Antenna

[0001] The present invention relates to an antenna with four or more helical radiating elements, such as a quadrifilar or octafilar helix antenna.

[0002] The input impedance of a quadrifilar helix radiating element is a function of resonant length, pitch angle and helix diameter. It is desirable to match the input impedance of the element to the output impedance of the feed network, which is typically 50 ohms. However, the helix diameter needed to provide a 50 ohm input impedance is typically larger than packaging constraints will allow. For example, a diameter of 1.5 inches is required for an antenna operating at the GPS L1 frequency of 1575 MHz. At diameters less than one inch (which are desirable for cosmetic and packaging reasons), the input impedance can be as low as 2-10 ohms.

[0003] One way of approaching this problem is to provide a widened transformer section in each antenna feed line, as shown in U.S. Pat. No. 5,828,348. The increased line width of the transformer sections results in packaging problems. An alternative method is described in U.S. Pat. No. 5,198,831 and U.S. Pat. No. 6,184,884. In this case, the feedpoint of each radiating element is elevated to a position where a 50 ohm impedance (or other higher impedance) is obtained.

[0004] An exemplary embodiment of the invention provides an antenna including a feed network; four or more helical radiating elements, and four or more impedance matching elements each coupling a respective radiating element to ground in parallel with the feed network.

[0010] FIG. 1 is a planar view of a quadrifilar helix antenna 1 constructed in accordance with the teachings of the present invention. The antenna 1 is made of a radiating segment 2 and a base segment 3. FIG. 1 shows the front side of the antenna, but hidden elements on the rear side of the antenna are also shown. The radiating segment 2 includes radiating elements 4-7 on the front side, and shunt inductors 20-23 on the rear side. The base segment 3 contains a microstrip hybrid junction power divider feed circuit 8 on the front side and a ground plane 9 on the rear side. Both segments 2,3 of the antenna 1 are made of one single section of dielectric substrate on which copper (or any suitable conductor) is deposited or etched to form the radiating elements 4-7, the hybrid junction power divider feed circuit 8, the shunt inductors 20-23 and the ground plane 9.

[0011] As is illustrated in FIG. 1, the radiating elements 4-7 are connected to the hybrid junction power divider feed circuit 8 at one end and are open circuited at the other end. The length of each of the four radiating elements is initially ¼ wavelength. However, after tuning and compensation for end effects, the resulting length is shorter than ¼ wavelength. Nevertheless, the elements operate in ¼ wavelength mode.

[0012] The feed circuit 8 is shown in detail in FIG. 2. The circuit 8 has a feed port 10, 0° hybrid port 11 and −180° hybrid port 12. A short antenna feedline 13 and a 90° phased line 14, extend from the 0° hybrid port 11. Similarly, a short antenna feedline 15 and a 90° phased line 16 extend from the 180° hybrid port 12. The radiating elements 4-7 are contiguous with the lines 13-16 respectively. The radiating elements are driven in phase quadrature, providing the phase relationships required by circularly polarized beam patterns.

[0013] The helical pattern is accomplished by designing the upper segment 2 as a parallelogram having vertical sides set at a predetermined angle (e.g., 50 degrees) above the horizontal line of the rectangularly shaped lower segment 3. The radiating elements are then disposed at the same angle. Thus, once the antenna is turned into a cylinder such that the angled sides of the parallelogram as well as the two vertical sides of the lower segment touch each other to form a seam, the radiating elements produce a helical pattern relative to each other. Note that the helical pattern is controlled by the pitch of the chosen angle. Hence, the more acute the angle, the more turns there will be in the helices formed by the radiating elements 4-7 upon the cylindrical transformation of the planar antenna of FIG. 1. (see FIG. 3.)

[0014] A 50 ohm line 17 extends downward from the feed port 10 to a connector (not shown). Although a 50 ohm line is used in this embodiment, it is not absolutely required. Therefore, in an alternative embodiment the connector may be placed adjacent to the hybrid junction power divider feed circuit 8 thereby circumventing the use of the 50 ohm line. Also, impedances other than 50 ohm may be employed if required.

[0015] To fabricate the quadrifilar helix antenna of the present invention, the planar antenna of FIG. 1 is bent inwardly into a cylinder as illustrated in FIG. 3. Note that in FIG. 3, the front side of the antenna 1 is located within the cylinder whereas the rear side is outside. This is done to protect the radiating elements 4-7 and feed circuit 8 from possible damage due to handling and thereby eliminating the need to later run performance tests. Thus, in an alternative embodiment, the planar antenna of FIG. 1 may be bent outward to expose the hybrid junction power divider feed circuit 8 and radiating elements 4-7.

[0016] The shunt inductors 20-23 are identical and an illustrative one 20 is shown in detail in FIG. 4. The inductor 20 is in the form of a U-shaped stub with a proximal end 30 contiguous with the ground plane 9, and a distal end 31 with a plated-through hole 32. The plated-through hole 32 passes through the base of the radiating element 7, spaced slightly from the junction between the radiating element 7 and the phased line 16.

