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Why are 50 ohm feedlines more common than 75 ohms when working with diples?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W0RIO, Sep 25, 2012.

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  1. W0RIO

    W0RIO Ham Member QRZ Page

    It seems that most feedlines used for HF dipole antennas are 50 ohm impedance such as RG8 and RG58.

    Assuming the dipole is 1/4 wavelength. above ground, its characteristic impedance should be around 75 ohms according to my
    ARRL handbook.
    I know that a 50 ohm line is not a huge mismatch, but wouldn't it be better to use a 75 ohm feedline and use an ATU in the shack
    to match to 50 ohms?

    Along those lines, I have a good source of foil-shielded RG6 CATV cable, the only issue I see is connecting to the
    aluminum shield wires. I think that a crimp-on connector and sufficient waterproofing should work for the antenna side.
    The lighter weight of RG6 vs. RG7 seems like a big advantage when constructing a dipole that's hung from the ends.

    I did find a good article on using RG6 for transmission lines here:
    http://vk1od.net/transmissionline/RG6/
    That says RG6 can handle 500W at 30Mhz and more at lower frequencies.
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    50 Ohms is more common as it's an ideal compromise characteristic impedance that balances best power handling with least attenuation for any given cable diameter. In fact, if you research the history of coaxial cable (back to the 1940s), that's exactly why it was settled on.

    If you are using HF (<30 MHz) and have a tuner, it generally doesn't matter if you use 50 Ohm or 75 Ohm cable. Note that because most of our test instruments are calibrated/normalized to 50 Ohms, if you use that equipment in a "perfect" 75 Ohm environment, none of it will read accurately.:p However, that's usually not really important, either.
     
  3. NL7W

    NL7W Ham Member QRZ Page

    A couple of reasons...

    - The history of coaxial cable relates to 50 ohms as a clear winner for RF systems' power carriage, http://www.microwaves101.com/encyclopedia/why50ohms.cfm

    - Generally speaking, the impedance of HF dipoles below 1/2 wavelength is lower than 75 ohms, and can match either impedance quite well

    I'm sure others will chime in...
     
    Last edited: Sep 25, 2012
  4. K8ERV

    K8ERV QRZ Member QRZ Page

    For a given OD, 52 ohms can carry the most power. 72 has the lowest loss.

    Or so I read in the Times.

    TOM K8ERV Montrose Colo
     
  5. AF6LJ

    AF6LJ Ham Member QRZ Page

    This is very true, which is why receiving systems of many types and the Cable TV industry settled on 75 ohm coax....

    As I remember the actual point of minimum loss for coaxial cable is somewhere between 72-73 ohms, but what's an ohm or two between friends.
     
  6. NL7W

    NL7W Ham Member QRZ Page

    You don't meditate, do you?!

     
  7. KA9UCN

    KA9UCN Ham Member QRZ Page

    You can buy adapters for the 75-ohm TV crimp on to a pl259 from Radio shack for about $2 each. I keep some on hand because I have bought several roles of several hundred feet at yard sales and the like. I have used this type of coax many times over the years. It works fine for the average 100-watt class transceiver.

    Joe
     
  8. AF6LJ

    AF6LJ Ham Member QRZ Page

    Seriously;
    I use to meditate, among other things.......

    I still have one foot in the other world.
    I should spend more time there, but there is too much happening here in this world.
     
  9. K0RGR

    K0RGR Premium Subscriber QRZ Page

    All I know is that I still have the 500 foot roll of RG-6 my wife found in a parking lot, and someday I will actually use it.
     
  10. WA8UEG

    WA8UEG XML Subscriber QRZ Page

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