Abstract: A wideband hybrid horn antenna in which a circular section horn (1) is fed by a circular feed guide (3). A dielectric polyrod (7) is cantilever mounted in the throat end (4) of the horn (1) and is tapered to provide a match to the guide (3). Its forward section is tapered from the guide diameter down to about 2 mm just outside the mouth (11) of the horn (1). The various dimensions--horn diameter at throat and aperture, flare angle, polyrod diameter taper and extent--are all chosen to produce a balance of opposing effects on the beamwidth and thus provide a fixed beamwidth substantially independent of frequency over a wide band. A large bandwidth, from 8 to 16 GHz in the particular case, is thus obtained with a substantially constant beamwidth over the band.

Abstract: Apparatus for testing a radio transceiver installation having an antenna includes a screening hood (20) adapted to be fitted over the antenna (10) of an installation to be tested; and a hood antenna (21) mounted within said hood (20) so that when the hood is in its operative position the two antennae (10, 21) form an over-coupled bandpass filter. The position of said hood antenna (21) is within adjustable hood (20) which should normally have a diameter which is approximately 0.15 wavelengths of the resonant frequency of an antenna (10) to be tested.

Abstract: A radar seeker transient suppressor for an anti-radar homing missile in which glint effects upset the smooth tracking of a target by apparently changing the target position spasmodically. A seeker output signal is derived which is at least substantially independent of body movement e.g. the target sightline angle signal rather than the look angle signal (i.e. relative to a space reference rather than a body reference). This signal is used to control the steering either filtered or unfiltered according to whether the difference exceeds a threshold value or not. Both azimuth and elevation channels are treated similarly and in unison. An excessive filtered/unfiltered difference in either channel causes use of the filtered signal in both. A double integrating filter may be employed to avoid the relative displacement of output and input signals when the input is, or contains, a ramp component.

Abstract: A closure arrangement comprises an aperture-defining frame, a closure panel member which fits removeably into the frame, and a radio-frequency seal between the frame and an edge of the panel member. The seal is divided into two complementary parts attached respectively to the panel member and to the frame, the parts extending over different sections of the periphery of the aperture and cooperating to cover the entire periphery only when the panel member is closed. To improve the seal, a sharp ridge is formed in the edge of the panel member to enhance the contact pressure exerted on the seal locally on a line extending along the seal.

Abstract: A mixer incorporating FET's (4,5) as the switching elements includes a resonant circuit connected as a tank circuit of the local oscillator (6) such that the inherence capacitance of the FET's is automatically compensated for. The harmonic recovery technique is also used by the inclusion of rejection filters at the input and output of the system, which rejection filters have reciprocal impedance characteristics, such as short circuit/open circuit or the like.

Abstract: An acoustic loading rod for use in testing transducers which are intended for use in a liquid medium. The rod comprises an elongated body (10) capable of simulating the normal acoustic impedance presented to a transducer to be tested by the medium in which the transducer is to operate, the rod (10) having a bore (30) in its main body portion containing a core of material having a close match of acoustic impedance with the material of the rod but a higher acoustic loss factor. Active piezo-electric transducer (22) is mounted between the ends of the rod.

Abstract: A transmission line switch allows a single output line (3) to be switched to one of two or more input lines (1,2) to which it is permanently connected at a common junction (4). Each input line (1,2) has an associated amplifier stage (10) which can be biased in a normal high gain (`on`) state, or in an isolation (`off`) state. Suitable biasing in the `off` state ensures that the amplifier stage output presents a low impedance to its own input line, the length (L) of which is chosen to reflect a high impedance at the junction (4) with the other lines. Correct design enables the return loss and insertion loss of the `on` path to be kept to low values while simultaneously offering a high insertion loss to the `off` path signals.

Abstract: An accelerometer capable of sensing acceleration in three orthogonal directions while being formed in a single silicon wafer comprises three cantilever beams (2) etched from the wafer, each having a mass (3) asymmetric to the neutral plane (A) of the beam, and formed by etching away the beam (2) on one face only of the wafer. The beams are arranged at 120.degree. to each other so that their sensing axes (S) are orthogonal or approximately so. Deflection of the beams is measured by piezoelectric, electrostatic or electrodynamic effect and force feedback is preferably employed, again using one of these effects.

Abstract: An optical alignment detector uses a Fresnel bi-prism (12) on the base of which an incoming beam of light is focussed. When the beam is in alignment, it strikes the base substantially normally and is brought to a focus adjacent a ridge (13) of the bi-prism (12). To cope with a second axis of alignment, a second bi-prism (14) having a ridge (15) is placed beneath the first, with the two ridges (13,15) arranged orthogonally so as to produce four possible pupil entrance images which can be received by four photodetectors (17,18,19,20).

