US 3573840 A
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United States Patent Inventors 7 Roger L. Gouillou Draveil; Guy F. Ringenbach, Villeneuve-le-Roy;
Jacques H. Delomini, Epinay-sur-Seinc,
 References Cited UNITED STATES PATENTS 2,l74,353 9/1939 Roberts 343/895X 2,495,399 1/1950 Wheeler 343/895X 3,449,752 6/1969 Spitz et al. 343/895 OTHER REFERENCES France AppL No. 782,337 Filed Dec. 9, 1968 Patented Apr. 6, 1971 Assignee Office National DEtudes Et De Recherches Aerospatiales Chatillion-Sous-Bagneux, France Priority Dec. 15, 1967, Feb. 26, 1968 France 132,155 and 141,294
SMALL BULK HELICALLY WOUND ANTENNAE AND METHOD FOR MAKING SAME 9 Claims, 19 Drawing Figs.
us. Cl. 343/895, 343/745, 343/843 1m. (:1 l-l0lq l/36 Field ofSearch 343/895, 745,843
Helical Beam Antenna by John D. Kraus Communications for September 1949, pages 6- 8 and 34 relied upon, 343- 895 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Saxficld Chatmon, Jr Attorney-Abraham A. Saffitz ABSTRACT: Small bulk cylindrical or conical antennae made of a helically wound conductor with a free end and a grounded end. The coupling to a power supply or a load circuit is effected along a short length of said conductor near to and terminating on said ground plane, either by direct shunt coupling 7 or by inductive coupling. The grounded end can be provided with a capacitive load. The antennae can be manufactured by sticking a metal strip to a dielectric sheet and rolling the latter to the desired shape, or by printing circuit technique and subsequent winding to said shape, in which they may be kept by clamping, glueing or any other method.
5 Sheets-Sheet 1 INVENTORS:
Roger L. GOUILLOU, Guy F. nmcsmmcn,
Jacques H. DE OMINI 1AM aw AT 0R "Y Patented 7 April 6, 1971 5 Sheets-Sheet 2 mvmrous:
Roger L. GOUILLOU, Guy 1-. amcsmmcn,
Jacques H. DELOMINI y flLLZdo M 4 5 Sheets-Sheet 5 Fig.8
Roger L. GOUILLOU, Guy F. RINGENBACH, Jacques H. DEL IN I BY Z" ATT N Patented April 6, 1971 3,573,840
5 Sheets-Sheet 4 INVENTORS:
Roger L GOUILLOU Guy F. RINGENBACH, Jacques H. DELO NI ATTO EY Patented April 6, 1971 5 Sheets-Sheet 5 INVENTORS UH II mm m LB N UE OGE T GND A M L Ha TF8 e 8 yu Ouq RGCJWW B J SMALL BULK HELICALLY WOUND ANTENNAE AND METHOD FOR MAKING SAME The invention relates to antennae for receiving or transmitting ratio-electric signals, more particularly to helical antennae grounded at one end, the dimensions of the antenna being very small, both axially and transversely, with respect to the average operating wavelength.
It is important to reduce the size of antennae until they are. very small with respect to the operating wavelength, since the antennae can then be disposed in a restricted space, their weight can be reduced, and their mechanical rigidity can be increased. Unless, however, the facilities according to the invention are used, the reduction in size results in a high negative reactance in the antenna impedance, in series with a very low radiation resistance. As a result, it is particularly difficult to match the impedance to that of a power supply or a load circuit; furthermore, the efficiency is low and the frequency pass band is very narrow.
The aim of the invention is to construct antennae having a single conductor wound into a cylindrical or conical helix and transmitting a wave whose electric field is linearly polarized in a direction parallel to the helix axis, giving a radiation polar diagram which is practically circular in a plane perpendicular to the axis, the antennae being extremely small compared with the operating wavelength in free space, but being capable of having a great varietyof external shapes and thus being very easy to adapt to a wide range of operating conditions though their efficiency is very nearly equal to that of a conventional half-wave dipole, the antennae operating at high efficiency when they are connected by a coaxial lineto a transmitter or receiver.
According to one method for improving the matching of an antenna with a power supply or a load circuit, the radiating antenna element is associated with nonradiating elements so as to form a unit resonating at the operating frequency. It is known, inter alia, to load the top. of a vertical antenna from a horizontal conductor, the bottom of the antenna being grounded.
