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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. Hi All,

    I am looking for ways of converting a 75R Log periodic TV antenna to 50R to
    provide a "best" match for my spectrum analyser.

    2 Antenna's need to be converted to 50R , and may require different 75R to
    50R converters.

    a) 80MHz to 500MHz

    b) 500MHz to 1GHz

    Low power Rx signals only.

    Any pointer where to start would be appreciated.

    Any pointers would be appreciated.
  2. Phil Allison

    Phil Allison Guest

    "Joe G (Home)"

    ** No need to do that.

    The source Z of a wide a band TV antenna is all over the place anyhow.

    ** Just use 50 ohm co-ax for the feeder.

    No useful 75 /50 transformation is possible or needed.

    ....... Phil
  3. I'll give it go.


  4. Hello Joe,

    you could do it with a resistive divider. Using the right values will give
    you the
    corerct termination resistance from both sides.

    R1=43.3, R2=86.6

    This R2 is simply twice the value of R1. So just use 3 resistors with 43
    It's important to have a very low lead inductance. SMT-resistors would be

    43 Ohm
    75 Ohm side ----|____|-----o-------------- 50 Ohm side
    | ____
    ----|____|--- GND

    86 Ohm (or 2*43 Ohm)

    Such a passive divider has a gain of 0.44 compared to 0.8 without this
    Z-match circuit.
    So you will loose half the signal, but you have the advantage of the cable
    impedance matching.

    Best regards,
  5. Phil Allison

    Phil Allison Guest

    "Helmut Sennewald"

    ** Like chucking out the baby with the bath water.

    When are these demented ham radio FOOLS going to learn that cable
    matching is something you do at LOAD end.

    ........ Phil
  6. Then why didn't you suggest a balun? Or do you think it wouldn't make a
  7. Phil Allison

    Phil Allison Guest

    "Anthony Fremont"

    "Anthony Fremont = Psycho **** from Hell "

    The most asinine, most ASD FUCKED pile of sub human

    YANK SHIT on all usenet !!!!!!!!!!!!

    ** Nope .....

    Satan would be soo jealous of losing your undivided attention.

    Sucking his tiny, cold dick is your full time occupation.

    ** Shame about all the UTTERLY ASININE FUCKWIT assumptions

    brain dead, ASD fucked shitheads who post on SEB make !!!!!!!!

    Oh - now I see my tiny oversight .... all those brain dead scumbags

    must be your six toed, pointy headed, dribbling, farting, puking relatives.

    Yet to come down from the trees.

    Still hanging by their tails & eating their own droppings.

    Just like YOU !!!!!


    ........ Phil

  8. Hello,

    Here is a picture from an adapter. Most probably using resistors as shown
    The specified insertion loss of 5.7dB less is exactly the same as calculated
    from the resistors above.

    IL = 20*log10(0.422/0,8)

    Best regards,
  9. You wan't the world to killfile me, but you won't do it yourself? Why not?

    Joe should find a broadband balun (unless one is built into the antenna) so
    that the coax shield is less of an antenna and more of shield. Odd that a
    log periodic would be 75 Ohms, IIRC they are usually in the vicinity of
    200 - 300 Ohms UNBALANCED.
  10. Cheers! A bit of searching and I also found Mini Circuits have these with
    5.7dB loss...

    At this stage I am tinkering... but will get serious later...

    Simple to build....

  11. Best regards,
    Oh... I see you are active on LTSpice... too.

  12. mpm

    mpm Guest

    You need something called a "Minimum Loss Pad"
    Lots of people make them. From cheap $ to very expensive $$$$.
    I know HP makes them. But that's the expensive variety. Or you can
    make one. Ebay, etc..
    For TV (US Terrestrial anyway) focus on 50 MHz to 700 MHz.
    Anything more that that would just be a waste of money.
    After the conversion to DTV, I believe the highest UHF channel will be
    51, with probably very few exceptions if any.

  13. Phil Allison

    Phil Allison Guest

    "Helmut Sennewald"

    = Some Asinine Kraut FUCKWIT !!

    ** Only an UTTER ASS uses resistors for any RF matching job.

    **** the HELL OFF !!!!!!!!!!!!!!!!!!


    ....... Phil
  14. Joerg

    Joerg Guest

    Or just place a 25ohms resistor in series with the coax. Then the
    antenna sees a 75ohms impedance. If the coax is properly terminated at
    the other end (by the spectrum analyzer) it should be fine.