Abstract: The level of signal noise caused by fluid flow at a pressure transducer, such as a hydrophone or geophone, is determined by measuring the electric field induced when the fluid flow through a magnetic field. The electric field, which is proportional to flow rate, is measured by electrodes positioned about the transducer. The pressure due to the flow is derived from the flow rate voltage by a digital filter which may be adaptive, to allow for varying flow conditions, or which may employ a predetermined electric field/flow noise relation. This flow noise is then subtracted from the received signal, the resultant signal then being substantially noise free.

Abstract: An ice breaker arrangement of forcing open the door of an enclosure such as a radio frequency shelter for electrical equipment comprises a threaded stud (3) which, when turned by a key, is made to project from the door frame (5) so as to force a reaction plate (2) attached to the door (1) away from the frame, thus opening the door (1). Appropriate apertures (21, 73) are provided at each end of the stud (3) to allow access to the stud (3) by the key.

Abstract: A gun sighting system in which a daylight (visual) sight and a thermal imaging (TI) night sight are mounted on the gun breech. The TI field of view is superimposed on the visual field of view, necessitating accurate alignment between their lines of sight and the gun muzzle boresight. Adjustment of the visual sight causes separation of the visual and TI displayed images which is indeterminate in the absence of distinct target features. In accordance with the invention a visual reference mark is injected on to the field of view, which reference mark is locked to the target scene. Separation of the visual and TI scenes causes corresponding separation of the reference mark and TI sight line marker thus permitting adjustment of the TI field of view to remove this separation and align the visual and TI scenes.

Abstract: A dynamic frame length slotted ALOHA communication system has a central station which monitors the traffic congestion and determines the optimum frame length. This is transmitted to the sub-stations in the form of a service information message when there is to be a change in the frame length but not, as is conventional, at the beginning of each frame. The amount of time available for the useful transmission of communication messages is thus optimized.

Abstract: In a dynamic frame length slotted ALOHA system the synchronizing signals are transmitted by a base station at different frequencies (each having a period equivalent to a multiple of the time slot period). The different frequencies denote different frame lengths and the central stations detect these frequencies and select a frame length accordingly.

Abstract: A microwave balun transformer provides an extension of operating frequency range particularly in conjunction with a cavity-backed spiral antenna (5). The balun cavity (13) has a dipole (15,17) extending between an unbalanced coax port (19) and an opposite end wall (29), the dipole junction (16) being connected to a balanced twin line (7). The length of the cavity is effectively controlled to make it closer to (two) quarter-wave stubs by inserting a frequency dependent reflector (21) at each end of the dipole. At low frequencies the reflectors are transparent thus giving the full length of the cavity, while at high frequencies the reflectors reflect and effectively shorten the cavity.

Abstract: In a tracking radar system target glint and multipath sometimes produce signals which falsely indicate high acceleration rates of the target. These signals cause the boresight of the antenna to be driven away from the direction in which the target actually lies.To avoid this problem, received signals from a receiver are fed through a switch which is opened whenever the target appears to have an acceleration or velocity above a threshold value. The threshold value is varied so that the proportion of the time during which the switch is open is kept approximately constant. Thus, if there is a period during which a large number of high acceleration or high velocity signals appear from the receiver (for example is there is a very high noise level) the switch still remains closed for a certain proportion of the time period. This ensures that the tracking system never loses the target while also ensuring that false tracking due to target glint and multipath is minimized.

Abstract: A method of automatically tuning a high power system by measuring the gardient of component value against efficiency for each component impedance in turn and, for each component, finding two points of equal and opposite efficiency-to-impedance gradient and setting the impedance of the component to a value half-way between these points.

Abstract: A close-packed divergent array of cavity-backed antennas for use in the radome of a missile. Each antenna cavity is tapered from its radiating face towards its base so that the antennas can be mounted more closely together while their radiating faces maintain conformity with the streamlined surface. The array can thus be moved farther into the apex of the radome with a consequent reduction of the antenna divergence angle and greater sensitivity in the boresight region.

Abstract: A method of preventing interference between `friendly` pulse doppler radars when in action against a common target or adjacents targets. Despite considerable separation of the radar r.f's the pulsed signals have repeated sideband pulses which may be taken by another radar as a doppler-shifted echo. This problem is to a large extent alleviated by frequency modulating the r.f. transmission at a very low cycle rate of the order of one cycle per second and at a modulation rate sufficient to indicate to a receiving radar that the source could not be a target accelerating at such a high rate.

Abstract: A microwave radiating system including an antenna composed of a plurality of vertically spaced elements and circuit members connected to feed the elements in a manner such that the amplitude of the energy distribution among the elements and the second derivative of the phase variation in the vertical direction of the antenna each have a maximum between the bottom and the center of the antenna.