There are also extremely high quality trombones / line stretchers in the test sets for old HP network analysers (HP8740A and HP8742A) - before they used computers, the way to do "port extensions" was... to extend the transmission line with a trombone. Since they didn't have computer error correction, they had to be really well made so that the measurements were still accurate and good to 12.5GHz.
These test sets can be had for maybe $50 on e-bay, which is a real bargain. It would be easy enough to take the line stretcher out of one of these (and if desired you could add a stepper motor to make it computer controlled), but it seems wrong to slaughter such a beautifully made instrument for parts.
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