EP0889541A1 - Helix antenna with variable length - Google Patents

Abstract

The antenna consists of first and second conducting coils with a common central axis (Y - Y) and fitted together so that their adjoining ends (HI1, HS2) overlap but without mechanical contact, and their outer ends (HI2, HS2) are separate. It also has a two-section insulated housing (CPI, CPS) with sections which move in a spiral relative to one another and vary the overlap between the two coils. The two coils are virtually identical, and the coils of their overlapping portions are spaced evenly relative to one another, e.g. so that the coils of one lie half way between the coils of the other.

Description

The present invention relates to an antenna in propeller to broadcast and receive waves radio, especially waves metrics for radio signals in the band FM sound broadcasting between 87 and 110 MHz, and more generally in a frequency band from 70 to 150 MHz approximately.

Currently, a transmit / receive antenna operating in this frequency band is a almost omnidirectional antenna in azimuth of type "whip" or "end of wire" consisting of a strand substantially vertical conductor having a length equal to half a wavelength, one to two about meters.

To improve the broadcasting of radio programs stereophonic and to ensure better comfort listening to listeners along motorways, transmitters synchronized at the same frequency mark out the motorways. The transmitters can also broadcast data signals to automatically tune receivers using of digital messages identifying the radio programs. Such a broadcast system frequency modulation is known by the acronym RDS (Radio Data System). Also within the framework of the dissemination of digital information, can be produced by microcomputers for be broadcast to receivers associated with tele-billboards in cities and in public transport vehicles, at terminals of parking, etc.

For all these applications, the antenna reception, or transmission / reception, should have small dimensions for reasons of space in situ and aesthetic, while retaining the character omnidirectional in a horizontal plane specific to the sound broadcasting. To meet the conditions previous, the antenna length must be reduced, but at the expense of bandwidth which is all the more diminished. In this case, the antenna will be granted over relatively bandwidth narrow, a few MHz.

To overcome dimension problems adapting the whip antenna, it could be replaced by a simple helical antenna, the length would be reduced to about twenty about centimeters, a gain in length of around 75%. By experimentally selecting the number of turns of the antenna propeller, the antenna would be tuned to the desired frequency and adapted to the impedance of a coaxial line at which it is connected.

Such an antenna can for example be fixed to the top of a bus stop sign, or at the top of an even higher tubular post for a parking staking system, or on the frame of an urban tele-display screen which can be more than five meters above the ground. All of these antennas are difficult to access.

When an operator of a private radio station or local decides to change the transmission frequency of the station, or when the operator is replaced by another, all the antennas of reception, and / or transmission / reception, must be set on the new transmission frequency of station by changing the antennas and performing new measures to tune these.

A helix antenna comprising four propellers parallel conductors extending around an axis common central in the same direction is disclosed in patent application WO 96/07215. A unique dielectric helix concentric with the common axis extends inside the four propellers, and has a length substantially equal to that of the propellers which are regularly spaced and fixed on the dielectric helix. A box contains the propellers and is attached to one end of the propeller dielectric. A tuning device is attached to the other end of the dielectric helix and can rotate relative to the housing, so that the rotation of the tuning device changes the pitch of the propeller dielectric and therefore not common to conductive propellers without significant variation in nominal diameter of the propellers. For example, when the number of turns per unit of length decreases, the length of the conducting propellers increases.

Another four-wire helix antenna or helical radiating "filaments" is disclosed in patent application WO 96/19846 and includes also means for rotating one end wires relative to the other fixed end of these so as to change the length and the pitch sons.

Such antennas are antennas of reception in a telecommunications system by satellite and radiate axially in polarization circular at very high higher frequencies at the gigahertz. The modification of the geometry of conductive propellers allows you to change the aperture of the antenna radiation pattern according to radiation directions close to the direction vertical axial.

The objective of the invention is to provide a correcting linear polarized helix antenna to the drawbacks of the above-mentioned known antennas, and more specifically, provide a helical antenna which can be tuned in frequency very easily and very quickly by any unqualified person, without using an in situ measuring device and without dismantle the antenna.

To this end, a helical antenna comprising first and second conductive propellers extending around a common central axis along a same sense, is characterized in that the first and second propellers have first portions nested one inside the other without being in contact mechanical and second non-nested portions, and the antenna includes means for moving helically by sliding on itself the second propeller relative to the first propeller.

