Conventional monopole antenna
The traditional monopole antenna consists of the radiation patch, the feed line and the circular base. The radiation patch for the conventional monopole antenna is usually designed at a quarter wavelength, i.e. 1/4 λg (where λg corresponds to a controlled wavelength). For use in a marine environment, the base station is usually located several meters above the antenna system built into the boats, thus requiring antenna structures with inclined beam patterns. Current distributions for dipole or monopole antennas with lengths of 1.5λ and 0.75λ, respectively, result in an upward slope of the maximum beam. Therefore, in this work, the length of the single-pole patch radiator is designed at 3/4 λg (derivative of 1/4 λg) to form an inclined radiation pattern beam. The conventional monopole antenna exhibits an impedance bandwidth of |S11| < ‒10 dB at 1.35 to 1.6 GHz and 2.1 to 2.4 GHz. Moreover, a small gain of less than 3 dBi is realized in the lower frequency range of the antenna.
To improve the impedance matching condition of a conventional monopole antenna, open-circuit rectangular stubs are added to the sides of the rectangular patch radiator as shown in Figure 2. Each stub is designed with a length l4 and width w3.
The results of simulated reflectance (S11) and voltage standing wave ratio (VSWR) are shown and compared with conventional ones in Figure 3a,b, respectively. Resistive Bandwidth |S11| < ‒10 dB and VSWR ≤ 2 in the lower frequency range 1.32 to 2.3 GHz, and the upper frequency range 2.6 to 2.9 GHz is achieved by adding stubs.
Cross plate design
One of the drawbacks of the conventional printed monopole antenna is that the azimuth radiation pattern (xoy-plane) does not have a uniform universal pattern. This is because the radiation pattern is more focused summer-plane relative to Xoz– an airplane. Furthermore, the omnidirectional radiation pattern of a conventional monopole antenna is distorted by frequency changes within the antenna’s operating range. One way to have a uniform all-round radiation pattern is to reduce the width of the patch radiator. However, this method reduces the bandwidth characteristics of the antenna.
To address this challenge, a cross plate is incorporated into the conventional monopole antenna as shown in Figure 4. The cross plate consists of a metal plate inserted perpendicular to the partial ground plane forming a cross-like structure6. The cross-plate structure located on the circular base of the antenna provides a reflective aperture to the electromagnetic waves radiated from the monopole antenna, which can prevent the azimuth radiation pattern from narrowing in a certain direction (i.e. summer-plane).
To further investigate the physical significance of the cross plate, the width of the cross plate, wcross which plays an important role in achieving a flat universal radiation pattern is varied. As shown in Figure 5a, the azimuth radiation pattern broadens to form a universal radiation pattern with increasing wcross. It can also be seen that the best flatness of the azimuth radiation pattern is already reached wcross= 44 mm. However, a variant of wcross does not appear to significantly affect the impedance bandwidth of the monopole antenna as shown in S11 and VSWR results in Figure 5b, c, respectively.
The azimuth radiation pattern results are given in Figure 6. For easy reference, the conventional monopole antenna with stubs added is labeled as structure Awhereas a conventional monopole with both stubs and a cross plate added is labeled as a structure B. From the azimuth results, it can be seen that a stable radiation pattern is achieved when the cross plate is added compared to a conventional monopole antenna. Furthermore, the 3D peak gain results shown in Figure 7 show a slight gain drop at higher frequencies when the cross plate is added. Nevertheless, a significant increase is observed for the lower frequencies.
Design of top hat structure
To increase the gain of a conventional monopole antenna, especially in the lower frequency range (ie the frequencies between 1.6 and 2 GHz), a top hat structure is added to the end of the monopole antenna. The configuration of the top hat structure is given in Figure 8. The top hat structure consists of three metal sheets arranged in a hat-like configuration. The slots at the top of the top hat structure provide insertion points for the Taconic electrical substrate.
The motivation behind using the metal hat is to present a large radiation aperture for the monopole antenna to increase directivity, without increasing the overall size of the antenna. However, after incorporating the top-hat structure, the gain of the monopole antenna deteriorates, although high directivity is achieved. This is mainly attributed to the antenna’s poor matching conditions after the addition of the top hat.
To reap the full benefits of the top hat structure while achieving good antenna matching conditions, a layer of frequency selling surface (FSS) consisting of three rectangular metal unit cells is placed behind the unipolar radiating patch. FSS structures are frequency dependent surfaces consisting of periodic structures that can control the properties of the electromagnetic waves passing through it24. The unique frequency-shifting capability of the FSS is used to improve the antenna’s matching conditions due to its simplicity and ease of fabrication. Moreover, the FSS structures have additional advantages of resonant properties that could be exploited to improve the impedance bandwidth characteristics of the antenna. The FSS unit cell can behave as a transmissive and reflective surface based on the dimensions. Therefore, each module cell is designed with an optimized 34 mm × 34 mm (0.3λ)g× 0.3 ming) to achieve the desired characteristics of the target range of the monopole antenna.
The top hat structure combined with the FSS unit cell layer results in a maximum gain of about 5.2 dBi in the target range as shown in Figure 9. Again, it can be concluded from the results in Figure 10 that the high gain feature is achieved by both the contribution of the FSS layer and the top hat structure. The |S11| and VSWR results are given in Figure 10a,b, respectively where the proposed antenna shows an impedance bandwidth |S11| < ‒10 dB and VSWR ≤ 2 at 1.35–2.1 GHz and 2.4–2.75 GHz.
A parametric study
To further investigate the effect of the key design parameters of the top hat structure, viz. length, width and spacing (LHAT, SHAT and WHATrespectively), a detailed parameter study with respect to the reflection coefficient (S11), VSWR and realized gain are given in Fig. 11a–c. With respect to the target operating range of the antenna (i.e. the lower frequencies), it can be seen that both gain and impedance bandwidths increase with increasing length (LHAT) as shown in Fig. 11a. The best gain and bandwidth performance is achieved immediately LHAT= 40 mm.
Additionally, increase the width of the top hat (WHAT) appears to increase the gain of the antenna in the target frequency range as shown in Fig. 11b. A trade-off between impedance bandwidth and gain is realized in the case of WHAT variation where the impedance bandwidth performance increases with decreasing WHAT. In this case, the best performance considering both gain and resistance is already WHAT= 30 mm.
Furthermore, the gap between the vertical metal plate of the top hat (SHAT) is varied to determine its effect on the bandwidth gain and resistance performance as shown in Fig. 11c. Again a similar observation can be made SHAT as is the case WHAT that a trade-off occurs between gain performance and bandwidth resistance. From the results in Figure 11c, the best gain and bandwidth performance are already achieved SHAT= 30 mm.
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