Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
1996-05-28
2002-05-07
Wimer, Michael C. (Department: 2821)
Communications: radio wave antennas
Antennas
Microstrip
Reexamination Certificate
active
06384785
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to antennas, used for transmitting and receiving radio signals in the frequency range of microwave to millimeter-wave bands, having integral power lines and high frequency circuits for signal processing. The present invention relates, in particular, to a microstrip antenna of compact and high-flexbility design, comprising a plurality of radiation elements, provided in a common unit, for use in different frequency bands.
Such an antenna would find applications in personal communication systems including entry-control and security cards, terminals for ATM wireless access (AWA), remote-access terminal and others. Because of its compact and low-profile features, the antenna is also suitable for use in interior LAN, needing a multi-band system to overcome the operational problems of fading and shadowing caused by multi-pass interference being experienced in such a network.
2. Description of the Prior Art
Microstrip antennas are being used for radar, mobile and satellite communication systems, because of their compact, thin and light weight features.
Microstrip antennas are particularly suitable for use as active antennas. Active antenna is an antenna having all of the necessary components, such as an antenna element, a feeding circuits, active devices or active circuits, integrally provided on a monolithic substrate, thus producing a compact, low cost and multi-function antenna equipment.
FIG. 14A
is a perspective view of an example of the configuration of the conventional microstrip antennas, and
FIG. 14B
is a cross sectional view through a plane at A—A in FIG.
14
A. The device includes an radiation element
102
, a ground plane
101
and a dielectric film member
100
. The radiation element
102
, the ground plane
101
and the dielectric film member
100
constitute a microstrip antenna
104
. A strip conductor
103
, together with the ground plane
101
and the dielectric film member
100
constitute a microstrip line
105
. A signal propagated through the microstrip line
105
couples with the antenna element
102
in accordance with the electromagnetic field generated by the microstrip line
105
and feeds the microstrip antenna
104
. The microstrip
104
is a type of resonators, and the generated radio waves are radiateted into the free space.
However, in this type of microstrip antenna, the microstrip line
105
locates in the direction of radiation of the waves with respect to the ground plane
101
, leading to an undesirable problem that the unnecessary radiation from the microstrip line adversely affects the radiation field of the microstrip antenna.
To resolve this problem, there has been a suggestion for a different configuration of microstrip antenna such as the one presented in
FIGS. 15A
,
15
B.
FIG. 15A
is a perspective view of another example of the conventional microstrip antennas, and
FIG. 15B
shows a cross sectional view through a plane at A—A in FIG.
15
A. The device includes an radiation element
112
, a ground plane
111
and a first dielectric film member
110
. The radiation element
112
, the ground plane
111
and the first dielectric film member
110
constitute a microstrip antenna
114
.
The device also has a strip conductor
113
and a second dielectric film member
115
. The strip conductor
113
, the second dielectric film member
115
and the ground plane
111
constitute a microstrip line
116
. Also, a slot
117
is fabricated on the ground plane
111
.
The signal propagated through the microstrip line
116
couples with the radiation element
112
in accordance with the electromagnetic field generated by the microstrip line
116
through the slot
117
, and feeds the microstrip antenna
114
.
The unnecessary radiation from the microstrip line
116
in the microstrip antenna shown in
FIGS. 15A and 15B
generates little adverse effects on the radiation field of the microstrip antenna
114
, because the ground plane
111
intervenes and blocks the parasitic signals from the microstrip line
116
(acting as the power line) affecting the performance of the radiation element
112
.
The properties of the microstrip antennas shown in
FIGS. 14A
,
14
B and
FIGS. 15A
,
15
B can be obtained from the dielectric constant, the dielectric dissipation factor (tan&dgr;) and the thickness (h) of the first dielectric film substrate
110
(or
100
), and the conductivity (&sgr;) of the radiation element
102
or
112
.
FIG. 16
is a graph showing the relationship between the radiation efficiency and no-load Q of a circular microstrip antenna without considering the effect of the surface wave (refer to K. Hirasawa and M. Haneishi, “Analysis, Design, and Measurement of Small and Low-Profile Antennas”, Artech House, Norwood, Mass. 02062).
In this case, if S is the voltage standing wave ratio (VSWR), the bandwidth BW of the microstrip antenna is given by the following equation:
BW=(S−1)/Q
0
S
0.5
.
Therefore, the bandwidth of an antenna is inversely proportional to no-load Q, and, to obtain high performance properties in an antenna (high radiation efficiency, wide bandwidths etc.),
FIG. 16
shows that it is preferable to have a thick dielectric film.
However, when the thickness h exceeds a certain value, the antenna performance becomes degraded because the loss caused by surface waves can no longer be ignored and higher order excitations are generated in the thickness direction. Therefore, in designing a high-performance antenna, the thickness of the dielectric film is the most important parameter. For microstrip antennas in general, the thickness of the dielectric film is chosen in the range of {fraction (1/50)}th to {fraction (1/20)}th of the free-space wavelength of the center frequency.
Although microstrip antennas are convenient and advantageous in many respects, they operate in a single frequency band and has a high Q-value, and therefore, if they are to find a wider application possibilities, increased bandwidth of the antenna is mandatory.
An attempt has been made to solve these problems by developing a dual-frequency microstrip antenna which is shown in
FIGS. 17A
,
17
B.
FIG. 17A
shows a plan view of a dual-frequency microstrip antenna, and
FIG. 17B
shows a cross sectional view through a plane at A—A in FIG.
17
A. The antenna shown in this drawing is made by laminating two dielectric films
100
having one radiation element
102
between the two films and another radiation element
102
of the same size above the top film
100
. Operating power is supplied to each of the radiation elements
102
through a power pin
200
formed on a ground plane
101
which is disposed opposite and away from the radiation element.
In this device, the boundary conditions of the electric and magnetic field components of the top and bottom dielectric films
100
coupled to each of the radiation elements
102
(disposed above and below the dielectric film member
100
) are different, thereby providing different equivalent dielectric constants to enable the antenna to perform as a dual-frequency microstrip antenna.
More specifically, denoting the dielectric constant for the upper dielectric film
100
by &egr;r1 and that for the lower dielectric film
100
by &egr;r2, the equivalent dielectric constant of the upper radiation element can be approximated by (&egr;r1+1)/2 and that for the lower radiation element by (&egr;r1+&egr;r2)/2.
For popular film base material such as teflon or polytetrafluoroethylene (PTFE) whose relative dielectric constants &egr;r are about 2.55, there is little change in the equivalent dielectric constant, and the two radiation elements resonate in a close range of frequencies.
Therefore, there are cases in which an optimum film substrate thickness for one radiation element is not optimum for the other radiation element, resulting in degradation in the radiation efficiency and bandwidth of the overall antenna.
Another type of microstrip antenna designed to overcome the problems discussed above is shown in
FIGS. 18A
,
Kamogawa Kenji
Tokumitsu Tsuneo
Finnegan Henderson Farabow Garrett & Dunner LLP
Nippon Telegraph and Telephone Corporation
Wimer Michael C.
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