1. Field of the Invention
The present invention relates to an antenna duplexer mainly used for a high-frequency circuit or the like of a radio system to share an antenna by a transmitter and a receiver.
2. Description of the Prior Art
Because mobile communication has recently advanced, an antenna duplexer is used for a lot of portable telephones and automobile telephones. An example of the above conventional antenna duplexer is described below while referring to the accompanying drawings.
FIG. 13 shows an exploded perspective view of a conventional antenna duplexer. In FIG. 13, symbols 1301 to 1306 denote dielectric coaxial resonators, 1307 denotes a coupling board, 1308 denotes a metallic case, 1309 denotes a metallic cover, 1310 to 1312 denote series capacitors, 1313 and 1314 denote inductors, 1315 to 1318 denote coupling capacitors, 1321 to 1326 denote coupling pins, 1331 denotes a transmission (hereafter TX) terminal, 1332 denotes an antenna terminal, 1333 denotes a receiving (hereafter RX) terminal, and 1341 to 1347 denote electrode patterns formed on the coupling board 1307.
The dielectric coaxial resonators 1301, 1302, and 1303 and the series capacitors 1310, 1311, and 1312, and inductors 1313 and 1314 constitute a TX band rejection filter. Moreover, the dielectric coaxial resonators 1304, 1305, and 1306 and coupling capacitors 1315, 1316, 1317, and 1318 constitute a RX band pass filter.
One end of a TX filter is connected to the TX terminal 1331 electrically connected with a transmitter and the other end of the TX filter is connected with one end of a RX filter and also connected to the antenna terminal 1332 electrically connected to an antenna. The other end of the RX filter is connected to the RX terminal 1333 electrically connected with a receiver.
Operations of the antenna duplexer constituted as described above are described below.
First, the TX band rejection filter shows a small insertion loss for a TX signal in a TX frequency band and makes it possible to transfer the TX signal from the TX terminal 1331 to the antenna terminal 1332 almost without attenuating the TX signal. Moreover, the TX band rejection filter shows an operation that RX signals input through the antenna terminal 1332 return to the RX band pass filter because the TX band rejection filter shows a large insertion loss for the RX signals in a RX frequency band and most input signals in the RX frequency band are reflected.
However, the RX band pass filter shows a small insertion loss for a RX signal in a RX frequency band and makes it possible to transfer the RX signal from the antenna terminal 1332 to the RX terminal 1333 almost without attenuating the RX signal. Moreover, the RX band pass filter shows an operation that TX signals coming through a TX filter are sent to the antenna terminal 1332 because the RX band pass filter shows a large insertion loss for TX signals in a TX frequency band and most input signals in the TX frequency band are reflected.
An antenna duplexer used for a high-frequency band of mobile communication has wide band characteristics. Therefore, to secure a necessary attenuation value in a wide band, it is necessary to further increase the number of stages of cascaded dielectric coaxial resonators.
In the case of the above structure, however, when the number of stages of resonators is increased to increase the attenuation value, the loss in a signal pass band width increases. To avoid the bad effect, it is considered to increase the unloaded Q of a dielectric coaxial resonator. However, to increase the unloaded Q, it is necessary to increase the size of the dielectric coaxial resonator. This is reciprocal to the recent antenna-duplexer downsizing trend.