I

I

Figure 9.3 Current in the connection between the balanced antenna and the coaxial cable.

9.2.1.2Measurement of balanced-fed antenna

In measurement of balanced-fed antennas, a balun is needed between the antenna and the coaxial cable linked to a measuring instrument. Figure 9.3 shows a branch of the current in the connection between the balanced antenna and the coaxial cable. The effect which reduces the current flowing on the outside conductor of the coaxial cable is provided by the balun. As the undesired current is suppressed by using a balun, the unwanted radiation by the current on the surface of a cable disappears. Moreover, since a balanced antenna is also excited in balanced mode, the measurement of the original antenna characteristics can be attained.

Figure 9.4 shows some typical balun configurations which have been used. There are a strip type, a Bazooka type, a quarter-wavelength coaxial type, and a half-wavelength coaxial type, and all of these are simple to produce. As each of these baluns has about 10% relative bandwidth, it is sufficient to manufacture a single balun for the center frequency in antenna measurement of mobile terminals. However, the above-mentioned baluns have a large fault in antenna measurement of mobile terminals, because they have sizes comparable to a wavelength. There is another balun, however, which consists of a 180-degree hybrid to use in a higher-frequency band [4]. The characteristic of this balun is improved by putting in some resistance. It is possible to make this balun easily on a printed circuit board and various sizes and frequencies can be chosen according to the purpose. Such are the advantages of this balun, shown in Figure 9.5.

9.2.2Method of measurement by using small oscillator

For radiation pattern measurement, use of a small, integrated oscillator which removes the influence of an external coaxial cable is very effective [5]. Although the measurement using a small oscillator is inexpensive and easy, it cannot measure phase information. The small oscillator, which operates from a battery in the metal case, is shown in Figure 9.6 [5]. Under the present circumstances, it is useful to insert a small fixed attenuation (6 dB for example) between an antenna and the small oscillator. Thereby, influence on the oscillator resulting from a mismatch of the antenna is minimized.

The system of measurement using a small oscillator is shown in Figure 9.7. Since measurement over a frequency band may require tuning the oscillator throughout a frequency band, it is useful to connect a spectrum analyzer to the receiving probe antenna. Quantitative measurements then are taken by comparing the spectrum analyzer

Blocking bushing

(c) Half-wavelength bypass line balun (Principle figure(left) Practical structure(right))

Slot

l

Parallel line

(d) Split balun

Figure 9.4 Typical Balun configuration: (a) Spertopf, (b) bypass conductor balun,

(c) half-wavelength bypass line balun, and (d) split balun.

readings when the oscillator is connected to a standard dipole antenna, to readings obtained when the same oscillator is connected to the unit under test.

The distance between the antenna of a personal mobile terminal in the presence of a human body at the time of measurement, and a receiving antenna is needed to be at least 2D2/λ [6]. D is the size of the portion considered to contribute to radiation of the antenna containing a human body, and λ is the operating wavelength. For example, when the spherical model with a diameter of 20 cm is used as a human body head model and there is 10 cm distance between a spherical model and an antenna, D is 30 cm.