Patent Application
20220045430
The present invention relates to an antenna with one or more spiral segment(s) for emitting radiation, in particular radiofrequency (RF) radiation with frequency which may be between 300 MHz (megahertz) and 30 GHz (gigahertz). It may relate in particular to an antenna of the “ultra-wideband” type, or UWB. In a known manner, a UWB antenna emits radiation of a determined frequency mainly from a restricted zone of this antenna, which is called the radiative zone for the frequency considered. This radiative zone varies depending on the frequency of the radiation emitted, and therefore depending on the frequency of each spectral component of the antenna feed signal.
More precisely, an antenna as considered in this description comprises at least one guide path for a traveling electromagnetic wave, from an electrical feed input to which the feed signal is applied. The radiative zones associated with different values of the frequency of the emitted radiation are distributed along the guide path of the traveling wave, depending on the shape of this path. In the following, “radiation” will be used to refer to electromagnetic radiation which is emitted by the antenna and which propagates in free space outside the antenna, for the purpose of transmitting signals over long distances. In contrast, the term “traveling wave” will denote the electromagnetic wave which propagates along the guide path of the antenna, being confined to this path. We will then call the “effective wavelength” of this traveling wave, its spatial period along the guide path, taking into account the constitution of the antenna, the electrical and dielectric parameters of the materials of which it is composed, and the possible presence of a reflective metal plate which is intended to limit the field of emission of the antenna to a half-space, with solid angle of 2π steradians.
In a known manner, for an antenna whose guide path is in the form of a spiral starting from a feed signal input located at the center of this spiral, the radiative zone which corresponds to the frequency value f is approximately superimposed on the circle which is concentric with the spiral and whose circumference length is a multiple of the effective wavelength of the traveling wave.
However, when the traveling wave reaches the outer end of the spiral guide path, it is at least partially reflected and the returning traveling wave again emits radiation. This delayed additional emission then partly interferes with the main radiation which is simultaneously emitted from the traveling wave which is propagating from the feed input towards the end of the guide path. To avoid this interference, it has been proposed to provide an absorbing material at the outer end of the spiral guide path, to absorb the traveling wave and thereby reduce the amplitude of its reflection. However, this results in a reduction in the transmission efficiency of the antenna, which affects in particular the frequency values whose radiative zones are located at the periphery of the spiral. These frequency values are located at the beginning of the antenna's transmission band, towards its lower frequency limit.
In addition, the article entitled “Self Matched Spiral Printed Antenna with Unidirectional Pattern”, by J. Massiot et al., 7th European Conference on Antennas and Propagation (EuCAP), 2013, IEEE, pp. 1237-1240, proposes reducing the reflection of the traveling wave on the outer end of each portion of the spiral-shaped guide path by providing an electrical resistor which connects the last two turns of this portion of the spiral path. This electrical resistor is placed at a distance from the outer end of the spiral path portion which is equal to a quarter of an effective wavelength value of the traveling wave, for a frequency value within the transmission band of the antenna. This solution is not optimal, however, and is not satisfactory for certain applications which require a good transmission efficiency of the antenna extending to the start of its transmission band, in other words for frequency values that are close to the lower limit of the antenna's transmission band, expressed in terms of frequency.
Based on this situation, one object of the invention consists of improving a spiral antenna of the type which has just been described, in order to increase its transmission efficiency at the start of the transmission band.
To achieve this or other objects, the invention provides a novel antenna for emitting radiation from at least one electromagnetic traveling wave which propagates along a guide path determined by a structure of the antenna, this guide path forming a transmission line dedicated to the traveling wave and having at least one path portion in the form of a spiral segment extending to a terminal end of this spiral segment. In other words, the antenna of the invention can be of the ultra-wideband type.
According to the invention, the guide path further comprises a continuous loop which surrounds each spiral segment, and the terminal end of each spiral segment is connected to the loop at a connection point of this spiral segment. Thus, an electrical signal which is transmitted to a feed input of the antenna produces a traveling wave which propagates along each spiral segment, then is transmitted to the loop at the connection point of this spiral segment. The portion of the traveling wave that is transmitted to the loop at each connection point then participates in the production of radiation. In other words, the loop constitutes at least a portion of a radiative zone of the antenna. In addition, this radiative zone corresponds to frequency values which are close to the lower limit of the antenna's transmission band, expressed in terms of frequency. The performance of the antenna at the start of the transmission band is thus improved.
According to additional features of the invention, intended to further reduce the portion of the traveling wave that is reflected at each connection point:
Preferably, the following additional features may be implemented:
When these additional features /2/ to /4/ are all implemented, the connection of the spiral segment to the loop forms a Wilkinson divider, which is arranged to be run along in a wave-joining direction by the traveling wave transmitted by this spiral arm.
