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Update September 2018: it turns out that this line stretcher idea dates back to at least 1956, as it appears in the "Handbook of Tri-Plate". Read about it on our history of stripline page.
This is an idea for a line stretcher, which we came up with based on a question on the ol' Microwaves101 message board. This line stretcher provides an analog stretch, it's infinitely variable between minimum and maximum values. It must be realized in stripline (at least the way we envision it!)
The image below shows the centerconductor in a the stripline, this metal is plated on the bottom dielectric.
The metal below is also the center conductor, but it's attached to the top dielectric. We used two slightly different colors so you'll be able to tell the difference when we stack them up. This metal is plated to a "wheel" of dielectric that can be twisted perhaps 90 degrees.
Now watch what happens when we stack the metals and arrive at a "radial trombone" when the two metals contact each other. The image below is for minimum phase shift. The metals in the image don't exactly overlap properly, we did this quickly in Powerpoint!
The image below the movable metalization has been twisted to the right by 90 degrees, thereby stretching the line.
Of course, the Devil is in the details of making making the top groundplane continuous, but trust us, this can be done. Further advice: tighten the crap out of the stucture afte you select the line length!
Another example of Microwaves101 Solid Gold, yours for free! Many more to come!