Thanks to the helical displacement of the second propeller relative to the first propeller, the propellers having constant geometric characteristics, the antenna is tuned to the desired frequency.

The propellers are not in mechanical contact, i.e. are galvanically isolated and are only coupled by electromagnetic flux. This galvanic insulation of propellers improves quality signals received and / or transmitted by the antenna when the antenna, or the part of the antenna surrounding the decoupled propeller of the coaxial power cable of the antenna inadvertently touches a wire electric or metallic ground.

In order to center the transformer impedance achieved by the first portions nested propellers, the first and second propellers are substantially identical, and preferably the turns of the first propeller portions nested are regularly spaced apart by compared to others, preferably substantially half a turn of the propeller.

According to a preferred embodiment, the means for move helically includes a first element of fixed insulating material and containing at least partially the first propeller, and a second insulating material which is screwed on the first element and which contains at least partially the second propeller. Screwing and unscrewing the second element to the first element respectively decreases and increases the length of the propeller portions radiant nested and so respectively increases and decreases the tuning frequency of the antenna. At least the second portion of the second propeller can be attached to the second element. The first and second elements constitute practice a protective cover for the antenna. The desired frequency can be selected easily by any unqualified person by turning the second element on the first element. For this purpose frequency graduation is provided along the said first element, cooperating with one end of the second element, for example by overlap partial variable. The screwing / unscrewing of the second element is stopped for example when the edge of the threaded end of the second element is opposite of the desired frequency marked on the scale.

To fix the antenna on a post, upright or any other seat, a base is provided metallic having a face to which is fixed a microwave line connector having a central conductor is connected to a strand of the second portion of the first propeller, and on the other side of which one end of said first element is applied. The tightness of the cover formed by the first and second elements can be provided by a sealing washer housed between said end of said first element and the base. A end of the second element can be closed, preferably by a removable hat, by which the two propellers are accessible.

To keep the propellers upright with a some flexibility in the event of shock or vibration communicated to the base or hood, it can be provided with a cylindrical piece of insulating material which is fixed and housed substantially axially in the first propeller, which first propeller can be fixed to the cylindrical piece; or a piece cylindrical in insulating material which is movable with the second propeller and housed substantially axially in the second propeller, which second propeller can be fixed to the cylindrical part.

• la figure 1 est un schéma de principe d'une antenne en hélice selon l'invention ; et
• la figure 2 est une vue en coupe axiale d'une antenne en hélice selon une réalisation préférée de l'invention.

Other characteristics and advantages of the present invention will appear more clearly on reading the following description of several preferred embodiments of the invention with reference to the corresponding appended drawings in which:

• Figure 1 is a block diagram of a helical antenna according to the invention; and
• Figure 2 is an axial sectional view of a helical antenna according to a preferred embodiment of the invention.

Referring to Figure 1, an antenna in propeller according to the invention comprises two propellers electrically conductive circulars HI and HS. The propellers have a common central axis YY and are substantially identical, i.e. have no constant and substantially equal turns P, constant and substantially equal diameters D, and constant and substantially equal lengths LH. The HI and HS propellers are the same direction; according to illustrated embodiment, the propellers are direct (at left), although according to another embodiment they may be downgraded (right).

HI and HS propellers have first portions HI1 and HS1 which are nested one inside the other and second non-nested portions HI2 and HS2 between which the first nested portions HI1 and HS1 extend to the center of the antenna.

The turns of the first HI1 portion are arranged parallel to and evenly spaced of the other first portion HS1 and are not not in mechanical contact with them. Each turn of one of the first HI1 and HS1 portions is substantially located half a P / 2 step from two turns successive of the other first portion.

In one of the propellers, for example the first HI propeller, the second HI2 portion complementary to the first HI1 portion has a BR end strand connected to the central DC conductor of an AC coaxial cable which is connected to receiving equipment, or transmitter receiver. In the other propeller, that is to say the second HS propeller, the second HS2 portion complementary to the first portion HS1 constitutes the free end of the helical antenna.

The YY axis of the helical antenna is in practice arranged vertically. For example the first propeller HI is located at the bottom of the antenna and the second HS propeller is located in the upper part of the antenna. The first nested propeller portions HI1 and HS1 are then respectively a portion upper of the first HI propeller and a portion bottom of the second HS propeller.

