Differential QPSK receiver
Coax cables are delay lines. Tiny sections of solid-shield coax were used in 150MHz adaptive equalizers, prior to dataslicing and phase-locking and data recovery.

For your purposes, cut the coax to exceed the switching-time between symbols, so you have some clean regions to feed into a mixer. You'll need to trim the delays rather tightly, to N cycles + 90/180/270 degrees, for best noise rejections.