Other known antennae are caused to radiate a circular polarized wave by winding the radiating conductor into a helix. As a rule, such helical. antennae are used to transmit or receive signals whose wavelength is much smaller than the length of the conductor when unwound, but attempts have also been made to use them at wavelengths which are equal to or greater than the unwound length.
Other known helical antennae radiate a wave whose magnetic field is polarized in a direction perpendicular to the helix axis, with a radiation diagram substantially circular in a plane perpendicular to the axis, one end of the helically wound conductor being isolated or connected to a conducting component forming a terminal capacitance with'respect to ground, whereas the other end is connected to the central conductor of a coaxial line whose external conductor is grounded. These antennae are not very efficient, however, since their impedance to the coaxial line is very different fromthe characteristic impedance of the line.
As a rule, small antennae of the aforementioned type give mediocre results, either because'the'magnification coefficient of the circuit containing them becomes very high with a narrow pass band, or owing toconsiderable losses, or because the impedance of the antenna at the terminals of the power supply or load is much too small compared with the impedance of the power supply and'the load themselves.
The present invention provides an antenna comprising a single conductor helically wound round a geometrical surface having a longitudinal axis, one end of the conductor being connected to a point on a conductive ground plane and the other end being insulated, the antenna comprising means for coupling it to a load circuit and being characterized in that, if )t denotes the wavelengthinfree space corresponding to the operating frequency and 1 denotes the total length of the wound conductor measured along said conductor, A and satisfy the relation and that the said coupling means are coupled to the conductor along a pan of the conductor having the said first end as one of its ends, the length of the said part being equal to a very small fraction of the wavelength.
The average length of a turn of the helically wound conductor and the axial length of the antenna both are much smaller that the said wavelength, and this all the more that there are more turns in the helical winding. In fact, the said average turn length is equal to the quotient by the number N of the turns of the length l, itself always smaller than 0.45 A
In a first embodiment of the invention, the said coupling means are a direct or shunt connection to the said load circuit between the aforementioned point and another point which is very near to the first point and situated on the conductor.
in a second embodiment of the invention, the said coupling means are a magnetic coupling device comprising a short length of auxiliary conductor parallel and very near to, but not in contact with, the said part of the said antenna conductor, the said length of auxiliary conductor having one end connected to the said ground plane and the output circuit being connected between the ground plane and the other end of the length of auxiliary conductor.
A number of types of antennae according to the invention will now be described. First, to clarify ideas, the expression extremely small size compared with the operating wavelength implies that the diameter of the helical windings can be as small as one-hundredth of the wavelength in free space, and the axial height can be as small as one-fiftieth of the wavelength. As a result, antennae according to the invention are much smaller than prior art antennae.
In the case of a cylindrical helix, experiments by the Applicants have shown that the height and diameter of the ideal cylinder on which the helical conductor is wound can be varied within wide limits, provided that the product of the number of turns N and the diameter d of the cylinder is between one-seventh and one-tenth of the wavelength A in free space at the operating frequency, i.e.:
). llOSNdSh /7 0.l0SNd/)\S 0.14 (1) if the pitch of the helix is considerably smaller than the cylinder diameter, the quantity (Nrrd) represents the length I of the antenna wire; in antennae according to the invention, therefore, the ratio l/A satisfies the condition:
If the helix is conical and not cylindrical, the quantity d should be replaced by the arithmetical mean of the diameters of the first and last turn in the helix.
The operation of the antenna and its improved efficiency will now be explained.
The helical conductor winding greatly reduces the apparent propagation velocity of electromagnetic energy measured in the direction of the helix's axis. It is also known that the propagation velocity of the electromagnetic energy along the conductor itself is slightly less than it would be if the conductor was rectilinear.
Since the total length of the helically wound conductor is greater, at the operating frequency, than a quarter of the wavelength in free space, andsince its upper end is insulated whereas its lower end is grounded (or connectedto a ground" conductor plane), the current distribution along the conductor has maximum intensity at a point near the lower end. Near the end, the local impedance varies very rapidly as the measuring point moves along the conductor. If k c denotes the wavelength measured along the conductor and x is the distance of the measuring point from the lower end, the impedance is expressed by a function of the type Z being a very high impedance. The function varies very rapidly in the neighborhood of x=0.
Accordingly, near the lower, grounded end of the antenna, there is a drive point where the apparent impedance, measured between this point and ground, is close to a predetermined, e.g. the characteristic impedance of a coaxial supply line.
