GB2096833A - Combined antenna and reflector - Google Patents

Abstract

A communication antenna comprises an electrically conducting cylinder (2) and a feeder cable (3) and has a plurality of retro-reflective units (4) formed thereon to reflect short- wave electromagnetic radiation (such as radar signals or laser beams). The antenna thus adds to its normal function as a communication receiver/transmitter, the function of a location indicator. <IMAGE>

Description

SPECIFICATION Improved communication device This invention relates to an improved communication device which serves the dual function of an antenna for radio waves and a reflector for electromagnetic waves of shorter wavelength than the radio waves. The invention has particular, but not exclusive, application to marine craft, or other buoyant devices but can be used in any situation where, for example, both radar or laser navigation on the one hand and radio communications on the other hand, are to be employed.

Communication antenna are known which comprise an electrically conducting cylinder as the radiating/receiving element. The length of the cylinder is selected in relation to the free-space wavelength of electromagnetic radiation at the centre of the frequency band for which the antenna is to be used. The cross-sectional size of the cylinder (i.e. its diameter in the case of a circular cylinder) is related to the 'Q' of the antenna (and thus determines the band width over which the antenna can be used). A larger cross-section gives a larger bandwidth for a given degree of mismatch with the feeder cable with which the antenna is to be used.

Radar reflectors and reflectors of visible light, infrared or ultra-violet radiation are known which operate on the retro-reflective principle and comprise a plurality of retro-reflective units each having a conical, pyramidal or part-spherical electrically conducting or reflective surface, the units being clustered together with the axis of symmetry of each surface directed towards a central region of the cluster and radiating from that region over an arc (which can be anything up to 3600) and which defines the angle (hereinafter referred to as the "effective angle") over which an incident signal will be retro-reflected.

In accordance with the present invention the conducting cylinder of a communication antenna incorporates a plurality of retro-reflective units forming a reflector for short wave electromagnetic radiation such as radar signals, light, infra-red or ultra-violet beams, whether incoherent or coherent as when emitted by a laser source.

The number and disposition of the retroreflective units will, inter alia, be determined by the size of the cylinder, the wavelength of the incident short wave signal and the intended effective angle of the reflector. Where the retro-reflective units are conical, pyramidal or partspherical surfaces indenting or projecting from the cylinder, there may be many such separate units regularly or randomly disposed over the surface of the antenna. The minimum transverse dimension of each such indent would normally exceed that of the wavelength of the incident short wave electromagnetic signals it is desired to reflect and thus for S-band (about 3 GHz) applications, a lower transverse dimension of some 100 mm would be used and for X-band (about 10 GHz) applications, a lower transverse dimension of some 30 mm would be required.

However, even retro-reflective indentations or projections which are considerably smaller than the wavelength of the short wave incident radiation will act as retro-reflectors, and can thus be used to reflect a proportion of the incident energy they receive.

The cylinder would typically be of circular cross-section (since this is easy to fabricate and gives good omnidirectional properties) but elliptic, other curvilinear, or non-curvilinear cross-sections are not ruled out.

The reflective array on the cylinder can be produced from a flat or curved sheet or tube, which is indented by a punching or other pressure-induced, or vacuum forming operation.

Alternatively, where a dielectric antenna or matching sleeve core is used, the dielectric core of the antenna can be preformed to exhibit the desired array of cavities or projections for the retro-reflective units and can then be plated with an electrically conducting reflective layer in accordance with any of the known conventional electrical or chemical techniques.

The conducting cylinder can be coated with or housed inside a surrounding layer of protective material transparent to the short wave electromagnetic radiation it is desired to reflect.

This material can also act as a filter if it is desired (e.g. for secrecy purposes) to prevent reflection of a particular band or short wave incident signals or to only permit reflection in one or more particular bands.

If desired, the retro-reflective units can be vibrated (e.g. electrically, mechanically or pneumatically) in order to impress information on the reflected incident short wave signals for example to provide an identity signal or to permit communication via the short wave signals. If the reflecting surfaces are backed by an appropriate material that exhibits piezo electric or other mechanical deformation under electrical stress then the information transmitted via the short wave signals can correspond to that being received by the device acting as an aerial.

The invention will be further described by way of example, with reference to the accompanying drawing, in which: Figure 1 is a partly sectional schematic view of one form of reflective communication device, Figure 2 is a schematic view of a second form of device, Figure 3 shows how the device of Figure 1 can be used in a practical installation, and Figure 4 shows how a retro-reflective unit can be modified.

The device shown in Figure 1 comprises a waterproof radome 1 housing a metallic cylindrical member 2 forming a communication antenna which is connected to a coaxial cable 3 in conventional manner. The surface of the member 2 is provided with indentations 4 which define retro-reflective units for radar signals of a wavelength at which the radome 1 is transparent. The length of the member 2 between one quarter and one half of the radio wavelength for which the antenna is designed.

