8.1 FSA (Functionally Small Antennas) |
313 |
|
|
y |
|
|
y |
|
(xN, yN) |
|
|
w5 |
|
|
|
|
(x2, y2) |
|
|
w1 |
(x1, y1) |
w1 |
|
|
|
|
w4 |
|
|
|
w3 |
w6 |
|
w6 |
z |
x |
x |
z |
|
w2 |
|
w2 |
(a) |
|
|
(b) |
Figure 8.58 Spline curved planar monopole antenna, indicating points for defining spline;
(a) front view and (b) back view ([38a], copyright C 2007 IEEE).
8.1.2.1.2.3.2.2 Spline-shaped antenna
A spline-shaped UWB antenna was synthesized [38a]. An innovative design approach based on the use of a spline description is applied to create a novel UWB antenna geometry. It is also used for formulation of the synthesis in terms of return loss at the input port and coupling properties of a system with identical antennas modeling the UWB communication. A suitable implementation of the PSO (Particle Swarm Optimization) has been integrated with spline-based shape generator and a MoM-based electromagnetic simulator.
The representative parameters to be optimized are shown in Figure 8.58, where the coordinates of the nth control points to be determined by the optimization are given as Pn (xn, yn), taken on the coordinate (x, y). Here n = 1,2, . . . N; N being the total number of the control points used to describe the antenna geometry.
The antenna is printed on the front side of the substrate (thickness 0.78 mm and εr = 3.38), having length w1 = 69.2 mm and half-width w2 = 10 mm, and the ground plane of length w6 = 51 mm is printed on the lower part of the back side of the substrate. The antenna geometry is characterized by the array of geometric variables
X = {(xn , yn ), n = 1, . . . , N ; w1, w2, . . . , w6}. |
(8.2) |
In the UWB communication system, impedance matching and distortionless conditions for the UWB bandwidth are imposed as the electrical constraints. As for the impedance matching over the UWB bandwidth,
|S11( f )| ≤ −10 dB.
314 |
Design and practice of small antennas II |
|
|
Figure 8.59 A proto-type dongle antenna: (a) front view and (b) back view ([38a], copyrightC 2007 IEEE).
And for the condition of distortionless system, the antenna is required to satisfy a condition pertaining to the magnitude of S21, |S21 (f)| to be
|S21| ≤ 6 dB.
And the group delay τ g,
τg ≤ 1 ns.
Here |S21| and τ g, respectively, denote the maximum variation in the whole frequency band of |S21| and τ g.
The antenna is required to be placed on the platform of the size 100 × 60 (in mm). By the optimization, coordinates of the control points are; p1 (6.9, 50.6), p2 (9, 55.5),
p3 (7, 62.7), p4 (2.7, 66.3), and p5 (1.9, 61.8) all in mm. As for the feeding line, w4 = 5.4, and w3 and w5, which define the range of contour variations along the y-axis, are 51.6 and 56, respectively. A prototype antenna is illustrated in Figure 8.59, (a) front view and (b) back view. Simulated and measured return loss is shown in Figure 8.60.
The spline-shaped UWB antenna is applied to integrate in a wireless USB (Universal Serial Bus) dongle [38b]. It has a miniaturized planar structure with maximum extension 39.2 × 19.2, within which the radiator occupies only an area of 16.2 × 19.2 mm. The antenna is printed on a two-sided dielectric substrate (εr = 3.38 and thickness 0.78 mm) and the geometry is defined by the set of values of the descriptive parameters; ϕ1 (length of the substrate) = 39.2, ϕ2 (half-width of the substrate) = 9.6, ϕ3 (half width of the feeding point) = 2.1, and ϕ4 (length of the ground plane) = 23.0 (in mm). Antenna geometry is determined by the spline-description, giving control points on the (x, y)
8.1 FSA (Functionally Small Antennas) |
315 |
|
|
|
0 |
|
|
|
|
|
|
|
|
|
|
|
|
|
−5 |
|
|
|
|
|
|
|
|
|
|
|
|
|
−10 |
|
|
|
|
|
|
|
|
|
|
|
|
|
−15 |
|
|
|
|
|
|
|
|
|
|
|
|
|
−20 |
|
|
|
|
|
|
|
|
|
|
|
|
|
−25 |
|
|
|
|
|
|
|
|
|
|
|
|
|
−30 |
|
|
|
|
|
|
|
|
Simulated data |
|
|
−35 |
|
|
|
|
|
|
|
|
Measured data |
|
|
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
|
1 |
Frequency (GHz)
Figure 8.60 Simulated and measured return loss of the dongle antenna ([38a], copyright C 2007 IEEE).
