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Why is 75 Ω cable commonly used for cable TV instead of 50 Ω?

  1. Oct 12, 2015 #1
    The standard characteristic cable impedance for the microwave industry is 50 Ω. Why is 75 Ω the standard for cable TV? Making a 75 Ω low loss cable with a 3 GHz rating pushes the limits of technology. Typically a 50 Ω cable would be less than half the diameter, lighter, more flexible and easier to install, cheaper, and have better electrical characteristics.
     
  2. jcsd
  3. Oct 12, 2015 #2

    berkeman

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    What is the input impedance of a dipole antenna? Why would that have an input on your question? :smile:
     
  4. Oct 12, 2015 #3
    I should have been clearer. I'm referring to the 75 Ω coaxial cable thats used a lot in USA residential construction. The 75 Ω cable looks quite bulky compared to 50 Ω coaxial cable.
     
  5. Oct 13, 2015 #4

    davenn

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    you come from a false assumption

    depends on the model of cable ... 75 Ω and 50 Ω come in all sorts of diameters

    RG59 (75 Ω) and RG58 (50 Ω) are both 1/4 inch dia
    RG6 (75 Ω) and RG8 (50 Ω) are similar size and lower loss
    and so on

    As to your actual; post question
    Berkeman's answer gives half the answer .....
    the other part is .....

    you have to go a little further .... A cable, with an air dielectric, has a maximum peak power handling capability at a given impedance.
    It also has a lowest loss figure for a given impedance.
    Peak power handling comes at 30 Ω impedance but lowest loss is at 77 Ω
    50 Ω coax is chosen particularly for cable systems where transmission and reception is involved as it is a compromise/trade-off between power handling and lowest loss. But for TV receive, power handling isn't a requirement so a nice round figure of 75 Ω is used to take advantage of the lower loss.

    These figures change a little with a more common Teflon, foamed etc dielectric.

    Dave
     
  6. Oct 13, 2015 #5

    Averagesupernova

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    Also interesting to note that 300 ohm twin lead which is very uncommon nowadays is exactly 4 times the impedance of 75 ohms working out well for the common 4:1 baluns.
     
  7. Oct 13, 2015 #6

    davenn

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    yes, stemming from back in the days when just a folded dipole was a common TV antenna and it has an impedance of 300 Ω
    The very first B&W Tv my mom and dad got way back in the late 60's even had a 300 Ω input ( no coax input)
    Even when they (TV antenna manufacturers) went to folded dipole Yagi's they kept the 300 Ω thought, even tho the addition of a reflector and directors meant that the feedpoint was no longer 300 Ω
    It dropped much closer to 200 Ω. where a 4:1 balun to 50 Ω coax would be more feasible
    But, TV receive being what it is, no one really seemed to worry too much about a bit of a mismatch :wink: :rolleyes:
    When transmit is also part of the process, matching becomes more important and my 10 element, folded dipole driven element Yagi on 2m (144MHz) ham band uses a 4:1 balun for good matching to a 50 Ω coax.


    Dave
     
  8. Oct 14, 2015 #7
    "RG59 (75 Ω) and RG58 (50 Ω) are both 1/4 inch dia
    RG6 (75 Ω) and RG8 (50 Ω) are similar size and lower loss
    RG59 (75 Ω) and RG58 (50 Ω) are both 1/4 inch dia
    RG6 (75 Ω) and RG8 (50 Ω) are similar size and lower loss"

    Thanks but 75 ohm cable installed in modern construction has about a 3 GHz rating and all the 75 ohm cables I've seen are quite large in diameter to handle this high frequency. RG59 and RG6 have frequency ratings a lot lower than 3 GHz.
     
  9. Oct 14, 2015 #8

    davenn

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    you didn't really read all of what I said ... did you :wink:

    Note that comment I made

    now if you want to be specific. then state the model/type of cable that you have regularly seen
    I only named a few to give you an idea, there are many, many types

    that is also a totally incorrect assumption .... increasing physical size is for 2 reasons
    1) power handling and
    2) lower loss

    Power handling for TV receive is pretty irrelevant ... lower loss if VERY relevant
    in fact as cable diameter increases its maximum useable frequency decreases. This is because a
    coax transmission line is basically a waveguide and if the waveguide is too large a diameter
    then multimoding occurs and that leads to additional losses

    RG58 and 59 are lower freq rated only because they are extremely lossy above around 500MHz or so


    Dave
     
  10. Oct 14, 2015 #9
    I don't have access to a sample cable right now, but all the 3 GHz low loss (low power) 75 ohm cable I've seen has been large diameter. Isn't it much easier to make a high frequency 50 ohm cable than a 75 ohm cable?
     
  11. Oct 14, 2015 #10

    davenn

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    all cables will operate to and well beyond 3GHz ( not sure why you are fixated with the 3GHz figure?
    the only difference is that some cables will have greater loss at 1 to 5 GHz. So the design of the cable determines the attenuation (loss) the cable has
    per 100 ft (30 metres). The loss is measured in dB. The spec's for a given cable will show a table of the losses for a range of frequencies / 30 metres
    lets choose my fav cable for the 144MHz to 1300MHz (1.3GHz) LDF4-50A

    t006_r00697_v5.jpg


    this is 1/2 inch diameter ( maybe a tad more)
    solid copper or sometimes copper clad aluminium, centre conductor, foam dielectric and a copper outer conductor

    Freq -- dB/100m -- dB/100ft
    150 ----- 2.673 ------ 0.815

    450 ----- 4.749 ------ 1.447

    1000 --- 7.284 ------- 2.22

    1500 --- 9.093 ------ 2.771


    you can see how the attenuation increases rapidly as the frequency rises.
    every 3 dB of loss you halve your power ( that applies to transmit or receive signals)

    I have also often used its big brother on commercial installations, LDF 5-50, 7/8 inch diameter ( close enough to 1 inch dia)

    the lowest attenuation is achieved by having an all air dielectric, but this is impossible in a coaxial cable as there needs to be something to support the inner conductor in the centre of the cable. One way around this is to use a spiral of Teflon. This keeps the 2 conductors uniformly separated but still achieves a dielectric that is >80% air. See the centre coax in the pic below

    upload_2015-10-15_8-21-53.jpeg

    for frequencies 5GHz and greater and for lengths more than a metre or so, waveguide is used. This has very low loss compared to any coax cable


    no, the manufacturing process is just the same

    do you understand what determines the impedance of a coaxial transmission line ? ......

    that should be εr on the bottom line .... not going to name the page it came from as I don't particularly like some of their other garbage

    Wiki gives a derivation of this formula ....




    cheers
    Dave
     
    Last edited by a moderator: May 7, 2017
  12. Oct 14, 2015 #11
    Here is some 4.5 GHz RG6 commercial cable: http://www.broadbandutopia.com/belden1694a.html
    I didn't know they could readily make 75 ohm cable that high but still think 50 ohm cable would probably be more practical. Don't 50 ohm cables generally have better performance than 75 ohm cables? Would it make sense to standardize most 75 ohm coaxial cable transmission equipment to 50 ohms?
     
  13. Oct 14, 2015 #12

    davenn

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    yes they can, and no not more practical

    again, no

    again, no

    Why ? ... read again what I wrote way back in post #4 ......

    that is why 75 Ω is much better for this type of service


    Dave
     
  14. Oct 14, 2015 #13
    Thank you for that good concise answer.
     
  15. Oct 14, 2015 #14

    davenn

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    any more questions ... ask away :smile:

    we all do our best to help


    D
     
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