WO1998027614A1

WO1998027614A1 - Antenna with diversity transformation

Info

Publication number: WO1998027614A1
Application number: PCT/US1997/023256
Authority: WO
Inventors: Peter Mailandt; Tan D. Huynh
Original assignee: Allen Telecom Inc.
Priority date: 1996-12-18
Filing date: 1997-12-15
Publication date: 1998-06-25
Also published as: TW382833B

Abstract

An antenna (100) comprising a first antenna array (101) having a first polarization, a second antenna array (103) having a second polarization different from the first polarization and orthogonal thereto. A quadrature hybrid (105) for transforming the polarizations of the first and second antenna arrays to third and fourth different polarizations that are orthogonal to each other and are different from the first and second polarizations. The third and fourth polarizations comprise +45 degree polarization and -45 degree polarization.

Description

ANTENNA WITH DIVERSITY TRANSFORMATION

FIELD OF THE INVENTION

This invention relates generally to antennas and in particular to antenna systems for polarization diversity, and is more particularly directed toward an antenna system configured for diversity operation at first and second polarizations and including a transformation to third and fourth polarizations.

BACKGROUND OF THE INVENTION

Antennas designed for operation in cellular radiotelephone systems and personal communication systems (PCS) are often configured to serve only a single sector of a cell . Such antennas are of en implemented as panel antennas because control of horizontal beamwidth is more easily implemented in a panel configuration, thus making the panel antenna nearly ideal for sectorized cell sites.

Because cell sites in both cellular and PCS applications serve primarily mobile and portable subscribers, relative motion between the cell site and the subscriber units must be taken into account. In urban environments, there may also be regions within each cell site where coverage is difficult because of tall buildings or other obstructions . Such obstructions can give rise to multipath transmissions and result in interference. One way to address such coverage difficulties is to provide diversity reception. Diversity refers to the concept of providing more than one antenna for communication between the cell site and the subscribers. Usually, diversity is provided for the receive path only, although transmit diversity can also be useful in addressing certain propagation problems. Diversity is also generally provided at the cell site only, but many mobile subscribers implement diversity reception via the provision of multiple vehicle antennas. The multiple antenna approach often observed on vehicles is generally a form of spatial diversity. Spatial diversity is the physical displacement of a plurality of antennas from one another to provide multiple paths for communication signals. Special switching arrangements are provided to sample the signals from each of the plurality of antennas and to decide which signal is the best by applying predetermined signal quality metrics.

When the best signal is found, the antenna associated with the best signal is used for reception until the next sampling and testing cycle. These sampling and testing cycles are repeated at frequent intervals, often separated by only a few milliseconds. Although, as noted above, transmit diversity is sometimes employed, receive diversity is the most commonplace. When spatial receive diversity is employed using two antennas, a single antenna is generally selected for transmission, and the two antennas are sampled in sequence for the better receive signal.

Spatial diversity is particularly useful in solving propagation problems resulting from multipath where there is interfering cancellation at one antenna, but not at another antenna displaced a short distance away from the first . Spatial diversity does not solve all types of propagation problems , however .

Other types of diversity, such as polarization diversity, can be brought into play to address other kinds of communication difficulties. Polarization, of course, refers to the orientation of the electric field (E-field) of a transverse electromagnetic (TEM) wave with respect to the earth's surface. A half-wave dipole oriented perpendicular to the earth's surface will exhibit vertical polarization, and the same dipole antenna rotated so that it is parallel to the surface of the earth is said to be horizontally polarized.

Antenna systems that include vertically and horizontally polarized arrays are known for cellular and PCS communication systems. Often, the vertically polarized array is used for transmitting, with diversity switching provided between the vertical and horizontal arrays to provide polarization diversity for the receive path. However, this configuration does not provide optimum coverage for some types of communication units.

