This apparatus - basically just a long piece of coax cable - is used to emulate the radar echo over a longer range within the relatively small confines of the lab bench - mainly because it's cold and don't feel like taking all the apparatus outside.

20 meters of RG-58 coax will introduce a delay of about 100 ns. 20m of coax is equivalent to 30m of free space (because of the velocity factor of about 2/3), so the 20 meter cable is equivalent to a radar return from a target at a range of 15m.

The loss in 20m of RG-58 at 6 GHz is non-trivial, but then again the free-space path loss isn't trivial either - we expect substantial attenuation in either case.

At a triangle-wave modulation frequency of 25 Hz, the half-period (the length of the positive-gradient part of the ramp) is 20 ms, so if the VCO modulation range is say 200 MHz, we expect to see a frequency difference of (200 MHz * 100 ns / 20 * 10^6 ns) about 1 kHz, which is what we'll see in the echo signal on the output of the IF amplifier and filter chain.