Patent Application
20080136720
The present invention relates to the field of communications, and more particularly, to loop type antennas, circular polarization, dual polarization antennas and related methods. It also relates to traveling wave antennas.
The use of satellite communications has increased the demand for circularly polarized antennas and for dual polarization antennas. For instance, many of the satellite transponders in use today carry two programs on the same frequency by using separate polarizations. Thus, single antenna structure may be called upon to simultaneously receive two polarizations, or perhaps to transmit in one polarization and receive in another. The single antenna structure must therefore separate the two polarization channels, to a high degree of isolation.
It is possible to have dual linear or dual circular polarization channel diversity. That is, a frequency may be reused if one channel is vertically polarized and the other horizontally polarized. Or, a frequency can also be reused if one channel uses right hand circular polarization (RHCP) and the other left hand circular polarization (LHCP). Polarization refers to the orientation of the E field in the radiated wave, and if the E field vector rotates in time, the wave is then said to be rotationally or circularly polarized.
Smaller, lighter, lower power receivers are now being developed to satisfy a variety of operational needs. For example, a small, lightweight, low-power, 4-channel satellite receiver (PCI computer card) will soon be fielded to meet the needs of many size-constrained platforms. Today, the antenna may be the only piece of associated equipment that remains to be miniaturized for use in various environments
An electromagnetic wave (and radio wave, specifically) has an electric field that varies as a sine wave within a plane coincident with the line of propagation, and the same is true for the magnetic field. The electric and magnetic planes are perpendicular and their intersection is in the line of propagation of the wave. If the electric-field plane does not rotate (about the line of propagation) then the polarization is linear. If, as a function of time, the electric field plane (and therefore the magnetic field plane) rotates, then the polarization is rotational. Rotational polarization is in general elliptical, and if the rotation rate is constant at one complete cycle every wavelength, then the polarization is circular. The polarization of a transmitted radio wave is determined in general by the transmitting antenna (and feed)—by the type of the antenna and its orientation. For example, the monopole antenna and the dipole antenna are two common examples of antennas with linear polarization. A helix antenna is a common example of an antenna with circular polarization, and another example is a crossed array of dipoles fed in quadrature. Linear polarization is usually further characterized as either Vertical or Horizontal. Circular Polarization is usually further classified as either Right Hand or Left Hand.
The dipole antenna has been perhaps the most widely used of all the antenna types. It is of course possible however to radiate from a conductor which is not constructed in a straight line. Preferred antenna shapes are often Euclidian, being simple geometric shapes known through the ages. In general, antennas may be classified as charge separation or charge conveyance types, corresponding to dipoles and loops, and line and circle structures.
Circular polarization for dipole antennas has been attributed to George Brown, which was described in the literature as “The Turnstile Antenna”, Electronics, 9, 15, April 1936. Approaches to circular polarization in loop antennas appear lesser known, or perhaps even unknown in the purest forms. For instance, the present edition “Antenna Engineering Handbook”, R. Johnson and H. Jasik editors, does not describe methods to obtain circular polarization from loop antennas. In spite of the higher gain of the full wave loop vs. the half wave dipole (3.6 dBi vs. 2.1 dBi), dipoles are commonly used for circular polarization needs, as for instance in turnstile arrays. Both the dipole turnstile and a single loop antenna are planar, in that their thin structure lies nearly in a single plane.
Many structures are described as loop antennas, but canonical loop antennas are a circle. The resonant loop is a full wave circumference circular conductor, often called a “full wave loop”. The typical prior art full wave loop is linearly polarized, having a radiation pattern that is a two petal rose, with two opposed lobes normal to the loop plane, and a gain of about 3.6 dBi. Reflectors are often used with the full wave loop antenna to obtain a unidirectional pattern.
Dual linear polarization (simultaneous vertical and horizontal polarization from the same antenna) has commonly been obtained from crossed dipole antennas. For instance, U.S. Pat. No. 1,892,221, to Runge, proposes a crossed dipole system. A dual polarized loop antenna could be more desirable however, as loops provide greater gain in smaller area. An existing, prior art approach to dual polarization in single loops does not come to mind.
U.S. Pat. No. 5,977,921 to Niccolai, et al. and entitled “Circular-polarized Two-way Antenna” is directed to an antenna for transmitting and receiving circularly polarized electromagnetic radiation which is configurable to either right-hand or left-hand circular polarization. The antenna has a conductive ground plane and a circular closed conductive loop spaced from the plane, i.e., no discontinuities exist in the circular loop structure. A signal transmission line is electrically coupled to the loop at a first point and a probe is electrically coupled to the loop at a spaced-apart second point. This antenna requires a ground plane and includes a parallel feed structure, such that the RF potentials are applied between the loop and the ground plane. The “loop” and the ground plane are actually dipole half elements to each other, and the invention is related to microstrip antennas.