Subscribers using portable communication units are often careless of the orientation of the antenna during a conversation. Those who are used to communicating over portable two-way radios, such as police, fire protection personnel, etc., will often consciously attempt to maintain a vertical antenna orientation during radio communication. This is not true for members of the general public, however, who use portable cellular and PCS units just as though they were landline telephone sets, and are often unaware that radio communication is occurring as a consequence of the telephone calls they place and receive with their cellular and PCS "phones . "

These users often have the antenna oriented at a 45 degree angle with respect to the earth's surface. This is something of a "natural" angle considering the design of modern portable units . With the unit held to the ear in normal conversation position, the antenna is oriented at about 45 degrees with respect to the horizontal.

Polarization diversity using vertically and horizontally polarized antennas does, in fact, address a significant number of propagation problems. However, for communicating with a subscriber population consisting largely of portable units, polarization diversity with antennas oriented at plus and minus 45 degrees is very effective. Accordingly, a need arises for an antenna design that is usable not only in a vertical and horizontal polarization diversity mode or in a plus-or-minus 45 degree polarization diversity mode, but also usable in a vertical polarization mode for transmission of RF

(radio frequency) signals and a plus-or-minus 45 degree polarization mode for receiving RF signals . Such an antenna should be easily reconfigurable from one polarization orientation to another, simple and economical to manufacture, and both light and sturdy for flexibility in mounting on towers or buildings, both inside and out.

SUMMARY OF THE INVENTION

These needs and others are satisfied by the antenna of the present invention, which comprises a first antenna array having a first polarization, a second antenna array having a second polarization different from the first polarization and orthogonal thereto, and means for transforming the polarizations of the first and second antenna arrays to third and fourth different polarizations that are orthogonal to each other and are different from the first and second polarizations. In one form of the invention, the first antenna array is a vertically polarized antenna array, and the second antenna array is a horizontally polarized antenna array . In one aspect of the invention, the means for transforming the polarizations of the first and second antenna arrays comprises a quadrature hybrid. The first and second polarizations are vertical and horizontal and the third and fourth polarizations comprise +45 degree polarization and -45 degree polarization. In the preferred embodiment of the invention, the antenna further comprises a first substrate having horizontally and vertically oriented antenna elements disposed thereon, and a second substrate spaced from the first substrate, the second substrate having apertures disposed on a first side thereof and antenna feed lines disposed on a second side thereof, such that the feed lines are coupled through the apertures to the antenna elements.

In one form of the invention, . the antenna further includes an extrusion of conductive material providing a backplane and having means for mounting the second substrate thereto. The first substrate is spaced apart from and held substantially in alignment with the second substrate by a plurality of spacers . The extrusion may further include first and second opposed extension panels for extending the horizontal beamwidth of the antenna.

The antenna may further include filter means interposed between the second antenna array and the means for transforming the polarizations. The filter means may comprise a ceramic block combline bandpass filter. The antenna may also include a duplexer means having a first path interposed between the first antenna array and the means for transforming the polarizations, and a second path interposed between the first antenna array and a transmit signal input . In a preferred form, the duplexer means comprises a first ceramic block combline bandpass filter having a first center frequency and coupled to a second ceramic block combline bandpass filter having a second center frequency. The quadrature hybrid may comprise a four-port network of four one-quarter wavelength microstrip transmission lines interconnected in a rectangular geometry.

In another aspect of the invention, an antenna configured for diversity operation and having one transmit signal input connector and two receive signal output connectors comprises a vertically polarized antenna array, a horizontally polarized antenna array, a quadrature hybrid having first and second input ports and first and second output ports, a bandpass filter interposed between the horizontally polarized antenna array and the first input port of the quadrature hybrid, and a duplexer having a bidirectional port, an input port, and an output port, the duplexer interposed between the vertically polarized antenna array and the second input port of the quadrature hybrid, such that the duplexer bidirectional port is coupled to the antenna array, the duplexer output port is coupled to the second input port of the quadrature hybrid, and the duplexer input port is coupled to a transmit signal input connector of the antenna. The quadrature hybrid first output port provides an antenna output signal having +45 degree polarization, and the quadrature hybrid second output port provides an antenna output signal having -45 degree polarization .

