An antenna including a feed network; four or more helical radiating elements, and four or more impedance matching stubs each coupling a respective radiating element to ground in parallel with the feed network. The shunt stubs provide a reactance in parallel with the radiating elements to rotate the desired frequency locus about a constant admittance curve. Specifically, the shunt stubs rotate the desired frequency locus from a low resistance, capacitive impedance to 50 ohms purely resistive. The length of the stubs can be adjusted to control the inductance.
1. An antenna including:
a feed network;
four or more helical radiating elements;
four or more impedance matching elements each coupling a respective radiating element to ground in parallel with the feed network; and
a substrate which carries the radiating elements on a first side and a ground plane on a second side, wherein the substrate carries the impedance matching elements on the second side and each impedance matching element couples a respective radiating element to the ground plane in parallel with the feed network.
2. The antenna of claim 1 wherein each impedance matching element is a stub.
3. The antenna of claim 1 wherein each impedance matching element is an inductive element.
4. The antenna of claim 1 wherein each impedance matching element is a stub coupled to ground via a conductive short circuit connection.
5. The antenna of claim 1 wherein the antenna is a quadrifilar antenna having four radiating elements.
6. The antenna of claim 1 including four feed lines each connected to a respective radiating element at a respective junction, wherein each impedance matching element is connected at a connection point located at or adjacent to the junction.
7. The antenna of claim 6 wherein the connection point is located on the radiating element adjacent to the junction.
8. The antenna of claim 6 wherein the connection point is located on the feed line adjacent to the junction.
9. The antenna of claim 1 wherein the antenna is configured to transmit and/or receive circularly polarized radiation.
10. The network of claim 1 wherein the feed network is a microstrip feed network.
11. The network of claim 1 , wherein the feed network includes a hybrid coupler.
12. The network of claim 11 , wherein the hybrid coupler has no terminated port.
13. The network of claim 11 wherein the hybrid coupler is a ring hybrid.
14. The antenna of claim 1 wherein the feed network includes a 180° hybrid coupler having a feed port, a 0° port; a 180° port having an approximately 180° phase difference with the 0° port; a first antenna port coupled to the 0° port; a second antenna port coupled to the 0° port via a respective phased line, the second antenna port having an approximately 90° phase difference with the first antenna port; a third antenna port coupled to the 180° port; and a fourth antenna port coupled to the 180° port via a respective phased line, the fourth antenna port having an approximately 90° phase difference with the third antenna port.
15. The antenna of claim 1 wherein the feed network has an output impedance between 45 and 55 ohms.
16. The antenna of claim 1 , wherein the radiating elements each have substantially the same length.
17. The antenna of claim 1 , wherein the radiating elements are each coupled to the feed network at one end, and open circuited at another end.
18. An antenna including:
a feed network;
four or more helical radiating elements;
four or more impedance matching elements each coupling a respective radiating element to ground in parallel with the feed network;
a substrate which carries the radiating elements on a first side and a around plane on a second side, wherein each impedance matching element couples a respective radiating element to the ground plane in parallel with the feed network and each impedance matching element includes a plated-through hole passing through the substrate.
19. The antenna of claim 18 wherein each impedance matching element is a stub.
20. The antenna of claim 18 wherein each impedance matching element is an inductive element.
21. The antenna of claim 18 wherein each impedance matching element is a stub coupled to ground via a conductive short circuit connection.
22. The antenna of claim 18 wherein the antenna is a quadrifilar antenna having four radiating elements.
23. The network of claim 18 wherein the feed network is a microstrip feed network.
24. The network of claim 18 wherein the feed network includes a hybrid coupler.