GB2292640A - Receiver protection device - Google Patents

Abstract

Device for protecting receivers of microwave frequency signals from the reception of high energy signals which could damage them. The basis of the invention is the fact that a circular guide (1) is used having a plurality of stubs (5, 6, 7) having specific functions and arranged in separate planes along the waveguide. The stubs (5) are fitted with limiting diodes which become conductive when the energy of the microwave frequency signals exceeds a certain threshold. The stubs (6) are used to compensate, at low level, for the interferences introduced by the stubs (5). Finally the stubs (7) are provided in order to absorb the microwave frequency energy. The invention is applicable to radars. <IMAGE>

Description

RECEIVER PROTECTION DEVICE The invention relates to devices which enable receivers to be protected from the reception of high energy signals which could damage them, particularly in the case of receivers of radar signals which are associated with transmitters suppling high frequency pulses of high peak value.

In radars, this protection is normally obtained by gas tubes disposed in the reception channels. These tubes switch on when the received signal has a value greater than a certain determined threshold and produce shortcircuits which reflect the received signal and the latter cannot reach the receiver. On the other hand, when the received signal is lower than the threshold, the tubes remain off and the signal propagates towards the re receiver.

These gas tubes have a certain number of disadvantages among which will be mentioned a rather long response time; because of this they take a certain time to switch on and therefore allow a large energy peak to pass on towards the receiver; also, after a transmission pulse which has switched them on, they take a considerable time to switch off and therefore prevent the reception of useful signals.

Another disadvantage is that their characteristics vary in the course of time and they therefore become inoperative after a certain operating time. Finally their input window, generally made of glass, restricts the mean power to be applied. It is therefore necessary to make use of other components placed upstream, such as a ferrite limiter.

The object of the present invention is therefore to produce a receiver protection device which has a short response time both at the start of the received signal and after the end of the Latter.

Another object of the present invention is to produce a receiver protection device which can withstand high powers and does not therefore make the use of addi tional components necessary.

Another object of the present invention is to produce a receiver protection device whose lifetime is practically unlimited.

According to the invention there is provided a device for the protection of a receiver of microwave frequency signals from high energy signals, comprising a circular guide from which emerge at least three coaxial lines or "stubs": - a first of the coaxial lines having limiting diodes mounted in it and being disposed in a first transverse plane of the circular guide, - a second of the coaxial lines having an absorbing load mounted in it and being disposed in a second transverse plane of the circular guide, separated from the first transverse plane by a distance d in the direction propagation of the microwave frequency waves, and - a third of the coaxial lines, called a compensating line, being disposed in the first transverse plane symmetrically with the first coaxial line with respect to an axial plane of the circular guide passing through the second coaxial line.

Preferably, each coaxial line or stub is duplicated by a second coaxial line of the same type which is disposed diametrically opposite to the first one in the same transverse plane.

The simplest protection device in which the invention is embodied comprises a first transverse plane and a second transverse plane containing the coaxial lines described above and the distance d separating these two planes is not critical but is in general less than a quarter of the group wavelength.

In order to improve the protection, arranging a succession of first and second transverse planes each con- taining the coaxial lines described above is proposed, the first transverse planes as ell as the second transverse planes being separated from each other by a distance equal to an odd whole number of group quarter wavelengths.

More precisely, the circular guide is generally excited in TE 11 mode and this results in that the co axial lines with limiting diodes are, in the transverse plane, inclined at 450 with respect to the direction of the electrical field.

Finally, the limiting diodes are disposed at a dis tance from the periphery of the circular guide which is equal to a whole number of half-wavelengths of the micro wave frequency signals.

Other characteristics and advantages of the present invention will appear on reading the following descrip tion of a particular embodiment, the said description being given with reference to the appended drawings in which: - Figure 1 is a perspective view of a receiver protection device in which the invention is embodied, - Figure 2 is a cross-sectional view along the line II-II of Figure 1, and - Figure 3 is a cross-sectional view along the line III-III of Figure 1.

As shown in Figure 1, a receiver protection device 10 essentially comprises a central circular guide 1 in which the protection is carried out by means of a certain number of elements which will be described hereafter with reference to Figures 2 and 3. As this is a circular guide whereas the received signal is transmitted, for example, by a rectangular guide (not shown), it is necessary to interpose a transition section between the two, which bears the reference 2 on the input side of the device 10 and the reference 3 on the output side in the direction of the receiver to be protected. The assembly between the various transmission elements is for example carried out by flange systems referenced 4.

