GB2153154A - Microwave switching arrangement - Google Patents

Abstract

A microwave switching arrangement used to route a microwave pulse from a magnetron (2) to an output port (1), and to isolate the magnetron from a receiver (9) when an echo of the pulse is received, consists of a PIN diode (7) mounted in a waveguide (3). The conductivity of the diode, which determines whether it acts to transmit or reflect microwave energy in the waveguide, is controlled by inductivity (14) coupling it to a lead (10) by means of which pulsed energy is supplied to the magnetron to bring it into microwave oscillation. <IMAGE>

Description

SPECIFICATION Microwave switching arrangement This invention relates to a microwove switching arrangement and is particularly suitable for use with a duplexer which is coupled to a magnetron. A duplexer of the kind which can be used in combination with a magnetron is described in our copending patent application 8235816.

A magnetron feeds into a side wall of a waveguide channel, one end port of which is, in use, coupled to a common receiving and transmitting antenna, and the other end port of which is coupled to a very sensitive receiver which receives weak echo signals from a target. A microwave switch device is located between this other end port and the magnetron feed point so as to isolate the receiver during transmission of a high power microwave pulse from the magnetron. Such switches must be very fast- acting, and provide excellent electrical isolation properties.

The present invention seeks to provide an improved microwave switching arrangement.

According to a first aspect of this invention a microwave switch includes a PIN diode mounted in a waveguide channel into which pulses having a microwave frequency are introduced, and means for altering the microwave frequency impedence presented to the channel by the diode from a low value to a higher value in synchronism with a signal which initiates generation of a pulse.

According to a second aspect of this invention, a microwave switching arrangement in ciudes a channel capable of transmitting microwave energy, having a first port at one end and a second port at the other end; a feed point intermediate the two points for feeding microwave energy into said channel: switcha bie means including a microwave diode positioned within said channel between said feedpoint and the second port for passing incident microwave energy when said means is in a first conductive state and for reflecting incident microwave energy when said means is in a second conductive state; and bias means synchronising the change from the first state to the second state with a signal which initiates generation of a microwave pulse which is fed to said feed point.

The arrangement does not rely on the electric field induced in the channel by the microwave energy to control the reflection pro-perties of the switch, but instead an externally applied control signal is used, in contrast to the previously used system. This invention therefore permits the diode to be chosen with regard to its microwave conduction and ref!ec- tion properties, and not to the way in which it responds to the electric field within which. it is situated. This enables diodes which are inherently slight slower acting to be used-this implies a greater internal capacitance and an improved reflection property.

Preferably the arrangement incorporates a magnetron oscillator which is coupled to said feed input, with the switching signal being derived from the high voltage pulse which is applied to the magnetron to bring it into oscillation.

Additional field-dependent diodes, can be provided in the channel to give enhanced reflection and transmission properties; and can be tuned to operate at the same frequency or at frequencies which are different to the magnetron oscillation frequency.

The invention is further described by way of example, with reference to the accompanying drawing, in which: Figure 1 is a sectional view of a microwave switching arrangement which incorporates a magnetron, and Figure 2 is an explanatory diagram.

Referring to Fig. 1, a radar system having a common antenna (not shown) connected to port 1 for transmitting and receiving signals includes a magnetron 2 which generates pulses of microwave energy. During transmission the magnetron output is coupled into a waveguide channel 3, via a co-axial line 4, probe 5 and glass dome 6. The energy propagates in both directions along the channel 3 and in one direction (to the right as shown) is incident on two PIN diodes 7 and 8 which can act either to pass or reflect the incident energy. Port 9 is connected to a very sensitive receiver.

The diodes 7 and 8 each form part of a respective resonant circuit whose resonance frequency is dependent on whether each diode is conductive or not. The resonant frequency of the diode 7 is brought to the microwave frequency of the magnetron when the diode is conductive, so that the diode presents a very high impedance and reflects incident microwave pulse energy from the magnetron to the port 1, where it is radiated.

The distance of the diode 7 from the probe 5, at which the microwave energy is fed into the channel 3, is chosen so that the reflected energy combines with that travelling from the feed point directly to the port 1.

