The development of radar is one of the biggest, and best covered threads of the technological history of World War II. Indeed, it is probably given disproportionate importance, especially compared with complementary technologies such as radio navigation and direction finding. That is probably in part because of the way that we prefer to tell the story of the Battle of Britain as the comeuppance of the mighty Nazi war machine, rather than of a smaller air force trying to beat a bigger one after handing it the home field advantage; but that just opens the first box.It goes all the way down, guys. We tell it in the context of the Battle of Britain because, long ago, Robert Watson-Watt gave us a packaged technological history of radar. Were it not for his deft guidance, we would soon be lost in the thickets. There are histories of radar (Guerlac and Brown) but they cover a subject that's even more abstruse than aeroengines. The details just don't sink in.
Inevitably, when these kinds of discussions come up, there is a professional historian about to denounce it as "rivet counting." Which it is. Let's be frank here; any history that is written out of the Journal of the Institute of Electrical Engineers is going to a bit on the technical side. Unfortunately, if you leave it to the technicians, you are helpless in the face of a highly polished argument neatly freighted with Vast Significance: to wit, that the Air Staff's strategic blindness is demonstrated by their willingness to fly bombers with centimetric radar sets over Germany months before the same technology was available to go to sea with ASW aircraft, thereby giving the Germans a precious opportunity to develop centimetric radar warning receivers for their submarines, which they fortunately did not take up.
But it's poppycock that requires you to know electrical engineering. Oy vey. I'm your guide here, and I'm not a very good guide, because electrical engineering is hard, both the math and the engineering; and our great-grandparents saw and mastered huge hunks of change over the 70 years that separate Hertz's experiments from the self-congratulation fest that was the JIEE's 1946 special conference on radiolocation.
Put it this way: the first radio signals were sent on long wave sets that broadcast discrete radio pulses 10 kilometers apart, although, due to the fact that light is very, very fast, this corresponds to 30,000 oscillations per second. The radars we are talking about here had pulses separated by approximately 9.2cm, corresponding to an oscillation rate of 30 billion cycles per second. Nothing mechanical moves fast enough to complete a rotation in 1/30,000th of a second. There are ways of faking it, but they break down well before you get to the gigahertz range. You need electrical oscillators, which will, if you're like me, put you in mind of those Inductance-Resistance-Capacitance circuits you used to build in the lab to do various logic circuit-y type things, or, more often, so that your instructors would have an excuse to torture you with applied linear systems of partial differential equations with the eigenvectors and the eigenvalues and the eigenwhatnot, oh my.
Circuit-based oscillators have their limits, too, and those well short of centimeter-length radio waves. Indeed, the next step in the development of what would come to be radar came in the first, fumbling days of radio, with the application of vacuum tube technology to the problem. (If you want to say that "John Fleming invented the vacuum tube in 1904," or that Lee De Forest invented the vacuum tube amplifier in 1907, I can't stop you.)
Fortunately, as Albert Hull of GE's Shenectady lab was the first to realise in 1920, per the lawsuit-proof patent history of the technology, musicians do not need a physical oscillator to produce waves in a given frequency. Cavity resonators will do as well.
*Paolo Rivera, Cover, Daredevil #12: "Radar Vision." Is it still actually sonar? I've lost track.