Miniaturized Antennas

Most compact antennas are resonant antennas. In resonant type wire antennas, length is a main factor in determining the antennas’ frequency of operation. For a conventional monopole, it requires a quarter wavelength to resonate and for a loop antenna, it requires one wavelength. To have a compact design, one can simply arrange the resonant path inside a small area or reduce the physical length by inductive or capacitive loading while keeping same electrical length. The conventional design of the resonant path alignment is to bend, fold or turn the path into a smaller size – even a spiral or helical shape is acceptable. Figure 1.1 shows a loop antenna that is folded to be compact for handset application [1].

For mass production, cost is always the first priority. The following examples are given with low cost printed antennas. The printed slot antenna consisting of a slot line, can be miniaturized using similar methods. For example, the most used method is to spiral the slot while keeping its resonant length as shown in Figure 1.2 [5]-[7].

In Figure 1.2(b), a compact open-ended meander slot antenna is shown [8]. In Figure 1.1(c) a slot ring has been miniaturized by fractal geometry [9]. The length of these spiraled or meandered slots have been effectively reduced by increasing antenna width. By bending the slot, the above methods have maintained the total electrical length which determine the resonant frequency.

According to transmission line theory, the inductive loading or capacitive loading can increase the equivalent electrical length which is useful in miniaturizing

capacitive loading, as realized by printed elements, shown in Figure 1.3(a) [10]. Chip capacitors can be loaded at the ends of the coplanar-waveguide-fed slot line for further size reduction, as shown in Figure 1.3(b) [8]. Figure 1.3(c) is the disk-loaded monopole. The antenna is considered as a series LC resonator when the disk acts as a capacitive load [11]. The simple LC model can show resonance characteristics of an antenna but does not include an impedance matching mechanism.

Aside from, a single-band compact antenna, there are many other compact dual-band antenna designs. Since 1999, the WLAN standards including the IEEE 802.11a/b/g systems were established by the IEEE 802.11 Group. In the U.S., the 802.11b/g WLAN standards are used in the frequency range of 2.4 to 2.4835 GHz while the 802.11a standards are used from 5.15 to 5.35 GHz and 5.725 to 5.825 GHz.

To enhance the communication capacity of a unit cell, a combo system with 802.11a/g or 802.11a/b/g standards have became popular. In this combo system, the antennas are like transceivers operating in both 2.45 and 5.25 GHz frequency bands; thus, a dual-band antenna with a single input port is needed for size reduction. Many dual-band antennas have been proposed in the literatures. Some of these designs use two similar resonators to achieve dual-band operations, such as a double inverted-F antenna [13] and an F shape monopole (double-L monopole), as shown in Figure 1.4(a) [14]-[17]. These are miniaturized by bending the strips into an L-shape. Also, some of these designs use a single block antenna with multiple resonant modes to achieve dual-band operations, such as a tapered bent folded monopole [18], as shown in Figure 1.4(b), meandered CPW-fed monopole [19], and, an L-shaped monopole [20].

Figure 1.1 Line alignments of a loop antenna for handset use.

(a) (b) (c)

Figure 1.2 (a)The spiraled microstrip-fed slot antenna. (b)The microstrip-fed meandered slot antenna.

(c)The fractal microstrip-fed slot ring antenna.

(a) (b) (c)

Figure 1.3 (a)The microstrip-fed slot antenna with inductive load in central portion. (b)The CPW-fed slot antenna with capacitive load at the open end. (c) The disk-load monopole antenna.

20 mm

8 mm

24 mm

19 mm 24 mm

19 mm Feed

Feed (a) (b)

Figure 1.4 (a)The compact coupled inverted-L dual-band antenna. (b)The compact CPW-fed dual-band antenna.

(a) (b)

Figure 1.5 (a)The monopole antenna printed on a dielectric slab. (b)The dielectric loaded monopole antenna.

To have enough electrical length under physical constraints, employing the use of a dielectric is an efficient solution. With a material of higher relative permittivity, one can have a larger phase constant, thereby reducing the physical resonant length [21]. Figure 1.5 shows a monopole printed on a ceramic piece [22]-[23]. In Figure 1.5(b) the antenna length is half than that in air by using the relative permittivity of 22 [23].

Unlike natural transmission line with a positive phase constant, metamaterial based transmission lines have a designable phase constant. This kind of artificial structure has been utilized by numerous guided and unguided wave applications. One notable example is Left-handed materials (LHMs) which possess negative refractive index and have drawn tremendous interests in both scientific and engineering fields.

The Left-handed transmission line (LH TL) is characterized by the phase advance whereas the Right-handed transmission line (RH TL) is characterized by phase delay along the power traveling direction. Metamaterial antennas possess different characteristics from conventional antenna designs that are based on the standard transmission line. RH TL and LH TL can be embedded into each other and named as Composite Right/Left- Handed Transmission Line (CRLH TL). [24] provides an practical application inserting LH TL into the host RH TL. Its equivalent circuit and phase constant are shown in Figure 1.6. The Zeroth-order Resonance (ZOR) makes use of the opposite phase properties of RH and LH TL and has been proved experimentally [25]. Small planar antennas utilizing the ZOR structure were published [26-27], as shown in Figure 1.7. Basically, they are just a section of synthesized CRLH TL. The physical size of such antenna can be arbitrary since its size is specified by the value of the capacitances and inductances instead of wavelength. The concept of the infinite wavelength resonant antenna was first demonstrated in [26].

(a) (b)

Figure 1.6 (a)The equivalent lumped circuit of CRLH TL. (b)The phase constant curve of CRLH TL which possesses both positive and negative values.

Feed

CRLH TL

(a) (b)

Figure 1.7 (a)The taped CPW-fed ZOR antenna. (b)The coupled microstrip-fed ZOR antenna.