1. Field of the Invention
The present invention relates generally to antennas reduced in size and having excellent reception characteristics and in particular to antennas receiving electric waves of ultrahigh frequency (UHF) band.
2. Description of the Background Art
Conventionally a variety of antennas have been proposed for different applications. In recent years, in particular, a variety of miniature antennas, omnidirectional antennas and the like intended to be installed indoors, mounted in mobile terminals and the like are proposed.
For example Japanese Patent Laying-open No. 2000-13130 discloses an antenna having a plurality of closed loop elements to be capable of reducing the size of a closed loop element for accommodating a desired frequency band and also of having large directivity.
Furthermore Japanese Patent Laying-open No. 2004-282319 discloses an antenna having a main body member implemented by a flat plate of metal in a strip having a prescribed width and thickness and a prescribed geometry.
Furthermore Japanese Patent Laying-open No. 2001-85928 discloses a folded dipole antenna having an antenna element of conductor formed in a folded square belt.
Furthermore Japanese Patent Laying-open No. 5-63435 discloses an antenna apparatus having a radiation element with an additional element arranged adjacent thereto to allow an antenna to cause complex resonance to achieve an increased band width. This antenna apparatus has the radiation element and the additional element with a reactance element loaded thereto to change a value in reactance to provide impedance-matching with the characteristic impedance of a feeder. The antenna apparatus can thus achieve sufficient reception over a wide band.
In Japan, digital terrestrial broadcasting started in 2003 and its viewable area is currently increasing. Accordingly, reception equipment with a digital high-definition (DH) reception mark attached thereto has been introduced in the market.
The DH mark is a symbol mark guaranteeing that equipment with the mark is reception system equipment registered with Japan Electronics and Information Technology Industries Association (JEITA) and at least having a specified level of performance. Products subject to registration for the DH mark include ultrahigh frequency (UHF) antennas receiving digital terrestrial broadcasts.
For an antenna receiving a television broadcast signal, Yagi antenna is often used. For Yagi antenna, such improvements are generally introduced as increasing the numbers of guidewaves or the area of the reflector to improve gain, front-to-back ratio, and other performance.
Furthermore, as another approach to enhance an antenna in performance the antenna is changed in configuration to be different than conventional. For example as an antenna capable of improving gain a stacked antenna having a plurality of antennas combined together is conventionally known.
FIG. 25 shows an example of a configuration of a stacked antenna.
With reference to the figure, a stacked antenna 100 includes antennas ANT11 and ANT12 each being a Yagi antenna receiving a UHF band electric wave which is in turn input via matchers 101 and 102 to a mixer 103 mixing the two electric waves together for output.
Antennas ANT11 and ANT12 each have an output impedance of 300Ω. Mixer 103 has an input impedance of 75Ω. If antennas ANT11 and ANT12 are directly connected to mixer 103, a loss associated with mismatching in impedance is increased. To match in impedance, matchers 101 and 102 are connected to antennas ANT11 and ANT12, respectively.
Typically, Yagi antenna is installed outdoors for use. As such, if the antenna is increased in size, it requires a larger area for installation and is also more susceptible to wind. In particular, if a Yagi antenna having a reflector increased in area to achieve enhanced performance receives wind, it is susceptible to damage.
In particular, the tendency to increase antennas in size significantly appears for wide band antennas capable of receiving analog and digital terrestrial broadcasts. If a single Yagi antenna is used with an increased gain to receive an electric wave of a UHF low channel band (13–44 channels) such as digital terrestrial broadcasting, the antenna is necessitated to be larger. Larger antennas, however, tend to cause problems associated with installation area, damage, and the like as described above.
Furthermore, for the stacked antenna shown in FIG. 25, the matchers and the mixers provide losses, and the performance is not improved as theoretically expected.
Furthermore, the above described miniature and omnidirectional antennas do not have performance suitable for receiving digital terrestrial broadcasts. First, conventional antennas reduced in size are reduced in gain and other performance. Furthermore, omnidirectional antennas receive not only an electric wave from a transmitting antenna but also that reflected from any obstacle existing therearound. Thus omnidirectional antennas are susceptible to multipath propagation.
For analog broadcasting, multipath propagation causes a ghost on a television screen. For digital broadcasting, if multipath propagation exceeding a level is caused no image will not be shown on a television screen. As such, conventional miniature and omnidirectional antennas are unsuitable for receiving digital terrestrial broadcasts.