7Design and practice of small antennas I
7.1Design and practice
This chapter describes practical design and examples of small antennas. The chapter is composed of four sections, which deal with four types of small antenna: ESA (Electrically Small Antennas), FSA (Functionally Small Antennas), PCSA (Physically Constrained Small Antennas), and PSA (Physically Small Antennas). Since this grouping is rather flexible according to the antenna structure, types of applications, and so forth, some antennas may be classified into more than a single group.
7.2Design and practice of ESA
7.2.1Lowering the resonance frequency
7.2.1.1Use of slow wave structure
7.2.1.1.1 Periodic structure
7.2.1.1.1.1 Meander line antennas (MLA)
Meander line antennas have been applied to various small wireless systems such as mobile phones, digital TV receivers, RFID, and so forth, as the antennas can be easily and more flexibly constituted in planar structure than simple wire types. The meander line structure has practically been employed with modified versions such as an asymmetric dipole for two-frequency bands, a composite with a line or a slot for multiband operation, a horizontal element of an IFA (Inverted-F antenna), and so forth. These antennas used as a part of another type of antenna like the horizontal element of a PIFA are designed partly as an MLA and integrated into the other type of antenna.
7.2.1.1.1.1.1 Dipole-type meander line antenna
The principle of application of a meander line structure to antennas is as mentioned in Section 6.2.1.1, application of SW (Slow Wave) performances, the main feature of which is the smaller phase velocity as compared with that of free space so that the resonance frequency is lowered, resulting in the antenna size reduction. As one way to evaluate the size reduction effect, the length-shortening ratio SR is proposed [1], and demonstrated
7.2 Design and practice of ESA |
93 |
|
|
|
500 |
|
|
|
|
|
|
|
400 |
2 Lax |
|
|
|
e |
|
) |
300 |
e |
|
(Ω |
|
|
|
|
|
|
|
in |
|
|
|
|
|
|
|
Z |
200 |
|
|
Input impedance |
Xin |
|
|
|
|
||
|
Rin |
|
|
100 |
|
|
|
0 |
|
|
|
|
|
|
|
|
|
Calculated |
|
|
−100 |
Experiment |
|
|
|
(e = 0.0133 λ.) |
|
|
−200 |
0.4 |
0.5 |
|
0.3 |
||
|
|
Axial length 2Lax (λ) |
|
Figure 7.1 Input impedance (Zin = Rin + j Xin) of MLA (from [1], copyright C 1984 IEEE).
by considering two models; a meander line antenna and a zigzag antenna. The SR is defined as
S R = (λ/2 − 2Lax )/(λ/2) |
(7.1) |
where Lax is the axial length of the antenna (the length taken directly from one end to another end of the antenna structure) and λ is the resonance wavelength. The phase velocity Vp in this case is expressed by using SR as
Vp = (1 − S R)c. |
(7.2) |
Here c is the velocity of light in free space.
The input impedance Zin (= Rin + jXin) of the antenna is calculated and depicted in Figure 7.1, where the inset illustrates the antenna model (a dipole type), and Zin is drawn with respect to the axial length 2Lax. This figure indicates that the resonance occurs with the length of 2Lax = 0.35 λ (the length L0 of the meandered wire extended into a straight line = 0.70 λ with e = 0.0133 λ), and Rres (Rin at the resonance) = 43 . In this case, SR is 0.3. The radiation pattern is similar to that of a half-wave dipole with gain of 1.95 dB and half-power beam width of ±47 degrees.
The dipole-type MLA is also described in [2], providing data on monopole type, dipole type, tapered type, folded type, and other types, including examples of practical application to mobile phones.
94 |
Design and practice of small antennas I |
|
|
d |
2a |
p
h
Feed point
Ground plane
Figure 7.2 Monopole-type MLA placed on a ground plane (from [3]).
0.10 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
N = 1 |
|
a/h = 0.0069 |
|
|
||||
|
|
|
|
|
|
|
|
|||||
|
|
|
|
|
|
|
|
|
|
|||
0.08 |
|
|
|
|
1.5 |
|
|
|
Calculated |
|
|
|
|
|
|
|
|
|
|
|
|
||||
|
|
|
|
2 |
|
|
|
Measured |
|
|
||
|
|
|
|
|
|
|
|
|
|
|||
|
|
|
|
|
|
|
|
|
|
|
|
|
0.06 |
|
3 |
|
|
|
|
|
|
|
|
||
|
|
|
|
|
|
|
|
|
||||
λ |
|
4 |
|
|
|
|
|
|
|
|
||
|
5 |
|
|
|
|
|
|
|
|
|
||
/ |
|
6 |
|
|
|
|
|
|
|
|
|
|
d |
|
|
|
|
|
|
|
|
|
|
||
0.04 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
0.02 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
0 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
0.05 |
0.10 |
0.15 |
0.20 |
0.25 |
|||||||
0 |
||||||||||||
h/λ
Figure 7.3 Relationship between antenna height h/λ and the width d (from [3] copyright C 1998 IEICE).
