AT&T Archives: Similiarities of Wave Behavior (Bonus Edition)

I've never understood why these old instructional films so often seem much better at getting things across and making them stick in my mind than more modern material. A couple other examples. 1. A 1959 film from Bell Labs on wave behavior that made standing wave, reflections, energy loss due to impedance mismatch, and impedance matching much clearer than anything else I've seen: [1]. 2. A 1937 explanation of how the differential in a car works: [2]. Here is another upload of that which is lower resolution, but has more from the same source in the sidebar [3]. I suppose it could be something akin to survivorship bias that makes old educational films seem so good--the bad ones don't get remembered 60 years later. [1] [this video] [2] [link] [3] [link]

Thank you for this link! The part starting here is particularly interesting to me: [this video] Why is that? Well, because a long time ago, I built an extremely low-power battery operated DC Tesla Coil. It worked, but the one thing I could not figure out was why the coil with the small number of turns, the coil that typically encircles the long coil with the many turns, why this coil with smaller turns -- had to be positioned a slight distance up the many turns coil, and not either directly at its end, nor at its middle. Well, this video, at the point I've noted, explains the answer why. It also explains the relationship between frequency, length of resonator, and ability to create larger amplitude waves via additive superposition, that is, the ability of one wave, travelling in one direction, has the ability to add to the amplitude/crests of another wave travelling in the other direction BUT IF AND ONLY IF THEY ARE IN PROPER ALIGNMENT ! Which, incidentally, is the principle behind how a laser works (in addition to a Tesla Coil!). Also, there's the knowledge here that we can change the frequency to match the resonating coil/cavity/resonator length/etc., OR we can change the resonator length/coil length (in a Tesla Coil this is accomplished by moving the smaller outer coil with less turns on it) to match the frequency. So we can choose one to adjust to match what's needed by the other. Now, if say, a Tesla Coil is storing energy when in operation by additive/constructive interferen

If you want to understand how TDR works, watch this classic tutorial from the mid-50s in the AT&T archive. The first 5 minutes, showing the reflection of an open and short circuit is enough. * AT&T Archives: Similiarities of Wave Behavior [this video] Many are surprised when they learned that it's possible to detect the location of a break in the mains wiring or an Ethernet cable, just by doing an electrical test from one end of the cable (some Ethernet cards even have builtin TDR support!). There's no closed loop, why is it possible to test anything electrically? Because at high frequency, the circuit becomes an electromagnetic waveguide. Basically like a radar, you send a fast electrical or optical pulse to the electrical cable or the optical fiber, the EM wave travels along the cable until it hits the break and reflects back. You record the time and listen for the echo. All discontinuities (such as damages) on the line will reflect and distort the original pulse. Use the speed of light to pinpoint the location of the fault.

Wifi radios are generally designed for 50 Ohm impedances. But household TV coax in the States (RG-59, RG-6, RG-11, and friends) is 75 Ohm. And impedance matters. How much it matters varies by application, but it can be very important. There's more going on in bit of coax than a shielded conductor[1]. (Relatedly, twisted pair cabling like Cat5e has more going on than a few pairs of wires that happen to be twisted.) But the concept of having a pair of radios connected together with coax does, of course, work fine as long as the I's are crossed and the T's are dotted. I've set up PtP Wifi-ish wireless links on the bench with attenuators and coax jumpers instead of antennas, but I kept the characteristic impedance consistent throughout. (Many-to-many networks are also possible, with similar caveats.) [1]: Similarities of Wave Behavior, as presented by Dr. J.N. Shive, 1959. [this video]