The elements which carry out the protection comprise coaxial lines distributed around the periphery of the circular guide 1 and disposed in several successive transverse planes. These coaxial lines, better known by the English term "stubs", can be grouped into three particular functions. Some of them, referenced 5, have the purpose of producing short-circuits and include socalled limiting diodes; others, referenced 6, have the purpose of absorbing the received power and include loads; finally, the third ones, referenced 7, have the purpose of compensating for the asymmetries introduced by the lines with diodes.In Figure 1, these coaxial lines are disposed in three successive pairs of transverse planes, the first plane of each pair (Figure 2) containing tuo coaxial lines with diodes and two compensating coaxial lines and the second plane of each pair (Figure 3) containing two coaxial lines with loads.

The planes of a pair of stubs with a diode (first planes) are separated from the next one by an axial distance which is equal to an odd whole number of bg/4 (bg is the group wavelength) and the same applies to the axial distance between the planes containing the coaxial lines with loads (second planes).

Furthermore, the second plane of each pair of transverse planes is separated from the first plane by a distance d which is not critical but which is generally less than one quarter of a group wavelength. This distance is not critical, because the purpose of the coaxial lines with loads contained in a plane is, as will be explained hereafter, to absorb the component of the electrical field created by the interference introduced by the coaxial line with diodes in the presence of a high energy signal. Specialists in the field will, of course, adjust this distance d experimentally in order to obtain the best possible absorption.

As shown in Figure 2, which is a cross-sectional view of the circular guide 1 at the location of the coaxial lines with limiting diodes and compensating coaxial lines, the lines 8 and 9 with diodes are diametrically opposite each other and the same applies to the lines 11 and 12, known as compensating lines.

Furthermore, the diameters thus defined are inclined at 450 with respect to a diametral plane of the circular guide 1 passing through the coaxial lines with loads 6 shown in cross section in Figure 3 and bearing the references 13 and 14.

More precisely, the inclination at 450 of the stubs is measured with respect to the direction of the electrical field E shown by the lines of force 20. This implies that the diametral plane corresponding to the stubs with toads is perpendicular to the lines of force of the electrical field.

As is known, each stub is a coaxial line comprising a central conductor 15 surrounded by a metallic envelope 16 which is separated from the conductor 15 by a dielectric. The coaxial line has one open end 17 which emerges at the periphery of the circular guide 1 and one end 18 which is closed by a short-circuit.

In order for the short-circuit disposed at the end 18 not to interfere with the propagation of the microwave frequency waves in the circular guide 1 when the microwave frequency signal must be transmitted to the receiver, the length of the stubs must be equal to an odd whole multiple of b/4, where b is the wavelength, in order to provide a high impedance at the periphery of the circular guide.

Each stub 8 and 9 comprises two diodes referenced 21 and 22 for the stub 8, each diode being connected between the outer envelope 16 and the central conductor 15. Thus the anode of the diode 22 is connected to the outer envelope while its cathode is connected to the central conductor. This connection is reversed for the diode 27.

These diodes are disposed at a distance b/2 from the end 17 or at a whole multiple of b/2.

In the case of the stubs 13 and 14, loads 23 and 24 (Figure 3) absorb the microwave frequency energy. The compensating stubs 11 and 12 do not comprise any particular arrangement and are as identical as possible to the stubs 8 and 9 except for the limiting diodes, in order to provide low level compensation for the interfering effect introduced by the stubs 8 and 9.

The functioning of the receiver protection device in which the invention is embodied is therefore as follows.

The transition section 2 enables the transition from the rectangular guide to a circular guide 1 which is excited in TE11 mode. When the energy of the microwave frequency wave which is propagating in the guide 1 is below a certain level, this mode propagates without interference up to the end of the guide 1 where, via the transition section 3, it is transformed into the rectangular guide propagation mode. When the energy of the microwave frequency wave exceeds a certain threshold, the diodes 21 and 22 become conductive and present a low impedance, i.e. a short-circuit is produced at the input of the stubs 8 and 9 connected at 450 with respect to the direction of the electrical field.These stubs, shortcircuited at their inputs, interfere with the propagation of the TE11 mode and transform the rectilinear polarization into an elliptical polarization. The loads 23 and 24 of the stubs 13 and 14 absorb the component of the electrical field which has undergone a rotation.

The phenomenon, which has just been described briefly, is repeated at each following pair of planes such that the microwave frequency energy is progressively absorbed by the loads of the stubs 6 (Figure 1) as it propagates in the circular guide 1.

The attenuation introduced by each pair of successive planes depends on the elementary interference due to the stubs 8 and 9 with limiting diodes and on the energy which can be absorbed by the loads. As this elementary interference and the absorption are limited, one is led to dispose the various stubs in such a way as to have a sufficient number of pairs of planes in order to obtain the overall attenuation required.

