Small dual frequency band hybrid mode feed

Claim 2

A dual frequency band, hybrid mode feed comprising: a hollow waveguide body (12) comprising an inner surface; and a helically wound wire structure (10) bonded to the inner surface of the waveguide body with a dielectric layer (14) having a predetermined thickness which permits a predetermined mode to propagate at a first frequency band; and a periodic tuned grid structure (16) which is embedded in the dielectric layer between the helically wound wire structure and the inner surface of the waveguide body at a predetermined depth from the helically wound wire structure to permit said predetermined mode to propagate at a second frequency band.

Claim 3

A dual frequency band hybrid mode feed according to claim 1 wherein the feed further comprises an entrance port at a first end thereof and an aperture (32) at a second end thereof; and the helically would wire structure comprises a configuration which, in the direction from the entrance port to the aperture of the feed gradually tapers outwards towards the inner surface of the hollow waveguide body for converting a TE 10 -rectangular mode or a TE 11 -circular mode in the first frequency band at said entrance port to a HE 10 -rectangular mode or a HE 11 -circular mode, respectively, at the aperture of the feed; and the periodic tuned grid structure comprises a configuration which, in the direction from the entrance port to the aperture of the feed, gradually approaches the helically wound wire structure for converting a TE 10 -rectangular mode or a TE 11 -circular mode in the second frequency band at said entrance port to a HE 10 -rectangular mode or a HE 11 -circular mode, respectively, at the aperture of the feed.

Claim 4

A dual frequency band hybrid mode feed according to claim 1 or 2 wherein the periodic tuned grid structure comprises a Jerusalem Cross grid arrangement formed on a dielectric substrate.