Antenna system with frequency dependent power distribution to radiating elements

An antenna includes a frequency dependent divider circuit configured to receive an input signal and generate an output signal, the output signal having a power level based on a frequency of the input signal and a radiating element that is responsive to the first output signal.

FIELD OF THE INVENTION

The present disclosure relates generally to radio communications and, more particularly, to multi-beam antennas used in cellular communications systems.

BACKGROUND

Wireless base stations are well known in the art and typically include, among other things, baseband equipment, radios and antennas. The antennas are often mounted at the top of a tower or other elevated structure, such as a pole, a rooftop, water towers or the like. Typically, multiple antennas are mounted on the tower, and a separate baseband unit and radio are connected to each antenna. Each antenna provides cellular service to a defined coverage area or “sector.”

FIG. 1is a simplified, schematic diagram that illustrates a conventional cellular base station10. As shown inFIG. 1, the cellular base station10includes an antenna tower30and an equipment enclosure20that is located at the base of the antenna tower30. A plurality of baseband units22and radios24are located within the equipment enclosure20. Each baseband unit22is connected to a respective one of the radios24and is also in communication with a backhaul communications system44. Three sectorized antennas32(labelled antennas32-1,32-2,32-3) are located at the top of the antenna tower30. Three coaxial cables34(which are bundled together inFIG. 1to appear as a single cable) connect the radios24to the respective antennas32. Each end of each coaxial cable34may be connected to a duplexer (not shown) so that both the transmit and receive signals for each radio24may be carried on a single coaxial cable34. In some implementations the radios24are located at the top of the tower30instead of in the equipment enclosure20to reduce signal transmission losses.

Cellular base stations typically use directional antennas32such as phased array antennas to provide increased antenna gain throughout a defined coverage area. A typical phased array antenna32may be implemented as a planar array of radiating elements mounted on a panel, with perhaps ten radiating elements per panel. Typically, each radiating element is used to (1) transmit radio frequency (“RF”) signals that are received from a transmit port of an associated radio24and (2) receive RF signals from mobile users and feed such received signals to the receive port of the associated radio24. Duplexers are typically used to connect the radio24to each respective radiating element of the antenna32. A “duplexer” refers to a well-known type of three-port filter assembly that is used to connect both the transmit and receive ports of a radio24to an antenna32or to a radiating element of multi-element antenna32. Duplexers are used to isolate the RF transmission paths to the transmit and receive ports of the radio24from each other while allowing both RF transmission paths access to the radiating elements of the antenna32, and may accomplish this even though the transmit and receive frequency bands may be closely spaced together.

To transmit RF signals to, and receive RF signals from, a defined coverage area, each directional antenna32is typically mounted to face in a specific direction (referred to as “azimuth”) relative to a reference such as true north, to be inclined at a specific downward angle with respect to the horizontal in the plane of the azimuth (referred to as “elevation” or “tilt”), and to be vertically aligned with respect to the horizontal (referred to as “roll”).

FIG. 2Ais a perspective view of a lensed multi-beam base station antenna200that can be used to implement the directional antennas32ofFIG. 1.FIG. 2Bis a cross-sectional view of the lensed multi-beam base station antenna200. The lensed multi-beam base station antenna200is described in detail in U.S. Patent Publication No. 2015/0091767, the disclosure of which is hereby incorporated herein by reference.

Referring toFIGS. 2A and 2B, the multi-beam base station antenna200includes one or more linear arrays of radiating elements210A,210B, and210C (referred to herein collectively using reference numeral210). These linear arrays of radiating elements210are also referred to as “linear arrays” or “arrays” herein. The antenna200further includes an RF lens230. Each linear array210may have approximately the same length as the lens230. The multi-beam base station antenna200may also include one or more of a secondary lens240(seeFIG. 2B), a reflector250, a radome260, end caps270, a tray280(seeFIG. 2B) and input/output ports290. In the description that follows, the azimuth plane is perpendicular to the longitudinal axis of the RF lens230, and the elevation plane is parallel to the longitudinal axis of the RF lens230.

