uses for 75 ohm coax in Amateur Radio

Discussion in 'General Technical Questions and Answers' started by KJ4GXU, Sep 5, 2009.

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  1. KA9MOT

    KA9MOT Ham Member QRZ Page

    Welcome to the hobby Russel!

    You can use it as is. Keep your runs short.

    Good quality RG6 has much lower losses than RG-58 and RG-8X, but is lossier than your better 50 OHm coax. It would be better on HF into a resonant antenna. If you can afford better coax, by all means, buy better coax (LMR-400). If you are like me and money is an obstacle, Run whatcha brung!

    I don't know what your support base is down there, but if you need to feel free email me. kc0rey@gmail.com

    I'll be moving to Macomb on Oct, 1st, I hope to hear you on the air.
     
  2. NN3W

    NN3W Ham Member QRZ Page

    Lots of applications can use 75 ohm coax - receive antenna systems, matching stubs, etc.

    What you have to worry about in using CATV coax is 1) ensuring you don't run too much power through it as much of it is foam dialectric and will melt with too much heat, and 2) ensuring your coax connectors are secure. Much CATV coax uses aluminum for the outer braid and aluminum does not solder well (if, at all).... Bad connections are worse than compromised coax.
     
  3. KA9MOT

    KA9MOT Ham Member QRZ Page

    Easy, using a UG-176 reducer, trim the braid so it is about 3/8ths of an inch long. Fan the braid out and push it back over the non threaded nose of the reducer. Thread your PL-259 on and using 2 pairs of pliers tighten the 2 parts together (Cheap PL-259 will break at the solder holes when you do this). Tight! So tight that you can not pull the assembly from the coax if you pull hard. Solder the center conductor. I like to fill my PL-259 connectors up with Dielectric grease and then seal them up. I do all my smaller coax like this and it is very rare that I have a failure.

    I've run 100w into TV coax with no problem.
     
  4. K4RKY

    K4RKY Ham Member QRZ Page

    I built a 135 ft' OCF Inverted V out of RG6 for our lake house and with a tuner I work 80 through 10 with no fuss. It works well without the tuner but I use a tuner just for peace of mind. I think it should be able to handle a few hundred watts at least. I run less than 500 my self.
    Don't throw it away!
     
  5. W8JI

    W8JI Ham Member QRZ Page

    You asked specifically about 2 meters and 70 CM, not HF.

    If you mix 75 ohm cable into a 50 ohm antenna system, the SWR at the radio can be as high as 2.25:1 or as low as 1:1 at the radio.


    How low or high SWR actually is at the radio depends on the exact load impedance and the length of the cable.

    A second effect is the mismatched cable can decrease bandwidth of the antenna.

    75 ohm cable is a whole lot easier to use at HF than VHF.
    This is because wavelength is longer at HF, because tuners are available, and because many antenna are above 50 ohms at HF. At VHF and higher it is often better to stay with the cable impedance specified for the system.

    73 Tom
     
  6. AE5JU

    AE5JU Ham Member QRZ Page

    Timely discussion... I was just given about 150 of very good looking RG-59. Nice bright copper shield.
     
  7. KC8VWM

    KC8VWM Ham Member QRZ Page

    Yeah that's another issue with many cable TV feedlines.

    In most cases the shielding is made out of aluminum or steel and you can't solder the darn stuff to anything.

    Not exactly sure how this fact might affect anyone's social security checks.
     
  8. W8JI

    W8JI Ham Member QRZ Page

    Be careful with these general statement of "Sure you can use it. It will work fine. It has low loss, etc etc."

    When 75-ohm cable is mixed into a 50-ohm system, the resulting SWR with a perfect 50-ohm match can be over 2:1. Very few radios will tolerate that without folding back.

    At HF, it is pretty common to use 75 ohm cables. This is because many antennas have an impedance above 50 ohms, because many systems have tuners, or because users can cut and tweak until the SWR is acceptable.

    I use 75 ohm F-6 (like RG6 but CATV/MATV type) cable myself on an 80 meter antenna system, BUT the length of cables are planned to allow me to combine two dipoles into a common feedpoint.

    I wouldn't hesitate to use it on a 80 meter dipole, if I had a tuner or could trim the length to provide a reasonable SWR.

    I would certainly not tell anyone it can be freely mixed into a 50 ohm system, especially on 2 meters or 70 cm like the original question. That is just wrong.

    If the 75 ohm cable has a 50 ohm load, the maximum impedance at the radio could be around 113 ohms. Very few radios, now that our old pi-network rigs are gone, would work very well with that. If the load is 40 ohms, the radio could see 140 ohms! That's almost 3:1 SWR the 50 ohm radio.

    The only correct statement would be.... "How it will work depends on the radio, the load impedance, and the 75 ohm line's length. It might work and it might not."

    It's just silly to say it will always work, and will never be a problem, because it commonly can be a big problem. This doesn't mean throw it away. It just means sometimes it works, and sometimes it does not.

    73 Tom
     
  9. AC0FP

    AC0FP Ham Member QRZ Page

  10. KA9MOT

    KA9MOT Ham Member QRZ Page

    Good info Tom! I've never seen a higher then 1.5 SWR with it but your points are valid and I for one will remember them.
     
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