7.2.1.1.1.1.2 Monopole-type meander line antenna
An MLA can be constituted in a monopole type and placed on a ground plane as shown in Figure 7.2, where a hatched part shows one segment of the antenna and the dimensional parameters are given [3]. In practice, this type of antenna has been often seen in small mobile terminals. The relationship between the antenna height h/λ and the width d/λ for the antenna in the resonance state is drawn in Figure 7.3, where the number N of segments is taken as the parameter and λ is the wavelength at the resonance. The h/λ varies linearly with d/λ and all lines in the figure concentrate at a point (0, 0.25), where d/λ = 0, corresponding to the resonance point of an ordinary quarter-wave monopole. An empirical equation for d/λ with respect to h/λ is given by
d/λ = (0.25 − h/λ)/1.70N 0.54. |
(7.3) |
Radiation resistance Rin in terms of h/λ is shown in Figure 7.4, where both calculated and experimental results are shown with respect to N. Rin does not vary substantially with N. When N = 3, Rin is given approximately by
Rin = 545(h/λ)1.82. |
(7.4) |
7.2 Design and practice of ESA |
95 |
|
|
Radiation resistance [Ω]
50 |
|
a/h = 0.0069 |
|
Calculated |
N = 1 |
Measured |
|
10 |
6 |
1 |
|
|
0.04 |
0.1 |
0.3 |
h/λ
Figure 7.4 Radiation resistance with respect to antenna height h/λ (from [3], copyright C 1998 IEICE).
100
a/h = 0.0069 Calculated
Quality factor
10
5 
0.04
Measured
N = 6
1 
0.1 |
0.3 |
h/λ
Figure 7.5 Antenna Q with respect to antenna height h/λ (from [3], copyright C 1998 IEICE).
Q is related with h/λ and shown in Figure 7.5, where both calculated and experimental results are provided, and given empirically for N = 3 as
Q = 0.777/(h/λ). |
(7.5) |
Radius a of the wire has almost no influence on the impedance characteristics.
Design parameters of monopole-type MLAs are also given in [4]. As a practical example, there has been a report on an MLA etched on both sides of a thin dielectric substrate [5]. The antenna model is depicted in Figure 7.6 with one side in (a) and the other side in (b). This antenna has wide enough bandwidth to receive the terrestrial TV broadcasting at UHF bands between 550 and 800 MHz, and gain of –3dB. The size is 80 × 100 × 1.6 mm3.
Another example is a monopole-MLA driven by a small loop mounted on a ground plane [6]. The meander line winding is optimized by using NEC (Numerical Electromagnetic Code) in conjunction with a Pareto GA (Genetic Algorithm). A further example
96 |
Design and practice of small antennas I |
|
|
61 mm 
61mm
y |
|
x |
|
z |
|
Via |
Via |
|
|
(a) |
(b) |
Figure 7.6 MLA etched on both side of thin dielectric substrate (from [5], copyright C 2006 IEICE).
Meander line
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
e2 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Z Y |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
||||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|||||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|||||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
||||||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|||||||||||
e1 |
|
|
|
|
|
|
|
Lax |
||||||||||||||||
|
|
|
X |
|
|
|
|
|
|
|
|
|||||||||||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
||||||
Sleeve |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
||||||||
|
|
|
|
|
|
|
|
|
|
|
l |
|||||||||||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
||||||||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Feed point
Figure 7.7 A small MLA with two sleeves located besides the antenna element (from [7]).
is a printed MLA [7], which is mounted on a ground plane and has two sleeves besides the meander line element as shown in Figure 7.7. The antenna impedance is calculated by using FDTD simulation and is shown to have operating bandwidth in the 0.9 to 3.0 GHz band, although the ground plane is comparatively small. As one more example, Figure 7.8 illustrates an MLA constituted in a loop structure in order to attain multiband operation [8]. In the figure (a) is the antenna mounted on the ground plane, (b) shows antenna structure (with bent part extended), and (c) illustrates the side view of the antenna mounted on the dielectric substrate. This antenna is designed as an internal antenna installed inside a mobile terminal used for GSM/DCS/PCS operations.
Tapered MLAs are described in [9a] and their applications to cellular units are introduced in [9b].