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Question about t.v. antenna wire

Discussion in 'Electronic Basics' started by bench, May 8, 2004.

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  1. bench

    bench Guest

    I have two questions:

    The quoted impedance for t.v. antenna/aerial wire, 75ohms,
    is it just 75ohms or is it 75ohms per meter. What is the
    significance of this parameter?

    2. Why do we use mixer boxes when we want to combine
    different vhf t.v. aerials into one wire, why can't we
    simply combine/bundle them all together, i.e. directly
    connect all the earths and similarly combine all the center
    wires, and then feed this to the t.v.?

  2. Rich Grise

    Rich Grise Guest

    Google is your friend:"transmission+line+fundamentals""impedance+matching"

  3. bench

    bench Guest

    sorry, no comprendo!
  4. L. Fiar

    L. Fiar Guest

    Simply 70 Ohms, regardless of length.
    The antenna, coax and TV input should all be the same impedance.
    If they do not all match, the signal will not be efficiently transferred
    from the antenna to the TV. The mismatch can create signals to be reflected
    up and down the coax, causing standing waves. In bad cases, this may even
    show up as "ghosting" - which is rather like double vision.
    As stated above, impedances must match.
    If you connect two TVs to one antenna, the impedance will not match. Look at
    it from one of the TVs... it sees the 75 Ohm antenna in parallel with the
    other 75 Ohm TV, making just 37.5 Ohms. This is the same looking from any
    direction, including the antenna seeing 2 TVs in parallel - 37.5 Ohms.
    So a splitter will include resistors so that each TV sees a 75 Ohm

    Naturally, with a splitter, the signal will be divided equally between the
    receivers (with some signal lost in the splitter). So, if the signal becomes
    too weak for the TVs, an amplifier is added.
  5. bench

    bench Guest

    Thank you very much!!!!!!!
  6. Art

    Art Guest

    Nominal impedance for the "FLAT TWIN LEAD" was 300 ohm, Coaxial Cables are
    manufactured with varied impedance determined by it's presumed usage. Some
    Coaxial Cable is 50 ohm, some 70 ohm, some 75 ohm etc. Most Coaxial Cable
    used for Home Antennae Services is 75 ohm impedance, normal types are the
    RG-59 and RG-6, specific parameters, regarding loss at specific frequency
    bands, etc also vary with the different manufactured items. You have
    already been pointed in the right direction with reference to the "Goggle"
    sites to garner a plethora of data.
  7. bench

    bench Guest

    well no, I was not offered any particlar usefull link, I was given a list of
    thousand links, of which none are really answering my question
    in any immediate way.
  8. bench

    bench Guest

    and nor does your reply either
  9. I don't believe you looked too hard. The first link from Rich Grise
    had, as entry 1, a link to Agilent. This one.
    It is a short course on transmission lines. It is definitely worth
    your time.
    It was certainly worth mine.
  10. bench

    bench Guest

    I don't believe you looked too hard. The first link from Rich Grise
    This is a pointless conversation hence this is my last reply on this
    issue. There was a good explanation from the 3rd poster. This
    is good for me, and all those who don't agree can follow the
    links posted. Thank you.
  11. L. Fiar

    L. Fiar Guest

    Sure, but the OP refrred to 75 Ohm coax for television.
    I have a list of coax types in one of my books here.
    For reception it is. For transceivers, 50 Ohm is common... RG8, RG58, RG213,

    I just spotted this typing error... I should have typed 75 Ohms, as the OP
    had. It appeared correctly further down my post.
    Sorry about that.
  12. L. Fiar

    L. Fiar Guest

    Please note that my reference to 70 Ohms was a typing error... I should have
    typed 75 Ohms.
    The stated impedance of any coax refers to any length of that coax type.

    If creating a splitter box with resistors...

    __/\/\/\__ TV1
    antenna __/\/\/\__|__/\/\/\__ TV2
    |__/\/\/\__ TV3

    All resistors are the same value, worked out by:

    R = ----- x Zo
    R = resistance in Ohms.
    N = number of TVs connected.
    Zo = TV impedance (75 Ohms)

    You can do this for any number of TVs from 2 upwards. All the TV outputs
    should either be connected, or go to a 75 Ohm load. So only add as many TV
    outputs as you need, and work it out for that number.
    If you later add another output, you need to change all resistors to the new
    If signal is weak, add an amplifier before the splitter.
  13. bench

    bench Guest

    I can spot a problem with this reasoning. When you say all outputs of the
    must be connected to a 75ohm load I suspect you mean that the coax cables
    be plugged into the televisions. Now, what happens if one neighbour
    a central antenna, splitted into several flats) unplugs the coax from
    his/her t.v. ,
    will that cause a mismatch in the splitter?
  14. bench

    bench Guest

    B.t.w. I had a look at that tutorial and it is rather good. It gives
    a good intuitive explanation and avoids the high level maths
    which is required for a complete explanation, i.e.Wave
    Equations (2nd order differentioal eaquations), but yes,
    it is really good. I was a bit hesitant at first as it is distributed
    as en exe and exe files can be worms/viruses, but I took
    the risk after I read your post. Thanks!!!
  15. Rich Grise

    Rich Grise Guest

    Actually, it does. There is a thing called a "terminator," which is
    just a dummy load, which you're supposed to put on the unused outputs
    of a splitter. I bought up a splitter once, and I'm pretty sure it
    came with a couple of terminators - I don't remember if I had to
    buy them separately.

  16. L. Fiar

    L. Fiar Guest

    A TV is one type of load, but a 75 Ohm resistor will present the same
    load. Resistive loads are commonly used for testing transmitting
    equipment, as you do not want to actually transmit and not using
    a load could damage the transmitter.
    With no "load" inserted in it's place, yes.
    There can be problems with splitting. Separate antennas is the
    best option, but not always practical.
  17. I disagree with some of the other poster's comments. If it is a good
    (2-way) splitter, you can leave one output un-connected without
    seriously compromising the quality of the signal to the other output.
    This effect comes from a property called isolation. The circuit in the
    splitter uses a ferrite transformer to couple the input to two
    separate outputs while achieving isolation between the two outputs.

    If you look inside a 75-ohm splitter, you will see a 150-ohm resistor
    connected between the two outputs. When both outputs are loaded, the
    resistor absorbs no power. If one output is unloaded, the resistor
    prevents a severe mismatch in the output that is left connected. IOW,
    the signal stays clean and free of "ghosts".

    A 2-way resistive splitter will lose twice as much signal as a ferrite
    splitter. The resistive version will also have less isolation.

    Besides the mismatch and ghosting reasons mentioned already, poor
    output isolation can cause interference from one TV to another. TV's
    generate radio signals via an internal oscillator that can interfere
    with another television tuned at about 90 MHz offset (in North

    Some inexpensive splitters do not maintain good isolation for all
    loads at any frequency. YGWYPF. In this case, it is best to use
    terminators as the others suggested.

    Frank Raffaeli
  18. bench

    bench Guest

    my splitter is a 4-way one, but you did not explain how the transformer
    works. Is it a step up voltage or step down. Also, I thought a transformer
    only isolates DC, why would it isolate 200MHz ?
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