Frequency selective antenna

A high frequency reflective antenna which is preferentially frequency selective at a design frequency of a given frequency or narrow range of frequencies, comprising

• a plurality of adjacent antenna sections, each section having a parabolic surface of different focal length, and each section being axially offset with respect to the next preceding section by an axial distance M, where M is greater than one-half wavelength of the design frequency of the antenna and M progressively increases for each succeeding section, and where M is defined by the following equation, ##EQU4## where X is the radial distance from the axis to a respective section,
• F.sub.L is the focal length of the respective (n.sup.th) section,
• n is the number of the section, and
• .DELTA.F.sub.L is the change in axial focal length from one section to the next and is an even multiple of one-half of the wavelength of the design frequency.

A frequency selective reflective antenna which is preferentially frequency selective at a given frequency or narrow range of frequencies, comprising

• a plurality of adjacent antenna sections, comprising a first circular central section, and succeeding sections in the form of concentric rings, each section having a parabolic surface of different focal length, and each section being offset with respect to the next preceding section in an axial direction by a distance M parallel to the axis of the antenna, where M is greater than one-half wavelength at said given frequency and M progressively increases for each succeeding section, and where the edges of the outer sections of the antenna are cut off to form a rectangular antenna, and where M is defined by the following equation, ##EQU5## where X is the radial distance from the axis to a respective section,
• F.sub.L is the focal length of the respective (n.sup.th) section,
• n is the number of the section, and
• .DELTA.F.sub.L is the change in axial focal length from one section to the next and is an even multiple of one-half of the wavelength of the design frequency.

A frequency selective reflective antenna which is preferentially frequency selective at a given frequency or narrow range of frequencies, comprising

• a plurality of adjacent antenna sections, comprising a first circular central section, and succeeding sections in the form of concentric rings, each section having a parabolic surface of different focal length, and each section being offset with respect to the next preceding section in an axial direction by a distance M parallel to the axis of the antenna, where M is greater than one-half wavelength at said given frequency and M progressively increases for each succeeding section, and where the edges of the outer sections of the antenna are cut off to form a rectangular antenna, and where M is defined by the following equation, ##EQU6## where X is the radial distance from the axis to a respective section,
• F.sub.L is the focal length of the respective (nth) section,
• n is the number of the section,
• .DELTA.F.sub.L is the change in axial focal length from one section to the next and is an even multiple of one-half of the wavelength of the design frequency, and
• feed horn means mounted with respect to the parabolic surfaces of said antenna and wherein the antenna reflects radiation to or from the feed horn means.

A high frequency reflective antenna which is preferentially frequency selective at a design frequency of a given frequency or narrow range of frequencies, comprising

• a plurality of adjacent antenna sections, a first circular central section and succeeding sections in the form of concentric rings, each section having a parabolic surface of different focal length, and each succeeding section being offset with respect to the next preceding section by a distance M in a direction substantially parallel to the central axis of the antenna, where M is greater than one-half wavelength but not a precise multiple of one-half wavelength or of one wavelength of the design frequency of the antenna and M progressively increases for each succeeding section, and
• said antenna has the following characteristic,
• F.sub.Lo /D.gtoreq.0.4,
• where F.sub.Lo is the prime focal length and D is the diameter of said antenna.