The magnetron commences oscillation when a voltage pulse applied to it from a high voltage transformer (not shown) rises above a threshold value VT. Typically, the threshold value is about 4kV. Although the rise time of the pulse is very fast, it may take 40 nS or so to reach the threshold value, and during this time charging current is flowing along the two input leads 10 which connect the magnetron to the transformer.

A typical voltage waveform is shown in Fig.

2 at line a, and the magnetron commences to oscillate when the threshold voltage VT is reached. Line b shows the current flowing in leads 10 due to the transformed voltage pulse. The initial current 11 is due to the charging of the internal capacitance of the magnetron, and the current rises sharply at 1 2 when microwave oscillation commences.

The microwave pulse 1 3 itself is shown at line C.

A small toroid 14 encicles the leads 10 at the point where they enter the magnetron 2.

The toroid consists of a ring of ferrite material having a toroidal winding 1 5-typically about ten turns are present. The toroid can be an integral part of the magnetron. The two ends of the winding are taken to the waveguide channel 3, one lead being connected at 1 6 to the waveguide itself to act as an "earth", and the other being connected to the base 1 7 of a probe 1 8 which extends across the channel 3, and carries the PIN diode 7 at its far end. The diode 7 is in contact with an adjustable bush 1 9 which completes the electrical path to the waveguide. The base 1 7 is held in position by a further adjustable bush 20, from which it is spaced apart by an insulating sheet 21.

The initial charging current 11 is sufficient to bias the diode 7 into conduction, thereby ensuring that its associated resonant circuit is in the reflective state by the time that the microwave pulse is injected into the channel 3. This permits the diode 7 to have a relatively large capacitance, thereby giving an effectively large reflection impedance.

The other PIN diode 8 has a much smaller capacitance and is thus very fast acting. It is brought into conduction by the electric field induced in the channel 3 when the magnetron generates a microwave pulse. When it comes into conduction it can reflect energy at the normal frequency of the magnetron or at a different frequency at which unwanted spurious signals may be generated. It is mounted in the channel 3 which is generally similar to diode 7.

On the cessation of the high voltage pulse from the transformer, microwave oscillation of the magnetron also ceases, and the diodes 7 and 8 revert to their previous state. This permits very weak signals to end at port 1, and to travel along the channel 3 to the port 9 which is coupled to the sensitive receiver.

Claims (8)

1. A microwave switch including a PIN diode mounted in a waveguide channel into which pulses having a microwave frequency are introduced, and means for altering the microwave frequency impedence presented to the channel by the diode from a low value to a higher value in synchronism with a signal which initiates generation of a pulse.

2. A microwave switching arrangement including a channel capable of transmitting microwave energy, having a first port at one end and a second port at the other end; a feed point intermediate the two points for feeding microwave energy into said channel; switchable means including a microwave diode positioned within said channel between said feedpoint and the second port for passing incident microwave energy when said means is in a first conductive state and for reflecting incident microwave energy when said means is in a second conductive state; and bias means synchronising the change from the first state to the second state with a signal which initiates generation of a microwave pulse which is fed to said feed point.

3. An arrangement as claimed in claim 2 and wherein the arrangement incorporates a magnetron oscillator which is coupled to said feed input, with a diode switching signal being derived from the high voltage pulse which is applied to the magnetron to bring it into oscillation.

4. An arrangement as claimed in claim 3 and wherein the diode switching signal is generated by means of an inductive coupling with a lead arranged to carry said high voltage pulse.

5. An arrangement as claimed in claim 4 and wherein said inductive coupling comprises a toroidal winding surrounding said loop, and which is connected to said diode.

6. An arrangement as claimed in any of claims 2 to 5 and wherein an additional diode is provided in said channel to give additional transmission and reflection properties, the additional diode being switchable in dependence on the electric field strength within which it is situated.

7. An arrangement as claimed in claim 6, and wherein said additional diode forms part of a frequency selective circuit which is tuned to a frequency which is different to that of the signal generated by the magnetron.

8. A microwave switching arrangement substantially as illustrated in and described with reference to Fig. 1 of the accompanying drawings.