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Properties of small antennas |
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Meanwhile, when a small magnetic dipole of the circumference 0.1λ with the same thickness as the above dipole is used, η is 9.28% if QC of the capacitive component is 1000. If QC of that capacitance is 4000, η becomes 21.1%. Because QC of the capacitive component is usually higher than QL of an inductive component, a small magnetic antenna element is advantageous to obtain higher radiation efficiency when about the same size of small antenna is considered.
As such, matching design will be very serious as the size of antenna becomes smaller.
3.1.4Gain
Gain G of an antenna is related to its directivity by accounting for efficiency and two different matching factors.
G = η DMp |
(3.17) |
where D is directivity, M is the impedance-matching factor, and p is the polarizationmatching factor. M is given by using reflection factor at the input terminals and VSWR S as
M = 1 − | |2 ≤ 1 |
(3.18) |
= 4S/(1 + S)2. |
(3.19) |
The polarization factor p is given by |
|
p = |ρH · ρE |2 = | cos ψp|2 |
(3.20) |
where ρH and ρE are unit vectors of the wave with horizontal and vertical polarization, respectively, and ψ p is the angle between the two vectors. Here the electric fields EH and EE of incoming waves Ei, respectively, are described as
EH = ρH Ei |
(3.21) |
EE = ρE Ei . |
(3.22) |
Gain of a very small antenna is determined almost by η and M, because D is around 1.5 independently of the size and p is usually taken as unity, since polarization matching can be good in almost all cases. In practical small antenna design, a condition M =1 will be often encountered. The M may take much lower values than unity, as the antenna size tends to be smaller. Gain reduction due to mismatching occurs in many cases with improper design of small antennas, and specific design techniques are needed to avoid degradation of antenna performance due to mismatching. Impedance matching problems will be discussed in the next section.
3.2Importance of impedance matching in small antennas
In communication systems, signal transmission through the network under the wellmatched condition is essential for no loss or no serious degradation in the signal quality
3.2 Importance of impedance matching in small antennas |
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Y/λ
0.2
0.1 
0.0
–0.1
–0.2 
INCIDENT PLANE WAVE
Y/λ
0.2
0.1
0.0
–0.1
–0.2
–0.2 |
–0.1 |
0.0 |
0.1 |
0.2 |
–0.2 |
–0.1 |
0.0 |
0.1 |
0.2 |
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X/λ |
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X/λ |
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(a) |
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(b) |
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Figure 3.8 Power flow of a plane wave incoming to a small dipole [11].
INCIDENT PLANE WAVE
or quantity. The same is true for antennas. Achieving good matching condition is particularly a serious problem in small antennas, because of difficulty in proper matching of small antennas to the load of 50 ohms. There are often cases where proper matching is impossible.
The importance of matching in a small antenna can be visualized by a model shown in Figure 3.8, where a small dipole antenna is placed in an environment with a plane wave incoming [11]. In the figure, the plane wave energy flow is illustrated by arrows, showing amplitude with the length and the direction with the arrow. The plane wave energy is intercepted by the antenna perfectly when the antenna is in the perfectly matched condition as Figure 3.8(a) illustrates. On the other hand, when the antenna is in mismatched condition the plane wave energy flow passes through the antenna as Figure 3.8(b) illustrates. This demonstrates that the antenna does not function as a radiator when it is in mismatched condition, and indicates how crucial a problem matching is for small antennas.
In practice, the smaller the antenna will be, the harder the matching to the load of 50 ohms becomes. In addition, a transformer necessary for transferring the very low resistive component of the antenna to the load 50 ohms adds additional loss so that the radiation efficiency will be further lowered as a consequence.
When an antenna is constructed as a structure within which self-resonance is possible, no reactance component is necessary for the matching, and the loss problem – at least in the matching circuit – is relieved. Hence higher efficiency may be achieved. Examples of such antenna structures are NMHA (Normal Mode Helical Antenna), meander line, zigzag line, fractal shape, and composite antennas, in which resonance condition at lower frequencies can be achieved by combining antenna elements.
The latest technology offers the novel matching method, which applies the NonFoster circuit and metamaterials to the matching circuits. Difficulty attributed to loss in the devices or the matching circuit can be overcome by applying the Non-Foster circuit to matching, and yet wideband matching may be expected.
20 |
Properties of small antennas |
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Figure 3.9 Image of a vertical monopole on a ground plane.
Applications of metamaterials have been shown to be useful for matching in space. Stuart has introduced a new concept [3], which demonstrated an application of negativeepsilon material to matching in the near field of a very small monopole antenna as Figure 1.9 illustrated. The principle is to compensate the reactive (capacitive) fields outside the antenna with an inductive component due to a negative epsilon material. By this means, an ultra small antenna is possibly created.
3.3Problems of environmental effect in small antennas
Antenna performance is affected by proximity effects due to materials existing near an antenna. A simple example is a monopole placed on a ground plane (GP) (Figure 3.9). When the size of the GP is infinite, a virtual monopole is produced beneath the GP by the image effect as shown with a dotted line below the GP in Figure 3.9. As a consequence the antenna is treated equivalently as a dipole. However, when the size of GP is finite, radiation current is induced on the GP by the excitation of the monopole, and the GP will act as a part of the antenna system. Thus the antenna together with the GP exhibits different performance from that of the dipole only.
