Electric wave filter employing interdigital line structures

Claim 2

An interdigital filter comprising a plurality of rod elements spaced between two outermost rod elements, means connecting one end of each of the rod elements to a ground plane, input and output connections to the filter coupling points on the ground plane to the outermost rod elements at regions thereof having an impedance close to the value of the characteristic impedance of a coupling cable, the outermost rod elements incorporating hollow ends connected to the ground plane to permit the passage of an insulated conductor of a cable through the respective rod to one of said regions, and a slot in the wall of the rod in said region permitting the passage of the conductor for connection to the ground plane at said point.

Claim 3

An interdigital filter as claimed in claim 1 in which the coupling cable is coaxial, in which the inner diameter of the hollow rod is that of the outer conductor of said coaxial cable and in which a conducting sleeve connects the outer conductor to the hollow rod at the junction of the rod and the ground plane.

Claim 4

An interdigital filter as claimed in claim 1, in which the case is of metal sections joined together only adjacent to the points where electrical connection is made to the case.

Claim 5

An interdigital filter comprising two ground planes formed by the metallic walls of a rectangular tube, a plurality of conductor elements arranged as interdigital conductor elements, support members for the interdigitated conductor elements of the filter, the support members closing the tube at each end, the conductor elements including round rods connected electrically and mechanically at one end to the respective support members, the two outermost rods in the tube having hollow sections at said connected ends, means for completing signal connections to the filter including a coaxial cable whose outer conductor dimensions corresponds to the internal diameter of the outermost rods, and the cable including an insulated inner conductor and insulator of the cable passing through the respective hollow rod to a slot in the wall of the rod and thence to a connection to a ground plane at a point adjacent the region of the slotted rod having an impedance close to the characteristic impedance of the cable, the outer conductor being connected to the respective rod at its junction with the ground plane.

Claim 6

An interdigital filter comprising two ground planes formed by the metallic walls of a rectangular tube, a plurality of conductor elements arranged as interdigital conductor elements, support members for the interdigitated conductor elements of the filter, the support members closing the tube at each end, the conductor elements including round rods connected electrically and mechanically at one end to the respective support members, the two outermost rods in the tube having hollow sections at said connected ends, means for completing signal connections to the filter including a coaxial cable whose outer conductor dimensions corresponds to the internal diameter of the outermost rods, and the cable including an insulated inner conductor and insulator of the cable passing through the respective hollow rod to a slot in the wall of the rod and thence to a connection to a ground plane at a point adjacent the region of the slotted rod having an impedance close to the characteristic impedance of the cable, the outer conductor being connected to the respective rod at its junction with the ground plane.

Claim 7

An interdigital filter as claimed in claim 4 in which the conductor elements are held at the correct spacing by a plurality of insulating blocks with bores tightly fitting on said elements.

Claim 8

An interdigital filter as claimed in claim 4 in which a trimming capacitor for a conductor element is formed by inserting a dielectric sleeve into the hollow outer end of the element and placing a conductor inside the sleeve, said conductor passing through the support member adjacent the outer end of the element, the length of the conductor within the sleeve being adjustable, and in which the conductor is held in electrical connection with the support member and in the correct adjustment by a setscrew in the support member.

Claim 9

An interdigital filter comprising a plurality of rod elements spaced between two outermost rod elements, means connecting one end of each of the rod elements to a ground plane, input and output connections to the filter coupling points on the ground plane to the outermost rod elements at regions thereof having an impedance close to the value of the characteristic impedance of a coupling cable, the free end of at least one rod being hollow and having an insulating sleeve therein and one end of a conductor in said sleeve being electrically connected to the ground plane to form a capacitor of which one plate is the conductor, the other plate the rod and the dielectric the sleeve.

Claim 10

An interdigital filter comprising a plurality of rod elements spaced between two outermost rod elements, means connecting one end of each of the rod elements to a ground plane, input and output connections to the filter coupling points on the ground plane to the outermost rod elements at regions thereof having an impedance close to the value of the characteristic impedance of a coupling cable, the free end of at least one rod being hollow and having an insulating sleeve therein and one end of a conductor in said sleeve being electrically connected to the ground plane to form a capacitor of which one plate is the conductor, the other plate the rod and the dielectric the sleeve.

Claim 11

An interdigital filter as claimed in claim 7, in which the conductor is adjustably clamped to the ground plane to form a trimming capacitor for tuning said at least one rod element to a desired frequency.

Claim 12

An interdigital filter as claimed in claim 7, in which said conductor is a wire clamped in a hole in said ground plane by a setscrew.

Claim 13

An interdigital filter as claimed in claim 7, in which said at least one rod is shorter than lambda /4 wavelength at the desired resonant frequency of the rod and is brought into resonance at said desired resonant frequency by said capacitor.