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Thread: Which RG-6 Satellite cable features are important, and which are not?

  1. #11
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    Quote Originally Posted by hbiss View Post
    The only balanced transmission line I have ever seen is 300 ohm twin lead and that was never good at rejecting interference. It had to be twisted to help with that.

    -Hal
    Actually IBM specified dual core coax (twinax) for data communications for its SNA/SDLC networks, interconnecting terminal equipment with mainframes.
    Not sure why IBM made this choice. The touted data rate of 1Megabit/second seems laughably low by current standards. But it was designed for relatively rough usage in uncontrolled noise environments. Since that time coax interconnection technology seems to have progressed in the direction of using better shielding instead of using differential signalling.

    In electrical terms, I think an analysis of its characteristics would be dominated by the twisted pair core parameters, with shielding added, rather than a coaxial analysis with exactly balanced currents on core and shield.
    Last edited by GoldDigger; 03-15-19 at 01:45 PM.

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    Quote Originally Posted by GoldDigger View Post
    Actually IBM specified dual core coax (twinax) for data communications for its SNA/SDLC networks, interconnecting terminal equipment with mainframes.
    Not sure why IBM made this choice. The touted data rate of 1Megabit/second seems laughably low by current standards. But it was designed for relatively rough usage in uncontrolled noise environments. Since that time coax interconnection technology seems to have progressed in the direction of using better shielding instead of using differential signalling.
    But why not use both? Maybe it's done in audio because the bandwidth is so much lower and 60Hz (the main culprit in audio interference) is in the audible frequency range. 60Hz is virtually DC to video frequencies.

    That's complete conjecture on my part, obviously.

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    Quote Originally Posted by ggunn View Post
    But why not use both? Maybe it's done in audio because the bandwidth is so much lower and 60Hz (the main culprit in audio interference) is in the audible frequency range. 60Hz is virtually DC to video frequencies.

    That's complete conjecture on my part, obviously.
    I am reasonably sure that line losses are higher with twinax compared to coax, especially at high RF frequencies.

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    Thinking about it, our CAT* cables are twisted pair and those handle RF. But you see what's happening there, trying to get more and more bandwidth out of that method is becoming a diminishing return. There is a CAT8 that will go to 2Ghz but it's said to be the end of the line. And with all CAT cables, there are 4 pairs and you are limited to 30 meters.

    The thing with coax is that it's cheap and works just fine. No finicky multi-pin connectors, bandwidth up to the point where you would begin using waveguides. Coax can simply be made larger with various dielectric materials and construction methods to cause less loss, and a simple amplifier placed in-line to compensate for it when necessary. Coax is regularly used to go many miles.

    -Hal

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    And the connectors needed for twinax are very low volume production, and AFAIK there are none suitable for tool-free or cheap-tool assembly, unlike connectors for coax or CAT.

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    Quote Originally Posted by GoldDigger View Post
    I am reasonably sure that line losses are higher with twinax compared to coax, especially at high RF frequencies.
    With balanced audio cables there is very little signal loss in twinax (is that what you call it?), but the frequencies are virtually DC compared to video, so I dunno. Mic level signal is down around, what, -60dB, and audio snakes are routinely 100' long and sometimes much longer.

    Rightly or wrongly, I have always thought that the length restrictions for CATx cable were more about crosstalk between twisted pair signal lines than signal loss.

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    Quote Originally Posted by ggunn View Post
    With balanced audio cables there is very little signal loss in twinax (is that what you call it?), but the frequencies are virtually DC compared to video, so I dunno. Mic level signal is down around, what, -60dB, and audio snakes are routinely 100' long and sometimes much longer.
    POTS telephone lines are audio twisted pair and how many miles can that go?


    Quote Originally Posted by ggunn View Post
    Rightly or wrongly, I have always thought that the length restrictions for CATx cable were more about crosstalk between twisted pair signal lines than signal loss.
    It might very well be, but the bottom line is for data 30 meters is the best current technology can deliver in trying to compete with fiber.

    -Hal

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    Satellite TV is an ingress only communication system. Your receiver has a one way path of communication from the satellite to your receiver, very similar to your AM/FM radio. Your receiver doesn't transmit to back to the satellite at all. Hence one conductor, and a shielding drain wire/sheath to dump any RFI to ground ... RG6 is definitely the best cable for this application.

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    Quote Originally Posted by BMANN06 View Post
    Your receiver doesn't transmit to back to the satellite at all. Hence one conductor, and a shielding drain wire/sheath to dump any RFI to ground ... RG6 is definitely the best cable for this application.
    You're not suggesting that coax isn't capable of two-way communication, are you?
    Code references based on 2005 NEC
    Larry B. Fine
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    Quote Originally Posted by GoldDigger View Post
    Actually IBM specified dual core coax for data communications for its SNA/SDLC networks, interconnecting terminal equipment with mainframes.
    Well, not for the mainframes (system370/30x3/etc) which used RG62 (93? ohm), but for the mid-range systems (system3x/as400); nobody liked the stuff and baluns for TP cable appeared quickly. We installed a fair amount of those in the 1980s.

    Although.... Direct Attach Copper (DAC) cables for 10G ethernet use twinax as do some other high-speed cables (e.g. SAS3), but it's a wee bit different from that used by mid-range systems.


    That said, coax is great for a lot of things, and ground loops aren't as much of a problem for multi-MHz "digital" signals as they are for baseband audio .

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