Dual-polarized radiating elements for base station antennas having built-in common-mode rejection filters that block common mode radiation parasitics
An antenna includes a radiator that is electrically coupled to a feed stalk having a common-mode rejection (CMR) filter therein. The CMR filter is configured to suppress common mode radiation from the radiator by providing a frequency dependent impedance to a pair of common mode currents within the feed stalk, which is sufficient to increase a return loss associated with the pair of common mode currents to a level of greater than −6 dB across a frequency range including a frequency of the common mode radiation.
1. A radiating element, comprising:
a cross-dipole radiator; and
first and second feed stalks, which are electrically coupled to said cross-dipole radiator and responsive to respective first and second radio frequency (RF) feed signals, said first and second feed stalks comprising respective first and second common-mode rejection (CMR) filters therein, said first CMR filter including a first impedance Z 1 =R 1 +jωL 1 +jωM(I 2 /I 1 ) and a second impedance Z 2 =R 2 +jωL 2 +jωM(I 1 /I 2 ), where L 1 and L 2 are the inductances of respective first and second inductors within the first feed stalk; L 1 ≈L 2 , where the expression designates an equality within ±20%; R 1 and R 2 are the resistances of the first and second inductors; M is a mutual inductance between the first and second inductors; I 1 and I 2 are first and second common mode currents in the first feed stalk; ω is the angular frequency of the first and second common mode currents; and M is sufficiently close in magnitude to L 1 and L 2 that a return loss associated with the first and second common mode currents is greater than −6 dB at the angular frequency ω.
2. The radiating element of claim 1 , wherein the first feed stalk comprises a doubled-sided printed circuit board having a pair of side-by-side inductors, as L 1 and L 2 , on a first surface thereof, and a feed trace with a U-shaped feed segment on a second surface thereof.
3. The radiating element of claim 1 , wherein the first feed stalk comprises a first doubled-sided printed circuit board having a pair of side-by-side inductors, as L 1 and L 2 , on a first surface thereof, and a feed trace with a U-shaped feed segment on a second surface thereof; and wherein the second feed stalk comprises a second doubled-sided printed circuit board having a pair of side-by-side inductors on a first surface thereof, and a feed trace with a U-shaped feed segment on a second surface thereof.
4. The radiating element of claim 1 , wherein the first and second feed stalks comprise respective first and second double-sided printed circuit boards having complementary grooves therein that interlock with each other.
5. The radiating element of claim 1 , wherein the first and second inductors L 1 and L 2 are configured as first and second spiral inductors, respectively.
6. The radiating element of claim 5 , wherein the first stalk comprises a double-sided printed circuit board (PCB); wherein the first and second spiral inductors are patterned on a first surface of the PCB; and wherein the first spiral inductor spirals inward in a counter-clockwise direction and the second spiral inductor spirals inward in a clockwise direction.
7. The radiating element of claim 2 , wherein L 1 and L 2 are spiral inductors.
8. The radiating element of claim 7 , wherein L 1 and L 2 are patterned as mirror images of each other relative to a center axis of the printed circuit board.
9. The radiating element of claim 8 , wherein the first and second feed stalks comprise respective first and second double-sided printed circuit boards having complementary grooves therein that interlock with each other along the center axis.
10. A radiating element, comprising:
a radiator having first and second radiating arms; and
a feed stalk having a common-mode rejection (CMR) filter therein, said CMR filter configured so that a first impedance therein, which is electrically coupled to the first radiating arm, is equivalent to Z 1 , and a second impedance therein, which is electrically coupled to the second radiating arm, is equivalent to Z 2 , where: Z 1 =R 1 +jω L1 +jωM(I 2 /I 1 ), Z 2 =R 2 +jωL 2 +jωM(I 1 /I 2 ), L 1 ≈L 2 ; R 1 and R 2 are the resistances of a first inductor and a second inductor, respectively; L 1 and L 2 are the inductances of the first inductor and the second inductor, respectively; M is a mutual inductance between the first and second inductors; I 1 and I 2 are the first and second common mode currents in the first impedance and the second impedance, respectively; ω is the angular frequency of the first and second common mode currents; the expression “≈” designates an equality within ±25%; and M is sufficiently close in magnitude to L 1 and L 2 that a return loss associated with the first and second common mode currents is greater than −6 dB at the angular frequency ω.
11. The radiating element of claim 10 , wherein the feed stalk comprises a dual-sided printed circuit board (PCB) having a hook-shaped feed line on a first surface thereof; and wherein the first and second inductors are configured as first and second spiral inductors on a second surface of the PCB.
12. The radiating element of claim 11 , wherein the first inductor is electrically connected to the first radiating arm via a first metal trace on the first surface of the PCB, and the second inductor is electrically connected to the second radiating arm via a second metal trace on the first surface of the PCB.
13. An antenna, comprising:
a radiator including a plurality of radiating arms electrically coupled to a common-mode rejection (CMR) filter, said CMR filter configured to suppress common mode radiation from said radiator by providing a frequency dependent impedance to a pair of common mode currents within the plurality of radiating arms, which is sufficient to increase a return loss associated with the pair of common mode currents to a level of greater than −6 dB across a frequency range including a frequency of the common mode radiation.
14. The antenna of claim 13 , wherein the CMR filter comprises a pair of spiral inductors.
15. The antenna of claim 13 , wherein the frequency of the common mode radiation is less than a frequency of differential mode currents within the CMR filter when the antenna is active and responsive to: (i) at least a first RF feed signal at the frequency of the differential mode currents, and (ii) radiation from an adjacent radiator, which is responsive to at least a second RF feed signal at the frequency of the common mode radiation.
16. An antenna, comprising:
a reflector;
a first radiating element responsive to at least a first feed signal, on the reflector;
a second radiating element responsive to at least a second feed signal, on the reflector, said second radiating element comprising:
a radiator including a plurality of radiating arms electrically coupled to a common-mode rejection (CMR) filter, said CMR filter configured to suppress common mode radiation from said radiator by providing a frequency dependent impedance to a pair of common mode currents within the plurality of radiating arms, said frequency dependent impedance being sufficient to increase a return loss associated with the pair of common mode currents to a level of greater than −6 dB across a frequency range including a frequency of the common mode radiation.
17. The antenna of claim 16 , wherein the pair of common mode currents are induced within the plurality of radiating arms in response to differential mode radiation from said first radiating element.
18. The antenna of claim 16 , wherein said CMR filter comprises a pair of spiral inductors.
19. The antenna of claim 18 , wherein a portion of a first of the pair of spiral inductors is separated from a portion of a second of the pair of spiral inductors by a dielectric material extending therebetween.