I'm worried this might be two separate questions — or perhaps not any coherent question at all? — but some recent thinking about transmission lines made me realize I still don't know how to think about transmission lines.
The "un-intuitions" I have are related to inline components like a feed-through terminator with a resistor from (in coaxial terms) the center conductor to the shield:
…and/or a DC-block coupler, which is basically a capacitor interrupting the center conductor:
Now I get in broad terms that e.g. the feed-thru's resistor essentially terminates the coax with a "perfect load" at that point, or that the DC-block's capacitor looks like a good conductor to high frequencies and a bad conductor to low frequencies.
But when I ask myself things like:
- what would happen if, instead of placing a feed-thru terminator directly on my oscilloscope, I had an additional (long) length of coax connected to the terminator's output?
- why is it that a DC-block coupler can have a ± flat insertion loss (increasing slightly with frequency!) when the impedance of a capacitor is this dramatically inversely-proportional curve?
[^^^^ n.b. the above example questions are NOT my core question here!]
…I don't know how to think about them in terms of transmission lines!
They both seem related to the ≈resistance at a particular point in the line itself. Like normally there's approximately 0Ω resistance from one stretch of the center or outer conductor to the next — but the DC block adds some resistance at one particular spot. Or normally there's approximately ∞Ω resistance between the inner and the outer conductors — but the thru-terminator removes most of that resistance at one particular spot!
Is there any particularly helpful way to analyze these sort of random "odd spot in the coax" things?