The RF lens230is used to focus the radiation coverage pattern or “beam” of the linear arrays210in the azimuth direction. For example, the RF lens230may shrink the 3 dB beam widths of the beams (labeled BEAM1, BEAM2and BEAM3inFIG. 2B) output by each linear array210from about 65° to about 23° in the azimuth plane. While the antenna200includes three linear arrays210, different numbers of linear arrays210may be used.

Each linear array210includes a plurality of radiating elements212. Each radiating element212may comprise, for example, a dipole, a patch or any other appropriate radiating element. Each radiating element212may be implemented as a pair of cross-polarized radiating elements, where one radiating element of the pair radiates RF energy with a +45° polarization and the other radiating element of the pair radiates RF energy with a −45° polarization.

The RF lens230narrows the half power beam width (“HPBW”) of each of the linear arrays210while increasing the gain of the beam by, for example, about 4-5 dB for the 3-beam multi-beam antenna200depicted inFIGS. 2A and 2B. All three linear arrays210share the same RF lens230, and, thus, each linear array210has its HPBW altered in the same manner. The longitudinal axes of the linear arrays210of radiating elements212can be parallel with the longitudinal axis of the lens230. In other embodiments, the axis of the linear arrays210can be slightly tilted (2-10°) to the axis of the lens230(for example, for better return loss or port-to-port isolation tuning).

The multi-beam base station antenna200may be used to increase system capacity. For example, a conventional 65° azimuth HPBW antenna could be replaced with the multi-beam base station antenna200as described above. This would increase the traffic handling capacity for the base station10, as each beam would have 4-5 dB higher gain and hence could support higher data rates at the same quality of service. In another example, the multi-beam base station antenna200may be used to reduce antenna count at a tower or other mounting location. The three beams (BEAM1, BEAM2, BEAM3) generated by the antenna200are shown schematically inFIG. 2B. The azimuth angle for each beam may be approximately perpendicular to the reflector250for each of the linear arrays210. In the depicted embodiment the −10 dB beam width for each of the three beams is approximately 40° and the center of each beam is pointed at azimuth angles of −40°, 0°, and 40°, respectively. Thus, the three beams together provide 120° coverage.

The RF lens230may be formed of a dielectric lens material232. The RF lens230may include a shell, such as a hollow, lightweight structure that holds the dielectric material232. The dielectric lens material232focuses the RF energy that radiates from, and is received by, the linear arrays210.

SUMMARY

In some embodiments of the inventive concept, an antenna comprises a frequency dependent divider circuit configured to receive an input signal and generate an output signal, the output signal having a power level based on a frequency of the input signal and a radiating element that is responsive to the first output signal.

In other embodiments, the frequency dependent divider circuit comprises a power divider that is configured to generate a first divided output signal and a second divided output signal responsive to the input signal, a delay line that is configured to generate a phase delayed output signal responsive to the second divided output signal, the phase delayed output signal having a phase delay based on a frequency of the second divided output signal, and a directional coupler that is configured to generate the output signal responsive to the phase delayed output signal and the first divided output signal.

In still other embodiments, the delay line comprises a transmission line configured to generate the phase delay directly proportional to the frequency of the second divided output signal.

In still other embodiments, the delay line comprises a transmission line coupled to a Shiffman phase shifter. The Shiffman phase shifter is configured to substantially maintain the phase delay independent of frequency.

In still other embodiments, the delay line comprises an inductive portion and a capacitive portion.

In still other embodiments, the antenna further comprises a stub circuit configured to generate first and second coupler input signals responsive to the phase delayed output signal and the first divided output signal. The directional coupler is configured to generate the output signal responsive to the first and second coupler signals.

In still other embodiments, the stub circuit comprises a pair of quarter-wave shorted lines.

In still other embodiments, the stub circuit comprises a pair of half-wave open ended lines.

In still other embodiments, an input impedance of the stub circuit is capacitive.

In still other embodiments, an input impedance of the stub circuit is inductive.

In still other embodiments, the directional coupler is a 90° hybrid branch-line coupler having an operational bandwidth of approximately 690 MHz-2700 MHz.

In still other embodiments, the power divider is a 3 dB multi-section Wilkinson power divider.

In still other embodiments, the radiating element comprises a linear array of radiating elements.

In still other embodiments, the output signal comprises a plurality of output signals associated with the linear array of radiating elements, respectively, and the frequency dependent divider circuit is further configured to generate the plurality of output signals so as to have increasingly tapered power levels at each end of the linear array.

