600,805. Directive radio systems. RUMSEY, V. H., SMITH, R. A., WOOD, K. A., PUTMAN, J. L., LUTKIN, F. E., MACRO, W. B., and EVANS, T. S. April 7, 1944, Nos. 6574/44, 9900/45 and 9901/45. (Classes 40 (vi) and 40 (vii)] An aerial arrangement adapted for beam switching comprises two or more slot aerials, of the type described in Specification 591,053 in the wall of a cavity which has means for feeding energy to or from the cavity and means for rendering any of the slots inoperative. The aerial may be employed in overlapping- beam systems giving guidance in the horizontal and vertical planes. The cavity is preferably resonant or may consist of a waveguide ; it may be resonant to one polarization but not operative with waves polarized in the perpendicular direction. For an azimuth beacon the aerial arrangement may comprise a box resonator #/2 square and approximately #/4 high with horizontal slots, energized by a vertical unipole, the slot aerials being selectively rendered operative by relay-operated contacts across their centres. For an equisignal glide-path beacon the aerial may comprise vertical slots arranged one above the other in a vertical wave guide, the occurrence of false equisignal zones being prevented by arranging the amplitude of radiation from the upper aerial to be smaller than that from the lower aerial. The equisignal surface is conical and if the aircraft is guided in azimuth to one side of the glide path beacon the descent path becomes hyperbolic. If the radiation from the upper slot is beamed so that laterally of the beacon the angle of the conical surface is distorted, the path becomes nearly a straight line to a touch down point behind the level of the beacon. Beacons according to the invention may be interrogated by pulse transmission from aircraft. The returned pulses simulate echoes and enable the range to the beacons, or to the end of a runway if a suitable. delay is imposed, to be measured in the craft. For the reception of the interrogating signal the beacon aerial switching may be inoperative and the beacon response may be selective in respect of the interrogating pulse repetition frequency. Azimuth approach beacon, Fig. 1. A rectangular box resonator 11 is mounted on a wooden frame 12 in a.corner reflector 13. Horizontal slots 14, effectively one half wave length long, are formed round the outer corners of the box and are fitted with switch devices 15 comprising relays W/2, X/2, Fig. 3 (not shown), mounted on metal plates shaped to fit the slotted corners of the box. The relays are fitted with contacts which short circuit the centre of the slot and prevent radiation when the relay is energized. Normal changeover springs sets W1, X1 are also fitted to the relays. Insulating material coverings 14a are provided over the slots to protect the interior of the box and relays. Mounted vertically from the top of the box by an insulator 19 is the quarterwave probe 16 consisting of an inner tube 16b over which telescopes the outer tube 16a which is located in one of three positions by a bolt 16c and enables the probe to be adjusted to a length appropriate to the operating frequency. The probe is coupled to the feeder 18 by an impedance matching transformer comprising the tapered section 17, the outer member 17b of which is secured to the top of the box by a flange, and the terminal member 20 of which 'consists of a short length of solid dielectric line. Means is provided for locating the box 11 in an optimum position along the axis of symmetry of the corner reflector and for adjusting the box in a direction at right angles to thisaxis in order to swing the beam through a limited angle. The polar radiation diagrams of the separate slots are substantially minor images of one another, Fig. 4 (not shown), giving an equisignal path while the two slots together produce a single lobe. Responder beacon circuits.-In the complete responder beacon installation of Fig. 5 the aerial is arranged to receive on both slots 14 until interrogating pulses received from an aircraft and fed to the receiver R cause the pulse generator PG to modulate the transmitter T and a pulse to be transmitted in response to each interrogating pulse. The beacon is arranged to transmit alternately long and short pulses (dots and dashes) from separate slots under the control of the master relay and switch circuits MR and S so that the position of the interrogating aircraft may be determined by observing the timing and relative amplitudes of the dot and dash signals, as described in Specification 600,804. Negative-going pulses from the receiver R are fed to the,paralysis-time circuit PT comprising a " flip flop " circuit VI, V2 in which the valve V1 is normally conducting due to the negative bias source B1, connected to valve V2. The sharp negative pulse produced from the applied pulse by the differentiating circuit comprising condenser C1 and resistance R1, drives the circuit into its unstable condition from