Further objects, features, and advantages of the present invention will become apparent from the following description and drawings .

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an antenna in accordance with the present invention in block diagram form; FIG. 2 is a top plan view of a quadrature hybrid;

FIG. 3 is a partial exploded perspective view of an antenna in accordance with the present invention;

FIG. 4 is a plan view of a circuit board including the antenna elements shown in FIG. 1; FIG. 5 is a bottom plan view of a circuit board including antenna feed circuitry in accordance with the present invention;

FIG. 6 is a top plan view of the circuit board of FIG. 5 ; FIG. 7 is a partial assembled perspective view of the antenna of FIG. 3; and

FIG. 8 is an end view of the antenna of FIG. 3. DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, an antenna with diversity transformation is described that provides distinct advantages when compared to antennas of the prior art . The invention can best be understood with reference to the accompanying drawing figures .

FIG. 1 illustrates antenna 100 of the present invention in block diagram form. A vertically polarized antenna array 101 is illustrated, having antenna elements 102 interleaved with a horizontally polarized array 103 , made up of individual antenna elements 104. In many systems, an antenna . of this type would be employed in a diversity arrangement, with the vertically polarized antenna array 101 used for transmit, and both the vertically and horizontally polarized arrays 101, 103 used in receive mode.

In order to accomplish the transformation, the antenna

100 uses a quadrature hybrid to transform a simple panel antenna having vertical and horizontal polarizations into an antenna system with equivalent polarizations of +45 degrees and -45 degrees. This is a particularly effective arrangement for systems having a large number of portable units, and where antenna orientation in use is seldom vertical. The vertically and horizontally polarized arrays 101, 103 are connected as shown to a quadrature hybrid 105. The hybrid 105 is connected through a bandpass filter 106 covering the receive bandwidth of the antenna system in order to provide isolation between transmit and receive signals and to compensate for any phase distortion introduced when the vertically polarized signal passes through an associated duplexer 107. Preferably, the bandpass filter 106 is a ceramic block combline bandpass filter, although other types of filter implementations, such as a microstrip line bandpass filter, or a cavity bandpass filter, may also be satisfactory. The duplexer 107 is of conventional design and is inserted between the transmit signal input connector 108 and the vertically polarized array 101. The reverse path through the duplexer goes to port 3 of the hybrid 105. Preferably, the duplexer may be formed from a first ceramic block combline bandpass filter having a first center frequency and coupled to a second ceramic block combline bandpass filter having a second center frequency, all in accordance with the prior art. The first center frequency is the transmit frequency of the antenna, while the second center frequency is the receive frequency. The point of connection of the two filters defines the bidirectional port of the duplexer, with the input and output ports at the distal ends of the first and second filters, respectively. Of course, the bandpass filters that form the duplexer may also be microstrip line bandpass filters or cavity bandpass filters, for example.

The quadrature hybrid 105 is illustrated in greater detail in FIG. 2. Quadrature hybrids are known and operate generally as described below. The hybrid 105 is designed to operate in a system where the characteristic impedance (Z_Q) of the interconnecting transmission lines is 50 ohms. The hybrid 105 is also preferably designed to divide signal power equally at the output ports, so the shunt transmission line sections 201, 202 are designed to have a 50 ohm impedance. The series transmission line sections 203, 204 have a 35 ohm impedance. Each of the transmission line sections 201-204 is one-quarter wavelength in electrical length at the frequency of interest, and is constructed using known microstrip transmission line techniques. The quadrature hybrid transmission lines are interconnected in a rectangular geometry as shown.