The device in Figure 2 has an oviform antenna, the upper part 2a of which is connected to the inner conductor of the cable 3, and the lower part 2b of which is connected to the screening of the cable. Part-circular indentations 4a define the short wave retro-reflective units.

Figure 3 shows the device of Figure 1 mounted high in the rigging of a yacht, the coaxial cable (not shown in Figure 3) being loosely linked to a stay 5 and leading to a radio receiver/transmitter (not shown).

Figure 4 shows one way in which a retro reflective unit 6 can be used to impose signal information on an incident laser beam. The unit 6 will be vibrated by an electromagnetic vibrator 7 when the required signal (in electrical form) is fed to the coil of the vibrator.

Claims (Filed on 13 April 1982) 1. A communication antenna having an electrically conducting cylinder as a radiating/receiving element and a feeder cable for conducting signals to/from the cylinder, wherein the conducting cylinder incorporates a plurality of retro-refiective units forming a reflector for electromagnetic radiation of a much shorter wavelength than that for which the cylinder of the antenna is designed to radiate/receive radio communications.

2. An antenna as claimed in claim 1, in which the retro-reflective units are conical, pyramidal or part-spherical surfaces indenting or projecting form the cylinder.

3. An antenna as claimed in claim 2, in which there are many such separate reflective units reguiarly or randomly disposed over the surface of the antenna.

4. An antenna as claimed in any preceding claim, in which the cylinder is of circular crosssection.

5. An antenna as claimed in any preceding claim, in which the reflective array of retroreflective units is produced by deforming sheet material from which the cylinder is constructed.

6. An antenna as claimed in any of claims 1 to 5, in which a dielectric core of the antenna is formed to exhibit the array of cavities or projections for the retro-reflective units, which core is then plated with a reflective layer.

7. An antenna as claimed in any preceding claim, in which the cylinder is housed inside a surrounding layer of protective material transparent to the short wave radiation it is desired to reflect.

8. An antenna as claimed in claim 7, in which the housing material acts to filter out reflections in a particular band of the short wave incident signals.

9. An antenna as claimed in any preceding claim, in which means is provided to vibrate the retro-reflective units in a controllable manner to thereby impress information on the reflected short wave signals.

10. A communication antenna substantially as hereinbefore described with reference to, and as illustrated in, Figure 1 or 2 of the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. is transparent. The length of the member 2 between one quarter and one half of the radio wavelength for which the antenna is designed. The device in Figure 2 has an oviform antenna, the upper part 2a of which is connected to the inner conductor of the cable 3, and the lower part 2b of which is connected to the screening of the cable. Part-circular indentations 4a define the short wave retro-reflective units. Figure 3 shows the device of Figure 1 mounted high in the rigging of a yacht, the coaxial cable (not shown in Figure 3) being loosely linked to a stay 5 and leading to a radio receiver/transmitter (not shown). Figure 4 shows one way in which a retro reflective unit 6 can be used to impose signal information on an incident laser beam. The unit 6 will be vibrated by an electromagnetic vibrator 7 when the required signal (in electrical form) is fed to the coil of the vibrator. Claims (Filed on 13 April 1982)

1. A communication antenna having an electrically conducting cylinder as a radiating/receiving element and a feeder cable for conducting signals to/from the cylinder, wherein the conducting cylinder incorporates a plurality of retro-refiective units forming a reflector for electromagnetic radiation of a much shorter wavelength than that for which the cylinder of the antenna is designed to radiate/receive radio communications.

2. An antenna as claimed in claim 1, in which the retro-reflective units are conical, pyramidal or part-spherical surfaces indenting or projecting form the cylinder.

3. An antenna as claimed in claim 2, in which there are many such separate reflective units reguiarly or randomly disposed over the surface of the antenna.

4. An antenna as claimed in any preceding claim, in which the cylinder is of circular crosssection.

5. An antenna as claimed in any preceding claim, in which the reflective array of retroreflective units is produced by deforming sheet material from which the cylinder is constructed.

6. An antenna as claimed in any of claims 1 to 5, in which a dielectric core of the antenna is formed to exhibit the array of cavities or projections for the retro-reflective units, which core is then plated with a reflective layer.

7. An antenna as claimed in any preceding claim, in which the cylinder is housed inside a surrounding layer of protective material transparent to the short wave radiation it is desired to reflect.

8. An antenna as claimed in claim 7, in which the housing material acts to filter out reflections in a particular band of the short wave incident signals.

9. An antenna as claimed in any preceding claim, in which means is provided to vibrate the retro-reflective units in a controllable manner to thereby impress information on the reflected short wave signals.

10. A communication antenna substantially as hereinbefore described with reference to, and as illustrated in, Figure 1 or 2 of the accompanying drawing.