9Evaluation of small antenna performance
9.1General
Generally, evaluation of small antenna performance tends to be difficult, as the size of an antenna becomes very small. The difficulty of small-antenna measurement originates in small dimensions, asymmetry of antenna structure, and the antenna characteristics sensitive to the influence of its environment. Determination of correct or precise antenna performance will become difficult as the antenna dimension becomes smaller. Evaluation of antenna performance is performed by means of EM simulation and measurement. In this chapter, measurement of small antenna performances is described, whereas simulation is dealt with in Chapter 10. There are no essential differences between small antenna measurement and ordinary antenna measurement. However, as the size of an antenna becomes smaller, special considerations are required in order to assure the reliable evaluation of small antenna performances. Typical attentions are paid for cases:
(1)When the size of an antenna is very small compared with its wavelength or the size of a nearby conductor.
(2)When antenna structure is asymmetric and complicated.
(3)When an antenna is located in a complicated environment.
Although particular considerations are given to the measurement of small antennas, there may be some cases where no effective method is found for the accurate determination of the antenna characteristics. For example, when the size of the antenna is extremely small, the resistance becomes too small to be determined. Another case is where the antenna structure is so complicated that precise measurement is very difficult to perform.
To measure very small input resistance of a small antenna, for example, disturbance due to the connection of the instrument, which might be much larger than the size of the input terminals of the antenna, would give rise to significant errors in the measurement, etc.
When an antenna structure is symmetric, the antenna characteristics equivalent to those of the full structure can be measured by the image theory using a ground plane with a size of several wavelengths. If a ground plane can be used, it will become easier to perform exact evaluation of antenna performances as compared with the measurement without using a ground plane. Therefore, when an antenna to be measured has a simple and symmetrical structure, the antenna structure may be halved by placing it
372Evaluation of small antenna performance
over a ground plane, and the use of a coaxial cable for feeding the antenna becomes easy. In measurement by using a coaxial cable and ground plane, a balun (Balanced-to- Unbalanced Transformer) becomes unnecessary. Moreover, the measurement becomes stable, since noise and interference are isolatable from the feed of the antenna. Furthermore, it is also an advantage that there is no unbalanced current, which would likely disturb measurements, by feeding an antenna with a coaxial cable from beneath a ground plane. As, for example, with portable mobile terminals having an unbalanced mode terminal as an antenna connection terminal, a coaxial cable can be used in measurement. However, in actual measurement, since a ground plane is not infinitely large, unwanted unbalanced currents arise on a ground plane, and these currents also flow around to the back side of a ground plane. As a consequence, exact measurement of the input impedance becomes difficult, and radiation patterns will be distorted. For this reason, in order to measure the antenna characteristics in an environment as near as possible to free-space conditions, a ground plane with the size of a few wavelengths is usually used. However, in radiation pattern measurement, even if the ground plane has any kind of size, the influence by a finite size of a ground plane is not avoided, owing to the scattered wave from the edge of a ground plane and radiation produced by the currents flowing to the back side of the ground plane.
Since the characteristics of a small antenna installed in small wireless equipment are strongly influenced by nearby materials including electronic components and equipment case, special considerations are required for measurement. The coaxial cable connected to an antenna for measurement becomes especially a problem. Since a coaxial cable is a metallic line, it needs to be arranged so that current may not flow on the outside conductor of the coaxial cable when it is connected to the antenna, and it is most important how this is realized for the small antenna measurement on a portable mobile terminal. Furthermore, since the antenna characteristics are greatly dependent on the spatial relationship of a human body and an antenna, it is also important to perform measurements with consideration of the antenna characteristics under their environment.
There is a serious problem in measurement when an antenna has a balanced terminal at the feed point and a coaxial cable is connected to the terminal, and unbalanced currents are induced on the outside surface of the feed cable. A similar situation is encountered when a small antenna has asymmetry, irregular, or complicated structure. In that case, a balun should be used, and careful means for avoiding coupling, proximity influence, etc. must be taken. Thus, since the image theory by PEC (perfect electric conductor) GP ground plane cannot be used in case of measurements under conditions close to actual use, a special device is needed. The method for reducing undesired current on the outside surface of a coaxial cable in the unbalanced-fed antenna is different from that in the balanced-fed antenna. In the case of unbalanced-fed antennas, ferrite beads may be attached to a coaxial cable, or a coaxial cable feed is arranged near the antenna to attain measurement error as low as possible. For example, a coaxial cable might be oriented so that it may intersect perpendicularly with linear polarization when radiation patterns are measured. A simple way to check the influence of the coaxial cable is to