Communications: radio wave antennas – Antennas – Microstrip
Patent
1996-07-24
1998-06-16
Wimer, Michael C.
Communications: radio wave antennas
Antennas
Microstrip
343702, H01Q 138
Patent
active
057678103
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a microstrip antenna device in which a radiating patch is disposed adjacent but opposite to a ground plate and an inner and an outer conductor of a coaxial feeder are connected to the radiating patch and the ground plate, respectively.
PRIOR ART
In FIG. 1 there is shown an example of a conventional microstrip antenna device. In the conventional microstrip antenna a radiating patch 11 is disposed on a ground plate 12 in adjacent but opposite relation thereto with a dielectric substrate 13 sandwiched therebetween, a coaxial feeder 14 has its inner conductor connected at one end to the radiating patch 11 substantially centrally thereof through small holes made in the ground plate 12 and the dielectric substrate 13 and has its outer conductor connected to the ground plate 12, the other end of the coaxial feeder 14 being connected to a transmitter or receiver 15. Here, the length L of the radiating patch 11 is about 0.5 .lambda.e. .lambda.e is a guide wavelength given by .lambda.e=.lambda.x 1.sqroot..epsilon..sub.r , where .lambda. is the wavelength in a vacuum and .epsilon..sub.r is the dielectric constant of the dielectric substrate 13. This microstrip antenna yields a main lobe in a direction perpendicular to the radiating patch 11, developing a current distribution which is maximum at the center of the radiating patch 11 lengthwise thereof (in the direction of the length L) and minimum at its both ends. That is to say, the conventional microstrip antenna has its length L defined by 0.5 .lambda.e and is used in a half-wave resonant state.
The antenna length of the microstrip antenna, that is, the length L of the radiating patch 11, could be decreased by increasing the dielectric constant of the dielectric substrate 13. However, an increase in the dielectric constant increases also the dielectric loss, and hence impairs the antenna efficiency. With a view to reducing the antenna length L, there has been proposed an antenna in which the radiating patch 11 has slits SL extending from its edges as shown in FIG. 2 ('84 National Conference of IECEJ Communication Department, No. 624: A Discussion about Miniaturization of an Inverted F Type Antenna). With the use of this scheme, it is possible to lower the resonance frequency by increasing the number of slits SL and their length without increasing the dielectric constant of the dielectric substrate 13, with the result that the antenna length L is reduced. It has been reported, however, that the slits SL disturb current and hence impair the antenna efficiency when the antenna length L is short, even if the dielectric substrate 13 is formed of a low-loss material.
In Japanese Patent Application Laid-Open Gazette No. 29204/83 (Feb. 21, 1983) there is proposed a microstrip antenna which has a variable capacitance diode connected between one end of the radiating patch in a direction at an angle of 45 degrees to the direction of resonance and the ground plate to make the resonance frequency variable, but this is intended to radiate circularly polarized waves and hence has nothing to do with the miniaturization of the antenna. In Japanese Patent Application No. 124605/90 (May 1, 1990) there is proposed a microstrip antenna which has a variable capacitance element disposed in a space made in the dielectric substrate between the radiating patch and the ground plate for interconnecting them to make variable the frequency band used. The half-wave microstrip antenna with a square radiating patch whose side is 60 mm, exemplified in the above-mentioned application, is said to have a 1.42 GHz resonance frequency. In this half-wave antenna, if the dielectric constant .epsilon..sub.r of the dielectric substrate is set at 2 to 3, the length of the side of the radiating patch reversely obtainable from the 1.4-GHz resonance frequency (the wavelength .lambda. in a vacuum is around 20 cm) is .lambda.e/2=.lambda./(2.sqroot..epsilon..sub.r )=70.about.60 mm, which is nearly equal to the length 60 mm; hence, the above-mentioned capacitance
REFERENCES:
patent: 4924237 (1990-05-01), Honda et al.
patent: 5148181 (1992-09-01), Yokoyama et al.
Hagiwara Seiji
Tsunekawa Koichi
NTT Mobile Communications Network Inc.
Wimer Michael C.
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