598,900. Wave-guides ; directive radio systems. WESTERN ELECTRIC CO., Inc. Oct. 31, 1944, No. 21301. Convention date, April 5, 1940. [Class 40 (v)] Radio course marking apparatus comprises a plurality of electromagnetic horns or groups of horns having varied reflection characteristics spaced apart to outline a course, a radio wavetransmitter mounted on a vehicle traversing the course, and a radio receiver also mounted on the vehicle for identifying the horns by the wave energy reflected therefrom, or by their absorption of wave energy which would otherwise be reflected from their background. Fig. 1 shows a navigable channel in which reflecting horns 1 arranged along the course reflect back to a ship 2 a transmitted directional beam of ultra high-frequency energy. The reflected energy may be selective as regards frequency, or may be provided with a characteristic modulation of amplitude or frequency to enable the particular horn to be identified. Fig. 5 shows a form of reflector comprising two like-directed horns connected by a section of wave-guide. Wave energy intercepted by either horn is returned by the other. In its simplest form the horn may be terminated at the throat end by a reflecting plate in which case it will reflect all frequencies. A frequency selective form is shown in Fig. 6 in which the horn is terminated by a resonant chamber formed by an iris 12 and piston 13 with a power dissipator 14. This horn will reflect all frequencies except that for which the chamber is resonant. Further iris diaphragms may be provided to give a bandpass effect. Fig. 8 shows a horn 10 connected by a pipe guide 15 to an absorption chamber 16 filled with material such as cotton or glass wool moistened with water or coated with colloidal carbon. Included in guide 15 is a section 18 of larger diameter resonant at a frequency in the operating range. At this frequency the horn will reflect while at all other frequencies the energy will be absorbed in chamber 16. The resonant chamber 18 may be replaced by a lateral stub. Alternatively the pipe guide may be plain and of such dimensions as to exclude waves below the cut-off frequency, which waves are thus reflected by the horn. A similar effect may be obtained by inserting a multiapertured plate in the guide 15 (Figs. 10, 11, not shown). Complete absorption of all frequencies may be effected by using a horn terminating in a pipe filled with energy absorbing material, (Fig. 13, not shown), which is useful in a course defining system when placed against a reflecting background such as a cliff. Another form of complete absorber is shown in Fig. 14 in which a horn 10 is coupled by a pipe guide to a horn 22 which is directed skywards. A selective absorber may comprise a horn terminated by a slab of dielectric which is onequarter or one-half wavelength in thickness, (Fig. 15, not shown). Fig. 16 shows an array of horizontal horns 10 arranged so as to reflect or absorb waves received from any direction. The horns are connected to a common pipe 25, closed at the lower end and connected at the upper end to a power absorber or to a skywardly directed horn 22. Any of the above described frequency selective devices may be employed to enable the array of horns to be identified. A butterfly valve 26 may be closed to operate the array as a simple reflector, or it may be opened and closed in a coded sequence for purposes of identification. When the valve is closed, the array may be used for the rebroadcasting of a modulated signal, such as a weather report, which is received by the array from a central station. Better adjustment of the coupling of the horns to the common pipe and to each other may be obtained by the use of pistons (Figs. 17, 18, not shown). Fig. 19 shows an arrangement for impressing a frequency modulation on the reflected beam. The received wave in pipe guide 31 is reflected by a piston 34 and retransmitted along guide 32. Rapid movement of the piston 34 as by an electromagnet 35 will vary the wavelength of the reflected beam. This arrangement may be replaced by a single horn terminated in a reflecting diaphragm which is vibrated either by mechanical impact or by an electromagnet or a piezo-electric crystal, (Figs. 21-24, not shown). Fig. 20 shows a modification of Fig. 19 in which a virtual piston is formed at the point 36 in a tapered guide 33 where the transverse dimension equals the cut-off value. The position of the virtual piston may be varied rapidly by distorting the side wall of the guide. Amplitude modulation may be introduced into the reflected beam by variably constricting a pipe-guide connecting the horn to an absorbing chamber, or by varying the tuning of a resonant chamber opening into the throat of the horn, (Figs. 25-30, not shown). Fig. 26 shows a tapered rectangular guide section in which two side walls are in the form of stretched metallic tapes 54, 55 mounted inside perforated side walls 52, 53. The tapes may be vibrated by any means to produce modulation of a wave in the guide. Constructions of multiple arrays of sheet metal horns branched from a single pipe-guide to give directional effects are also described, (Figs 33-42, not shown), and formulµ are given for the optimum shapes of horns. Fig. 43 shows a form of receiver carried on a vehicle or craft using the system. Modulated waves are received by a horn 10 and demodulated at 86. After amplification, the audio-frequency signal is applied to an electromagnet 88 operating an acoustic diaphragm, thus using the horn as a loud-speaker. The diaphragm 87 is spaced one-quarter of the carrier wavelength from detector 86. The horn is made of metallised wood so as to satisfy both acoustic and electromagnetic considerations. Specifications 466,063, 472,725 and 519,766 are referred to. The Specification as open to inspection under Sect. 91 also describes an oscillator, Fig. 44 (Cancelled), with an acoustic link in the feedback path. The anode lead 94 of a triode 93 passes diametrically across a wave-guide 90 closed at both ends to form a chamber, whose length is resonant at a desired radio frequency. A detector 86 spaced a quarter wavelength from one end feeds a telephone transmitter 91 which co-operates with a telephone receiver 92 connected by transformer 95 to the grid of the oscillator 93. The device produces radiofrequency oscillations at a frequency for which the chamber 90 is resonant, these oscillations being modulated at an audio frequency for which the chamber 90 is acoustically resonant. This subject-matter does not appear in the Specification as accepted.