In still other embodiments, the radiating element comprises one of a plurality of radiating elements.

In still other embodiments, the frequency dependent divider circuit is further configured to adjust a taper of an aperture of the output signal based on the frequency of the input signal.

In still other embodiments, the frequency dependent divider circuit is further configured to adjust an insertion loss of the antenna based on the frequency of the input signal.

It is noted that aspects described with respect to one embodiment may be incorporated in different embodiments although not specifically described relative thereto. That is, all embodiments and/or features of any embodiments can be combined in any way and/or combination. Moreover, other apparatus, methods, systems, and/or articles of manufacture according to embodiments of the inventive subject matter will be or become apparent to one with skill in the art upon review of the following drawings and detailed description. It is intended that all such additional apparatus, systems, methods, and/or articles of manufacture be included within this description, be within the scope of the present inventive subject matter, and be protected by the accompanying claims. It is further intended that all embodiments disclosed herein can be implemented separately or combined in any way and/or combination.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are set forth to provide a thorough understanding of embodiments of the present disclosure. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In some instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present disclosure. It is intended that all embodiments disclosed herein can be implemented separately or combined in any way and/or combination. Aspects described with respect to one embodiment may be incorporated in different embodiments although not specifically described relative thereto. That is, all embodiments and/or features of any embodiments can be combined in any way and/or combination.

Some governmental jurisdictions place limits on antenna gain at one or more frequencies. For example, a governmental jurisdiction may define a power threshold for one or more frequency ranges and service providers may be required to ensure that transmission power is at or below this threshold. Some embodiments of the inventive concept stem from a realization that a frequency dependent power divider circuit may be used between, for example, a beam forming network and a radiating element to reduce the power directed to that radiating element based on signal frequency. The radiating element may represent an entire antenna, a linear array of radiating elements that comprises part of an antenna, and/or a single radiating element that is part of a larger array of radiating elements in accordance with various embodiments of the inventive concept. The frequency dependent power divider circuit may reduce the power directed to the radiating element at other frequencies than those for which a power reduction is desired. A stub circuit may be used to reduce the amount of power diverted away from the radiating element at frequencies for which a lesser power reduction is desired.

FIG. 3is a block diagram of a frequency dependent power divider circuit300according to some embodiments of the inventive concept. The frequency dependent power divider circuit300comprises a power divider305having a first output that is coupled to a first input of a directional coupler315. A second output of the power divider305is coupled to a second input of the directional coupler315via a delay line310. The power divider305splits the signal from the beam into two signals. The power divider305may divide the power approximately equally between its two output terminals. The delay line310imposes a phase delay to the signal received from the power divider305and provides this phase delayed signal as an input signal to the directional coupler315. The delay line310may have a fixed length, which results in the phase delay applied to the signal output from the power divider305to vary with frequency. For a given time delay, higher frequency signals experience more phase delay than low frequency signals. The directional coupler315receives equal amplitude signals as input signals where the signal received from the delay line310experiences increasing phase delay with increasing frequency. The directional coupler315outputs equal phase, variable amplitude signals where the amount of amplitude difference depends on the phase delay between the inputs, where the phase delay increases with increasing frequency. In accordance with some embodiments of the inventive concept, the directional coupler315may be 90° hybrid branch-line coupler with an operable bandwidth of approximately 690 MHz-2700 MHz. The power divider305may be, for example, a 3 dB multi-section Wilkinson power divider.

FIG. 4is a table that illustrates operations of the frequency dependent power divider circuit300ofFIG. 3according to some embodiments of the inventive concept. When the delay line310provides a phase delay φ of 0°, then the signal power at output terminals A and B of the directional coupler315is split approximately evenly with each terminal receiving ½ power. When the delay line310provides a phase delay φ of 90°, then the signal power is directed approximately in its entirety to terminal A with terminal B receiving approximately zero signal power. When the delay line310provides a phase delay φ of −90°, then the signal power is directed approximately in its entirety to terminal B with terminal A receiving approximately zero signal power.