Furthermore, the above reasoning shows that, allowing for the electric image with respect to the antenna ground plane, the antenna behaves substantially like a half-wave antenna, whose favorable properties are well known.
Furthermore, if the drive point is chosen in the manner described, the impedance can be transformed so as to transform the aforementioned radiation resistance to a value which facilitates matching with a coaxial supply line.
It is also well known that the maximum power which can be radiated by an antenna energized by a given power supply is independent of the ratio of its physical dimensions to the wavelength in free space, except to the extent that this ratio makes it difficult to match the supply line to an excessively small radiation resistance, in which case there would be considerable losses through dissipation in the passive resistances always present in the system.
Since the device according to the invention performs the aforesaid matching satisfactorily, it can efficiently transform the energy from the power supply into radiant energy.
The invention will be more clearly understood from the following description and accompanying drawings, in which:
FIG. I shows a conically-wound antenna according to the invention, a part of which is shunted by an adjustable capacitor, to facilitate tuning:
FIG. 2 shows an antenna wound around a circular cylindrical surface, according to the invention;
FIG. 3 shows an antenna similar to that of FIG. 1, but without the adjustable capacitor;
FIG. 4 shows an antenna similar to that of FIG. 2, but provided with an adjustable capacitor;
FIG. 5 shows an antenna similar to that of FIG. 2, but coupled in a different manner to its load or power supply circuit.
FIGS. 6 and 7 show antennae similar to that of FIG. 2, but provided with an extra capacitive load (hereinafter called roof) at their nongrounded end;
FIGS. 8 and 9 show other forms of cylindrically-wound antennae provided with a capacitive load having a mechanical shape differing from that of FIGS. 6 and 7.
FIGS. 10 and 11 illustrate a method for manufacturing a cyIindrically-wound antenna by rolling a sheet of insulating material on which a conducting strip has been stuck;
FIGS. l2, l3 and 14 illustrate processes for the manufacturing of a cylindrically-wound antenna having a capacitive load, by a method similar to that used in the case of FIGS. 10 and 11;
FIGS. 15a, 15b and 16 illustrate a variant of the manufacturing process used in the case of FIGS. 10 and 11; and
FIGS. 17 and 18 illustrate a process for the manufacturing of a conically-wound antenna by rolling a sheet of insulating material on which a conducting strip has been stuck or preferably printed.
An example of antenna constructed by the Applicants, which is a special case of the class of antennae according to the invention, can be disposed in a missile head and is shown in FIG. 1. A conductor 41, whose unwound length is approximately 0.4 )t is wound into a conical helix, the cone containing the central line of the conductor, which has a base approximately A /40 in diameter and a height of approximately )t /25. The conductor is driven at point 7, near the grounded end 46, by the inner conductor of coaxial line 43, whose outer conductor 44 is connected to ground 45. An adjustable capacitor 48 is used for tuning, one plate being connected to point 42 on conductor 41 and the other plate being connected to ground 45 The antenna has a pass band at 3 db. of attenuation of 3 percent of the operating frequency, and the efficiency is less than 0.5 db. belowthat of a half-wave dipole.
Experiments by the Applicants have shown that the size and shape of helical antennae can be varied within very wide limits without affecting their optimum performance at wavelengths much greater than their sizei.e., the antennae can be adapted in the most efficient manner to the conditions for particular applications, e.g. can be given the required rigidity and appearance and can be disposed in a required space, etc., provided that the conditions expressed by the relation (1) between the product Nd and the wavelength are precisely adhered to.
FIG. 2 shows a first type of cylindrical antenna according to the invention.
A silver-coated brass conductor wire 51, 2 mm. in diameter, is rolled into a helix on a support 52. The support consists of two plates 53, 54 perpendicular to one another and made of laminated plastics such as epoxy glass. The two plates form a sort of cylinder, coaxial with the antenna. The cylinder on which the helix is wound is 50 mm. in diameter and 100 mm. high. There are five turns and the length of wire is approximately cm. The support is fixed on a brass baseplate 55 which acts as a ground plane and is 2 mm. thick and 70 mm. in diameter.
The antenna is designed to operate at a frequency of 137 MHz., i.e. at a wavelength A 2,18 m. It can be seen that The inner conductor of the coaxial supply line 56 is connected to the antenna wire at a point 57 approximately I cm. from the grounded end 59 of the wire. As a result, the distance between the grounded end and the drive point is approximately one two-hundredths of the wavelength.
