US 3449752 A
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June 10, 1969 a. SPlTZ ET AL 3,449,752
HELICAL ANTENNA ELECTROMAGNETI'CALLY COUPLED To RESONANT LINE Sheet Filed Oct. 6, 1966 June 10, 1969 E. SPITZ ET AL 3,449,752
HELICAL ANTENNA ELECTROMAGNETICALLY COUPLED T0 RESONANT LINE Filed Oct. 6, 1966 v v Sheet 2 I of 2 FIG 5 United States Patent 1 3,449,752 HELICAL ANTENNA ELECTROMAGNETICALLY COUPLED T0 RESONANT LINE Erich Spitz and Ren Romeas, Paris, France, assignors to CSFCompagnie Generale de Telegraphic Sans Fil, a corporation of France Filed Oct. 6, 1%6, Ser. No. 584,829 Claims priority, application France, Oct. 8, 1965,
Int. Cl. Htllq 1/36 us. or. 343-s9s 7 Claims The present invention relates to antennae with longitudinal radiation, comprising a helical radiating structure fvhich is electromagnetically coupled with a propagation The travelling wave excitation of the helical structure makes it possible for these antennae to radiate within a broad frequency band. However, to obtain a substantial gain and directivity, the transverse and longitudinal dimensions have to be rather large.
In certain applications, such as radiotelephony, there arises the problem of reducing the dimensions of the antenna, if need be at the cost of an acceptable reduction of the pass-band.
As is well known, the gain and the directivity of an antenna depend directly on its dimensions expressed in wavelengths so that any reduction of transverse and longitudinal dimensions is normally limited by considerations relating to gain and directivity.
It is an object of the invention to solve this question in an acceptable manner.
According to the invention there is provided an endfire radiator element comprising: an helical radiating structure having an axis; a bifilar resonant line extending coaxially with said structure; means for coupling said line to a feeder; said line having two parallel conductors having respective first and second ends; said first ends being short-circuited and short-circuiting means for shorting said second ends for adjusting the resonance frequency of said line.
For a better understanding of the invention, reference will be made to the drawings accompanying the following description and in which:
FIG. 1 is a first embodiment of an antenna according to the invention;
FIG. 2 is an end view of an antenna according to the invention;
FIGS. 3 and 4 are further embodiments of an antenna according to the invention; and
FIG. 5 shows the directivity diagram of an antenna according to the invention.
FIG. 1 shows a radiating structure 1, essentially consisting of a conducting wire wound in the form of a helix and supported on insulators 5. Coaxially to this structure extends a propagation line consisting of conductors 2, interconnected by means of transverse short circuit bars 3. The conductors 2 are supplied at A and B by means of a symmetrical line 4 which supplies electromagnetic energy to the resonant line 2-3 substantially at one of the resonance frequencies of the line 2-3 which frequencies are given by the relation:
wherein J is a whole number;
V is the phase velocity of the waves propagated by the line;
L is the distance between the short circuits provided at the ends of the line.
The electromagnetic wave generated in the line is the "Ice sum of all TEM waves which are reflected successively from the short-circuited ends 3. Since the supply frequency is near to one of the frequencies f one obtains a system of standing waves with very high amplitudes.
FIG. 2 shows the law governing the distribution of the electric field lines corresponding to the TEM mode, surrounding the conductors of the excitation line 2. In the absence of a radiating structure 1, the line does not radiate. Its Q factor is very high and losses are negligible. However, by providing according to the invention a radiating structure in the form of a solenoid, which surrounds the excitation line, currents i are generated by electromagnetic coupling in the windings of the structure. These induced currents generate in turn an electromagnetic wave whose electric field has a non-zero component in the longitudinal direction which enables the assembly to radiate in the same way as an array of sources, distributed along the structure. The electromagnetic interaction between the excitation line 2 and the structure 1 modifies the standing wave mode in the line, which has lower phase velocity and Q factor.
Experience shows that the transfer of energy between the resonant line 2 and the radiating structure 1 could be effected very satisfactorily in spite of the small transverse dimensions of the spiral which may be between M30 and M12, wherein is the wavelength of the radiated wave. However, the length of the antenna may be reduced to half the wavelength of the wave propagated along the line.
