1. Field of the Invention
The present invention relates to compact antenna apparatuses such as those used for portable radio terminals.
2. Discussion of the Background
In recent years, it has been important for portable radio terminals to be compact and thin for improved portability. As portable radio terminals have been made compact, however, the effectiveness of the antenna used therein may deteriorate. This tendency becomes especially prominent with a built-in antenna such as an inverted F antenna.
FIG. 11 shows a conventional compact antenna apparatus in which a line-shaped inverted F antenna is disposed on a conductor plate. A line-shaped inverted F antenna 1101 is connected to a conductor plate 1102 by a short-circuited line 1103 and a power-feed line 1104. A portable radio terminals having an operational frequency of 800 MHz is used as an example. The radio terminals, is shown simply by a conductor plate. The longitudinal length L of the conductor plate 1102 is set to one fourth the wavelength of a signal having the specified operational frequency, in the example, one fourth of the wavelength of an 800 MHz signal.
FIG. 12 is a graph showing the radiation efficiency of the line-shaped inverted F antenna 1101 shown in FIG. 11. The horizontal axis indicates the longitudinal length L of the conductor plate 1102 shown in FIG. 11. It is understood from this graph that the efficiency becomes -3 dB or less when the longitudinal length of the conductor plate becomes one fourth the wavelength or less. This indicates that about half the power supplied to the antenna 1101 is lost in the antenna 1101. This deterioration may occur in a compact antenna such as a line-shaped inverted F antenna as a result of the compact conductor plate.
As described above, as a portable radio terminals have been made compact, antenna performance may deteriorate. Especially with a compact antenna apparatus such as an inverted F antenna mounted on a conductor plate having a length of about one fourth the wavelength, the degree of deterioration is high and stable communication may be impeded.