According to the invention, one of the HI and HS propellers is movable helically relative to the other propeller. According to the illustrated embodiment, the first HI propeller is fixed and the second HS propeller is helically movable by sliding on itself, that is to say by undergoing a translation of the pitch propeller P for a rotation of one turn on itself. In other words, the HS propeller can be simultaneously rotated around the YY axis of the antenna and translated along the antenna axis YY, the helical displacements, i.e. rotations and simultaneous translations of the HS mobile propeller can be sometimes according to a direction, sometimes following opposite direction, as indicated by the double arrows R and T. When the second HS propeller is moved helically, the length LA of the antenna decreases, respectively increases, and in the same proportions, the IM nesting length of first portions of HI1 and HS1 propeller increases, respectively decreases, and the useful length antenna (LH-IM) of each of the second portions HI2 and HS2 propeller decreases, increases respectively.

The antenna according to the invention is thus roughly equivalent to a helical antenna which has two radiating portions HI2 and HS2 in length variable (LH-IM) to receive and / or send electromagnetic waves, and a central part of variable length IM constituted by the first HI1 and HS1 end portions. The central part antenna is analogous to a transformer impedance whose transformation ratio is always equal to 1 since the numbers of turns coupled between the first HI1 and HS1 portions of propellers are always equal. The transformer impedance is an impedance centralizer which is the seat of a current belly. The antenna behaves as a dipole which is variable in length and at center of which the current is variable. Impedance input of the helical antenna is thus a function of number of turns "reduced" by the first portions nested HI1 and HS1 of the propellers and impedance of the helical antenna which is mainly composed of the impedances of the second HI2 portions and HS2 propellers and brought to the foot of the antenna, at strand level BR.

So, by helically moving the second HS propeller compared to the first HI propeller and, in consequence, by varying the coupling electromagnetic propellers, the impedance of the antenna can be adapted to the impedance characteristic of the AC power coaxial cable and at the desired frequency including a station radio.

According to a preferred embodiment shown as example in Figure 2, the HI and HS propellers are housed substantially in a lower CPI and a CPS upper part of a cylindrical cover hollow insulating material.

The lower part of the CPI is fixed with glue or screws on an EM metal base by through a sealing washer in RO rubber. The HI propeller is at least partially housed in the lower part of the CPI cover and is held coaxially with this part of the CPI cover around a cylindrical guide in GU insulating material which can be longer than the first HI propeller, even almost as long as double the length of each HI, HS propeller. At least turns of the second HI2 portion of the first propeller can be glued to the GU guide. The GU guide is fixed, by example by gluing, on the upper side of the metal base EM through the washer RO. At center of the external underside of the base metallic EM is attached a cable CO connector coaxial CA. The BR strand at the lower end of the lower portion HI2 of the first HI propeller is soldered to the central conductor CC of the CO connector to through a suitable hole TR. EM base is by square or circular example and is fixed at the top a pole or a remote-display mast or any other support, or serves as a base to be fixed on the roof of a vehicle, or on the edge of a table, or on the case of a microcomputer.

The upper part of the CPS hood has a lower end tapped AND so as to be screwed around a threaded upper end EF of the lower cover part CPI. The steps of ET and EF threaded and threaded end threads are equal to the helix pitch P in order to maintain the turns of the substantially parallel propellers and equidistant between them when screwing the part CPS to the CPI section.

In the upper part of the bore of the upper part of CPS hood is provided with a groove helical GH so as to fix it, for example by bonding, at least a few turns at the end upper portion of the second HS2 portion of the second HS propeller. The upper base of the hood part GPS is closed by a monolithic background with the CPS part, or by a CH cap fixed so removable on the hood part CPS.

So when the top cover of CPS is screwed on or unscrewed from the bottom of CPI hood, the second HS propeller descends or rises in the first HI propeller, and more precisely the variable length IM of the first propeller portions nested HI1 and HS1 increases or decreases so to tune and adapt the helical antenna to a higher or lower desired frequency. The relative displacement of one HS of the propellers by compared to the other HI propeller, and more precisely the second part of the CPS mobile hood relative to the first part of the fixed cover CPI is indicated by GR graduation along the upper end the part of the CPI cover which is covered partially by the hood part CPS. The graduation at divisions of 5 or 10 MHz for example in order to tune manually and precisely the antenna, without using a measuring device. The graduation is established experimentally in the factory, and is for example engraved on the cover part CPI.

