Help matching 50 to 75 ohm coax

Discussion in 'General Technical Questions and Answers' started by VK2ZYZ, Jun 16, 2008.

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  1. VK2ZYZ

    VK2ZYZ Ham Member QRZ Page

    I asked about using 75ohm coax and WB6BYU mentioned this

    "use 1/12 wave matching sections of 50 and 75 ohm coax
    at each end to transform up to 75 ohms and back to 50 ohms"

    I was just wondering how to go about doing this? and has anyone else tried it?

  2. W8JI

    W8JI Ham Member QRZ Page

    Read this:

  3. W5DXP

    W5DXP Ham Member QRZ Page

    Most experienced hams don't worry about the mismatch between 50 ohm coax and 75 ohm coax. The difference in SWR for 50 ohms is 1:1 vs 1.5:1 and both SWRs are perfectly acceptable in the average coax run at HF.
  4. WA9SVD

    WA9SVD Ham Member QRZ Page

    It depends upon your setup and what bands.

    For one thing, that is NOT a broadband or an all band solution; it CAN work on a single band or harmonically related bands. But the expense and hassle may not even be worth it. If your antenna can be adjusted for a 75 Ohm match AT THE FEED POINT, (rather than 50 Ohms) a simple run of 75 Ohm coax right to your radio will have only a 1.5:1 SWR, which is normally very acceptable; it won't cause any SWR protection to kick in, and the feedline loss will not be significantly increased over that of regular coax. As already said, it's often not worth worrying about.

    If you can, give more specific examples of your use.
  5. KI4TWB

    KI4TWB Ham Member QRZ Page

  6. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Hi Charley
    Actually a quarter wave with a cable having a Zo of 61.2 ohms would do it.
    (Square root of 50 x 75). Halfwave lines will repeat the impedance seen at either end.
    I use this arrangement inside my coaxial vertical for 70 cm.
    I can't argue that a 1.5 : 1 SWR is not a problem but tinkering with this stuff is part of the fun and even more than operating the radios.
    73, Pete
  7. VK2ZYZ

    VK2ZYZ Ham Member QRZ Page

    I have a roll of Quad shield 75 ohm coax left over from my satalite tv days and I was wanting to use that for my 2 meter FM antenna, I need all the power I can muster to hit one of the local repeaters
  8. W8JI

    W8JI Ham Member QRZ Page

    The problem isn't the ideal SWR on the 75 ohm line.

    The problem is the SWR presented to the rig by the 75 ohm line when it isn't a short 1/2 wave multiple, and what might happen when the load is off.

    It is NOT 1.5:1 at the radio, it is 1.5:1 on the 75 ohm line.....
    In the case of a 75 ohm line between a perfect 50 ohm load and a 50 ohm rig, the SWR at the rig can be as high as 2.25:1. That's a problem!

    Secondly, as the line get long in terms of wavelengths the SWR bandwidth between the low SWR and high SWR frequencies at the radio gets narrower.

    Third, if the antenna is below 50 ohms things get worse pretty fast.

    There are many reasons to use a transformer. The non synchronous transformer uses two line sections of standard impedance, no need to use difficult to find or complex to create 1/4 wave long 61 ohm lines. You can use two standard line sections of 50 and 75 ohms to build a matching section.

    73 Tom
  9. WA9SVD

    WA9SVD Ham Member QRZ Page

    I haven't crunched the numbers, but you'd probably lose more in the matching sections and connections than you would in the SWR mismatch. If you can adjust the antenna for a 75 Ohm match, your best bet is to use the cable as is. More of concern is that many satellite TV cables have ALUMINUM shield braid, so you won't be able to use ordinary, solder-on coax connectors.
  10. W8JI

    W8JI Ham Member QRZ Page

    Not true. Plus using the transformer assures the SWR at the radio is reasonable, and not 2:1 as it can be when NOT matching the line.
  11. WA9SVD

    WA9SVD Ham Member QRZ Page

    But won't the SWR be 1.5:1 IF the antenna is adjusted to 75 Ohms?
    I see a major problem being the (probable) aluminum braid in the coax...
  12. W8JI

    W8JI Ham Member QRZ Page

    Sure, I did that with all my Yagi antennas years ago so I could use a 75 ohm system.

    Back then the rigs were all pi networks so the 75 ohm load didn't matter.

    He is talking about something else.

    By the way, standard crimp F connectors and RG6 type (F6) cable can handle legal limit on low bands if the connector is kept dry. My M2 six meter yagis have half-power on the matching lines at the feed, and they use F connectors and F-6 (RG6 size with aluminum shield) CATV cable. I run 1300 watts into one antenna, so that's about 600W through the cable on six meters!

    73 Tom
  13. VK2ZYZ

    VK2ZYZ Ham Member QRZ Page

    My RG6 has two layers of aluminium foil and two layers of wire for shielding. The wire is not aluminium as I have soldered it before.

    I'm using a copper J-Pole antenna, I am wondering if I can adjust the attachment point of the coax to adjust the impeadance of the antenna to 75 ohms and just live with the difference at the radio. Max wattage I will be running on this setup will be 25 watts.

    Thanks for all the help guys

  14. WA9SVD

    WA9SVD Ham Member QRZ Page

    The short answer is yes, you can adjust a J-Pole for 75 Ohm impedance, which will match the satellite coax feedline, and you will only have the 1.5:1 mismatch at the radio end. And that shouldn't result in a significant amount of loss, even at 2 Meters.
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