Impedances and transmission line effects...

[Gryphon]

That gets us on to transmission line effects. That's a book in itself, but the short version is that for higher frequency work, transmission line effects start to become significant, so we need to terminate correctly at each end of the line. So a 50 ohm coax cable needs to "see" 50 ohms at each end - so the source impedance is 50 ohms, and the input impedance is set to 50 ohms. When done, the cable is working as well as it can be.

This is my favourite video explaining transmission line effects: https://www.youtube.com/watch?v=I9m2w4DgeVk

It's from Tek, made about 300 years ago! Enjoy

I appreciate that you're summarising the world of transmission lines for the purpose of showing that these considerations are not often necessary in the baseband audio world, but...

Terminating both ends of the line with the characteristic impedance is a small-signal world answer to the issue of reflections.  The purpose of terminating the sending end is to suppress any reflections due to a mismatch at the receiving end and prevent the energy from embarking on a third trip down the line.

Another way of looking at the 6dB loss to which you refer is that half the power generated is dissipated in the sending-end terminating resistance and only half reaches the load.  If you generate 2V of video and connect a terminated 75 ohm line though a 75 ohm resistor, this isn't a huge problem: the sending-end resistance dissipates about 13mW.  If you have an r.f. amplifier and are trying to get, say, 100W to your antenna, the idea of generating 200W but that 100 of them never leave the sender is, shall we say, less attractive.  You need to design your amplifier to work satisfactorily into whatever load your transmission line presents, but to have as low an output impedance as you can contrive, to minimise the sending end losses.

This implies that the load at the far end must be well-matched to line, so as to prevent reflections from occurring there.  I would be the first to acknowledge that this isn't often as easy as it sounds.

Thank you for the link to the Tek video, which I'd not seen before.  Like many, perhaps, my first encounter with the mysteries of transmission lines came from reading "Foundations of Wireless" late at night.  It's interesting, to carry on from above, that, although the video deals extensively with what happens at the far end of the line, conditions at the generator are rather skated over.

Something else about the video interested me.  At about two minutes in, we see what is clearly a video/r.f. jackfield.  Doesn't the connector look like a P.O No. 1 MUSA, but "the wrong way round"?