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Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KJ4OHB, Oct 10, 2017.

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  1. KJ4OHB

    KJ4OHB Ham Member QRZ Page

  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    You can find it by modeling, if you know the wire lengths, height above ground, and frequency of operation.

    Easiest way.
  3. N1OOQ

    N1OOQ Ham Member QRZ Page

    You can also guesstimate. Search around the web with questions like "dipole feedpoint impedance". You should get lots of graphs and tables of impedance vs height, vs wire size, vs length, etc. Plenty of info to make an educated guess.
  4. KU3X

    KU3X Ham Member QRZ Page

    I am not trying to be a smart ass, but based on your question it sounds like you need to purchase an antenna hand book. I'd suggest the ARRL Antenna Handbook.

    You can also use an antenna analyzer that has the OSL Calibration option, like the SARK 110.
  5. G3YRO

    G3YRO Ham Member QRZ Page

    If a dipole is resonant, and each leg is a quarter-wavelength long, the impedance in the centre will typically be between 50 and 80 ohms (depending on its height, etc)

    But if it's NOT resonant, ie you are using it on a variety of different frequencies, the impedance at the centre can vary from 50 ohms to about 5000 ohms. Similarly, the impedance at the ends of an open-wire type feeder will vary almost as much.

    That is why, with a multiband doublet antenna, you really can't predict what balun would give the best match into an ATU . . . you are much better using a proper balanced ATU that doesn't require a balun.

    Roger G3YRO
    N2SUB and N1OOQ like this.
  6. AI3V

    AI3V Ham Member QRZ Page

    You MUST MEASURE it.

    The very best way, with current state of the art gear, is to purchase a "antenna analyzer" that can be mounted at the feedpoint of your antenna, and sends the reading to your computer on a blue tooth link.

  7. G3YRO

    G3YRO Ham Member QRZ Page

    Yes but I suspect he's asking the question because he wants to use some kind of doublet on different bands . . .

    So measuring it wouldn't help ! It would just demonstrate how wildly the impedance varies from one band to the next.

    That's why, for multi-band use, no single ratio balun will give you a good match on all bands.

    And why it's better to use a proper balanced ATU.

    Having said that, I've always built my own ATUs, so have no idea what commercial balanced ATUs are available these days. (the only ones I'm familiar with are the old KW E-Z match and the KW107.)

    Roger G3YRO
  8. K5RT

    K5RT Ham Member QRZ Page

    Dan - KJ4OHB - Can you tell us what you are trying to do? Are you wanting to install a dipole for a single band?
    You've got a bunch of guys who are eager to help but need to understand what you are doing in order to do so

    Vy 73
  9. K8JD

    K8JD Ham Member QRZ Page

    Roger said it well, above.
    Are we confusing a fixed ratio BalUn with an antenna tuner ?
    Using a simple wire dipole, fed with coax cable, as a multiband antenna, is like the old alchemists trying to change lead and tin into gold. Good Idea but it doesn't work very well.
    A dipole cut for a halfwave on a frequency and used on that freq is close enough to 50 Ohms to work without a BalUn with 50 Ohm coax.
    In problem installations (metal obstructions nearby) you may need a 1:1 balun if there is feedline radiation and RFI in the house/ Usually not.
    Multiband antennas ?
    take a trip to the antenna handbooks.
    Last edited: Oct 13, 2017
  10. KJ4OHB

    KJ4OHB Ham Member QRZ Page

    thanks I was putting up an antenna and herd I should have a balun I know what It dose so I didn't see the point for a balanced wire so I asked the question thanks for all the input

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