FIELD OF INVENTION
The field of the invention relates to broadband antennas. More specifically, the invention relates to compact printed helical antennas.

BACKGROUND OF THE INVENTION
It is well-known that efficiency of mobile communication device banks on their antenna design as antenna design plays a key role in transmission/reception of signals of such devices. Design of an antenna depends on several factors including without limitation compactness of antenna, gain of antenna, directional vs omnidirectional radiation pattern, impedance matching, etc.
[0003] It is challenging to design compact antennas with high gain in a limited volume. When compactness of an antenna is focussed upon, characteristics such as reflection coefficient, radiation pattern, impedance bandwidth, etc. change. Further, it is difficult to achieve a high value of gain in all directions, thus resulting in a trade-off between radiation pattern and gain. In case, a directional antenna is used, the gain is higher but as the name suggests, such antennas radiate predominantly in specific directions.
[0004] Many researchers have undertaken work on compactness of antenna design at 800 – 900 MHz ISM or sub GHz band. The popular antenna in this frequency range is an Inverted F antenna (IFA) or planar inverted F antenna (PIFA). Normally, today’s mobile industry uses PIFA because the radiation pattern of a PIFA is omni-directional (complete circle in a plane). Various designs of PIFA have been proposed in the aforesaid frequency band, but all these designs require large volume to achieve high gain. Further, designing of such PIFA is marred by many other complexities to achieve the desired characteristics and performance.
[0005] Other antennas like chip antennas, monopole antennas, spiral (made with copper) antennas, flex PCB antenna etc. have high gain but due to their large size or substantially degraded performance, they are not suited for compact applications.
SUMMARY OF THE INVENTION
[0006] An object of the present invention is to provide a printed non-uniform helical antenna that provides an omni-directional radio frequency (RF) radiation pattern for ultra-high frequency (UHF) band.
[0007] Another object of the present invention is to provide a printed helical antenna that is compact in size or has a small form factor.
[0008] Yet another object of the present invention is to provide a printed helical antenna that provides high gain.
[0009] Yet another object of the present invention is to provide a printed helical antenna that provides increased ground clearance.
[00010] Yet another object of the present invention is to provide a printed helical antenna with reduced separation between the printed helical antenna and associated electronic circuitry.
[00011] Yet another object of the present invention is to provide a printed helical antenna that eliminates the need of a coaxial cable assembly to connect traces leading to minimal cable and connector losses which share the same plane as a ground on which the antenna is printed.
BRIEF DESCRIPTION OF DRAWINGS
[00012] Particular embodiments of the present disclosure are described herein below with reference to the accompanying drawings, however, it is to be understood that the disclosed embodiments are merely examples of the disclosure, which may be embodied in various forms. Well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present disclosure in virtually any appropriately detailed structure.
[00013] Figs. 1, 1a and 1b illustrate a perspective view, top view and bottom view respectively of a printed helical antenna assembly in accordance with exemplary embodiments of the present invention.
[00014] Figs. 2a, 2b and 2c illustrate different designs of the printed helical antenna with RF traces in accordance with exemplary embodiments of the present invention.
[00015] Figs. 3a, 3b and 3c depict simulated reflection coefficient, VSWR (voltage standing wave ratio) and input impedance respectively of the printed helical antenna with ground clearance of 0.3mm in accordance with an exemplary embodiment of the present invention.
[00016] Figs. 4a, 4b and 4c depict simulated reflection coefficient, VSWR and input impedance respectively of the printed helical antenna with ground clearance of 9.5mm in accordance with an exemplary embodiment of the present invention.
[00017] Figs. 5a and 5b depict simulated radiation patterns of the printed helical antenna in accordance with an embodiment of the present invention.
[00018] Fig. 6 illustrates measured reflection coefficient of the printed helical antenna in accordance with an embodiment of the present invention.
[00019] Figs. 7a and 7b depict radiation patterns measured in the anechoic chamber of the printed helical antenna in accordance with an embodiment of the present invention.
