US2785398A - Radio antennae - Google Patents

Description

March 12, 1957 B. B. JACOBSEN ETAL 2,785,398

RADIO ANTENNAE Filed July 3. 1952 Inventor B- 8. JACOBSEN D. L, THOMAS m ww Atlorney United States Patent RADIO ANTENNAE Bent Bulow Jacobsen and David Lane Thomas, London, England, assignors to International Standard Electric Corporation, New York, N. Y.

Application July 3, 1952, Serial No. 297,088 Claims priority, application Great Britain July 9, 1951 4 Claims. (Cl. 343-781) This invention relates to antenna arrangements of the type comprising a concave mirror having a co-operating antenna located at or adjacent to the focal point of the mirror. More particularly, it relates to arrangements of the said type in which the construction is such that one or more reflecting surfaces obstruct in part the mouth of the mirror and reflect back thereto some of the energy which would otherwise be radiated by the mirror into space. Such reflection-back effects the impedance matching of the co-operating antenna and renders it more difiicult to avoid standing waves, variable with frequency, in the antenna feeder, this effect being of particular importance in the case of antenna arrangements for wide-band systems such as television or multi-channel links. It is accordingly an object of the present invention to provide an antenna arrangement of the type described above, adapted for minimising the effect of reflection-back from surfaces obstructing radiation from the mouth of the mirror.

According to its most general aspect the invention resides in a radio antenna arrangement comprising a concave mirror, a co -operating antenna located at or adjacent to the focal point of said mirror, and at least one member so located as to obstruct radiation from said mirror, characterised in this, that said obstructing member is so formed as to reduce the effect of reflection back to the mirror of radiation therefrom intercepted by said member.

The invention is illustrated and explained in connection with the accompanying drawings, in which:

Fig. 1A illustrates the undesired reflection phenomenon in an antenna arrangement which does not incorporate the present invention; and

Fig. 18 illustrates an embodiment of the invention.

in these figures the same reference numerals are used to designate corresponding parts. Only those parts of the antenna arrangements are shown which are essential to the understanding of the invention.

One particular antenna arrangement in which the said refiection-back effect has been found to be of importance is illustrated in Fig. 1A of the accompanying drawing and comprises a paraboloidal mirror 1 having at its focus "a co-operating antenna in the form of a horn 2 which is coupled to an energy translating device (not shown) such as a transmitter or receiver by means of an H-bend 3 followed by a linear wave guide 4 of rectangular crosssection, which wave guide lies radially across the mouth of the mirror with one wall of the guide parallel to but slightly spaced out from the aperture of the mirror. In this arrangement energy projected from the mirror parallel to the generating axes impinges normally on the wall 5 of the linear guide and is reflected therefrom back to the mirror surface and thence to the horn antenna along paths a and b for example. Since the total reflection path from any point on the reflecting wave guide surface to the horn antenna via the mirror surface is approximately constant, the' contributions from difierent points 2,785,398 Patented Mar. 12, 1957' along the length of the .guide are approximately cophasal, the total being of appreciable magnitude.

One method of reducing the reflection-back is to provide an additional reflecting surface the reflection from which is equal in magnitude but opposite in phase to that just mentioned. This may be done, in one embodiment of the invention, by adding an auxiliary reflecting memby extending the wave guide linearly beyond its junction with the l-l-ben-d completely across the mouth of the mirror by means of a dummy portion of wave guide as illustrated at 6 in Fig. 1B, the dummy portion having a flat reflecting surface 7 facing the mirror, and of the same dimensions as the wave-guide proper, but displaced nearer the mirror by a distance of one quarter of the mean operating wavelength. This displacement is conveniently obtained by the superposition of a conducting strip 8 on the wall of the dummy portion of waveguide, this strip being of the same width as the waveguide wall, and one quarter wavelength in depth. Since the reflecting surface of the waveguide proper and the spacing strip are equal and symmetrically located with respect. to the antenna horn, very effective cancellation of the reflected back energies is obtained. The dummy portion of wave guide may also have a mechanical advantage in that it may be used as a member for improving the rigidity of the hornand-mirror assembly.

