1. Field of the Invention
The present invention relates to an excitation device for an ultra-high frequency corrugated or grooved source of revolution, operating in two remote frequency bands. These remote band corrugated sources, for example X and KU or KA with central frequencies 1, 17 and 35 GHz, are used in a particularly interesting way in dual band radar systems in which the narrow beam of the high band radiation pattern is used for tracking low elevation targets.
2. Description of the Prior Art
After a brief reminder about corrugated sources and the construction and the operation thereof, their presently known excitation devices will be described along with the disadvantages thereof.
By a grooved or corrugated source is meant a wave-guide having generally a constant or increasing circular section, which has transverse grooves formed therein the grooves are of a given depth and are spaced apart from one another by a distance d.sub.O, also called the period of the corrugated source.
There also exist so-called bi-periodic corrugated sources having two types of alternating grooves.
So as to better understand the operation of a corrugated guide, the notion of mode will be recalled according to which the electromagnetic energy is propagated, that is to say of electric field E and magnetic field H configuration in the guide. It is on this configuration that the radiation of the guide depends. In a guide of revolution with a smooth internal wall, the modes existing are of the well known transverse electric TE or transverse magnetic TM type. In a guide of revolution whose internal wall comprises grooves, the modes which are propagated are of the hybrid type, that is to say they are linear combinations of the two modes TE and TM, of the same phase speed. Before going further into the details, let us first of all be quite clear about the notion of hybrid balance. An operating point of a guide, defined by a frequency f and a propagation constant B, is called a hybrid balance point when, for any cross section of this guide, it presents the following characteristics:
the electromagnetic field is cancelled out at the inner edges of the corrugated source; PA1 the field is scalar, described by a real parameter; PA1 it is of revolution; PA1 the ratio between the electric field .vertline.E.vertline. and the magnetic field .vertline.H.vertline. is constant at any point of a cross section of the corrugated guide and equal to the impedance of the wave being propagated in the vacuum, (characteristic impedance .eta. of the propagation of the wave in free space): .vertline.E.vertline.=.eta..vertline.H.vertline..
For these hybrid balance points, the radiation patterns of the corrugated sources have the same properties, presenting more particularly the advantage of having weak lateral lobes since the electromagnetic field is cancelled out at the edges of the sources and an equality of patterns in the E and H planes. Another very interesting advantage is that there is no cross polarization in the radiation patterns.
Now, these hybrid balance points are very particular operating points of a corrugated guide, the whole of all the operating points forming the dispersion curves of the guide, which curves represent the different hybrid propagation modes. These hybrid modes already defined above comply, so as to exist in a guide, with certain conditions at the limits, i.e. at the level of the internal wall of the guide, more particularly with this one condition: the electric E.phi. and magnetic H.phi. components situated in a cross section of the guide and perpendicular to the radius thereof, are equal to zero. Now, in a wave guide it is precisely the non zero component H.phi. which induces longitudinal currents along the internal wall. This is why, so as to fulfil the condition H.phi.=0, these currents must be eliminated by placing obstacles in the internal wall of the guide, grooves for example which prevent any current flow.
FIG. 1 is an example of dispersion curves of a simple corrugation source, only comprising a single type of groove. These dispersion curves represent the propagation constant B of the wave which is propagated in the corrugated guide as a function of the propagation constant k of the wave in a vacuum or in free space. Curve C.sub.1 represents the hybrid mode EH.sub.11, curve C.sub.2 the hybrid mode HE.sub.11 which each present a hybrid balance point, referenced respectively P.sub.1 and P.sub.2. About a hybrid balance point, the operating passband is less than an octave. The periodicity of the curves is (2.pi./d), d being the period of the grooves in the guide.
FIG. 2 shows the dispersion curves of a bi-periodic corrugation source, operating in two different frequency bands, remote from one anothe (X and KU). The alternation of the two series of grooves allow the two modes of the simple corrugated sources to be coupled together. This alternation promotes the appearance of hybrid balances. It can be seen that the dispersion curves C.sub.3, C.sub.4 and C.sub.5 corresponding to the lowest operating band has a period (2.pi./d') about twice as small as that of the dispersion curves C.sub.1 and C.sub.2 corresponding to the same operating band for a simple corrugated guide, whose repetion period d of the corrugations is twice as small as the d' of the bi-periodic source. New hybrid balance points P.sub.3, P.sub.4 and P.sub.5 appear, on the one hand, in the lowest band, thus resulting in a better stability and, on the other hand, in the highest band (P.sub.6 and P.sub.7).
The excitation devices known at present for these corrugated sources are formed by a smooth circular guide opening directly into the mouth of these sources. The dimensions of such an excitation guide must be sufficiently small for only the fundamental mode TE.sub.11 to be propagated, whose electric field lines, in a cross section of the guide, shown in FIG. 3b, are the closest to those of the hybrid mode propagating in a corrugated source, the hybrid mode HE.sub.11 for example shown in FIG. 3a. It can be seen that these lines are almost rectilinear and parallel to each other in the center of the guide, but curved towards the edges. This curvature of the field lines shows that the matching between the smooth excitation guide and the corrugated guide is not perfect. To improve this matching, the first grooves of the corrugated guides are given more or less empirically different values from those assigned by the theory of corrugated structures.
For the corrugated sources operating in a single frequency band, this device for exciting by means of a smooth guide only excites the hybrid mode HE.sub.11, all the other possible modes being evanescent at the nominal frequency.
For corrugated sources operating in two remote frequency bands, several parasite hybrid modes are excited at the same time as the useful hybrid mode. When the two frequency bands are sufficiently close to each other, the ratio between the central frequencies being 1.5 or 1.6, these parasite modes are evanescent in these two bands and do not disturb the normal operation of the source.
But in so far as the sources are concerned operating in remote bands (X and KU or KA), that is to say whose central frequency ratio is greater than or equal to two, several propagative parasite modes may coexist with the desired useful mode and are even generated in the presence of the single fundamental mode TE.sub.11 in the smooth excitation guide. In fact, the incident mode TE.sub.11 is broken down into an infinite series of modes in the corrugated source, the first two or three of which modes are propagative in the high band.
In addition to this electrical disadavantage, there is the disadvantage presented by the successive mounting of the two smooth excitation guides, each attributed to one of the two operating bands. On the other hand, when a source is to operate simultaneously in two remote frequency bands with hybrid modes, the construction achieved, shown for example in FIG. 4, is very space-consuming. Such a source is formed from two monoband corrugated sources 40, 41, each excited in accordance with a knon procedure, by a smooth guide for example 42 and 43. Source 40 radiating in the lowest frequency band has larger dimensions than the source 41. They are placed so that their respective propagation axes A.sub.0 and A.sub.1 are perpendicular. A frequency spatial filter 44 is disposed at the the output of the two sources, at 45.degree. to the two axes A.sub.0 and A.sub.1. This filter 44 lets the low band wave pass and reflects the high band wave at 90.degree.. Such a dual band source is space consuming and costly.