In the configuration illustrated in FIG. 2, if an input signal is applied to port 4, it will be coupled to ports 1 and

2, but port 3 will be isolated. Since the hybrid is symmetrical, if an input signal is applied to port 3, it will be coupled to ports 1 and 2, but port 4 will be isolated. For an input signal applied to port 4, the output signal at port 2 will lag the output signal at port 1 by 90 degrees. For an input signal applied to port 3 , the output signal at port 2 will lead the output signal at port 1 by 90 degrees. The phase shifts and cancellations are due to the 90 degree phase shift induced by the interconnected quarter-wave sections of the hybrid.

In the preferred embodiment of the invention, the input signal to port 4 of the hybrid 105 is derived from the horizontally polarized array 103 , while the signal applied to port 3 of the hybrid 105 is derived from the vertically polarized array 101. These two input signals are out of phase with one another by 90 degrees by virtue of a quarter-wave section 205 of the transmission line of the port 4 input that induces a 90 degree phase shift.

Because of the phase shift around the quadrature hybrid 105, the composite output signal at port 1 is the sum of the input signals at ports 4 and 3. This is because the components of the input signals applied to ports 3 and 4 are in phase at output port 1. The sum of the vertically polarized input signal and the horizontally polarized input signal is a signal having a 45 degree polarization.

The components of the input signals applied to ports 3 and 4 are 180 degrees out of phase at port 2. Thus, the composite output signal at port 2 is the difference between the vertically polarized input signal and the horizontally polarized input signal, or, in other words, a signal having a -45 degree polarization. This operation of the hybrid 105 effectively transforms the polarizations of the signals received by the vertically and horizontally polarized antenna arrays 101, 103 to polarizations of +45 degrees and -45 degrees .

It should be noted that, because of the duplexer, the transmit signal is applied only to the vertically polarized antenna array, so the transmit signal remains vertically polarized.

As shown in FIGS. 3-7, in the preferred form of an antenna of the invention, the antenna 100 is an aperture coupled planar antenna system including both horizontally and vertically polarized arrays. The antenna elements 102, 104 for both the horizontally and vertically polarized arrays 101,

103 are rectangular resonator elements disposed on an insulating substrate 302 that is aligned with, but spaced apart from, a substrate comprising a circuit board 301 that includes the feed lines and apertures.

The substrate 302 that includes the antenna elements 102,

104 is preferably formed from a polyvinylchloride (PVC) sheet about .062 inch in thickness, with the antenna elements 102, 104 disposed thereon. Preferably, the antenna elements 102, 104 are deposited on the PVC substrate 302 by a screening process using a silver-based conductive ink, although other methods of depositing the antenna elements may also be acceptable . Each set of antenna elements 102, 104 is associated with one particular antenna within the array, and each antenna element 102 for the vertically polarized antenna array 101 is identical, as is each antenna element 104 for the horizontally polarized antenna array 103. As will be noted, the antenna elements 102 for the vertically polarized array 101 and the antenna elements 104 for the horizontally polarized array 103 alternate with one another along the length of the substrate 302.

Each antenna element 102 for the vertically polarized antenna array 101 includes a central rectangular resonator element 702 sized to be resonant near the upper frequency limit of the array 101, flanked on either side by two somewhat larger rectangular resonator elements 701 that are designed to resonate near the lower frequency limit of the antenna array passband. Note that the lower circuit board 301 in the antenna system (as shown by FIGS. 5 and 6) includes antenna feed lines 801, 802 and apertures 401, 402 for coupling to the antenna elements 102, 104 that form the actual antenna arrays 101, 103. This lower substrate or circuit board 301 is preferably a fiberglass-filled Teflon material having a thickness of about .062 inch. The feed circuitry is disposed on the side of the circuit board 301 that is closer to the backplane of the extrusion 303. Extrusion 303 provides both the antenna system backplane and a support structure to keep the component circuit boards in place with proper alignment .