As described above, some governmental jurisdictions place limits on antenna gain at one or more frequencies. The frequency dependent power divider circuit300ofFIG. 3may be configured to divert power towards one of the output terminals A or B at a frequency at which transmitted signal power is to be reduced and may be configured to divert power towards another one of the output terminals A or B at a frequency at which transmitted signal power is to be maintained. Embodiments of the inventive concept may be illustrated by way of example. A communication system may operate by transmitting in frequency bands 1710 MHz-1880 MHz, 1910 MHz-2170 MHz, and 2496 MHz-2690 MHz. A governmental regulation may limit the antenna gain at 2560 MHz to a threshold of no more than 17.0 dB. Thus, it may be desirable to reduce the gain at 2560 MHz without adversely impacting the gain in the other frequency bands of 1710 MHz-1880 MHz and 1910 MHz-2170 MHz. Using a frequency of 1940 MHz, which is at the center of the bands 1710 MHz-1880 MHz and 1910 MHz-2170 MHz, the frequency dependent power divider circuit300can be tuned so that the delay line310generates a phase delay φ of approximately −90° at 1940 MHz, this results in approximately all of the signal power being diverted to terminal B. When delay line310is configured to generate a phase delay φ of approximately −90° at 1940 MHz, then the following phase delays may be generated at frequencies 1750 MHz, 2170 MHz, and 2560 MHz:

Thus, at 2560 MHz, the frequency dependent power divider circuit300divides the signal power approximately equally between terminals A and B. The frequency dependent power divider circuit300can be used in an antenna system to adjust the signal power directed to a radiating element to ensure the antenna gain does not exceed a defined threshold as will be described below with reference toFIG. 5.

FIG. 5is a schematic of an antenna system500including a frequency dependent power divider circuit510according to some embodiments of the inventive concept. The antenna system500comprises a beam forming network (BFN) that receives a beam and distributes the signal to five different radiating elements515A,515B,515C,515D, and515E. Each of these radiating elements515A,515B,515C,515D, and515E may represent an entire antenna, a linear array of radiating elements that comprises part of an antenna, and/or a single radiating element that is part of a larger array of radiating elements in accordance with various embodiments of the inventive concept. As shown inFIG. 5, a frequency dependent power divider circuit510is used as an interface to the radiating element515E. Specifically, the frequency dependent power divider circuit510receives an output signal from the BFN505and diverts a portion of the signal power through terminal B to the radiating element515E and another portion of the signal power through terminal A, which is coupled to an impedance element, such as resistor R1shown inFIG. 5. The frequency dependent power divider circuit510may be implemented using the frequency dependent power divider circuit300ofFIG. 3. Applying the example described above to the example antenna system500ofFIG. 5, the frequency dependent power divider circuit510diverts approximately half of the signal power away from the radiating element515E to ground through resistor R1at a signal frequency of 2560 MHz. This may reduce the gain of the antenna system500by reducing the energy directed to the radiating element515E. The reduced energy results in an increase in the taper of the aperture of the signal driving radiating element515E. The energy diverted to the resistor R1represents an increase in the insertion loss of the antenna.

In the present example, it is generally desired to avoid decreasing the gain in the 1710 MHz-1880 MHz and 1910 MHz-2170 MHz frequency bands. The phase delay φ at 1750 MHz is approximately −116° and the phase delay φ at 2170 MHz is approximately −57°. As shown inFIG. 4, when the phase delay φ is −90°, then virtually all of the signal power is directed to output terminal B of the frequency dependent power divider circuit510. Because the phase delay φ in the desired frequency ranges is not precisely −90°, the frequency dependent power divider circuit510will divert some of the signal power to the resistor R1through terminal A. While it will not be a full half-power reduction in signal power, the gain of the antenna system500will nevertheless be marginally reduced due to the reduction in signal power directed to the radiating element515E. To reduce the impact of power attenuation in the desired frequency bands, the frequency dependent power divider circuits300and510may incorporate a stub circuit as described below with respect toFIG. 6.