The antenna in FIG. 2 is very nearly as efficient as a halfwave dipole, the difference being between 0.5 and I db., and the pass band, of the order of 1 percent of the average operating frequency, is considerably wider than in prior-art small antennae.
In the variant shown in FIG. 3, the surface containing the central line of the conductor is not a cylinder but a truncated cone, which flares out more than in FIG. I. The antenna is as efficient as that shown in FIG. 2, provided that relation (l) is adhered to, where d is the arithmetical mean of the base diameters of the truncated cone. In the antenna shown in FIG. 3, 61 denotes the antenna wire, 65 the earth level, 66 the inner conductor of the coaxial supply line, 67 the drive point, 68 the coaxial line and 69 the earthed bottom end of the antenna.
Experiments have also shown that helical antennae having a great variety of shapes (the surface containing the central line of the conductor can, for example, be a truncated cone having elliptic bases) give equally satisfactory results provided that relation I is observed, where d is the average diameter defined as: p
:1 and d being the axes of the ellipse forming the major base, d and d being the axes of the ellipse forming the minor base, and d and d the axes of the ellipse halfway between the bases. The antennae can have auxiliary apparatus for adjusting the resonance frequency or impedance, or for reducing the dimensions without affecting the performance. The auxiliary facilities, which are described below, are applied for simplicity to the cylindrical antenna only, as shown in FIG. 2, but they can of course also apply to the variant shown in FIG. 3.
According to a first auxiliary arrangement, shown in FIG. 4, a fixed or variable capacitor 91 is connected at one end to a point 92 on the conductor acting as the antenna, and at the other end to ground 55. Independently or in combination, another capacitor 93 can be inserted in series in conductor 51.
The capacitors can be used to adjust the resonance frequency of the antenna in its operating range.
FIG. 5 shows a second auxiliary facility. this time for driving the antenna by magnetic coupling. The central conductor of coaxial line 58 is parallel to, but not connected to, conductor 51 for a length 101 which can be adjustable and is connected to earth 55 at point 102. The distance between length 101 and conductor 51 can also be adjustable. The antenna impedance can also be matched with the impedance of the coaxial line. FIG. shows telescopic separating members for keeping the required distance between wires 51 and 101.
FIG. 6 shows another auxiliary facility. The end 157 of conductor 51 is connected to the center of a nonradiating roof Diameter of minor base of cone: d
Arithmetical mean of diameters: d Numberofturns: N
Height of truncated cone: h Resonance frequency: f Resonance wavelength: A Quotient: Nd/A Quotient: d/lt Quotient: h/k
for a number of conical helical antennae constructed-according to the invention.
TABLE II N 4.5 4.5 e 5.75 5.5 7.5 7.5 7.5 Nd 292 292 292 288 270 254 253 209 203 h, 45 90 135 60 120 175 so 150 210 f,MHZ 131.5 129 129 132 129 129 132.5 132.5 128 Mmn 2,300 2,300 2,300 2,300 2,300 2,300 2,300 2,300 2,300 Na/x 0.127 0.127 0.127 0.125 0.120 0.115 0.114 0.114 0.114 (ti/M10 2. 33 2. s3 2. s3 2. 09 2. 09 2. 09 1. 52 1. 52 1. 52 (ll/D10 2 4 0 2. 01 5.20 7.01 3.43 5.52 9.10 (l/x)10 3s 3s 3s 3s 3s 39 3s 3s 38 111 consisting of radial conductors 112 and a circular conductor 113 disposed in a plane parallel to ground level. The roof" enables the antenna to operate at a longer wavelength, if required, than in the absence of a roof.
FIG. 7 shows a variant of the last-mentioned facility, in which the too instead of being a network of conductors, is a metal plate 121 connected and disposed in the same manner as in FIG. 6.
FIG. 8 shows an advantageous method of constructing a roof having a variable area, which can be used to vary the resonance wavelength from the operating wavelength for a helical antenna without a roof to a wavelength approximately 30 percent greater. The roof is made up of independent blades 121 which can rotate round a shaft 122 and are held between abutments 123 and 124. The blades can open out like a fan. The lower abutment 124 is connected to the end 157 of the helical antenna. If n is the number of blades, each blade can cover a sector of a circle over an are extending slightly more than 360/n. By opening and closing the resulting fan, very large variations can be made in the capacitance between the antenna roof and earth level.
