Coaxial cable - Signal on oscillosope?

[Edgardo]

Hello all,

I once wanted to plug two coaxial cables together (tv - antenna),
and when I touched the coaxial cable coming from the antenna,
I got an electric shock (a weak one, like if you get an electric shock if
you are charged and touch metal).

My questions:

1) Why did I get an electric shock (is it because a coaxial cable
is a capacitor)? And when do I get those electric shocks?
I often touched the coaxial cable and nothing happened.

2) If you take an oscilloscope and measure the voltage of the antenna
coaxial cable, do you get a signal? Do you also measure current?
What does the signal look like?
Does the signal from a DVD look different than the signal from the antenna?

 

[Gokul43201]

Was the shield on the coax grounded ?

 

[Edgardo]

Was the shield on the coax grounded ?

I don't know :shy:
How do I see if it was shielded? It was a usual coaxial cable,
that you use to connect your tv to the antenna on the roof.

 

[GENIERE]

In a coax(ial) cable, you have the outer sheath of insulation and protection, below that is the shield composed of braided thin strand wire. Below that is the dielectric material, usually white, surrounding the center conductor.

The shield should be grounded to the same place as the meter for your household electricity. If not, it is a danger (lightning strike) to you and to whatever is connected to it.


The likely cause of your shock was a static discharge due to your body acquiring a charge (walking on a rug?) and then being discharged to ground through the antenna cable or through a capacitve path. Besides making sure the cable shield is grounded, I wouldn’t worry about it.

 

[Edgardo]

2) If you take an oscilloscope and measure the voltage of the antenna
coaxial cable, do you get a signal? Do you also measure current?
What does the signal look like?
Does the signal from a DVD look different than the signal from the antenna?

Can anyone with an oscilloscope at home do this?

 

[GENIERE]

Originally Posted by Edgardo
“2) If you take an oscilloscope and measure the voltage of the antenna
coaxial cable, do you get a signal? Do you also measure current?
What does the signal look like?
Does the signal from a DVD look different than the signal from the antenna?”

The TV broadcasting station transmits a high frequency carrier wave. The TV information (image, color, audio and synchronization) is superimposed upon the carrier wave, termed modulation.

To visualize the TV signal on an “o” scope would require some previous amplification and a detector device to exclude the carrier wave.

The DVD signal may be available on a jack labeled “video out”. That signal would be the same as above but without the high frequency carrier wave. It could be viewed directly on a scope.

To keep it simple, imagine a black and white, home security TV camera (no audio, no color, no carrier wave) is imaging a black and white chessboard at a distance so the chessboard completely fills the image area. The output signal connected to the scope would reveal:

2 tall horizontal sync pulses
8 not quite as tall pulses between the sync pulses representing the black squares.
8 gaps between the black pulses representing the white squares.
If there was a gray shaded square, its amplitude would lie somewhere between the gap and tall black representing pulses.

Depending on the broadcast standard, every so often there would be a vertical sync pulse.

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