Callsign
ad: ARScom-1
Page 1 of 2 1 2 LastLast
Results 1 to 10 of 14

Thread: Help matching 50 to 75 ohm coax

ad: L-HROutlet
ad: l-assoc
ad: l-Waters
ad: l-innov
ad: l-rl
ad: l-tentec
ad: l-WarrenG
ad: l-gcopper
  1. #1

    Default Help matching 50 to 75 ohm coax

    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?

    CHarley

  2. #2
    Join Date
    Oct 2003
    Location
    Middle Georgia USA
    Posts
    7,722

    Default

    Quote Originally Posted by vk2zyz View Post
    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?

    CHarley
    Read this:

    Link

  3. #3

    Default

    Quote Originally Posted by vk2zyz View Post
    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?
    CHarley
    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.
    73, Cecil, www.w5dxp.com
    Can CO2 emissions save us from the coming ice age?

  4. #4

    Default

    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. #5

    Default

    Here's something I just found along those lines.
    http://www.tuc.nrao.edu/~demerson/twelfth/twelfth.htm

  6. #6

    Default

    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. #7

    Default

    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. #8
    Join Date
    Oct 2003
    Location
    Middle Georgia USA
    Posts
    7,722

    Default

    Quote Originally Posted by w5dxp View Post
    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.
    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. #9

    Default

    Quote Originally Posted by vk2zyz View Post
    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
    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. #10
    Join Date
    Oct 2003
    Location
    Middle Georgia USA
    Posts
    7,722

    Default

    Quote Originally Posted by WA9SVD View Post
    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.
    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.

Page 1 of 2 1 2 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •