Min,
Don't think about lumped vs. continuous circuits as?an either-or.? The continuous view is always correct; the lumped one may be close enough but it does not replace the continuous one.
That's true for transmission lines too, e.g., coax.? A real transmission line has continuously distributed L, R, and C.? Regardless of its length, we could instead represent it as lumps of cascaded LRC sections, and it's good enough if the frequency is low enough.? There are reflections on long transmission lines even if you model it as a series of LC or LRC lumps.
For these RF signal generators, it is all the same to them whether they drive (say) a 150 ohm resistor directly, or a mismatched load on the end of a very long cable.? In either case, the impedance seen is X, the SWR is Y, the reflection coefficient is so much, etc.
At the RF device itself, it can't tell whether the thing it's driving has a forward-moving wave and a reflected wave.? That doesn't matter.? We can talk about things like VSWR even if there are no standing waves.? It's just a measurement tool.? Similarly, we can talk about "reflected power" even if there isn't any transmission line..? Anyway, those are abstractions.? What really exists, are voltages and currents, as a function of time and location along the line..? The electrons don't see forward or reflected waves, they just respond to the E-M fields acting on them, instant by instant.
At the output of the RF device, it sees a load impedance.? You can express it in different ways.? A useful one is reflection coefficient.? Even a 150 ohm resistor, connected to a "50 ohm" circuit by soldering it directly to the output pins (hence a lumped circuit model), has something we can measure and call "reflection coefficient".??We can also measure "reflected power", which takes into account both the magnitude of the reflection coefficient, and the output power from the RF device.? And as it happens, the reflected power is a useful metric for telling us how much mismatch the RF device can tolerate.
It might help to think of "reflected power" this way.? It means the same conditions as a long piece of transmission line with a mismatched load, having the same input impedance.
But we are waaay off-topic for this group -- even though the problem of modeling transmission lines (lumped vs. continuous) is important in SPICE and LTspice.
Andy