The feed circuitry 801, 802 is conventional in design and is laid out using known microstrip transmission line techniques . The lengths of the transmission lines are selected to provide the proper phase relationship between the antenna elements 102, 104 and between the horizontally and vertically polarized antenna arrays 101, 102 themselves. Each of the antenna feed lines 801, 802 is transversely aligned with a rectangular aperture 401, 402 on the opposite side of the circuit board 301. The side of the board 301 on which the rectangular apertures 401, 402 are formed is substantially covered with copper or other conductive material, which has been removed in selected zones, preferably by an etching process, to form rectangular apertures 401, 402 that are substantially devoid of conductive material.

As can be appreciated from an inspection of FIGS. 4-6 in particular, the apertures 401, 402 are transverse in orientation with respect to both the feed lines 801, 802 and the antenna elements 102, 104. Using the vertically polarized antenna array 101 as an example, the feed lines 801 and the antenna elements 102 are substantially parallel to one another, and are aligned such that the longitudinal axis of the feedline 801 is substantially coincident with the longitudinal axis of the interior resonator element 702. Due to space constraints, the feedlines 802 for the horizontally polarized antenna elements 104 each includes a 90 degree bend in the line, which does not affect operation since the bend occurs between the unterminated end of the feed line 802 and the nearest edge of the corresponding aperture 402. This portion of the transmission line 802 serves as a stub that helps compensate for inductance contributed by both the aperture 402 and the antenna element 104 to which it couples, thus resulting in a real input impedance for each antenna element 104. An advantage of the aperture coupling approach is that the antenna elements 102, 104 that form the radiating elements of the antenna are completely isolated from the feed. Spurious radiation from the feed does not degrade side lobe levels or increase cross polarization. For proper coupling between the feed and the antenna elements, the apertures 401, 402 are made slightly larger than resonant dimensions for the frequency band of interest. Of course, aperture resonance cannot be too far from the antenna passband, or insufficient coupling would result . In the preferred embodiment of the invention, the apertures 401, 402 are about 54.6 mm (millimeters) in length (resonant dimension) and about 12.2 mm wide.

It should be appreciated that the quadrature hybrid 105 is implemented in microstrip transmission lines on the same surface of the substrate 301 that includes the feed lines 801, 802 (FIG. 5) . Although the filter 106 and duplexer 107 are not illustrated in FIG. 5 for the sake of clarity, it is intended that the filter 106 and duplexer 107 be mounted mechanically in the vicinity of the quadrature hybrid 105 to facilitate electrical connection. Of course, other mounting contingencies for these components are also workable.

If it is desired to provide an antenna system having vertical and horizontal polarization diversity, the filter 106 and duplexer 107 can simply be eliminated from the antenna 100, and the quadrature hybrid 105 can be bypassed by the installation of coaxial cable jumpers. Thus, the antenna 100 is easily reconfigurable for vertical and horizontal polarization diversity.

Turning now to FIGS. 7 and 8, the extrusion 303 is preferably constructed from a conductive material that has good structural properties but is relatively light in weight, such as aluminum. Slots 307 are preferably provided to accommodate the interior circuit board 301. The PVC circuit board 302 with the antenna elements 102, 104 is held spaced apart from the interior circuit board 301 and in proper alignment by a series of non-conductive spacers (304 in FIGS. 3 and 8) which may be made of nylon. The two circuit boards 301, 302 are preferably maintained substantially parallel to one another at a spacing of about 0.625 inch. In its preferred form, the antenna system has a horizontal beamwidth of about 90 degrees. In order to achieve this beamwidth, extension panels 305, 306 of conductive material are disposed from either side of the extrusion 303. These extension panels 305, 306 act to extend the ground plane proximate to the interior circuit board 301, helping to minimize back lobe radiation and extending the main lobe so that the 3 dB beamwidth reaches 90 degrees. It has been determined that without the wing-shaped extension panels 305, 306, the antenna system has a 3 dB horizontal beamwidth on the order of 65 degrees.