When the effective wavelength of the traveling wave which serves as a reference for the two lengths of the intermediate portions is between 0.75 and 1.25 times the length of the loop, the connection of each spiral segment to the loop is sized to increase transmission efficiency of the antenna near the lower limit of its transmission band, expressed in terms of frequency.
It is possible for the antenna to be structured to define several guide path portions which are identical and each in the form of a spiral segment. Each spiral segment extends to a terminal end where it connects to the loop separately from the other spiral segments. Then the antenna may be configured so that all the guide path portions in spiral segments simultaneously transmit respective traveling waves to the loop.
Furthermore, for such a configuration with several spiral segments which feed the loop simultaneously with traveling waves, each spiral segment may be connected to the loop tangentially at the corresponding connection point. Furthermore, it may also be connected to the loop by a respective bridging structure, separately from each other spiral segment, and each spiral segment with the corresponding bridging structure can advantageously reproduce the features which have been indicated above, independently of every other spiral segment.
In various embodiments of the invention, the following other additional features may also be implemented, separately or with several of them combined:
Other features and advantages of the invention will be apparent from the following description of some non-limiting embodiments, with reference to the accompanying drawings, in which:
For clarity, the dimensions of the elements shown in
In accordance with
According to the invention, the antenna 100 comprises an additional slot segment 13 in the form of a loop which surrounds the spiral segments. For clarity, the additional slot segment 13 is referred to directly as a loop throughout the remainder of this description, and each spiral-shaped slot segment is referred to as a spiral segment. Preferably, the loop 13 is circular. Spiral segment 11 is connected to the loop 13 at connection point PR1, and spiral segment 12 is connected to the loop 13 at connection point PR2.
In the remainder of this description, it will be assumed that the antenna 100 has only two spiral segments, but it is understood that it can have any number of them: one, three, four, etc. In light of the description which follows, those skilled in the art will understand that when several spiral segments are connected to the loop 13 at connection points which are distributed along this loop 13, these spiral segments must be supplied with respective electric currents, at the feed input E, which are out of phase with respect to one another in a manner which is consistent with the distribution of the connection points on the loop 13. In the case of the antenna shown in
Then, each slot segment 11-13 constitutes a guide path portion for a traveling electromagnetic wave, this wave comprising variable electric currents which appear on the edges of the slot. Such an antenna 100 produces a coupling between the traveling electromagnetic waves which are guided in the slot segments 11-13, and an electromagnetic radiation external to the antenna 100. This coupling is maximal in areas of the antenna 100 which depend on the frequency value common to the traveling waves guided in the slot segments, and equal to the frequency value of the emitted radiation. These areas are called radiative zones. That one which corresponds to the frequency value f is superimposed on the circle that has the midpoint of the feed input E as its center, and that has a circumference length substantially equal to a multiple of the effective wavelength of each traveling wave having the frequency value f. The reference ZR designates such radiative zone, which is indicated with dotted lines in
The shape of the spiral segments may be selected according to the efficiency profile that is desired for the antenna 100 within its spectral band of transmission. For example, each slot segment may have an Archimedean spiral shape, whereby the radial distance increases linearly with the angle of the polar coordinate.
The loop 13 is supplied with traveling waves by the two spiral segments 11 and 12 at the connection points PR1 and PR2, so that a resulting traveling wave propagates along the loop 13 when an electrical signal is injected into the two spiral segments 11 and 12 at the feed input E. The loop 13 then constitutes a radiative zone for a frequency value of the emitted radiation which is close to the lower limit of the transmission band of the antenna 100, since it surrounds the spiral segments 11 and 12.
For reducing a reflection which could affect the traveling wave guided by each spiral segment 11, 12 at the corresponding connection point PR1 or PR2, it is advantageous that each spiral segment 11, 12 be connected to the loop 13 tangentially, or substantially tangentially, with respect to the loop.
