About this episode
Foundations of Amateur Radio
Recently I explored the use of a radio device aptly described by a fellow Aussie Electronics Engineer, "ozeng", as "Absolute witchcraft." .. I'm talking about an "RF circulator", one of which is sitting quietly on my desk, roughly 60 mm square, 30 mm thick, weighing in at just under half a kilogram, unexpectedly with a 200 year history.
Let the spelunking commence ..
The moment you start reading the "Circulator" Wikipedia page, you'll see this sentence: "Microwave circulators rely on the anisotropic and non-reciprocal properties of magnetised microwave ferrite material.", with a helpful reference to "Modern Ferrites, Volume 2: Emerging Technologies and Applications", a 416 page reference that promises to dig into the nitty-gritty, showing 55 hits for the word "circulator".
Anisotropic you ask? It's the property that describes velvet, rub it one way, it's smooth, rub it the other way and the hair stands up on the back of your neck. Wood is another example, easier to split along the grain than across it.
While we're at it, reciprocity in physics is the principle that you can swap the input and output of a linear system and get the same result.
If you know me at all, it should come as no surprise that I went looking for an inventor. There's over twelve-thousand patents referring to a "circulator", including more than a handful relating to Nuclear reactors. In 1960, a prolific Jessie L Butler came up with patent US3255450A, "Multiple beam antenna system employing multiple directional couplers in the leadin", which states: "This circulator has the characteristic that energy into one port will leave another port to the exclusion of a third."
If you recall, that's the exact phenomenon I used to describe the "RF circulator" on my desk.
So, job done, we have our inventor. Not so fast. The patent goes on to say: "Circulators of this type are discussed in an article 'The Elements of Nonreciprocal Microwave Devices' by C.Lester Hogan in Volume 44, October 1956, issue of Proceedings of the IRE, pages 1345 to 1368." The IRE is the Institute of Radio Engineers.
I found a copy of that tome, thank you worldradiohistory.com, which includes the following sentence: "Until a few years ago, all known linear passive electrical networks obeyed the theorem of reciprocity. Today several different types of passive nonreciprocal microwave networks are in practical use".
A footnote refers to an article by Lord Rayleigh, "On the magnetic rotation of light and the second law of thermodynamics" and includes images of an optical one-way transmission system from 1901.
In that 1901 article, Lord Rayleigh in turn refers to a paper published sixteen years earlier in which he observed that light polarisation can be made to violate the general optical law of reciprocity, using a system that consists of two so-called Nicol prisms, a crystal that can convert ordinary light into plane polarised light, invented by Wi