Each of the HI and HS propellers consists of a copper wire wrapped in a thin sheath plastic or coated with a layer of insulating protection, such as varnish, for example.

Advantageously, the antenna according to the invention offers a certain flexibility which makes it insensitive to vibrations communicated by the support of the antenna through the base and the cover, thanks to the absence of mechanical contact between the propellers.

As an indication, a helical antenna for receive or transmit radio waves in frequencies between approximately 70 MHz and 150 Approximately MHz includes a length of lead wire for each helix of the order of a quarter wave, i.e. each propeller has a length substantially between 7 and 12 cm, a diameter D of 3 to 5 cm approximately and a pitch P of the order of a centimeter. The cover CPI-CPS and the other elements in matter insulating G and D are for example in Plexiglas. The cover has a length which varies between approximately 15 cm and 25 cm approximately.

The helical antenna according to the invention radiates in linear polarization and presents a quasi diagram omni-directional in azimuthal directions substantially perpendicular to the YY axis, i.e. substantially parallel to the ground.

The helical antenna of the invention described above advantageously does not present parts superfluous mobiles, direct propeller contacts with the outside and risk of water infiltration. The antenna is insensitive to mechanical vibrations. No tools are required to tune the antenna.

Although the invention has been described according to a preferred embodiment, other achievements may be adapted by a person skilled in the art in function of frequency tuning and adaptation desired impedance. So the dimensions of two propellers making up the antenna can be different in particular in diameter and length. The GU inner guide can be fixed under the CH bottom the upper part of the CPS hood instead of being fixed on the EM base, and at least turns of the second portion HS2 of the second propeller can be glued to the guide instead of being glued in the GU groove according to the illustrated embodiment. According to one another variant, the threads of the lower parts and upper cover CPI and CPS are respectively internal and external so that the CPS hood part is screwed into the part of the CPI cover.

Other assemblies of the screw-nut assembly type to move helically by sliding on itself the second propeller compared to the first can be expected. The means including the first and second cover parts can further be associated with a remotely controllable motor so as to automatically move the second propeller and tune the helix antenna to the frequency desired.

Claims (11)

Helical antenna comprising first and second conductive helices (HI, HS) extending around a common central axis (YY) along the same sense, characterized in that the first and second propellers have first portions (HI1, HS1) nested one inside the other without being in contact mechanical and second non-nested portions (HI2, HS2), and the antenna includes means (CPI, CPS) to move helically by sliding on itself the second propeller (HS) relative to the first propeller (HI). Antenna according to claim 1, in which the first and second propellers (HI and HS) are substantially identical. Antenna according to claim 1 or 2, in which the turns of the first portions nested propellers (HI1, HS1) are regularly spaced from each other, from preferably substantially at mid-distance (P / 2). Antenna conforms to any of claims 1 to 3, wherein the means for move helically includes a first element fixed insulating material (CPI) containing at least partially the first propeller (HI), and a second insulating material (CPS) which is screwed to the first element (CPI) and which contains at least partially the second propeller (HS). Antenna according to claim 4, in which at least the second portion (HS2) of the second propeller (HS) is attached to the second element (CPS). Antenna according to claim 4 or 5, comprising a metal base (EM) having one face to which a line connector is attached microwave (CO) having a central conductor (CC) is connected to a strand (BR) of the second portion (HI2) of the first propeller (HI), and on the other side of which one end of said first element (CPI) is applied. Antenna according to claim 6, comprising a sealing washer (RO) housed between said end of said first element (CPI) and said base (EM). Antenna conforms to any of claims 4 to 7, wherein one end said second element (CPS) is closed, preferably with a removable hat (CH). Antenna conforms to any of Claims 4 to 8, including a graduation in frequency (GR) along said first element (CPI) and cooperating with one end of said second element (CPS). Antenna conforms to any of claims 1 to 9, comprising a part cylindrical in insulating material (GU) which is fixed and housed substantially axially in at least the first propeller (HI). Antenna conforms to any of claims 1 to 9, comprising a part cylindrical in insulating material which is movable with the second propeller (HS) and is housed substantially axially in at least the second propeller.

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