-direction in Fig. 1), this ratio is lower than −25 dB in all frequencies. In the lower frequencies, the ratio is even better (less than this value). In addition, these ratios, in other directions, were on average lower than −20 dB, which confirms the performance of the proposed antenna.

Fig. 6.

E-plane (xoz) and H-plane (yoz) radiation patterns of the antenna at different frequencies.

Fig. 7.

Cross-polarization to co-polarization ratio: (a) E-plane; (b) H-plane.

Realized gain and group delay variations, according to frequencies, are shown in Fig. 8. The gain increased rapidly in low frequencies; for example, the gain values for the frequencies 3, 5, and 6 GHz are 4, 6, and 7.9 dB, respectively. At frequencies greater than 8 GHz, the gain values are nearly 10 dB. The maximum gain was 11.3 dB at 10.8 GHz, which is high for a compact-size antenna. To verify the time-domain characteristics, two prototype antennas were located over a distance of 370 mm in the face-to-face position. For optimal wave transmission and reception, the antenna must exhibit non-dispersive behavior, which corresponds to phase linearity and constant group delay. The group delay was investigated in Fig. 8. As the results show, the measured group delay was about 1.8 ns with the variations of ±0.4 ns in most frequencies, which describes the low distortion of the received signal in respect to the transmitted signal across the whole working frequency band.

Fig. 8.

Realized gain and group delay versus frequencies.

To compare the performance of the proposed antenna with recently published research that described other antennas, we have summarized the results in Table 3. According to this table, our proposed antenna presents an acceptable bandwidth and gain in comparison with others. Although the proposed antenna in [Reference Wang, Yin, Wu and Lian14] shows comparable bandwidth and even better gain in lower frequencies than our antenna, it has worse cross-polarization to co-polarization values. The proposed BAVA antenna in [Reference Molaei, Kaboli, Mirtaheri and Abrishamian17] showed very good time and frequency characteristics. The gain in [Reference Molaei, Kaboli, Mirtaheri and Abrishamian17] was better than our gain, but the antenna's size was much bigger, and comprised a complicated three-layer structure. The proposed antennas in [Reference Wu, Zhao, Nie and Liu8, Reference Pandey, Verma and Meshram13] were smaller in size, but they presented worse bandwidth and gain in comparison with ours. Moreover, the cross-polarization value was not reported in either of those studies [Reference Wu, Zhao, Nie and Liu8, Reference Pandey, Verma and Meshram13]. The proposed AVA antenna in [Reference Moosazadeh and Kharkovsky15] had the broadest bandwidth, as shown in Table 3, but its low frequency was higher than others. Unfortunately, the parameters of group delay and cross-polarization were not evaluated in that study [Reference Moosazadeh and Kharkovsky15]. Therefore, our proposed antenna presented not only acceptable frequency characteristics in a medium compact size but also provided good group delay and cross-polarization to co-polarization values. However, the drawback of our proposed antenna was its low gain within the low-end frequency band.

Table 3.

Summary of the results in recently published literature.

Nr*, not reported.

IV. CONCLUSION

In this study, we proposed a miniaturized CPW-fed TSA with a TSE structure. In this new proposed design, rather than using a conventional microstrip to slot-line transition, a modified CPW to slot-line transition with an air-bridge was employed. The proposed antenna has a compact size of 50 × 70 mm². The results of simulations and measurements showed that the antenna offers broad bandwidth impedance (2.6–20 GHz), good directivity, good gain, and low cross-polarization. A measured group delay showed that the performance of the antenna was satisfactory in the time-domain. All these properties make this antenna suitable for UWB applications, particularly for microwave imaging systems.