The very small dimensions of this antenna would not permit to achieve a sufficient coupling between the line and the helix, if the ends of the line were not shortcircuited according to the invention. Actually, the wave generated in the line would then be a travelling wave which could pass only an insufficient amount of energy to the helix. The use of a line with resonance excitation makes it possible to increase substantially the electromagnetic coupling, due to the total reflexions which occur at the short-circuited ends. Everything happens as if the antenna were the seat of a travelling Wave over a length substantially greater than its actual length.
Although one must not expect to obtain a broad bandwidth with a resonant line, experience shows that, with a correctly matched supply, a standing wave ratio of less than 6 can be achieved, i.e., an attenuation of the transmission of less than 3 db in a frequency band of the order of -2 percent.
The antenna shown in FIGS. 1 and 2 radiates longitudinally along two opposed lobes. However, according to the invention an antenna, which radiates at one end only, is also provided.
To this end, as shown in FIG. 3, the invention provides an antenna comprising a plane reflector 6 which forms one of the short circuited ends of the line 2. The other short-circuiting element 2 is slidably mounted into the line conductors so as to permit the tuning of the resonant line. The supply of the line 2-3 is effected by means of a coaxial connection 7 whose core forms a coupling loop 8 which is coplanar with the conductors of the line 2.
FIG. 4 shows a further embodiment of the antenna according to the invention, differing from the preceding one by the manner in which energy is fed to the line. A coaxial cable 9 passes through the reflector 6 and is coupled at A and B to the line 2 at the distance I from one end thereof. Also a coupling as shown in FIG. 1 may be used.
In FIG. 5 is shown, in solid lines, the directivity diagram in the plane xoz of an antenna according to the invention, equipped with a plane reflector.
By way of comparison, the dotted lines on the diagram show the directivity of a dipole whose elements are dis posed parallel to o-x. In spite of a much smaller transverse dimension, it can be seen that the antenna according to the invention has a gain and a directivity substantially better than the dipole.
By way of non-limitative example, an antenna according to the invention may have the following characteristics in accordance with the diagram of FIG. 3:
The helix comprises 27 turns of wire with a diameter of (2.0) mm.
The conductors of the line are tubes with 3 mm. diameter. The plane reflector consists of a disc of 300 mm. diameter, forming a lattice Whose meshes have a side length of 50 mm.
Such an antenna has, at 780 mc./s., a bandwith of 11 mc/ s. Its main lobe has an aperture at 3 db, equal to 30. Its secondary lobes and the rear lobe are 19 db below the main radiation lobe. The gain is better by 9 db than that of a dipole radiator at 780 mc./ s.
Of course, an antenna according to the invention may be used as a primary source or for building up an array of radiating elements grouped in parallel on a common plane reflector, or aligned along a common longitudinal axis. It is therefore possible to make use of this antenna in all circumstances where a bandwidth of a few percent is regarded as suflicient. At the cost of this restriction, which is all the less important, as carrier frequencies are higher, a light antenna with a simple construction and small dimensions is obtained, Without sacrificing anything with regard to gain and directivity.
What is claimed is:
1. An end fire radiator element comprising: a helical radiating structure having an axis; a bifilar resonant line extending coaxially with said structure; means for coupling said line to a feeder; said line having two parallel conductors having respective first and second ends; said first ends being short-circuited and a short-circuiting means for shorting said second ends for adjusting the resonance frequency of said line.
2. An end fire radiator element as claimed in claim 1, wherein said coupling means comprise a bifilar feeder connected across said conductors near one of said ends.
3. An end fire radiator element as claimed in claim 1, wherein said coupling means comprise a coaxial feeder having an inner conductor and an outer conductor respectively connected to said conductors near one of said ends.
4. A end fire radiator element as claimed in claim 1, wherein said coupling means comprise a coaxial feeder inductively coupled to said line near one of said ends.
5. An end fire radiator element as claimed in claim 1, wherein said means for shorting said second ends comprise a slider.
6. An end fire radiator element as claimed in claim 1, wherein said helical structure has a transverse dimension smaller than M12; A being the free-field radiated wavelength.
7. An end fire radiator element as claimed in claim 1, further comprising reflector means positioned at one of said ends.
References Cited UNITED STATES PATENTS 3,263,233 7/1966 Spitz 343-895 ELI LIEBERMAN, Primary Examiner.
US. Cl. X.R.