[0017] The shunt inductors 20-23 provide a Smith Chart impedance matching technique. That is, they provide a reactance in parallel with a load to rotate the desired frequency locus (load) about a constant admittance curve. Specifically, the shunt inductors 20-23 rotate the desired frequency locus (load) from a low resistance, capacitive impedance to 50 ohms purely resistive. The length of the stub inductors can be adjusted to control the inductance, and the length of the radiating elements can be adjusted for proper frequency.

[0018] To manufacture the antenna of the present invention, the hybrid junction power divider feed circuit 8 has to first be designed to provide impedance matching and 0 to 180° phase shift while fitting into a particular chosen area. Secondly, the 0° and 180° phase shift locations of the hybrid junction power divider feed circuit 8 have to be located. Thirdly, the correct length and impedance of the 90° phased lines 14, 16 must be established to allow for both n/4 wavelength mode of operation and phase quadrature between the antenna ports. Once the steps above are accomplished, the correct configuration of all pertinent parts of the antenna is simply etched or deposited onto a dielectric substrate. The dielectric substrate can be made of glass, fiberglass, Teflon or any other material or combination thereof. However, in this case a pliable dielectric substrate is used to facilitate the shaping of the planar antenna of FIG. 1 into a cylinder.

[0019] Once the deposition or etching of the copper on the dielectric substrate is completed, the antenna is bent into a cylinder. The antenna is then fastened in that shape by taping the edges of the upper section of the antenna together and by soldering or joining the edges of the ground plane 9 with conductive tape. Finally, a connector is soldered to the end of the input line 17.

[0020] Note that with this method, many antennas can be deposited or etched on a large section of dielectric substrate. After the deposition, each antenna can be die cut, rolled into a cylinder, soldered or joined at the right locations and be ready for use. Note also that the soldering is minimal (i.e., one or two soldering connections) and done on non-sensitive parts of the antenna (i.e., ground plane and connector).

[0021] In an alternative closed-circuit antenna (not shown), the radiating elements 4-7 may be connected by a shorting ring with ends which are joined together during assembly by conductive tape or solder. In this case the elements 4-7 are increased in length to operate in ½ wavelength mode.

[0022] In the illustrated embodiment, shunt inductors are used, but in alternative embodiments (not shown), the inductors 20-23 may be replaced by capacitive elements depending on the original complex impedance.

[0023] The shunt inductors 20-23 are compact and efficient, and enable the output impedance to be close to 50 ohms without requiring bulky line transformers It has also been found that the initial tuning of the helix structure is much simplified over the raised feed design described in U.S. Pat. No. 6,184,844. Adjustment of the design in U.S. Pat. No. 6,184,844 requires simultaneous changes to the element length and feedpoint. In contrast, the use of shunt inductances only requires a single adjustment of the inductance, which remains stable over a reasonable frequency range. The length of the radiating elements 4-7 can be adjusted for frequency somewhat independently. The design shown in the figures has at least equal gain and pattern performance to raised feed designs, whilst enabling a compact 50 ohm feed network to be used. Additionally, with the choice of inductance as the reactive element, the effective length of the helix is modestly shortened. The helical structure remains realizable using simple P.C.B. fabrication on thin flexible substrates.

[0024] The present invention has been described herein with reference to particular embodiments for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof.

[0025] For example, an amplifier may be inserted between the hybrid junction power divider feed circuit 18 and the 50 ohm line 17. The microstrip feed network 8 may be replaced by a waveguide or stripline coupler. The ring hybrid may be replaced by a coupled line hybrid. For instance in an alternative feed network, the 180° ring hybrid may be replaced by a 90° coupled line hybrid with a 0° degree port and a 90° port, and a 90° Schiffman phase shifter coupled to the 90° port. Thus it can be seen that this circuit configuration will produce a 180° phase difference between the 0° port and the output of the Schiffman phase shifter. Thus the 90° hybrid and Schiffman phase shifter can together be considered to constitute a 180° hybrid coupler. The radiating elements may be aperture fed as in U.S. Pat. No. 6,172,656.

[0026] The radiating elements can be phased to operate in either endfire or backfire mode, either in the open-circuit configuration of FIG. 1 or in closed circuit configuration. Although the feed network is shown with meandering lines to save space, it will be understood that straight lines may be used instead.

[0027] It will be appreciated by a skilled person that the actual phase difference between the ports may differ slightly from the approximate values given above: in practice the phase difference may vary by up to 2%, or even in extreme cases up to 5% either side of the approximate value.

[0028] A preferred application for the antenna is for receiving satellite Global Positioning System (GPS) signals.

[0029] In the case of a GPS system, the radiating elements operate in receive mode. However it will be appreciated that the invention may be applicable to an antenna operable only in transmit mode, or operable in both transmit and receive modes. Therefore it will be understood that the term “radiating element” relates to an element which can receive and/or transmit radiation.

[0030] It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention.