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75R to 50R converter

Discussion in 'Electronic Design' started by Joe G \(Home\), Mar 20, 2007.

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  1. Phil Allison

    Phil Allison Guest

    "Rich Grise"

    ** The maker's rating.


    ** Nonsense - TV antennas are all 75 ohms.

    The OP's miss used the term " Log periodic "..

    VHF TV antennas look a bit like a periodic - but are not.

    UHF ones may be of that type if intended for a single band.




    ........ Phil
     

  2. Because you want a Un/Un, or unbalanced to unbalanced transformer. I
    made decent ones with six hole ferrite beads, and a 5 to 6 turn ratio
    transformer. 50 Ohms * 1.2 * 1.2 = 72 Ohms. Amidon is one source for
    the six hole beads.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  3. A TV antenna (or log periodic) is a balanced antenna, just like a dipole. A
    TV antenna (or real log periodic) is not likely to have feed point impedance
    of 75 Ohms, it is more likely to be in the vicinity of 200-300 Ohms. It
    could be that the OP has already attached the 4:1 balun that came in the box
    with it. In that case, the antenna would then have an unbalanced feed point
    impedance of roughly 75Ohms.

    BTW, I thought you plonked me.
     
  4. joseph2k

    joseph2k Guest

    Not in the least.
    Mini-Circuits has been making 50 MHz to 5000MHz transformers for over 40
    years, not new nor all that difficult.
    Only if used uncharacterized for a VNA situation, a mere spectrum analyser
    will be quite untroubled about group delay and a few 10ths of a dB will not
    often matter much very often.
    This sounds like someone seriously addicted to VNAs. Of course, if you are
    making amplifiers for 50 MHz to 5000MHz (and higher frequencies) you should
    be, that is where most of your performance parameters are measured.
    Just guessing from the frequency boundaries and the equipment already in use
    i think that OP is doing EMC compliance measurements.
    Spectrum analyzers are also great for for doing third order intercept
    measurements.
     

  5. Yes, cheap TV antennas are 300 ohms, balanced, but a few types are 75
    ohm unbalanced, for MATV/CATV service. These have the shield connected
    directly to the boom. I have built and used the 50 to 75 ohm VHF
    transformers to use better test equipment to repair CATV headend
    equipment and to align custom filters with 50 ohm test equipment.


    Just on sci.electronics basics, so far.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  6. mpm

    mpm Guest

    Joe -

    Can you give me a part#?
    I didn't find a single part that meets with your above assertion.

    http://www.mini-circuits.com/products/transformers_sm_c.html#config_c

    Also, I don't know anyone who measures EMI/EMC without a calibrated
    antenna system.
    I submit to you that if you just hook a 75 ohm antenna to a 50 ohm
    instrument, or vice versa, (and making no other measurements or
    calculations), then you are simply wasting your time and your client's
    money.

    I would not place much confidence in the results.
    ESPECIALLY for "off-air" field measurements, which is what I believe
    the OP is trying to accomplish.

    -mpm
     
  7. Tom Bruhns

    Tom Bruhns Guest

    Gee, I figured the OP was just trying to get a look at what sort of
    signals are out there. I can't imagine anyone trying to do calibrated
    measurements without a good known antenna. It sounded like he's just
    using a couple old TV antennas that happened to be kicking around.
    It's really unlikely that he'll notice any difference for off-the-air
    work with antennas that are essentially unknown anyway, if he just
    connects them directly to the SA. He can use either 75 ohm or 50 ohm
    line; it won't make a significant difference.

    Perhaps Joseph slipped a decimal place on that MiniCircuits
    transformer frequency range, at least as separate transformers. In
    any event, the TC1.5-1 is rated 0.5 to 2200MHz, well in excess of what
    the OP needs. Never mind that it has more power loss than if you just
    connect the 75 ohm antenna (assuming it really does look like a 75 ohm
    source) to the 50 ohm load.

    Cheers,
    Tom
     
  8. Yagis, dipoles, logs etc are typically balanced antennas by nature. I
    haven't seen any type of TV antennas that were fed straight from coax, but
    that doesn't mean they don't exist in the commercial world. I do a have a
    2mtr beam that has a gamma match built into it so you can feed it straight
    from coax, but that's only because the matcher is built in. It's still a
    balanced antenna.
    I've done a little antenna matching/tuning/construction myself. :)
    That's cuz you misunderstood what I was trying to say, I wasn't "boasting
    about being plonked allot". At any rate, keep your head in the sand over
    there if you like, but I think the whole plonk thing is silly.
     
  9. joseph2k

    joseph2k Guest

    Are you asserting that you have "worked" 2 m band and have never heard of a
    "J-beam"? How about "5/8 wave whip"?
     
  10. I never said any such thing. What do you mean?
     

  11. It expires before long. Some newsgroups are 75% noise, and need a
    lot of filtering to look for anything worth reading.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
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