DETAILED DESCRIPTION OF DRAWINGS
[00020] Prior to describing the invention in detail, definitions of certain words or phrases used throughout this patent document will be defined: the terms "include" and "comprise", as well as derivatives thereof, mean inclusion without limitation; the term "or" is inclusive, meaning and/or; the phrases "coupled with" and "associated therewith", as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; Definitions of certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art will understand that such definitions apply in many, if not most, instances to prior as well as future uses of such defined words and phrases.
[00021] Wherever possible, same reference numbers will be used throughout the drawings to refer to same or like parts. Moreover, references to various elements described herein are made collectively or individually when there may be more than one element of the same type. However, such references are merely exemplary in nature. It may be noted that any reference to elements in the singular may also be construed to relate to the plural and vice-versa without limiting the scope of the disclosure to the exact number or type of such elements unless set forth explicitly in the appended claims.
[00022] The following discussion of the embodiments of the invention directed to a printed helical antenna for wireless and telematic applications is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.
[00023] The present invention relates to an omnidirectional printed helical antenna having high gain in a limited volume. In an embodiment, the gain achieved by the antenna of the present invention is +2dBi. In an exemplary embodiment, the space in which the antenna is printed is 25 mm × 25 mm × 10 mm. However, it is to be noted that the gain depends on the size of the ground plane and ground clearance. The gain can be enhanced by increasing the size of the ground plane. Thus, the desired values of gain can be achieved in accordance with the teachings of the present invention by varying the size of the ground plane. The shift in frequency can then be optimized by a matching network.
[00024] Referring to FIG. 1 illustrates a perspective view of a printed helical antenna assembly 100 in accordance with an exemplary embodiment of the present invention.
[00025] The helical antenna assembly 100 includes a printed helical antenna 10 (or helical antenna), a feed point 20 and at least two ground planes 30. In an embodiment of the present invention, the helical antenna 10 may be a conducting wire wound in the form of a spring or helix.
[00026] The printed helical antenna 10 includes a plurality of microstrip lines 10a(1…n) (or traces) looped around a substrate 40 through a plurality of vias 50 (described later). It is to be noted that n denotes the number of turns of the helical antenna 10. The plurality of microstrip lines 10a are parallel to each other and is made of a conducting material say alumina or copper.
[00027] In an exemplary embodiment, the plurality of microstrip lines 10a are printed copper strips having a width (‘w’) of 1mm. Alternately, the width of microstrip lines 10a may vary depending on the application for which the printed helical antenna 10 is to be used. It is to be noted that the dimension (diameter/width) of the helical antenna 10 is smaller than the wavelength thereof to achieve the desired gain. The wavelength is evaluated using the design frequency on which the helical antenna 10 operates in order to provide omnidirectional radiation patterns.
[00028] In a preferred embodiment, the helical loops formed by the microstrip lines 10a are non-uniform, namely, the coil radius of each loop is different from the other. Alternately, the helical loops may be uniform.
[00029] The helical antenna 10 is printed on the substrate 40. In an embodiment, the helical antenna 10 is printed on a dielectric substrate 40 for example, FR4. FR-4 is a composite material composed of woven fiberglass cloth with an epoxy resin binder that is flame resistant (self-extinguishing). In an embodiment, the dielectric constant of FR-4 is 4.3 and thickness of the dielectric substrate 10b is 0.8 mm. It should be noted that other conventionally available dielectric substrates materials may also be used as per the teachings of the present invention with a possibility of minor variations in the characteristics of the antenna. The technique used for printing the helical antenna 10 without limitation includes photolithography, milling, etc.
[00030] The substrate 40 may be circular in shape as depicted in Figs. 1-1b or of any other shape including square, triangle, etc. The substrate 40 may include a top surface 40a and a bottom surface 40b. The top surface 40a as well as the bottom surface 40b of the substrate 40 may be divided into at least two sections. The division of such sections may be equal or may be unequal. In an embodiment as shown in FIG. 1, the top surface and the bottom surface 40a, 40b include two equally divided sections i.e. a first section 40c and a second section 40d.