The same principle may be applied in other ways. For example, the dummy waveguide may be dispensed with, and a quarter wave strip applied over a sufficient length of the wave-guide proper to give cancellation between the reflection from the strip and that from the uncovered part of the wave-guide. Moreover, it is not necessary that the spacing strip be so much as one quarter wavelength thick; a thinner strip may be used provided that the mounting means is such that the proper spacing out from the reflecting surface, or from the plane thereof, is obtained. Using a thin strip, the mounting mean may be made adjustable, and the shape of the strip need not be limited to one of the same width as the guide Wall, so long as the area of the strip is such that the total reflection therefrom balances the remaining reflection. Moreover, the strip may be divided into two efiective halves spaced respectively one quarter wavelength forward and one quarter wavelength backward from the plane of the offending surface, such disposition giving compensation over a very wide band.

Instead of using the principle of reflection-cancellation to remove the unwanted effect, the construction may be made such that the total reflection from an obstructing surface is small ab initio. For example, the obstructing surface may be coated with a layer of material of surface resistance such that the high frequency energy impingent thereon is absorbed instead of being reflected. Alternatively, the surface may be so shaped that components of reflection from different points add in random phase instead of cophasally for example by using a curved surface instead of a flat one for the outer wall of the guide, or by curving the axis of the guide, in a plane through the axis of generation of the paraboloid.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made by way of eX ample and not as a limitation on the scope of the invention.

What we claim is:

1. A radio antenna arrangement comprising a paraboloidal mirror and a co-operating horn antenna located at or adjacent to the focal point of said mirror, a rectangular metal wave guide coupling said antenna to an energy translating device, a major part of said wave guide being linear and located in front of said mirror with one wall of greases the guide parallel to the aperture plane of the mirror, the surface of which Wall reflects energy radiated thereon by said mirror, and a collinear extension of said coupling wave guide beyond the junction with the horn and extending in front of said mirror, the external surface of said extension facing said mirror having a flat radiationreflecting surface in a plane parallel to that containing the reflecting surface of said one Wall but displaced therefrom by a distance of one quarter of the operating wavelength, whereby reflection from part of the surface of said dummy Wave guide cancels the reflection from a corresponding part of the surface of said one wall.

2. A radio antenna arrangement comprising a mirror, a cooperating antenna located in front of said mirror and adapted to radiate energy in the direction of said mirror, transmission means for feeding said antenna, a portion of said transmission means being located in front of said mirror and having a surface which reflects energy radiated thereon by said mirror, a reflecting member forming a collinear extension with said transmission means extending in front of said mirror, the reflecting member having a radiation-reflecting surface, spaced at a different distance from said mirror than said transmission means to cancel the effects of reflection from a corresponding part of said transmission means.

3. The arrangement according to claim 2 wherein said collinear extension is displaced from said transmission line a quarter wavelength of the operating frequency in the direction of said mirror.

4. A radio antenna system comprising a curved mirror, an antenna positioned in front of said mirror substantially at its focus, a transmission means connected to said antenna and having a reflecting surface positioned in front of said mirror, whereby disturbing reflections back to said antenna are produced, and means for substantially cancelling the elfects of these disturbing reflections, comprising a reflecting means having substantially the same refleeting properties as said reflecting surface, positioned in front of said mirror and extending in a direction substantially symmetrically with respect to the transmission means, at a spacing from said mirror differing from the spacing of said transmission means by substantially the equivalent of a quarter wave length at the operating frequency.

References Cited in the file of this patent UNlTED STATES PATENTS 2,429,60l Biskeborn et al Oct. 28, 1947 2,607,010 Kock Aug. 12, 1952 2,67l,855 Van Atta Mar. 9, 1954 FOREIGN PATENTS 603,676 Great Britain Oct. 25, 1946

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