The following parameters are shown in Table l:
Diameter of helix: d
Number of turns: N
Pitch of helix: p
Height of helix: h
Resonance frequency: f
Quotient: Nd/A Quotient: d/)t Quotient: h/k
These parameters are for a number of cylindrical helical antennae constructed according to the invention.
The tables show that: a. the quotient Nd/A in each case is between 0.10 and 0. l4,
according to condition (1); b. the quantity I/)\ is always between 0.30 and 0.45, according to condition (2), and c. if conditions l and (2) are observed, the ratio d/k of the cylinder diameter or of the arithmetical means of the end diameters of the truncated cone, to the wavelength, and the ratio h/ of the height of the antenna to the wavelength, can be varied within relatively wide limits with practically no adverse effect on performance. The limits are as followg 0.0l s dllt s 0.04 0.02 s h/A s 0. I25
i.e. d must be between )1 I and )t /25 and h must be between )1 I50 and A 8.
As has been stated, the distance between the drive point and the grounded end of the antenna is about one two-hundredths of the wavelength.
Up to now, the nature of the ground plane has been left undefined. The ground plane can, of course, be the metal wall of a vehicle or missile, etc.
A description will now be given of a particularly efficient and economical method of manufacturirg helical antennae according to the invention: 1
Known helical antennae are made sufficiently rigid for operating conditions, either through the intrinsic rigidity of the conductor of which they are made, or by auxiliary elements such as masts, stays and braces or by being wound round an insulating former. These elements complicate the structure, are unsightly, increase the weight and expense of the antennae, and are difficult to manufacture, especially in a mass-production process, when the antennae need to have small dimensions.
One aim of the invention is to construct light, shapely anten- TAB LE I Table 11 gives the following parameters: Diameter of major base of cone: d,
nae which are extremely rigid and which have perfectly reproducible characteristics.
Antennae manufactured by the method according to the invention can comprise matching and adjusting elements such as roofs and variable or fixed capacitors, and can be embedded in a material such as polymerisable resin, having a suitable color and external shape. so that the resulting appearance is suitable for each particular application.
According to the invention, a conical or cylindrical helical antenna is manufactured by rolling a flexible sheet of a dielec tric, to whose surface a thin metal conductor tightly adheres.
In the embodiment of a cylindrical antenna shown in FIGS. 10 and 11, a conducting strip 211 cut from a thin sheet of metal, e.g. copper, is tightly glued or stuck by any other known method of manufacturing printed circuits on to a rectangular sheet 210 of flexible dielectric material such as laminated fibre glass and epoxy resin. The sheet 211 is then rolled into a right cylinder. The resulting cylinder is held in position by clamping, riveting, glueing or any other suitable method.
The dimensions of sides 212, 213, 214 and 215 of dielectric sheet 210, the length of conductor 211 and its slope 216 are chosen so that the cylinder obtained by rolling the sheet round an axis parallel to sides 214 and 215 comprises a number of successive layers equal to the number of turns in the helix being manufactured. In FIG. 11, for example, the antenna body comprises three layers of dielectric sheet 210, so that conductor 211 forms a helix having three turns.
FIGS. 12 and 13, which correspond to FIGS. 10 and 11,
show a similar method of manufacturing a cylindrical helical antenna having a number of matching and adjusting components.
Conductor 221 which is similar to conductor 211 is extended along part of sides 222 and 223 of dielectric sheet 220 by end bars 224 and 225, and has a connecting leg 226 extending perpendicularly towards bar 225. A terminal strip 227 is disposed near end bar 225.
Dielectric sheet 220 is rolled in the same manner as sheet 210 to give the tubular antenna body shown in FIG. 13.
In FIG. 13, end bar 224 of conductor 221 is connected to a brass nonradiating component or roof 231 by a circular weld 232. End bar 225 is similarly connected to a brass baseplate 233. The antenna is driven by a coaxial line whose inner conductor 234 is connected, e.g. by welding, to terminal strip 227, and the external conductor 235 is connected to baseplate 233 by a connection 236.
Terminal strip 227 is connected to end bar 225 by a conductor 237.
An adjustable tuning capacitor 238 is connected between leg 226 and baseplate 233.
FIG. 14 is a cross section of the antenna diagrammatically shown in FIG. 13.