which the circuit reverts after a time, determined by condenser C3 and resistances R1 and R3, which is chosen according to the maximum pulse repetition frequency (about 9000 per sec.) to which the beacon is to be allowed to respond, beacon reception being paralysed until the circuit PT reverts, to its normal condition. A diode V3 prevents the build up of excessive bias on the grid of V2 and so maintains the sensitivity of the circuit at high repetition rates. The drop in potential at the anode of V2 is fed to trigger the pulse generating circuit PG through a variable delay line DL, which provides an artificial delay on the beacon response which causes the beacon to appear to be sited at a standard distance, say 10,000 feet, beyond the near end of the runway. The circuit PG is a " flip flop '' circuit comprising valve V4 and valves V5, V6 in parallel, the output pulse from the cathode of V6 being fed to modulate the transmitter T to cause the emission of short or long pulses (dots and dashes) depending upon which condenser C6 or C7 is connected in the internal feed-back circuit by contacts y1, zl or relays Y/1, Z/1. Operation of the system is normally prevented by contacts U1 and U2 in the switch and master relay circuits S and MR, which are held open due to energization of relay U/2 in the discriminator circuit D. The negative pulses from the circuit PT are fed to the discriminator circuit and charge up condenser C13 through the diode V7, and when the interrogating pulse frequency exceeds a critical value (75 per sec.) set by the potential divider VR1, the grid of valve V9 goes negative and relay U/2 is released. The master relay circuit MR comprises a self-running phantastron oscillator V11 of the type described in Specification 582,758, the screen wave-form of which is fed to the grid of valve V10 and has a frequency of 8<1>/3 c.p.s. and is of sufficient amplitude to overcome the negative bias B1, so that the relay U/1 is operated at the low switching frequency, the mark-to-space ratio being adjusted to unity by variable resistance VR2. The make-before-break contact ul of relay U/1 connects the relays W /2, X/2 alternatively across the low tension supply LT1, and these relays, which are the slot switching relays, thus cause the slot aerials to be inoperative alternate, while their change over contacts w1, x1 cause relays Y/1, Z/1. to be energized alternately and close contacts yl, zl alternately to permit the pulse generator PG to operate and modulate the transmitter with dots and dashes. Alternative aerial system.-In a modified aerial arrangement for an azimuth approach beacon employing horizontally polarized waves shown in Fig. 1 (9901/45), a metal box 2 is mounted on a wooden frame 9 in a corner reflector 1, vertical slots 6, 7 being formed in the parallel lateral sides. A partition 5 divides the box into two compartments 3 and 4 and supports the exciting probe 8, the feeding cable being enclosed in compartment 4 so that the amount of loose cable exposed in the reflector, which would cause distortion of the indicated beam, is a minimum. Glide-path beacon, Figs. 6 and 7.-A resonant cavity in the form of a rectangular wave-guide 20 with two longitudinal radiating slots 21a, 21b is mounted vertically near the landing end of the runway and displaced to one side. The wave-guide is energized by a probe 26 mid-way between the back and front and equidistant from the slots, and the whole is mounted on a wooden base 23 provided with vertical guides 24. A winch enables the aerial to be raised or lowered with respect to the ground to control the distribution of the radiation polar diagram lobes produced by the slots, and thus the inclination of the equisignal glide path P. The wave-guide is constructed of a light wooden framework, adapted to give way to the impact of an aircraft, lined with sheets of insulating material and with an inner lining of copper foil. The slots 21a and 21b are switched and associated with transmitting, receiving and switching circuits similar to those shown in Fig. 5. A separate aerial 29 may be mounted on top of the wave-guide for the reception of the interrogating signals, both slots being then shorted during reception. In order to prevent a second ambiguous equisignal glide path occurring due to intersection of the lobe produced by the lower slot with the second lobe produced by the upper slot, the relative amplitude of the radiation pattern from the upper slot is reduced as indicated in Fig. 8 (not shown) by the insertion of an attenuator 30, comprising a shallow prism consisting of a sheet of carbon-impregnated material on a wooden former, between the probe 26 and top slot 21a. Mismatch between the impedances presented by the slots to the probe due to the presence of the attenuator is corrected by making the shorted end sections of the guide of different lengths. The equisignal surface produced by the arrangement of vertically displaced slots is substantially conical a