U.S. Pat. No. 5,838,283 to Nakano and entitled “loop antenna for radiating circularly polarized waves” is directed to a loop antenna for a circularly polarized wave. Driving power fed may be conveyed to a feeding point via an internal coaxial line and a feeder conductor is transmitted through an I-shape conductor to a C-type loop element disposed in spaced facing relation to a ground plane. By the action of a cutoff part formed on the C-type loop element, the C-type loop element radiates a circularly polarized wave, Dual linear, or dual circular polarization are not however provided.
U.S. Pat. No. 6,522,302 to Iwasaki and entitled “circularly-polarized antennas” is directed to a circularly polarized antenna array rather than a single circularly polarized loop element. A circle is among the most elemental of antenna structures, and it is the most fundamental single geometry capable of circular polarization.
There is a longstanding requirement then, to obtain circular polarization from a single loop antenna, such as the full wave circumference circular conductor, and to identify a polarization method for loops dual to the turnstile for dipoles. A method is also needed, to obtain dual linear or dual circular polarization from a single loop antenna. Finally, there has been a practical need for a relatively compact loop antenna with dual polarization, linear or circular, such as to meet the requirements of today's multiplexed satellite communications.
In view of the foregoing background, it is therefore an object of the present invention to provide a circularly polarized loop antenna, a dual linearly polarized loop antenna, and a dual circularly polarized loop antenna with an increase in gain and decrease in size.
This and other objects, features, and advantages in accordance with the present invention are provided by a circularly polarized loop antenna comprising a loop electrical conductor, and two signal feedpoints along the loop electrical conductor and separated by one quarter of a length of the loop electrical conductor for a signal feedpoint phase angle input difference of 90 degrees. Each of the signal feedpoints preferably includes a series signal feedpoint so that at least one signal source coupled thereto provides circularly polarization for the loop electrical conductor.
The series signal feedpoint preferably defines a discontinuity in the loop electrical conductor. The loop electrical conductor may comprise a circular electrical conductor. At least one of the signal feedpoints may further include a two-wire transmission line connected in series at the respective series signal feedpoint. Also, a feed structure may include a direct leg and a phase inducing leg connected in parallel to the series signal feedpoints. The loop electrical conductor may have a length equal to one wavelength.
Another aspect of the invention is directed to a circularly polarized loop antenna including a loop electrical conductor, and a plurality of signal feedpoints along the loop electrical conductor. Each of the signal feedpoints may include a series signal feedpoint so that at least one signal source coupled thereto provides circular polarization for the loop electrical conductor. The series signal feedpoint may define a discontinuity in the loop electrical conductor, and the loop electrical conductor may be a circular electrical conductor.
Also, each of the signal feedpoints may further comprise a two-wire transmission line connected in series at the respective series signal feedpoint. A feed structure may include a direct leg and a phase inducing leg connected in parallel to the series signal feedpoints. The antenna may be circularly polarized, and the loop electrical conductor may have a length equal to one wavelength.
A method aspect of the invention is directed to making a circularly polarized loop antenna forming an electrical conductor into a loop, and forming a plurality of signal feedpoints along the loop electrical conductor. Each of the signal feedpoints comprises a series signal feedpoint so that at least one signal source connected to the series feedpoints provides circular polarization for the loop electrical conductor.
Forming the plurality of signal feed points may include forming discontinuities in the loop electrical conductor, and forming the loop electrical conductor may comprise forming a circular electrical conductor. Providing the plurality of signal feed points may further comprise connecting a two-wire transmission line in series at the respective series signal feedpoint. A feed structure may be connected including a direct leg and a phase inducing leg connected in parallel to the series signal feedpoints. The loop electrical conductor may have a length equal to one wavelength.
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in alternative embodiments.
Referring initially to
Each of the signal feedpoints 14, 16 may be a series signal feedpoint so that at least one signal source 18, 20 coupled thereto provides circular polarization from the loop electrical conductor 12. In other words, the signal feedpoints 14, 16 are in series in the loop electrical conductor 12. Each of the series signal feedpoints 14, 16 preferably defines a discontinuity in the loop electrical conductor 12. Each of the signal feedpoints 14, 16 may have two terminals 40, as is typical.
An example of a signal feeding approach will be described with additional reference to
Both polarization senses are possible with this invention by controlling the phasing (e.g. reversing the phasing). The circularly polarized loop antenna has a current distribution that is uniform in amplitude and linear in phase, and the present invention is an antenna of the traveling wave type. The uniform current distribution produces a high, e.g. maximum, possible gain for area, without superdirectivity. An example of the performance of a circularly polarized loop antenna in accordance with the present invention is summarized in the table below.
The radiated field of the present invention in the Z direction (normal to the loop plane) has a constant magnitude over time which is described by
E=(cos2 ωt+sin2 ωt)1/2=1
which is the necessary condition for circular polarization.
A method aspect of the invention is directed to making a circularly polarized loop antenna 10 by forming an electrical conductor into a loop 12, and forming a plurality of signal feedpoints 14, 16 along the loop electrical conductor. Each of the signal feedpoints 14, 16 comprises a series signal feedpoint so that at least one signal source 18, 20 connected to the series feedpoints provides circular polarization for the loop electrical conductor 12.
