The antenna prototypes at WLAN and Ka-band are fabricated to validate the simulated results. Figure 7 depicts the top and bottom view of an electrically small WLAN antenna which is realized on an FR-4 substrate. Later the antenna is interfaced with the slotted waveguide antenna optimized for Ka band operation. The integrated prototype is displayed in Fig. 8. It can be observed that the overall antenna system achieves a very compact structure.
Figure 9 presents the measured results for the reflection coefficient when the antenna is excited at ports 1 and 2, respectively. When port 1, connected to the slotted waveguide antenna, is excited, the experimental results show an impedance bandwidth (IBW) that is nearly identical to the simulated results. However, additional peaks appear due to fabrication tolerances in the waveguide structure. For the electrically small monopole antenna excited at Port 2, the measured reflection coefficient differs from the simulated results. This variation is primarily attributed to the challenges in connector interfacing, given the antenna's small footprint.
As the antennas are integrated within the same structure, it is crucial to analyze the isolation between the ports, as high mutual coupling can adversely affect performance in individual bands. Figure 10 shows the isolation between the ports at the Ka and WLAN bands, demonstrating that isolation levels are consistently below − 35 dB. Additionally, current distribution plots were analyzed as observed in Fig. 11, revealing no coupling between the antenna elements.
The radiation patterns are measured in the anechoic chamber for the antenna system as shown in Fig. 12 and Fig. 13, respectively. The slotted waveguide antenna displayed directional radiation patterns at 26 GHz, 28 GHz and 30 GHz, focusing energy in specific directions, making it ideal for applications requiring concentrated signal strength, such as radar and satellite communications. In contrast, the monopole antenna exhibited an omnidirectional radiation pattern, radiating energy uniformly in all directions perpendicular to the antenna. This characteristic makes the monopole antenna suitable for applications needing broad coverage, such as mobile and Wi-Fi communications. The distinct radiation patterns of these antennas allow them to be tailored to specific applications, leveraging the waveguide's directionality for focused signals and the monopole's omni-directionality for widespread coverage.
The gain of the integrated antenna system is shown in Fig. 14 (a-b). It is observed that the slotted waveguide array has high gain, meaning it can send strong, focused signals in specific directions. On the other hand, electrically small monopole antennas have low gain because they are very small and spread their signal evenly in all directions. This makes their signal weaker. While the slotted waveguide array is powerful and effective for directed signals, the electrically small monopole antenna struggles with low gain, which limits its performance in situations that need strong or long-range signals. The performance of the proposed integrated antenna system is compared with other similar structures as shown in Table I.
Table I. Comparison with other similar works
Ref
|
Frequency (GHz)
|
No. of input ports
|
IBW# (%)
|
Gain (dBi)
|
Low freq
|
High Freq
|
Low freq Band
|
High Freq Band
|
Low freq Band
|
High Freq Band
|
6
|
2.6
|
25
|
2
|
7
|
7.6
|
8
|
16
|
7
|
23.6
|
28.17
|
2
|
23.45
|
9.76
|
10.4
|
14.6
|
8
|
10
|
28
|
2
|
4.51
|
9.625
|
13.8
|
23.6
|
9
|
3.7
|
28.5
|
2
|
3.4
|
4.5
|
8.98
|
19.2
|
10
|
3.5
|
60
|
2
|
2.3
|
5.3
|
7.3
|
24
|
11
|
5.8
|
29.2
|
2
|
3.5
|
4.6
|
10.9
|
18.7
|
12
|
5.2
|
24
|
2
|
1.93
|
3.21
|
5.86
|
6.32
|
13
|
3.5
|
28
|
2
|
7.4
|
12.1
|
6.9
|
13.6
|
Proposed*
|
5.5
|
28
|
2
|
15.27(5.08–5.92)
|
14.56(26.48–30.64)
|
1.1
|
14.02
|
*Electrically small antenna at lower band, #IBW: Impedance Bandwidth |
As observed from the table, the key contributions of this work are:
1. High gain with high radiation efficiency achieved through the slotted waveguide array and beamforming techniques facilitated by all-metallic 3D printing, ensuring optimal performance at high frequencies.
2. An electrically compact structure is essential for microwave frequency operations when the antenna is to be integrated with mmwave antenna. The design features an electrically compact structure at the mmwave band that integrates seamlessly into the two-port assembly.
3. The two-port design maintains the pattern integrity of the antenna operating in the mm-wave bands, ensuring reliable and consistent performance.
4. The design ensures minimal energy leakage from one port to another, maintaining efficient and isolated operation of both antennas.