Communications: radio wave antennas – Antennas – Microstrip
Reexamination Certificate
1990-01-11
2001-01-30
Blum, Theodore M. (Department: 3662)
Communications: radio wave antennas
Antennas
Microstrip
Reexamination Certificate
active
06181277
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to antennas and more particularly to microwave antennas that are used in a global positioning system.
A global positioning system (GPS) requires the use of a dual frequency circularly polarized antenna. GPS antennas range from fixed beam antennas to phased arrays to adaptive arrays. A common feature for all GPS antenna systems is an antenna element with the following requirements: dual frequency operation of a low frequency F
l
having a low central frequency of 1.227 Ghz, a high frequency F
h
having a high central frequency of 1.575 Ghz.
The bandwidth for both F
l
and F
h
is 20 Mhz. Polarization is right hand circular, axial ratio is not specified, the coverage is hemispherical, and the gain at 0° is 0.0 dBci and at 80° a −6.0 dBci.
Antenna elements that can meet these requirements are (1) dual frequency microstrip patch elements, (2) spiral elements and (3) volute elements. In some applications, there is an additional requirement for an antenna element with a low monostatic scattering in the region of 45° to 90° from zenith. Typical frequencies for the low monostatic scatterings are 500 Mhz to 18 Ghz.
The volute antenna element with a high profile has unacceptable monostatic scattering and therefore, does not satisfy the above stated requirements.
FIGS. 7 and 8
are illustrations showing a dual frequency microstrip patch antenna element of the prior art system and in particular a ground plane
7
is provided over which a low profile antenna element that includes a first microstrip patch
1
and a second microstrip patch
3
is mounted. Terminals
9
and
5
connect the dual frequencies between the antenna element and a receiver (not shown). In the embodiment of
FIG. 8
, the microstrip patch
1
is separated from the ground plane
7
by a honeycomb spacer
11
. Similarly, the microstrip patch
3
is separated from the microstrip patch
1
via honeycomb spacer
13
. The overall assembly is covered by a nonconductive radome
15
.
The antenna element in
FIGS. 7 and 8
has a moderate profile above the ground plane
7
and accordingly this antenna does not have low monostatic scattering.
The prior art spiral element antenna has marginal gain and does not meet the scattering requirement. A commonly used approach to reduce monostatic scattering for an antenna element is to recess the element into the ground plane and to cover the element with sufficient microwave absorbing material. However, this approach excessively reduces the antenna gain.
SUMMARY OF THE INVENTION
A dual frequency antenna is mounted into a ground plane with an opening having a first predetermined length and width and under which a container having a second length, width and also depth is mounted. The dimensions of the opening and container are selected according to the operating characteristics of the antenna. The cavity includes four sides and a bottom plate. A first microstrip element is mounted in the container in alignment with the ground plane in the opening and it has a length and width less than the first predetermined length and width. The second length and width of the microstrip element is selected to optimumly perform at a first operating frequency. A second microstrip element is mounted in the container and is separated from the bottom plate by a preselected difference and separated from the first microstrip element by a second preselected distance and its operating frequency is selected to be less than the first operating frequency. A plurality of feed probes having an appropriate phase arrangement are connected to the first microstrip element. A power divider network is operatively connected to the feed probes and either conducts to or receives power from the feed probes.
The dual frequency antenna has a dielectric material in the container between the first microstrip element and second microstrip element and between the second microstrip element and the bottom plate. The dual frequency feed probes are brought in to maintain a 90° phase shift between each feed probe.
The first microstrip element is centrally positioned in the opening in the ground planes such that the separation between the edge of the first plate and the opening is equal to one half the difference between the first length and the second length.
It is the object of the invention to provide a dual element antenna that reduces scattering from the structure of the antenna and provides for an antenna that is flush mounted within a conductive ground plane and uses higher dielectrics than air in the inner part of the antenna element which is preferably higher than 2.0 dielectric coefficients.
It is another object of the invention to provide a dual element antenna which reduces monostatic scattering and includes a dual element antenna in which the dual elements are designed to have a narrow bandwidth with each element being design at an optimum frequency rather than a single band that covers both frequencies.
It is yet another object to provide additional monostatic scattering reduction in an antenna system in which a dual element antenna is flushed mounted within a conductive ground plane and is covered with a thin layer of magnetic microwave absorber.
These and other features and advantages of the invention will become more apparent from a reading of the specification in conjunction with the drawings in which:
REFERENCES:
patent: 4138684 (1979-02-01), Kerr
patent: 4170013 (1979-10-01), Black
patent: 4218682 (1980-08-01), Yu
patent: 4320402 (1982-03-01), Bowen
patent: 4369447 (1983-01-01), Edney
patent: 4510498 (1985-04-01), Mori et al.
patent: 4605932 (1986-08-01), Butscher et al.
patent: 4660048 (1987-04-01), Doyle
patent: 4733245 (1988-03-01), Mussler
patent: 4783661 (1988-11-01), Smith
patent: 0174068 (1986-03-01), None
patent: 2005922 (1979-04-01), None
patent: 0059605 (1983-04-01), None
Kesler Oren Byrl
Watson Thomas Stanley
Baker & Botts L.L.P.
Blum Theodore M.
Raytheon Company
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