As FIG. 14 shows, the end bar 224 of conductor 221 is connected to brass roof 231 by circular weld 232, and end bar 225 is connected to baseplate 233. Plate 233 is a solid brass disc provided with a circular groove 330 for receiving the lower part of the tubular antenna body. Baseplate 233 is fixed by locking or welding bar 225 in groove 330.
The opposite end of the baseplate is prolonged by a threaded shank 331 for attaching the antenna to a support by tightening a nut 332. Threaded shank 331 is formed with a borehole 333 terminating in a device 334 for gripping and holding the coaxial line supplying power to the antenna. The inner conductor 234 of the coaxial cable is connected to terminal strip 227, and the outer conductor 238 is connected to baseplate 233.
An opening 241 through the successive layers of dielectric sheet 220 gives access to the component for adjusting the tuning capacitor 238 disposed inside the antenna body. After being adjusted, the said component and the connection inside the antenna body can be held in position by embedding them e.g. in a polymerisable resin 242.
In a variant shown in FIGS. 15a and 15b, parallel conductors 251 are disposed on each side of a dielectric sheet 250. The conductors on each surface are, respectively, symmetrical with respect to the plane passing through the middle of the thickness of the dielectric.
The slope 254 of the conductors is such that, after the dielectric sheet has been rolled into a single turn, the ends of the conductors on one surface overlap the ends of the conductors on the other surface so as to form the helix shown in FIG. 16. The conductors are connected by welding at points 255.
Rectangular spare lengths 252, 253 without conductors are disposed at each end of sheet 250 so as to hold the antenna body in position, e.g. by clamping or glueing.
FIGS. 17 and 18, which are similar in principle to FIGS. 10 and 11, show two essential stages in the manufacture of a conical helical antenna. FIG. 17 shows the unfolded surface of a truncated cone having a circular base and made from a dielectric sheet 260 having a conductor 261 stuck to its surface. FIG. 18 shows the truncoconical antenna formed by rolling dielectric sheet 260 until the ends overlap.
The examples described are cylindrical or conical helical antennae having a circular base and a constant pitch. These embodiments, of course, are in no way limitative and the cited method can be applied to the manufacture of cylindrical or truncoconical helical antennae having a noncircular, e.g. elliptical, base and/or a variable pitch. The cited extension of the process is an integral part of the invention.
More particularly, in the case of FIGS. 17 and 18 where, owing to its nonrectilinear shape, the conductor cannot be manufactured in the form of a metal strip stuck to a sheet of dielectric material, part of the surface of the sheet can be coated with metal, e.g. in the manner used for printed circuits.
1. An antenna comprising a plurality of turns of a single conductor helically wound around a geometrical surface having a longitudinal axis, one end of said conductor being connected to a conductive ground plane and its other end being free, said antenna comprising means for coupling it to a load circuit and being characterized in that, A denoting the wave length in free space corresponding to the operating frequency and by I the overall length of the wound conductor measured along said conductor, A and l satisfy the relation:
0.3). s I s 0.45k
said coupling means including a short length of auxiliary conductor parallel and very near to, but not in contact with, said part of he said antenna conductor and being coupled to said conductor along a part thereof close to and terminating on said ground place, the length of said part being equal to a very small fraction of said wavelength, the said length of auxiliary conductor having one end connected to the said ground plane and the said load circuit being connected between the said ground plane and the other end of said length of auxiliary conductor.
2. An antenna according to claim 1, wherein an adjustable capacitor is connected between one point on the conductor and the said ground plane.
3. An antenna according to claim 1, wherein the end of the conductor which is not connected to the ground plane is connected to a conducting element acting as a capacitive load with respect to ground.
4. An antenna according to claim 3, wherein the conducting element comprises linear conductors disposed along the circumference and radii of a circle.
5. An antenna according to claim 3, wherein the said element is a plane, metal conducting plate.
6. An antenna according to claim 3, wherein the said element comprises a number of plates in the shape of sectors of a circle, which can be rotated round a common axis.
7. An antenna according to claim 1, wherein said conductor consists of a thin metal strip which adheres to a flexible sheet of dielectric material rolled in the shape of said surface.
8. An antenna according to claim 7, wherein said conductor is a thin metal strip printed on the surface of said dielectric sheet.
9. An antenna according to claim 7, wherein, it comprises auxiliary matching elements held in position by immersion in a dielectric substance.