y
|
P7 |
P4 |
|
P5 |
|
P6 |
|
P3 |
|
|
|
|
P2 |
|
|
P1 |
ϕ1 |
ϕ3 |
|
ϕ2
z
x
Figure 8.61 Geometry of a UWB dongle antenna front view ([38b], copyright C 2008 IEEE).
coordinates as: p1 = (2.1, 25.0), p2 = (6.6, 29.5), p3 = (8.6, 29.4), p4 = (7.3, 35.9), p5 = (6.9, 34.9), p6 = (2.2, 32.4), and p7 = (0.0, 33.8). These points are given on the antenna geometry illustrated in Figure 8.61.
Measured and simulated return loss is given in Figure 8.62. It shows bandwidth of 2 GHz from 3 GHz to 5 GHz for –10 dB return loss. In terms of |S21|, it is 5 dB, which is smaller than the requirement, with average |S21| it is about –23 dB.
316 |
Design and practice of small antennas II |
|
|
–5 
Simulated data Measured data
–10
–15
–20
–25
Frequency (GHz)
Figure 8.62 Return loss of a UWB dongle antenna ([38b], copyright C 2008 IEEE).
8.1.2.1.2.3.3 UWB antennas with slot/slit embedded on the patch surface
Slots/slits are embedded on the patch surface in order to lengthen the current paths and increase the number of currents so that multiple resonances occur, leading to production of ultra wide bandwidth. Use of slots/slits has another objective; that is, to produce stop bands within the UWB system band for avoiding interference against other wireless systems, for instance, WLAN (5-GHz bands). For another purpose, a slit is used on the patch to reduce the current on the ground plane so that contribution of the ground plane to radiation is reduced. This leads to reduction in the size of the ground plane at the same time.
Various shapes of slots/slits are applied to patch antennas, depending on the purposes. Examples of such patch antennas with band-notch performance are U-slot on a square patch [39], circular/elliptical patch [40a], circular/ elliptical slot [40b], circular/elliptical slot with U-shaped tuning stub [40c], H-shaped plate and rectangular slots [41], rectangular patch with a notch and a strip [42], pentagon shaped-slot [43], tapered ring slot [44], and octagonal wide slot with square ring [45].
8.1.2.1.2.3.3.1 A beveled square monopole patch with U-slot
A beveled square monopole patch is introduced in the previous section [20] as one of the useful UWB antennas. In order to avoid interference from other wireless systems operating in the UWB band, a slot/slit (notch) is embedded on the patch surface. Figure 8.63(a) depicts a square beveled monopole patch along with S11 characteristics, and the patch, in which a thin U-slot is employed, is shown in Figure 8.63(b), which also provides dimensional parameters [39]. Figure 8.63(c) is the S11 characteristics, giving a stop band within the UWB band as a result of a U-slot application. On a beveled square monopole patch antenna, four mode currents J0, J1, J2, and J3 flow on the surface as Figure 8.64 illustrates. J0 is loop current, which is a special non-resonant inductive mode, J1 is vertical current flowing along the monopole, associated with resonance at
|
|
|
|
L |
|
|
L |
|
|
|
0 |
|
|
l2 |
|
L |
|
|
|
−5 |
(b) |
L |
|
|
−10 |
l1 |
|
h0 |
|
|
|
|
|
−15 |
|
h0 |
t |
|
−20 |
|
hs |
|
|
|
|
|
−25 |
|
|
|
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Frequency (GHz) |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
0 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
−5 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
(dB) |
−10 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
(c) |
−15 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
11 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
S |
−20 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
−25 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
−30 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
Frequency (GHz)
Figure 8.63 (a) Geometry of a beveled square planar monopole (L = 19 mm, h0 = 0.2 mm) with S11 characteristics, (b) geometry of a beveled square monopole with a resonant U-slot (L = 19, l1 = 10, l2 = 8, t = 1, hs = 4, and h0 = 0.2 (in mm)), and (c) reflection coefficient of the antenna ([39], copyright C 2010 IEEE).
|
Mode J0 (3 GHz) |
Mode J1 (3 GHz) |
1 |
|
|
0.9 |
|
|
0.8 |
|
|
0.7 |
|
|
0.6 |
|
|
0.5 |
Mode J2 (7 GHz) |
Mode J3 (11 GHz) |
0.4 |
|
|
0.3
0.2
0.1
Figure 8.64 Normalized current distributions on the beveled square monopole antenna for the first four characteristic modes ([39], copyright C 2010 IEEE).