The device which has just been described has the following main advantages: - it can withstand a high power by using several successive planes some of which include coaxial lines with limiting diodes and compensating coaxial lines and the others of which include coaxial lines with Loads; - the microwave frequency energy which propagates in the circular guide is absorbed by the loads instead of being reflected as would be the case with gas tubes disposed in a coaxial propagation line; - the limitation is instantaneous as soon as the signal exceeding the defined threshold is applied; - the recovery time is very short compared with that of the gas tubes currently used; - the use of solid state diodes enables a lifetime of the device to be obtained which is longer than for a device with gas tubes.

Claims (9)

1. A device for the protection of a receiver of microwave frequency signals from high energy signals, comprising a circular guide from which emerge at least three coaxial lines or "stubs": - a first of the coaxial lines having limiting diodes mounted in it and being disposed in a first transverse plane of the circular guide, - a second of the coaxial lines having an absorbing load mounted in it and being disposed in a second transverse plane of the circular gulde, separated from the first transverse plane by a distance d in the direction of propagation of the microwave frequency waves, and - a third of the coaxial lines called a compensating line, being disposed in the first transverse plane symmetrically with the first coaxial line with respect to an axial plane of the circular guide passing through the second coaxial line.

2. A device according to claim 1, comprising at least six coaxial lines arranged in such a way that the first transverse plane contains two diametrically opposite coaxial lines with limiting diodes and two compensating coaxial lines also diametrically opposite while the second plane contains two diametrically opposite coaxial lines with loads.

3. A device according to claim 1 or claim 2, comprising several successive first transverse planes each containing at least one coaxial line with limiting diodes and at least one compensating coaxial line and the same number of second transverse planes each containing at least one coaxial lin with a load.

4. A device according to claim 1, claim 2 or claim 3, wherein the circular guide is excited in TE 11 mode and the coaxial lines with limiting diodes are, in the transverse plane, inclined at 450 with respect to the direction of the electrical field.

5. A device according to claim 1, claim 2, claim 3 or claim 4, wherein the distance between each first transverse plane and each second transverse plane is equal to an odd whole number of group quarter wavelengths of the microwave frequency signals.

6. A device according to any one of the preceding claims, wherein the length of the coaxial lines is equal to an odd whole number of quarter wave-lengths of the microwave frequency signals.

7. A device according to any one of the preceding claims, wherein the limiting diodes are disposed at a distance from the periphery of the circular guide which is equal to a whole number of half wave-lengths of the microwave frequency signals.

8. A device according to any one of the preceding claims, wherein the distance d between the first transverse plane and the second transverse plane which follows it is less than a quarter of the group wavelength of the microwave frequency signals.

9. A device for the protection of a receiver of microwave frequency signals, substantially as described hereinbefore ith reference to and as illustrated in the accompanying drawings.

9. A device for the protection of a receiver of microwave frequency signals, substantially as described hereinbefore with reference to and as illustrated in the accompanying drawings.

Amendments to the claims have been filed as follows

1. A device for the protection of a receiver of microwave frequency signals from high energy signals, comprising a circular guide from which. emerge at least three stubs.

- a first of the stubs having limiting diodes mounted in it and being disposed in 2 first transverse plane of the circular guide, - a second of the stubs having an absorbing load mounted in it and being disposed In a second transverse plane of the circular guide, separated from the first transverse plane by a distance d in the direction of sronazation of the microwave frequency avers, and - a third cf the stubs called a cornpenssting line, being disposed in the first transverse plane syrmetrically with the first coaxial stub with respect to an axial plane of the circular guide passing through the second stub.

2. A device according to claim 1, comprising at least six stubs arranged in such a way that the first transverse plane contains to diametrically opposite stubs with limiting diodes and ixo compensating lines also diametrically opposite while the second plane contains two diametrically opposite stubs with loads.

5. A device according to claim 1 or claim 2, comprising several successive first transverse planes each containing at least one stub with limiting diodes and at least one compensating line and the same number of second transverse planes each containing at least one stub with a load.

4. A device according to claim 1, claim 2 or claim.

5, wherein the circular guide is excited in TE 11 mode and the stubs with limiting diodes are, in the transverse plane, inclined at 450 with respect to the direction of the electrical field.

5. A device according to claim 1, claim 2, claim 3 or claim 4, wherein the distance between each first transverse plane and each second transverse plane is equal to an odd whole number of group quarter wavelengths of the microwave frequency signals.

6. A device according to any one of the preceding claims, wherein the length of the stubs is equal to an odd whole number of quarter wave-lengths of the microwave frequency signals.

7. A device according to any one of the preceding claims, wherein the limiting diodes are disposed at a distance from the periphery of the circular guide which is equal to a whole number of half wave-lengths of the microwave frequency signals.

8. A device according to any one of the preceding claims, wherein the distance d between the first transverse plane and the second transverse plane which follows it is less than a quarter of the group wavelength of the microwave frequency signals.