    However, a TV antenna will show a real 75ohms only at one or two small
    frequency bands, if that.
  15. Tom Bruhns

    Tom Bruhns Guest

    There's probably not much real need to do it, but a reasonably low-
    loss way is to use a transformer. MiniCircuits ADT1.5-122T or
    ADT1.5-17 will do a good job over the whole frequency range you
    mention. A log-periodic has at least a chance to maintain a
    reasonably constant impedance over a range of frequencies, but you'd
    do well to check what the impedance really is, if you have access to a
    network analyzer, before messing with matching it to your analyzer.

    Before you do any of that, though, consider that there is only 0.18dB
    loss of power incurred by connecting a 50 ohm load to a 75 ohm source,
    as compared with connecting a 75 ohm load to the 75 ohm source.
    That's clearly not worth worrying about when the component you add
    introduces more loss than that, and where the gain of the transducer
    (antenna) is a function of frequency, and isn't known to even ten
    times that inaccuracy. Where it could be worth considering is in a
    system with rapid variations over time, where echos at mismatches
    cause reflections, and only then in some specific instances. That
    doesn't seem to match what you've described.

  16. mpm

    mpm Guest


    Impedance Power Loss (dB) Voltage Loss (dB)
    75 to 50 -5.72 -7.48
    50 to 75 -5.72 -3.96
    Round Trip -11.44 -11.44

    Note that power losses are the same regardless of which way you're
    Voltage losses differ depending on the direction of the impedance
    As you would expect.

  17. Tom Bruhns

    Tom Bruhns Guest

    Huh indeed. 75 ohm generator, 2Vrms open circuit. Connect a 75 ohm
    load. Power delivered to the load is 1V at 75 ohms, or 13.33
    milliwatts. Do you accept that that's the most power you can get from
    that source? Connect to a 50 ohm load instead; the load voltage is
    2V*50/(75+50) = 0.800 volts. 0.8V at 50 ohms is 12.80 milliwatts.
    12.8 milliwatts is 0.96 times 13.333 milliwatts. 10*log10(0.96) is

    The power loss attributable to such a modest impedance mismatch is
    generally not enough to worry about.

    I really don't know where you got your numbers, but they don't seem to
    stand up to analysis. Show us the math and maybe we'll understand
    where they came from.

  18. Tom Bruhns

    Tom Bruhns Guest

    Oh, I see--your numbers are for a minimum loss pad. But what I posted
    was that you connect the 50 ohm load directly to the 75 ohm generator,
    NOT through a pad! Unless you can convince me that the antenna
    performance suffers in some way because it's terminated in 50 ohms
    instead of 75 ohms, there simply is no need for the pad. In any
    event, if you do have some hangup about terminating the antenna into
    75 ohms, don't use a pad--use a transformer. The loss will be much
    lower. Pads are nice for DC-to-daylight performance, but for the
    modest one decade or so of frequency range the OP wanted, the
    transformer will be fine.

  19. Rich Grise

    Rich Grise Guest

    How do you know the Z of the antenna is 75R? I've just done a web search,
    and all of the designs that have turned up so far are 50. In this case, a
    1:1 balun would be all you need.

    Good Luck!
  20. mpm

    mpm Guest

    Sorry about that.
    In any event, I'm glad you deciphered that post because I was
    definitely not looking forward to typing in those equations using
    plain text format. :)

    I don't know? The OP asked for 1 GHz. That's quite a lot.
    I think a simple transformer would introduce unacceptable bandwidth
    Certainly there would be group delay and response issues.

    Also as I'm sure you're aware, most useful "answers" are not displayed
    on the spectrum analyzer screen itself, but are instead calculated
    from the displayed results. I'm not sure I would (or could?) trust
    calculations where I was not controlling for RF Impedances.

    Now of course, for un-amplified off-air measurements (if that's what
    the OP intends to do), the loss of an L-Pad might be too high, in
    which case, he needs a network analyzer too so he can properly
    characterize the impedance of his setup. Ditto if he ever adds a

    But if he's just gonna check center-freq and maybe some simple CW
    deviations, I see your point.

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