News - A review of transmission line antennas based on metamaterials (Part 2)

2. Application of MTM-TL in Antenna Systems
This section will focus on artificial metamaterial TLs and some of their most common and relevant applications for realizing various antenna structures with low cost, easy manufacturing, miniaturization, wide bandwidth, high gain and efficiency, wide range scanning capability and low profile. They are discussed below.

1. Broadband and multi-frequency antennas
In a typical TL with a length of l, when the angular frequency ω0 is given, the electrical length (or phase) of the transmission line can be calculated as follows:

Where vp represents the phase velocity of the transmission line. As can be seen from the above, the bandwidth corresponds closely to the group delay, which is the derivative of φ with respect to frequency. Therefore, as the transmission line length becomes shorter, the bandwidth also becomes wider. In other words, there is an inverse relationship between the bandwidth and the fundamental phase of the transmission line, which is design specific. This shows that in traditional distributed circuits, the operating bandwidth is not easy to control. This can be attributed to the limitations of traditional transmission lines in terms of degrees of freedom. However, loading elements allow additional parameters to be used in metamaterial TLs, and the phase response can be controlled to a certain extent. In order to increase the bandwidth, it is necessary to have a similar slope near the operating frequency of the dispersion characteristics. Artificial metamaterial TL can achieve this goal. Based on this approach, many methods for enhancing the bandwidth of antennas are proposed in the paper. Scholars have designed and fabricated two broadband antennas loaded with split ring resonators (see Figure 7). The results shown in Figure 7 show that after loading the split ring resonator with the conventional monopole antenna, a low resonant frequency mode is excited. The size of the split ring resonator is optimized to achieve a resonance close to that of the monopole antenna. The results show that when the two resonances coincide, the bandwidth and radiation characteristics of the antenna are increased. The length and width of the monopole antenna are 0.25λ0×0.11λ0 and 0.25λ0×0.21λ0 (4GHz), respectively, and the length and width of the monopole antenna loaded with a split ring resonator are 0.29λ0×0.21λ0 (2.9GHz), respectively. For the conventional F-shaped antenna and T-shaped antenna without a split ring resonator, the highest gain and radiation efficiency measured in the 5GHz band are 3.6dBi - 78.5% and 3.9dBi - 80.2%, respectively. For the antenna loaded with a split ring resonator, these parameters are 4dBi - 81.2% and 4.4dBi - 83%, respectively, in the 6GHz band. By implementing a split ring resonator as a matching load on the monopole antenna, the 2.9GHz ~ 6.41GHz and 2.6GHz ~ 6.6GHz bands can be supported, corresponding to fractional bandwidths of 75.4% and ~87%, respectively. These results show that the measurement bandwidth is improved by approximately 2.4 times and 2.11 times compared to traditional monopole antennas of approximately fixed size.

Figure 7. Two broadband antennas loaded with split-ring resonators.

As shown in Figure 8, the experimental results of the compact printed monopole antenna are shown. When S11≤- 10 dB, the operating bandwidth is 185% (0.115-2.90 GHz), and at 1.45 GHz, the peak gain and radiation efficiency are 2.35 dBi and 78.8%, respectively. The layout of the antenna is similar to a back-to-back triangular sheet structure, which is fed by a curvilinear power divider. The truncated GND contains a central stub placed under the feeder, and four open resonant rings are distributed around it, which widens the bandwidth of the antenna. The antenna radiates almost omnidirectionally, covering most of the VHF and S bands, and all of the UHF and L bands. The physical size of the antenna is 48.32×43.72×0.8 mm3, and the electrical size is 0.235λ0×0.211λ0×0.003λ0. It has the advantages of small size and low cost, and has potential application prospects in broadband wireless communication systems.

Figure 8: Monopole antenna loaded with split ring resonator.

Figure 9 shows a planar antenna structure consisting of two pairs of interconnected meander wire loops grounded to a truncated T-shaped ground plane through two vias. The antenna size is 38.5×36.6 mm2 (0.070λ0×0.067λ0), where λ0 is the free space wavelength of 0.55 GHz. The antenna radiates omnidirectionally in the E-plane in the operating frequency band of 0.55 ~ 3.85 GHz, with a maximum gain of 5.5dBi at 2.35GHz and an efficiency of 90.1%. These features make the proposed antenna suitable for various applications, including UHF RFID, GSM 900, GPS, KPCS, DCS, IMT-2000, WiMAX, WiFi and Bluetooth.

Fig. 9 Proposed planar antenna structure.

2. Leaky Wave Antenna (LWA)
The new leaky wave antenna is one of the main applications for realizing artificial metamaterial TL. For leaky wave antennas, the effect of the phase constant β on the radiation angle (θm) and the maximum beam width (Δθ) is as follows:

L is the antenna length, k0 is the wave number in free space, and λ0 is the wavelength in free space. Note that radiation occurs only when |β|<k0. When this condition holds, the antenna structure tends to radiate, and the propagation constant (i.e., the wave number in free space) determines the radiation angle. Since β is frequency-dependent, a frequency sweep is possible. For a CRLH TL, the propagation constant can be controlled so that the total frequency sweep includes positive (forward), null (sideways), and even negative (reverse) beams.

3. Zero-order resonator antenna
A unique property of CRLH metamaterial is that β can be 0 when the frequency is not equal to zero. Based on this property, a new zero-order resonator (ZOR) can be generated. When β is zero, no phase shift occurs in the entire resonator. This is because the phase shift constant φ = - βd = 0. In addition, the resonance depends only on the reactive load and is independent of the length of the structure. Figure 10 shows that the proposed antenna is fabricated by applying two and three units with E-shape, and the total size is 0.017λ0 × 0.006λ0 × 0.001λ0 and 0.028λ0 × 0.008λ0 × 0.001λ0, respectively, where λ0 represents the wavelength of free space at operating frequencies of 500 MHz and 650 MHz, respectively. The antenna operates at frequencies of 0.5-1.35 GHz (0.85 GHz) and 0.65-1.85 GHz (1.2 GHz), with relative bandwidths of 91.9% and 96.0%. In addition to the characteristics of small size and wide bandwidth, the gain and efficiency of the first and second antennas are 5.3dBi and 85% (1GHz) and 5.7dBi and 90% (1.4GHz), respectively.

Fig. 10 Proposed double-E and triple-E antenna structures.

4. Slot Antenna
A simple method has been proposed to enlarge the aperture of the CRLH-MTM antenna, but its antenna size is almost unchanged. As shown in Figure 11, the antenna includes CRLH units stacked vertically on each other, which contain patches and meander lines, and there is an S-shaped slot on the patch. The antenna is fed by a CPW matching stub, and its size is 17.5 mm × 32.15 mm × 1.6 mm, corresponding to 0.204λ0×0.375λ0×0.018λ0, where λ0 (3.5GHz) represents the wavelength of free space. The results show that the antenna operates in the frequency band of 0.85-7.90GHz, and its operating bandwidth is 161.14%. The highest radiation gain and efficiency of the antenna appear at 3.5GHz, which are 5.12dBi and ~80%, respectively.