[00031] As clearly depicted in FIG. 1a, the first section 40c of the top surface 40a includes the microstrip lines 10a (1…n) (n = number of turns of helical antenna) having the feed point 20. Similarly, as shown in FIG. 1b, the first section 40c of the bottom surface 40b includes the plurality of microstrip lines 10b (1…n) (n = number of turns of helical antenna). In an embodiment, the substrate 40 includes a plurality of vias 60 extending from the first sections 40c of the top surface 40a and the bottom surface 40b for the passage of the microstrip lines 10a.
[00032] The plurality of microstrip lines 10a (or traces) printed on the top surface 40a are connected to the respective plurality of microstrip lines 10b on the bottom surface 40b by means of the plurality of vias 60 to form electrical connections between the microstrip lines (10a, 10b) provided on both surfaces of the substrate 40, thereby forming the printed helical loop pattern.
[00033] The plurality of microstrip lines 10a, 10b is bonded to the dielectric substrate 40. As depicted in Fig. 1a, the plurality of microstrip lines 10a are non-uniformly printed on the dielectric substrate 40 to provide a compact helical antenna 10. Further, the said non-uniformly printed helical antenna design in the above manner is able to provide high gain for various applications of mobile networks such as ISM band, sub Ghz frequency in a small form factor.
[00034] In an embodiment, an end (s1) of a first microstrip line 10a1 on the top surface 40a is connected to the feed point 20. This results in low insertion loss and ensures minimum power dissipation within the feed point 20 to restrict delta rise in temperature average input RF power.
[00035] The second section 40d of the top surface 40a and bottom surface 40c forms a surface for the ground plane 30 as depicted in Figs. 1-1b. The ground plane 30 is corresponds to a layer of a metal foil, say copper, on a printed circuit board (PCB or substrate) connected to the ground point i.e. one terminal of a power supply (no shown). The ground plane 30 serves as a return path for current from flowing through the antenna 10.
[00036] In an embodiment, the ground plane 30 on both surfaces is semi-circular shaped as depicted in Figs. 1-1b having a radius ‘r’. Various other shapes of the ground plane 30 can also be considered.
[00037] The ground clearance (d) i.e. the distance between the ground plane 30 and the plurality of microstrip lines 10a, 10b (helix turns) may vary depending on the application, for example, 0.3mm to 9.5mm. It is to be noted that increase in distance between the ground plane 30 and the microstrip lines 10a, 10b reduces interference with the electronic circuitry 70 because the ground plane 30 is shared by both the printed helical antenna 10 and electronic circuitry. Since interference is reduced, therefore effective radiated power of the printed antenna design is increased. Hence, the efficiency of the helical antenna 10 is increased. However, the increased ground clearance between the ground plane 30 and the microstrip lines 10a, 10b may result in reduced gain.
[00038] Figs. 2a-2c depict various designs of the printed helical antenna assembly 100. Fig. 2a depicts a front view of the helical antenna 10 printed on the substrate 40 having a ground clearance ‘d’ of 0.3mm. In the said figure, the microstrip lines 10a are printed on the first sections 40c of the top surface 40a while an electronic circuitry 70 is printed on the ground plane 30. In an embodiment, electronic traces 70a are printed along with the electronic circuitry 70 to avoid requirement of co-axial cable. In the depicted embodiment, the electronic circuitry 70 is printed over almost the entire ground plane 30. Given the ground clearance between the printed helical antenna 10 and the ground plane 30 is minimal, such configuration acts as a good receiver.
[00039] Fig. 2b depicts the helical antenna 10 printed on the substrate 40 having a ground clearance ‘d’ of 0.3mm and a channel 120 of 25mm. An electronic circuitry 70 is printed on the channel such that part of the circuitry 70 overlaps with the ground plane 30 while part of the ground plane 30 is exposed. The electronic trace 70a runs through the electronic circuitry 70 as depicted.
[00040] Fig. 2c depicts the helical antenna 10 printed on the substrate 40 having a ground clearance ‘d’ of 9.5mm and a channel 120 of 5mm. The electronic circuitry 70 is separated from the antenna 10 by a distance of 9.5mm. Due to this, low interference is observed and the antenna 10 in this configuration acts as a good transmitter.
