Antenna

Figure 9.1 Unbalanced-fed antenna.

Antenna

Il = IR

Figure 9.2 Balanced-fed antenna.

9.2.1Measurement by using a coaxial cable

9.2.1.1Measurement of unbalanced-fed antennas

It is possible to measure the input impedance characteristics in the tip of a coaxial cable by connecting the outside conductor of a coaxial cable to the case of a portable terminal and connecting a central conductor to the antenna for the unbalanced-fed antenna. Moreover, if the case structure is designed so that a coaxial connector may be attached in a case and the central conductor of the coaxial cable may serve as a feed line to an antenna, that is convenient for measurement. In this case, by short-circuiting the center pin and case of a coaxial connector linked to the antenna side, the electrical contribution from the measuring side at the tip of a coaxial cable can be known and compensated. Besides, it is necessary to choose carefully the position and direction for alignment of the coaxial cable connecting to the case so as to not disturb the current flowing on the case, and to align the coaxial cable in a direction that does not affect the radiation pattern [1]. Moreover, the balun method of preventing unwanted currents on the outside of the coaxial cable, including the dual-band balun of the same structure as the Spertopf, or bazooka balun of a sleeve antenna is also effective [2][3]. If the optical fiber explained in Section 9.2.3.1 is used instead of a coaxial cable, ideal measurements will be attained even when current flows on a whole metal case.