For further reducing the reflection which could affect the traveling wave guided by each spiral segment 11, 12 at the corresponding connection point PR1 or PR2, it is also advantageous that this spiral segment 11, 12 be connected to the loop 13 by a Wilkinson divider structure, or by a connection structure whose structural and electrical features are close to those of a Wilkinson divider. Such Wilkinson divider is well known to those skilled in the art, so its efficiency in suppressing reflection does not need to be demonstrated again here. Each Wilkinson divider structure is implemented as indicated in
A bridging structure SP1 is added to connect the spiral segment 11 to the loop 13, upstream of the connection point PR1 relative to the propagation direction of the traveling wave guided by the spiral segment 11 and originating from the feed input E. The link formed by the bridging structure SP1 between the spiral segment 11 and the loop 13 is effective for transmitting between them a portion of the traveling wave guided by the spiral segment 11 or the loop 13. For this purpose, and as can be seen in
The bridging structure SP1 and the connection point PR1 thus demarcate two intermediate guide path portions: the intermediate portion 11i along the spiral segment 11, and the intermediate portion 13i along the loop 13. The intermediate portions 11i and 13i preferably each have a length which is substantially equal to a quarter of a determined effective wavelength value, which is relative to the traveling wave guided in the antenna 100. This effective wavelength value can correspond to the radiation which is mainly emitted by the loop 13 as a radiative zone. Thus, the common value of the length of the two intermediate zones 11i and 13i may be substantially equal to a quarter of the circumference length of the loop 13. More generally, it may be equal to L13/(4·n), where L13 is the circumference length of the loop 13, and n is a positive integer.
Furthermore, for further reducing the reflection of the traveling wave on the end of the spiral segment 11, the bridging structure SP1 may be designed to produce a determined impedance value for the traveling wave portion that it transmits. To achieve this, the spiral segment 11 and the loop 13 each have the same characteristic impedance value Zo out of the intermediate portions 11i and 13i. For example, the respective slot segments which constitute the spiral segment 11 and the loop 13 have geometric, electrical, and dielectric parameters which are identical. From these parameters, a person skilled in the art knows how to determine the characteristic impedance value of a slot segment, for the traveling wave that it transmits. On this subject, one can refer in particular to the thesis entitled “Comparison of slotline characteristics” by Yong Seok Seo, Institutional Archive of the Naval Postgraduate School: Cahloun, Monterey, Calif., June 1990, accessible at the Internet address http: //hdl.handle.net/10945/34829. When the only slot antenna parameter that is varied is the slot width, the characteristic impedance of a slot segment is an increasing function of that slot width. Then, the impedance value of the bridging structure SP1 may advantageously be selected as equal to approximately 2×Z0. The impedance value which is thus desired for the bridging structure SP1 can be produced by arranging an appropriate electrical resistance R1 between the opposite edges of the additional slot segment of this bridging structure SP1. The electrical resistance R1 may be equal or substantially equal to 2×Z0. It may consist of a discrete component which is attached to the antenna 100, for example by soldering its two terminals, each to one of the two edges of the additional slot segment of the bridging structure SP1. Alternatively, the electrical resistance R1 may also consist of a segment of resistive film of a commercially available type, which is attached locally between the two edges of the slot.
Again for further reducing the reflection of the traveling wave on the end of the spiral segment 11, the characteristic impedance values of the intermediate portions 11i and 13i, which are effective for the traveling wave guided by each of them, may be adjusted. Thus, when the spiral segment 11 and the loop 13 each again have the common characteristic impedance value Zo out of the intermediate portions 11i and 13i, these latter portions may preferably each have a characteristic impedance value which is substantially equal to 21/2×Z0. Such an adjustment of the characteristic impedance value can in particular be performed by increasing the slot width in the intermediate portions 11i and 13i, in comparison to the slot width value common to the spiral segment 11 and to the loop 13 out of the intermediate portions 11i and 13i.
The adjustments which have just been described, for the impedance of the bridging structure SP1 and for the characteristic impedances of the intermediate portions 11i and 13i, are performed for the same effective wavelength value as that used to adjust the length of the two intermediate portions 11i and 13i. Under these conditions, the antenna 100 has a Wilkinson divider structure between the spiral segment 11 and the loop 13. This structure makes it possible to inject traveling wave 2 (see
References PR2, SP2, 12i and R2 respectively correspond to references PR1, SP1, 11i and R1, for spiral segment 12 in place of spiral segment 11.
A second metal surface, for example another metal plate 20 as shown in
By using the invention, the inventors have obtained a gain of at least 7 dB (decibel), or even of more than 12 dB, in the electrical reflection coefficient of the antenna 100, commonly designated by S11 and measured at the feed input E. This gain is effective near the lower frequency limit of the transmission band of the antenna 100.
It is understood that the invention can be reproduced while modifying secondary aspects thereof relative to the embodiments detailed above. In particular, the following features of the antenna can be changed:
| Number | Date | Country | Kind |
|---|---|---|---|
| 18 00953 | Sep 2018 | FR | national |
This application is the U.S. national phase of International Application No. PCT/EP2019/073830 filed Sep. 6, 2019 which designated the U.S. and claims priority to FR 18 00953 filed Sep. 13, 2018, the entire contents of each of which are hereby incorporated by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/073830 | 9/6/2019 | WO | 00 |
