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RF & AC different?

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Dr_Doggy

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just as i though i understood the difference between a AC electric wave and an RF wave, I get sent to cable school, where the prof says that there is no voltage on the cable line but just an RF wave( measured in db's) to which grounding out the wire doesn't short the signal???

can someone explain this, he says there is no voltage/current on the line ever, and we use voltage @ the amp to step up the signal, but the signal is RF?

but then a db = v^2 / R so wouldn't that mean there is current present?
ALSO my scope can pick up a cable signal so isnt that an AC voltage?

Verry confused now, can someone explain this out for me>?

PS, I am talking about a TV cable line here....
 
Ask prof what powers the cable line amps and how is it supplied.
 
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The prof is using confusing terminology.

IF the frequency of AC waveform on a cable is such that a wavelength is much less than the cable length then you have voltage on the line that will disappear if you short the output.

If the frequency of the AC waveform is such that the wavelength is less than the length of the cable then it will have RF properties and the signal will still appear on the line, even if you short the output.

But I don't understand his statement about current and voltage. Certainly there is current and voltage on an RF line. High power RF lines can actually have voltage breakdown in the cable dielectric if it gets too high. It's the current and voltage that determine the RF power. Of course, the current and voltage are related to the EM wave that propagates down the cable, but there is still current and voltage.
 
yes I am refering to CATV lines all through Canada, and 5 - 800 MHz signals, so I think our lines go over few hundred feet and the amp is powered @ 90V.

good, that EM is still VI@f that was my main concern that there was more than electrons travelling down the line.

The reason this came in to question was that we were talking that TX for the digital box comes in on its frequencies in the CENTER pin, which is fine by me, but then when the digital box RX(es) it comes out on the braided wire(around center pin) which would also be fine, BUT on every house we are required to run a ground line to protect from surging (lightning) which connects to that braided wire, however from my perspective that ground protection would short out the RX data that leaves the digital box back to the fiber optic converter.

(grounding occures at each and every home) and RX data is closer to 5 MHZ
 
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That signal on the center pin is relative to what?

Answer this and you have part of your answer.

RF and fiber signals occupy a "slice" of available bandwidth. Information is transmitted as long as that "particular slice" is available for travel.

Where did you find this guy?
 
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There are several standards and new high bandwidth optical trunks change a lot of things.

The most common is -48 vdc fed up the center of coax to power the amps. Some systems converted to AC power due to issue with DC electrolysis that grows green crud when moisture intrudes connectors.

For data, one or more RF channels are dedicated for downlink data carriers. Return data carriers are less then 50 MHz. Trunk amps have diplexer to split forward (>50 MHz) and reverse (<50 MHz) RF paths to amplify them. Most systems use 16QAM modulation. As DSP's have become more powerful, some newer modems are using up to 64QAM to increase data rate.

Outside steel cable is primarily for strength to be able to string coax between utility poles. It can also provide some grounding for lightning protection. No signal is carried on the steel support cable.

The standard RF measurement in cable industry is dBmV which is db relative to 1 mVrms voltage.
 
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To give you an interesting picture, suppose there is a CATV line and channel 6 comes from 1 end of a 10 mile cable and channel 10 from the other to serve customers along it's length.

Generally it wouldn't happen this way because of attenuation and amplification, but it gives you something to think about especially the term "reverse channel"
 
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