Communications: radio wave antennas – Antennas – High frequency type loops
Reexamination Certificate
2001-05-24
2001-11-27
Phan, Tho G. (Department: 2821)
Communications: radio wave antennas
Antennas
High frequency type loops
C343S741000, C343S745000
Reexamination Certificate
active
06323814
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to high frequency, loop antennas and, particularly, to such antennas having a series reactance in the loop.
BACKGROUND OF THE INVENTION
In the past, efficient antennas have typically required structures with minimum dimensions on the order of a quarter wavelength of the lowest operating frequency. These dimensions allowed the antenna to be excited easily and to be operated at or near a resonance, limiting the energy dissipated in impedance losses and maximizing the transmitted energy. These antennas tended to be large in size at the resonant wavelength, and especially so at lower frequencies.
DISCUSSION OF THE RELATED ART
In order to address the shortcomings of traditional antenna design and functionality, the meander line loaded antenna (MLA) was developed. One such antenna is disclosed in U.S. Pat. No. 5,790,080 for MEANDER LINE LOADED ANTENNA, issued to John T. Apostolos, the inventor of the present application, the contents of which are hereby incorporated by reference.
The aforementioned U.S. Pat. No. 5,790,080 describes an antenna that includes two or more conductive elements acting as radiating antenna elements, and a slow wave meander line adapted to couple electrical signals between the conductive elements. The meander line has an variable physical length which affects the electrical length and operating characteristics of the antenna. The electrical length of the meander line, and therefore the antenna, may be readily controlled.
A typical MLA
100
, as shown in
FIG. 1
includes two, spaced-apart vertical conductors
102
and a horizontal conductor
104
. The vertical and horizontal conductors are separated by gaps
106
, which are bridged by meander lines
108
. Meander lines
108
include a slow wave structure having sequential sections with alternating high and low impedance values, which structure provides an electrical length that is greater than its physical length.
Meander line
108
is characterized by a plurality of series connected sections
110
,
112
. Sections
110
,
112
are alternately sequentially connected and are designed to have respective high and low characteristic impedance values, which impedance values are consequently alternated by the alternating sequential connection. These alternating impedance values create a slow wave structure having an effective electrical length that is greater than the actual physical length. This impedance structure may be formed by a transmission line having sections which alternate in their separation from a ground plane. In
FIG. 2
, high impedance sections
110
are suspended above the top surface of a dielectric sheet
114
and low impedance sections are formed as conductors directly on the top surface of dielectric sheet
114
. Placing the dielectric sheet against a planar conductor creates the different impedance values because the planar conductor acts as an effective ground plane. In the antenna
100
of
FIG. 1
, the vertical conductors
102
are used to create that ground plane for meander lines
108
.
Meander lines
108
are also designed to allow adjustment of their length. The slow wave structure permits lengths of the meander line to be switched in or out of the circuit quickly and with negligible loss, in order to change the effective length of the antenna. This switching is possible because the active switching devices are always located between the high and low impedance sections of the meander line. This keeps the current through the switching device low and results in very low dissipation losses in the switch, thereby maintaining high antenna efficiency.
FIG. 3
, shows four typical operating modalities for the MLA
100
in combination with the meander line
108
. The operating frequency and meander line lengths are alternatively shown as quarter wavelength, 1/2&lgr;, 1&lgr;, and 3/2&lgr;. The simple, basic MLA can be operated in a loop mode that provides a “Figure eight” coverage pattern. Horizontal polarization, loop mode, may be obtained when the antenna is operated at a frequency such that the electrical length of the entire line, including the meander lines, is a multiple of a full wavelength. The antenna can also be operated in a vertically polarized, monopole mode, by adjusting the electrical length to an odd multiple of a half wavelength at the operating frequency. The meander lines can be tuned using electrical or mechanical switches to change the mode of operation at a given frequency using a given mode.
The MLA allows the physical dimensions of antennas to be significantly reduced while maintaining an electrical length that is still a multiple of a quarter wavelength. Antennas and radiating structures built using this design operate in the region where the limitation on their fundamental performance is governed by the Chu-Harrington relation. Meander line loaded antennas achieve the efficiency limit of the Chu-Harrington relation while allowing the antenna size to be much less than a quarter wavelength at the frequency of operation. Height reductions of 10 to 1 can be achieved over quarter wave monopole antennas while achieving comparable gain.
The prior art MLA antennas have relatively narrow instantaneous bandwidth. Although the switchable meander line allows the antennas to have a very wide tunable bandwidth, the bandwidth available for simultaneous use is relatively limited. Thus for multi-band or multi-use applications and for applications where signals can appear unexpectedly over a wide frequency range, existing MLA antennas are somewhat limited.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to provide a meander line loaded antenna (MLA) having a wide instantaneous bandwidth.
It is a still further object of the invention to provide an MLA having an instantaneous bandwidth of 7:1.
Accordingly, a wide band, meander line loaded antenna includes a first planar conductor extending orthogonally from a ground plane, a signal coupling device connected to the first planar conductor proximally to the ground plane, a second planar conductor substantially parallel to the ground plane and separated from the first planar conductor by a gap, a meander line interconnecting the first and second planar conductors across the gap, and a third conductor connecting the second planar conductor to ground.
The meander line loaded antenna may also include a fourth conductor connected to the second planar conductor and extending toward the first planar conductor for enhancing capacitance there between.
Alternatively, the present antenna may be arranged in opposed pairs, and also as two orthogonally opposed pairs for enabling circular polarization.
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Asmus Scott J.
BAE Systems Information and Electronic Systems Integration INC
Maine Vernon C.
Phan Tho G.
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