[00041] In an embodiment, the channel includes radiofrequency (RF) traces 70a for electrostatic coupling between the helical antenna 10 and the electronic circuitry 70. The radiofrequency (RF) traces 70a help in achieving desired compactness of the design (reducing the volume) and eliminate the use of a co-axial cable assembly to connect to the ground. Further, the traces minimize losses incurred due to a cable and connector assembly and share the same plane as a ground where antenna is printed.
[00042] Compared to the conventional co-axial cable assembly which increases antenna size, the printed helical antenna 10 having RF traces 70a is a simplified structure which results in reduced antenna manufacturing cost.
[00043] In order to provide omni-directional radiation pattern, the printed helical antenna 10 is operated in a normal mode. It is to be noted that when the circumference of the helix is significantly less than a wavelength and its pitch (axial distance between successive turns) is significantly less than a quarter wavelength, the antenna 10 is said to operate in a normal mode and such antenna is called a Normal-Mode Printed Helical Antenna (NMPHA). A NMPHA has very small dimensions, and therefore, it is a key antenna in all wireless communication engineering sectors where the physical size (radius < ?/20) of the radiator plays a significant role.
[00044] The aforesaid NMPHA antenna was simulated using electromagnetic (EM) software. In an embodiment, the NMPHA is operated at a frequency of 868 MHz, the simulation and measured result illustrated a circular ground plane which is small. Due to this, the NMPHA did not require any external impedance matching network. The resonance condition of NMHA is achieved by appropriately selecting dimensions say, clearance/pitch of the antenna, diameter (2r1, 2r2) of one or more printed microstrip lines (coil) and the number of turns of printed helical antenna 10 (n).
[00045] The printed helical antenna assembly 100 as per the teachings of the present invention can be used for various applications. For example, it can be used as a transceiver in smart metering & smart city applications, energy distribution applications, agricultural applications, healthcare applications, home automation, wearable electronics, industrial automation, tracking application & radio frequency identification tags (RFID- UHF 860 MHz band) etc.
[00046] In an exemplary embodiment, the substrate 40 of the helical antenna 10 is wrapped around a cylindrical support (not shown), for example, a button. Thereafter, the antenna 10 is optimized with a matching circuit to further boost the gain value by fine tuning and eliminating small losses in the design. The aforesaid antenna was simulated using electromagnetic (EM) software. Input impedance was kept around 50O at a frequency of 868 MHz. Following findings were obtained.
[00047] Referring to Figs. 3a, 3b and 3c depict reflection coefficient, VSWR and input impedance respectively of the printed helical antenna with a ground clearance of 0.3mm. As can be seen from the graphs, reflection coefficient is good and VSWR in near to 1, i.e. the antenna 10 is optimally matched to the transmission line and more power is delivered to the antenna and no power is reflected from the antenna. Further, no standing waves appear which indicates that good match is provided between the printed helical antenna 10 and the respective feed point 20. Similarly, as depicted the input impedance is well matched to the design frequency. Input impedance is around 50O at 868 MHz.
[00048] Figs. 4a, 4b and 4c depict reflection coefficient, VSWR and input impedance respectively of the printed helical antenna 10 with a ground clearance of 9.5mm in accordance with an exemplary embodiment of the present invention. The magnitude of reflection coefficient is below -10 dB at the design frequency, which confirms a good match between the antenna 10 and feed point 20. Also, it can be seen from the input impedance plot that the antenna 10 has an input impedance close to 50 O at design frequency.
[00049] Figs. 5a and 5b depict simulated radiation patterns of the printed helical antenna 10 in accordance with an embodiment of the present invention. While Fig. 5a provides simulated radiation pattern of the antenna 10 placed vertically, Fig. 5b provides simulated radiation pattern of the helical antenna 10 placed horizontally. Fig. 5a depicts radiation pattern at phi =0 degrees while Fig. 5b depicts radiation pattern at phi = 90 degrees. As depicted in the graphs, the said antenna radiates RF power in all the directions uniformly.
