Innovation Insights: It starts with the physics

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Innovation Insights with Richard Langley

IT’S ALL PHYSICS. How things work, that is. Well, maybe a little chemistry too in some cases. I might be a little biased in my opinion given that I’m an applied physicist by training. Radio? Satellite navigation? Yes, the principles of their operation are all governed by physics. Many physicists of my generation started out as radio tinkerers. I’ve recounted in this column before that I built my first radio (from a kit) when I was 14 (not counting the crystal radio that my father helped me to put together when I was 8 or 9). I built a few more during high school, got into radio astronomy as an undergraduate and did a Ph.D. in the application of very long baseline (radio) interferometry to geodesy.

The great American physicist Richard Feynman was also a radio tinkerer in his youth. He recounts in one of his autobiographical books how he used to fix radios. Since he would approach the task of repairing each non-functioning set by first contemplating why it wasn’t working, he got the reputation of fixing radios by thinking!

One of Feynman’s special abilities was in explaining how things worked. In fact, he has been called “The Great Explainer.” He authored what is arguably the best physics textbooks ever produced: The Feynman Lectures on Physics. The three-volume set, developed from his Caltech lectures to undergraduates between 1961 and 1964, covers mechanics, radiation, electromagnetism, matter and quantum mechanics. Many students and practicing physicists have learned or relearned aspects of physics from the famous “red books.” Many more will now thank Caltech, which recently put the Lectures online for anyone to read.

In the February 2016 column, we learned about the development of a microprocessor-controlled multi-element GNSS antenna array for interference rejection. While there are many textbooks that describe how multi-element antennas work, Feynman explains their operation in his Lectures from first principles–from the principles of physics. The phenomenon governing the behavior of antennas with multiple elements is called interference.

If we combine two electromagnetic waves, they will interfere with each other with a result that depends on the relative phase (or phase difference) of the waves. The waves might reinforce each other leading to a larger net amplitude, called constructive interference, or partially or fully null each other out, called destructive interference. When we apply this concept to the signals transmitted by a pair of antennas making up an array in a horizontal plane, we find that the array has directionality. That is, if we space the antennas by one-half wavelength of the signal to be transmitted and feed the antennas in phase (zero phase difference), we will transmit a strong signal in the directions perpendicular to the baseline connecting the antennas (say east-west) and no signal in the orthogonal directions (north-south). If we use this antenna pair for receiving, we will have a null in the reception pattern to the north and to the south and will be insensitive to signals arriving from those directions. And as Feynman describes in his lectures, by adding more antennas to the array and “some cleverness in spacing and phasing our antennas,” we can have a fairly narrow pattern null in a chosen direction. In the case of a GNSS antenna array, that direction might be that of a jamming signal and so we can null out the jammer and maintain a positioning capability.

There is more to it in developing a practical microprocessor-controlled GNSS antenna array, but it starts with the physics.