Similar proximity effects are often observed in an antenna system located near some influential materials. The proximity effect is either advantageous or disadvantageous for an antenna system, depending on the dimensions of the antenna element and the materials. In general, one common situation is variation or degradation of the antenna performance due to the proximity effect. Contrarily, an advantage derived from the proximity effect is often observed for cases where the antenna performance is enhanced by the existence of the proximity effect. A practical example of such advantage is a PIFA installed inside a mobile terminal. The PIFA itself is an antenna of inherently low gain and narrow band, say about 1%–2% when the antenna is placed on an infinite or comparable size of GP; however, a PIFA installed on a finite size GP, for example that in a mobile phone terminal, exhibits higher gain and wider bandwidth than the antenna in free space [12]. This improvement is a result caused by the assistance of the GP, on
References 21
(a) |
(b) |
Figure 3.10 Image of a horizontal monopole on a ground plane; (a) PEC and (b) N-PEC.
which the radiation current excited by the PIFA flows, and the performance of the entire antenna – that is, the PIFA plus the GP – is improved. In practice, the PIFA to be mounted on the mobile phone is designed intentionally to include the GP as a part of the antenna system so as to meet the system specification such as gain and bandwidth.
Another good example is a small ceramic-chip antenna encapsulated in a fewmillimeter cube, which is a typical ESA and also PSA as well. When designing a small ceramic-chip antenna, dimensions of the mobile terminal unit, location to install the antenna, space to be occupied by the antenna, as well as the size of the antenna, should all be included in the antenna specifications.
It should be noted that a small antenna that is appropriately designed to satisfy the system requirements should not be placed near certain materials so that designed antenna performance is substantially harmed. It can be said generally that the smaller the antenna becomes, the larger the influence of such nearby materials is likely to be.
Recently, non-perfect electrically conducting (N-PEC) surface has drawn much interest worldwide to be used as a platform on which an antenna is placed. When a linear current source is placed near a PEC surface in parallel as shown in Figure 3.10(a), an image of the inverse phase is produced beneath the surface. However, when an N-PEC surface is used, the image is the same phase as the original source as shown in Figure 3.10(b). By using the N-PEC surface, a current source antenna can be placed very near to the surface, implying that such an antenna can have low-profile structure. Typical N-PEC surface can be composed by using EBG (Electronic Band-Gap) material, which is realized by either periodical structure or physically constructed material. Application of EBG surface has now become one of the popular topics in the field of small antennas. It is considered as a useful means to make an antenna low-profile [13], to reduce the coupling between two closely spaced antennas [14], and to miniaturize an antenna [15].
References
[1]R. W. P. King, The Theory of Linear Antennas, Harvard University Press, 1956, pp. 184–192.
[2]R. W. P. King and C. W. Harrison, Antennas and Waves: A Modern Approach, The MIT Press, 1969, pp. 549–552.
[3]C. A. Balanis, Antenna Theory, 2nd edn., John Wiley and Sons, 1997, pp. 209–216.
[4]C. A. Balanis, Antenna Theory, 2nd edn., John Wiley and Sons, 1997, pp. 236–239.
[5]R. M. Fano, Theoretical Limitations on The Broadband Matching of Arbitrary Impedance,
MIT Research Lab of Electronics, Technical Report, no. 41, 1948, pp. 56–83.
22Properties of small antennas
[6]R. M. Fano, A Note on the Solution of Current Approximation Problems in Network Synthesis, Journal of The Franklin Institute, vol. 249, March 1950, pp. 189–205.
[7]G. L. Matthaei, E. M. T. Jones, and L. Young, Microwave Filters, Impedance-Matching Networks and Coupling Structure, McGraw-Hill, 1964, sections 4.09 and 4.10.
[8]R. C. Hansen, Phased Array Antennas, John Wiley and Sons, 1998, section 5.4.3.
[9]L. G. Linvill, Transistor negative impedance converters, Proceedings of IRE, vol. 41, 1953,
pp.725–729.
[10]R. M. Rudish and S. Sussman-Fort, Non-Foster Impedance Matching improves S/N of Wideband Electrically-Small VHF Antennas and Arrays, Proceedings of the Second IASTED International Conference 19–21 July 2005, Banff, Alberta, Canada.
[11]K. Ishizone, Poynting Vector flow in the vicinity of a receiving dipole antenna, IEICE Technical Report AP-86-38, 1986, pp. 35–41.
[12]K. Satoh et al., Characteristics of a Planar Inverted-F antenna on a Rectangular Conducting Body, IEICE Transactions B, vol. J-71-B, 1971, no. 11,71, pp. 1237–1243.
[13]F. Yang and Y. Rahmat-Sami, Reduction Phase Characteristics of the EBG Plane for Low Profile Antenna Applications, IEEE Transactions on Antennas and Propagation, vol. 51, 2003, pp. 2691–2703.
[14]E. Saenz et al., Coupling Reduction Between Dipole Antenna Elements by Using a Planar Meta-Surface, IEEE Transactions on Antennas and Propagation, vol. 57, 2009, no. 2,
pp.383–392.
[15]J. L. Volakis et al., Antenna Miniaturization Using Magnetic-Photonic and Degenerate BandEdge Crystal, IEEE Antennas and Propagation Magazine, vol. 48, October 2006, no. 5,
pp.12–28.