Forming the plurality of signal feed points 14, 16 may include forming discontinuities in the loop electrical conductor 12, and forming the loop electrical conductor may comprise forming a circular electrical conductor. Providing the plurality of signal feed points 14, 16 may further comprise connecting a two-wire transmission line 32 in series at the respective series signal feedpoint. A feed structure 30 may be connected including a direct leg 37 and a phase inducing leg 38 connected in parallel to the series signal feedpoints 14, 16. The loop electrical conductor 12 may have a length equal to one wavelength, that is, the circumference of loop electrical conductor 12 may be one wavelength.
Referring now to
The dual linear loop antenna 10′ includes a loop electrical conductor 12′, e.g. a circular electrical conductor. The loop electrical conductor 12′ may be a conductive wire, tubing, trace etc., and the circumference is preferably equal to one wavelength. Two signal feedpoints 14′, 16′ are along the loop electrical conductor and separated by one quarter of a length of the loop electrical conductor. One signal feedpoint 14′ may be referred to as the vertical polarized port and include a signal source 18′ connected in series in the loop electrical conductor 12′. The other signal feedpoint 16′ may be referred to as the horizontal polarized port and include a signal source 18′ connected in series in the loop electrical conductor 12′.
Each of the signal feedpoints 14′, 16′ is a series signal feedpoint and the signal sources 18′, 20′ coupled thereto provide the simultaneous vertical and horizontal polarization for the loop electrical conductor 12′. Each of the series signal feedpoints 14′, 16′ preferably defines a discontinuity in the loop electrical conductor 12′. Each of the signal feedpoints 14′, 16′ may have two terminals 40′, as is typical.
Referring now to
The
Referring again to the
The
If a 0 degree type hybrid is substituted for quadrature hybrid 60″, in the
The circularly polarized loop antenna radiates a circularly polarized wave because the traveling wave current distribution is conveyed circularly. That is, although loop electrical conductor 12 does not physically rotate, the charge distribution upon it rotates. It does so linearly in time at an angular rate of 2πf radians/second. Prior art traveling wave antennas generally have been linear rather then circular structures, radiating linear rather than circular polarization. The conventional dipole turnstile has a rotating charge separation between the dipole arms.
A unidirectional radiation is desirable for many purposes.
Cup reflectors are well suited for the present invention as well, as are common with dipole turnstiles. Loop reflectors and loop directors may also be configured, with the present invention being the driven element, to form a circularly polarized Yagi-Uda type parasitic array.
The elevation (XZ plane) cut radiation pattern for the circularly polarized loop antenna embodiment of the present invention is compared with that of a conventional ½ wave dipole turnstile antenna in
Exact resonance in the present invention circularly polarized loop antenna occurs at slightly larger than 1 wavelength nominal circumference. For small diameter loop conductors, it has been found to be 1.03 to 1.04 wavelengths. This is in reverse to thin half wave dipoles, where exact resonance may occur at slightly shorter than nominal length, such as 0.47 or 0.48 wavelengths.
Although 1 wavelength circumference is a preferred embodiment for loop 12, the invention continues to produce circular polarization for smaller loop diameters.
The present invention is not so limited as to require discontinuities in the loop conductor at signal feed points 14, 16, and other signal feed approaches may be used, as for example, shunt feeding. The gamma or Y match are suitable shunt feeds, as are common in dipole and yagiuda antenna practice.
Inset feed approaches may also be used to form signal feed points 14, 16. For instance, loop electrical conductor 12 may be made of coaxial cable, and the radiating current a common mode current on the outside of a coaxial cable loop. The coax cable braid may be spread, but not severed, to bring the center conductor out at the desired location, and the signal feed points 14, 16 formed by a discontinuity the coaxial cable loops outer conductor.
Other loop shapes may be substituted in the present invention, with qualitatively similar results. For instance the full wave circular loop may be made square, with ¼ wavelength sides, or even triangular. If used with a square reflector or directors, the present invention can form circularly polarized version of the Quad Antenna, as is common in amateur radio.
Three embodiments of the present invention have been described then: the circularly polarized loop antenna, the dual linearly polarized loop antenna, and the dual circularly polarized loop antenna. As will be apparent to those skilled in the art, various switching schemes may be employed to reconfigure the embodiments of present invention, so that linear or circular polarization, with single or dual orthogonal channels, can be obtained at will from a single loop radiating element.
Accordingly, a multiple polarization loop antenna is provided with an increase in gain and decrease in size. The antenna according to the present invention uses two feedpoints in series in the loop conductor. The antenna uses quadrature phasing (zero and ninety degrees) at the feedpoints and is a true loop antenna that does not require a ground plane. The antenna preferably has a traveling wave current distribution, providing a high, e.g. maximum, possible gain for the area.
Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.
