Electronics > Beginners
50 ohm termination again
radiolistener:
--- Quote from: Phaedo on January 21, 2020, 06:56:04 pm ---Just one question more :
Why is the impedance of the co-ax not included in the current calculations ?
--- End quote ---
Because this is the input impedance of the coax line before reflected wave will be returned back. It has some impedance, because coax line can be represented as a lot of LC circuits in series. Distributed inductance of coax line limits current change speed, this is why coax line has some impedance. But this is not heat loss resistance. When reflected wave will be returned back it will affect the source, so the input impedance of the coax line will be changed and will depends on coax line length and coax line load.
In other words electric wave has a limited speed of distribution in the cable, the wave needs some time to travel from cable input side to cable output side. At this time period, the input of the cable will looks like simple resistor equals to cable impedance. For example if coax cable is 75 Ohm, it will works like 75 Ohm resistor until wave is traveling from input side to output side.
But the things may be changed when the wave reach the output side of the cable. At this point the wave may be reflected back if the load on the cable is not equals to coax cable impedance.
When reflected wave will return back to the input side of the cable it will start to affect the source. It equals to change of the cable input impedance. This change will depends on the frequency, cable length and the load impedance on the output of coax cable.
So, when you enable generator, first you will see 75 Ohm on the input of the coax cable, but after some short period of time (which is needed for wave propagation through cable back and forth) the impedance will be changed.
If the load impedance is equals to the coax impedance, it will consume all wave energy and reflection will not occurs. So the cable will always work with 75 Ohm on the input side.
This 75 Ohm is not a heat loss resistance, this is a reactive resistance which consumes energy needed to charge the coax line with RF energy. This energy is stored in the cable, the same as it happens with LC circuit at resonant frequency.
Phaedo:
Thanks a lot folks !! Most helpful and erudite set of replies. :clap:
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