There has been described herein an antenna with diversity transformation that offers distinct advantages when compared to antenna systems of the prior art. It will be apparent to those skilled in the art that modifications may be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited except as may be necessary in view of the appended claims .

Claims

What is claimed is :

1. An antenna comprising: a first antenna array having a first polarization; a second antenna array having a second polarization different from the first polarization and orthogonal thereto; and means for transforming the polarizations of the first and second antenna arrays to third and fourth different polarizations that are orthogonal to each other and different from the first and second polarizations.

2. The antenna of claim 1, wherein the first antenna array is a vertically polarized antenna array.

3. The antenna of claim 1 , wherein the second antenna array is a horizontally polarized antenna array.

4. The antenna of claim 1, wherein the means for transforming the polarizations of the first and second antenna arrays comprises a quadrature hybrid.

5. The antenna of claim 1, wherein the first and second polarizations are vertical and horizontal and the third and fourth polarizations comprise +45 degree polarization and -45 degree polarization.

6. The antenna of claim 1, further comprising: a first substrate having horizontally and vertically oriented antenna elements disposed thereon; and a second substrate spaced from the first substrate, the second substrate having apertures disposed on a first side thereof and antenna feed lines disposed on a second side thereof; such that the feed lines are coupled through the apertures to the antenna elements.

7. The antenna of claim 6, further including an extrusion of conductive material providing a backplane and having means for mounting the second substrate thereto.

8. The antenna of claim 6, wherein the first substrate is spaced apart from and held substantially in alignment with the second substrate by a plurality of spacers .

9. The antenna of claim 7, wherein the extrusion further includes first and second opposed extension panels for extending the horizontal beamwidth of the antenna.

10. The antenna of claim 1, further including filter means interposed between the second antenna array and the means for transforming the polarizations.

11. The antenna of claim 10, wherein the filter means comprises a ceramic block combline bandpass filter.

12. The antenna of claim 1, further including a duplexer means having a first path interposed between the first antenna array and the means for transforming the polarizations, and a second path interposed between the first antenna array and a transmit signal input.

13. The antenna of claim 12, wherein the duplexer means comprises a first ceramic block combline bandpass filter having a first center frequency and coupled to a second ceramic block combline bandpass filter having a second center frequency.

14. The antenna of claim 4 , wherein the quadrature hybrid comprises a four-port network of four one-quarter wavelength microstrip transmission lines interconnected in a rectangular geometry.

15. An antenna configured for diversity operation and having one transmit signal input connector and two receive signal output connectors, the antenna comprising: a vertically polarized antenna array; a horizontally polarized antenna array; a quadrature hybrid having first and second input ports and first and second output ports; a bandpass filter interposed between the horizontally polarized antenna array and the first input port of the quadrature hybrid; a duplexer having a bidirectional port, an input port, and an output port, the duplexer interposed between the vertically polarized antenna array and the second input port of the quadrature hybrid, such that the duplexer bidirectional port is coupled to the antenna array, the duplexer output port is coupled to the second input port of the quadrature hybrid, and the duplexer input port is coupled to a transmit signal input connector of the antenna; such that the quadrature hybrid first output port provides an antenna output signal having +45 degree polarization, and the quadrature hybrid second output port provides an antenna output signal having -45 degree polarization.

16. The antenna of claim 15, further comprising: a first substrate having horizontally and vertically oriented antenna elements disposed thereon; and a second substrate spaced from the first substrate, the second substrate having apertures disposed on a first side thereof and antenna feed lines disposed on a second side thereof ; such that the feed lines are coupled through the apertures to the antenna elements .

17. The antenna of claim 16, further including an extrusion of conductive material providing a backplane and having means for mounting the second substrate thereto.

18. The antenna of claim 16, wherein the first substrate is spaced apart from and held substantially in alignment with the second substrate by a plurality of spacers.

19. The antenna of claim 17, wherein the extrusion further includes first and second opposed extension panels for extending the horizontal beamwidth of the antenna.