A feature of the '50s-era radars I worked on in the '60s was MTI, or
moving target indication. It was done simply by subtracting the return
from the previous pulse from the current one and displaying only things
which had changed. This was done to reduce clutter from fixed objects,
and it was effective against some jamming techniques. Doing something
like that is trivial today, but it wasn't back then. The pulse
repetition time of the long-range radars was over 2 ms (round trip time
for 400 miles or so), so a delay of that time with a bandwidth of a few
MHz was required.

The sets I worked on used a large piezoelectric quartz slab with many
faces, and transducers on two of the faces. One transducer would convert
the electrical signal to a mechanical wave which would enter the slab,
bounce around from one face to another, until at the right time it would
hit a face below the critical angle and exit. And that face was, of
course, where the other transducer was. To keep the timing precise, the
slab was used to control the radar pulse interval.

Not long before I was involved, a mercury delay line was used. This was
a tube of mercury with a transducer at each end, and a mechanical wave
was sent from one end to the other. I never saw one, but heard many
stories about how fussy they were and that it would take hours or days
for waves to settle if it was bumped or jiggled. Folks who had dealt
with them considered the quartz delay line to be a big improvement.

Roy Lewallen, W7EL