1. Field of the Invention
The present invention relates to a planar antenna and an antenna array applicable to mobile communication equipment, a compact information terminal and other radio devices containing a planar antenna, and which may also be used in an application of millimeter wave communication, such as a wireless local area network (LAN).
2. Description of the Background
According to the development of technology, there has been an increase in the use of millimeter-wave communication systems, such as a portable product for a wireless local area network (LAN), a movable communication apparatus, millimeter-wave imaging arrays for remote sensing, radio astronomy, plasma measurement, etc. These apparatuses provide for using high frequency radiowaves with wavelengths in a range of a millimeter or submillimeter. For example, such systems may be used in approximately a 60 GHz frequency range. As a result of these communication systems which use a high frequency range, there is interest in a planar antenna element. A planar antenna is able to be designed to be compact for planning such communication systems. Furthermore, a planar antenna is easy for integrating with other planar devices of electric circuits, such as a high frequency electric circuit of a receiver or a transmitter. Therefore, a planar antenna may be used in many applications including a portable product for a wireless LAN system, or a movable communication apparatus, and so on. A tapered slot antenna is one of a typical implementation of a planar antenna.
A tapered slot antenna in one form of a plane antenna is provided with a structure in which a slot width of a slot line is widened by inclining (tapering), wherein an electromagnetic wave is radiated in a direction parallel to an antenna surface (in a progressive direction of the slot line). Since a tapered slot antenna has a same structure as the slot line, a tapered slot antenna does not need a ground conductor on a back surface thereof in a same way as a microstrip line. Accordingly, a tapered slot antenna can be easily integrated with a feeder and a matching circuit having a uniplanar structure. Hereinafter, a tapered slot antenna is simply referred to as a plane or planar antenna.
In applications of millimeter-wave integrated circuits, if it is not possible to provide an impedance matching of an antenna apparatus, a power of radiowaves is decreased through the antenna element so as to be reflected either during a radiating or a receiving period. Therefore, the antenna apparatus has to consider impedance matching which provides sufficient characteristics for high efficiency of millimeter-wave communication.
Examples of background tapered slot antennas are disclosed in "The Tapered Slot Antenna--A New Integrated Element for Millimeter-Wave Applications" by K. S. Yngvesson et al, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol. 37, No. 2, February 1989.
This disclosure recites several tapered slot antenna apparatuses which have taper patterns which are relatively simple for implementation. For example, a "Vivaldi" which has an exponential taper pattern, a "LTSA" which has a linear taper pattern, and a "CWSA" which provides a constant width near an aperture portion of the slot pattern, are described therein. However, considering a millimeter-wave communication system, such as using a high frequency of 60 GHz, these tapered slot antennas are hard to implement in a compact structure since a length of the slot is almost three or four wavelengths long. These disclosed patterns of a tapered slot would not be able to provide sufficient characteristics for directivity in a short length of the slots.
Although a tapered slot antenna apparatus has just a one dimensional structure in a direction of wave radiation, a tapered slot antenna apparatus is known to radiate radiowaves which has nearly a circular shape with sufficient directivity in millimeter wave communication apparatuses. For radiating nearly circular waves in a millimeter wave communication apparatuses, a thickness of the antenna substrate would be configured in a range described by the following expression which is derived experimentally: ##EQU1## wherein .epsilon. is a dielectric ratio of a material which composes the antenna substrate, t is a thickness of the antenna substrate, and .lambda. is a wavelength in a vacuum.
However, according to the above referenced expression, a thickness of an ideal antenna substrate would be less than 0.1 millimeter when the tapered slot antenna radiates a radiowave which is approximately at 60 GHz of frequency. Consequently, in this planning of a thickness of a tapered slot antenna, it is too thin to provide a sufficient mechanical strength for implementing with a millimeter wave communication apparatus.
Furthermore, if another dielectric device is in a neighborhood of the tapered slot antenna apparatus, characteristics of the antenna apparatus deteriorate because of a dielectric loss of the antenna circuit. Therefore, in a case of implementation, the antenna apparatus would be provided with some spatial separation in a neighborhood of the antenna apparatus thereof. This provides another problem for implementing and integrating a millimeter-wave communication system.