I recently put cable TV in my room (we already had cable TV before, but not in my room). The technician from my TV provider, through a split, ran a coaxial cable up to the TV in my room. When he did this he did not measure the length of the cable or anything like that, in fact, on the contrary, he told me that he could run the cable wherever I wanted and so it was. He also did not measure the impedance of the TV (i.e. the impedance of the load) and, in spite of this, the TV looks fine and everything is functional.

It is not a particular issue of the technician, because I have always seen that all technicians work in the same way.

Why don't they need to measure these parameters before? Wouldn't it be convenient to measure these parameters to verify that the line is correctly balanced? PS: My TV is not satellite.

  • \$\begingroup\$ A transmission line (coax or other) terminated at each end with the correct impedance can be of arbitrary length. Special magic can be caused by adding open or far-end shorted "stubs" but that's another issue. \$\endgroup\$ – Russell McMahon Jun 7 at 11:50

All TVs are designed with a standard input impedance of 75 ohms. No need to measure.

The cable is also 75 ohms, the length doesn't matter. Except the signal will get weaker if it has to travel further.

As @Hearth points out older TVs may also have a 300 Ohm balanced feeder antenna connection. Very rare these days though, especially on a cable-ready set.

  • \$\begingroup\$ Nitpick: old TVs that used ladder line instead of coax had 300 Ω input impedance. You don't see those anymore ever, but it's not always 75 ohm coax. \$\endgroup\$ – Hearth Jun 5 at 22:30
  • \$\begingroup\$ @Hearth - You're right I was forgetting about those. \$\endgroup\$ – Kevin White Jun 5 at 22:31
  • \$\begingroup\$ I see, that implies that the magnitude of the reflection coefficient is going to be zero, so it doesn't matter what length of cable you choose. Thank you very much! \$\endgroup\$ – Rodrigo Pozo Jun 5 at 23:34

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