[00050] Fig. 6 illustrates the reflection coefficient of the printed helical antenna 10 in accordance with an embodiment of the present invention. The reflection coefficient of the helical antenna 10 (as shown in FIG. 2c) was measured by using vector network analyzer and radiation patterns were measured in the anechoic chamber by using a wideband double-ridged horn antenna (Gain is known) as a receiver and proposed antenna as a transmitter. The proposed antenna was placed 225 cm far from the horn (load) to satisfy the far-field requirement. As depicted in the graph, the reflection loss was found minimum at the operating frequency (design frequency) of antenna.
[00051] Figs. 7a and 7b depict radiation patterns measured in the anechoic chamber of the printed helical antenna 10 in accordance with an embodiment of the present invention. 2D radiation patterns were measured in two principal planes (phi = 0 and 90 degrees) at 868 MHz. It is clear from Figs. 7a and 7b that the proposed antenna radiates uniformly in these two planes.
[00052] The aforesaid antenna finds application in Internet of Things market where low power devices are required to communicate efficiently to radiate RF power. The proposed antenna can be used as a transceiver for below applications but not only limited to smart metering & smart city applications, energy distribution applications, agricultural applications, healthcare applications, home automation, wearable electronics, industrial automation, tracking application & radio frequency identification tags (RFID- UHF 860 MHz band) etc.
[00053] The antenna of the present invention offers significant advantages including:
1. Higher gain with smaller size,
2. Small form factor,
3. Omni-directional radiation pattern,
4. Repeatability of performance during volume production by printing the track on a printed circuit board,
5. Tuning control for precise matching,
6. Significant reduction in cost – compared to external antenna or any other method which uses cable and connector, etc.
[001] The scope of the invention is only limited by the appended patent claims. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings of the present invention is/are used.

CLAIMS:
WE CLAIM:
1. An omnidirectional helical antenna assembly (100) having a high gain and small form factor, the antenna assembly (100) comprising:
a. a substrate (40) having a top surface (40a) and a bottom surface (40b), each of the top surface (40a) and the bottom surface (40b) including a first section (40c) and a second section (40d);
b. a plurality of microstrip lines (10a, 10b) placed parallel to each other being printed on the first section (40c) of the top surface (40a) and the bottom surface (40b) of the substrate (40) respectively, each microstrip line (10a) on the top surface (40a) being coupled to the respective microstrip line (10b) on the bottom surface (40b) via one or more vias (60) forming a turn of a helix; and
c. a layer of metal foil (30) provided on the second section (40d) of the top surface (40a) and the bottom surface (40b) of the substrate (40) and being operatively coupled to the microstrip lines (10a, 10b), wherein distance between the microstrip lines (10a, 10b) and the layer of metal foil (30) ranges from 0.3mm to 9.5 mm.
2. The omnidirectional helical antenna assembly (100) as claimed in claim 1 wherein an end (s1) of a first microstrip line (10a1) of the plurality of microstrip lines (10a) is coupled to a feed point (20).
3. The omnidirectional helical antenna assembly (100) as claimed in claim 1 wherein circumference of the helix is significantly less than a wavelength.
4. The omnidirectional helical antenna assembly (100) as claimed in claim 1 wherein radius of each turn of the helix is non-uniform.
5. The omnidirectional helical antenna assembly (100) as claimed in claim 1 wherein axial distance between successive turns of the helix is significantly less than a quarter wavelength.
6. The omnidirectional helical antenna assembly (100) as claimed in claim 1 wherein width of the microstrip lines (10a, 10b) is 1mm.
7. The omnidirectional helical antenna assembly (100) as claimed in claim 1 wherein the layer of metal foil (30) is made of copper.
8. The omnidirectional helical antenna assembly (100) as claimed in claim 1 wherein separation of the microstrip lines (10a) from an electronic circuitry (70) range between 5mm to 25mm.
9. The omnidirectional helical antenna assembly (100) as claimed in claim 1 wherein the antenna (10) is coupled to RF traces (70a) provided on the substrate (40) avoiding a coaxial cable for connection with an electronic circuit.