what is 75 ohm impedence?

here what I know (or what I beleave to know): impedence is the difficult electric current encounter to pass trough a wire / device. It is possible to measure that keeping 2 terminals. Now, the coaxial cable is 75 ohm impedence wire, but where can I measure this impedence?between what point and what point of wire I can measure it? More, the impedence is variable in depending on the frequency alimentation: 75 ohm is refered at what range of frequency?

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Here is an easy way to think of cable impedance. Suppose you live in a world where 1 nanoSecond seems like a long time, and you can get several things done in such a time. You pick up a meter long piece of 75 Ohm twinax and hook your Ohm-meter up to it, at one end. It applies a voltage and measures the current, or applies a current and measures the voltage, computes the ratio, and then displays "impedance". You observe "75.000 Ohms", after a few picoSeconds of measurement. You start the next phase of whatever project induced you to make this measurement, leaving the meter connected and running. Then, about 10 nS later, a strange thing happens: The reading changes! The current or voltage applied by the meter when you hooked it up has traveled down the cable, bounced off the far end because there was no 75.000 Ohm terminator there, and come back so as to modify the observed terminal conditions.

If you think through this story, and imagine that a circuit can indeed exist in that fantasy world, provided it delays its displays enough for us slower folk to perceive them, you should find your questions answered.

--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
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Larry Brasfield

Impedance is a two dimensional version of DC resistance (volts per ampere). When you deal with AC signals instead of DC, the current is not necessarily in phase with the voltage, so one way to have the math take care of all possibilities is to have one dimension take account of current that is either in phase or exactly out of phase with the voltage (+- real current) and the other dimension take account of the component of the current that is +- 90 degrees out of phase with the voltage.

Transmission lines carry AC energy as traveling waves, and the impedance of the transmission line tells you the ratio of voltage to current for those waves. Since those waves are a form of AC, the two dimensional measure of that ratio is needed, so it is called impedance, not resistance.

If you wish to launch 7.5 volt waves into a transmission line of 75 ohms impedance, the source will have to supply .1 ampere of current to the line. When the waves reach the other end, if there is not a 75 ohm load absorbing that ratio of voltage to current, some of the energy will be launched back into the line from that end (reflected off the mismatched load). If the line is much less than a wavelength long, the reflections bounce back and forth so often and overlap so many times that it gets hard to measure the line impedance.

John Popelish
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John Popelish

An easy way (for my simple mind ;-) to understand cable impendence is to picture the resistance to motion of my hand by a rope that's attached to, say, a block of concrete sitting on the floor.

If the rope is short, say a foot or so, and the back-and-forth motion of my hand is also slow then the resistance is mostly a function of the load: how heavy the block and how much friction with the floor.

But if the rope is longer, say twenty feet, and I move my hand very quickly then the resistance to motion that I feel is almost all due to moving the rope and not the load.

If the "wavelength" of my hand is very short with respect to the rope, I see the characteristic impedance of the rope. If the wavelength is very long (say "DC" or a constant pull) then I see the resistance of the load.

When you measure the resistance with a multimeter, you're seeing the results of a constant pull on the rope

Rich Webb   Norfolk, VA
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Rich Webb

IOW, if the cable is of an infinite length, it will show 75 ohms impedance; if the cable is of a short length *and* terminated with a

75 ohm resistance, it will show 75 ohm impedance. Also, the chatracteristic impedance of coaxial line is *not* frequency-dependent. But the cable loss *will* vary with length.
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Miles Harris

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