1. How does a signal go through the cable
Coax cable
How does a signal go through the cable
I won’t discuss line transmission theory here. That is a bit too much. But there are a few common misunderstandings concerning coax.

2. Coax cable 50 Ohm Coax 100mA 100mA 5V AC Load 100mA 100mA
A fast current through a coax goes like a traveling wave. So the current on one spot can be completely different from that on another spot.
The shield carries the return current, and follows the current through the core with exactly the same speed: almost the speed of light. So a wave front is traveling through the wire.
Wave front moving with almost the speed of light
A coax can be thought of as a infinite series of coils and capacitors
Initially a RF current will flow through the coax regardless the load on the end: this is determined by the impedance of the cable, 50 Ohm is most common.

3. A signal in coax cable 50 Ohm Coax 100mA 100mA 5V AC Load 100mA 100mA
A fast current through a coax goes like a traveling wave. So the current on one spot can be completely different from that on another spot.
The shield carries the return current, and follows the current through the core with exactly the same speed: almost the speed of light. So a wave front is traveling through the wire.
Wave front moving with almost the speed of light

4. A signal in coax cable 50 Ohm Coax 100mA 0mA +5V Load - 100mA 0mA
A fast current through a coax goes like a traveling wave. So the current on one spot can be completely different from that on another spot.
The shield carries the return current, and follows the current through the core with exactly the same speed: almost the speed of light. So a wave front is traveling through the wire.
Wave front moving with the speed of light

5. A signal in coax cable 50 Ohm Coax 100mA 0mA 0mA 100mA +5V - 100mA 0mA
Load
50 ohm -
100mA
0mA
0mA
100mA
A fast current through a coax goes like a traveling wave. So the current on one spot can be completely different from that on another spot.
The shield carries the return current, and follows the current through the core with exactly the same speed: almost the speed of light. So a wave front is traveling through the wire.
Animation: no current,
current in the first part
current in the second part
current in the load.
Wave front moving with the speed of light

6. What a coax is not: just a shield for a conductor.
50 Ohm Coax
100mA
+5V
Load -
Often a coax is thought of being just a shield for a conductor to keep capacitive coupling away from the inner conductor.
And the return current goes through a undefined path called ground.
Too often “electric ground” is thought of as a black hole, or dumping ground of electrical signals.
100mA

7. Coax cable Shield carries return current, so sum of currents is zero.
A traveling wave goes through the coax
Initial current through the coax is determined by the impedance of the cable
If the current is not determined by the voltage and cable impedance alone, there must be reflection.