Adjustable passband filter

Claim 1

An adjustable passband transmission line filter having a preselected substantially fixed center frequency comprising, in combination: an input section of transmission line to which a signal to be filtered may be applied; a second section of transmission line; and a network coupled between said input and said second sections, said network comprising: a third section of transmission line matched in impedance to the input and second sections and coupling said input section to said second section; a reentrant transmission line network in shunt with said third section of transmission line; said reentrant transmission line network comprising first, second and third transmission paths connected in series with one another in the order named, said first and said third transmission paths being connected at one end to spaced points along said third section, and at their other end to opposite ends of said second path, said first, second and third paths and said third section each having a length of substantially nλ/4 where n is an odd integer and λ is the wavelength of said center frequency; and means including first and second controllable switching devices connected between said other end of said first and third paths, respectively, and a point of reference potential for switching said reentrant transmission line network between a first condition in which said reentrant transmission line network appears like an open circuit to said third section whereby said filter passes a first band of frequencies centered at said fixed center frequency and a second condition in which said reentrant transmission line network appears like a substantial value of capacitance load on said third section whereby said filter passes a second relatively narrower band of frequencies centered at said fixed center frequency.