FIG. 6is a block diagram of a frequency dependent power divider circuit600including a stub circuit620according to some embodiments of the inventive concept. The frequency dependent power divider circuit600comprises a power divider605, a delay line610, and a directional coupler615that are configured as shown and may be implemented as described above with respect to corresponding elements inFIG. 3. The frequency dependent power divider circuit600differs from the frequency dependent power divider circuit300with the addition of a stub circuit620between the delay line610and the directional coupler615and the power divider605and the directional coupler615. The stub circuit620may include one or more stubs or resonant stubs connected to the transmission lines input to the directional coupler615. A stub or resonant stub is a length of transmission line or waveguide that is connected at one end only. The free end of the stub is either left as an open-circuit or is short circuited to a reference terminal or plane. The input impedance of the stub is reactive—either capacitive or inductive—depending on the electrical length of the stub and whether it is configured as an open or short circuit. A stub may function as a capacitor, inductor, and/or a resonant circuit at radio frequencies. The stub circuit may provide, for example, phase compensation stubs to drive the phase delay φ closer −90° to allow more of the energy to be diverted to terminal B of the directional coupler615. In some embodiments, two quarter-wave shorted lines622may be used as the compensation stubs to compensate 90° and/or two half-wave open ended lines624may be used as the compensation stubs to compensate 180°. Thus, in some embodiments, the frequency dependent power divider circuit510ofFIG. 5may be implemented using the frequency dependent power divider circuit600ofFIG. 6to increase the power diverted to the radiating element515E ofFIG. 5through terminal B of the directional coupler615to reduce the amount of gain reduction for the antenna system500in the 1710 MHz-1880 MHz and 1910 MHz-2170 MHz frequency bands.

The delay line310ofFIG. 3and the delay line of610ofFIG. 6may be implemented in different ways according to various embodiments of the inventive concept.FIG. 7Aillustrates an example of a delay line in which the phase delay is directly proportional to frequency and can be used to implement the delay line310ofFIG. 3and the delay line610ofFIG. 6according to some embodiments of the inventive concept. The delay line ofFIG. 7Amay comprise a 50 Ohm microstrip line with a length d, where the phase delay φ=[2πd(εeff1/2)]/λ0, where εeffis the effective dielectric constant of the substrate material and λ0is the wavelength in free space.

FIG. 7Billustrates an example of a delay line comprising a regular transmission line combined with a Shiffman phase shifter. A Shiffman phase shifter may provide substantially constant phase over the frequency band. As a result, the phase for the delay line ofFIG. 7Bmay change more slowly with frequency as compared to the embodiment ofFIG. 7A.

FIG. 7Cillustrates an example of a loaded delay line comprising narrow sections (series inductances) in combination with wide sections (parallel capacitances), which may provide about 15%-30% faster phase change as compared to the embodiment ofFIG. 7A.

The selection of a particular type of delay line to implement the delay line310ofFIG. 3and the delay line610ofFIG. 6may be based on a desired relationship between signal amplitude and frequency and a desired beam position, width, and/or sidelobes.

Some embodiments of the inventive concept may, therefore, provide an antenna system with a frequency dependent power divider circuit that may be used to reduce the amount of power directed to one or more radiating elements in the antenna system. When it is desired to reduce the antenna gain at a particular frequency, the frequency dependent power divider circuit may be tuned so as to divert a desired amount of power away from, for example, one of the antenna's radiating elements. In the example described above, half of the signal power is diverted away from a radiating element at the target frequency. The delay line used in the frequency dependent power divider circuit may be tuned such that more or less signal power is diverted away from a radiating element at the target frequency. In some embodiments, virtually all of the power may be diverted away from the radiating element effectively eliminating that element from the antenna at the target frequency. Moreover, in some embodiments, the frequency dependent power divider circuit may be duplicated so as to be inserted in the paths of multiple radiating elements so that the signal power is reduced for multiple ones of the radiating elements thereby decreasing the directivity of the antenna system at the target frequency to an even greater degree. For example, frequency dependent power divider circuits may be inserted in the paths of respective radiating elements at either end of an array of radiating elements so as to reduce the signal power directed to radiating elements at the array ends. In some embodiments, the reduction in signal power may be greater the closer a radiating element is to the end of an array. To reduce the impact of the signal power reduction at the target frequency on other frequency bands, a stub circuit may configured for use in the frequency dependent power divider circuit to reduce the amount the signal power diversion away from a radiating element for a particular frequency or band of frequencies.

FURTHER DEFINITIONS AND EMBODIMENTS

The terminology used herein to describe embodiments of the invention is not intended to limit the scope of the inventive concept.