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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. KJ4GXU

    KJ4GXU Ham Member QRZ Page

    After going through the local cable company, and 2 separate satellite services and finally settling on a FTTH provider for internet and TV (No coax runs for TV or Internet) I've got probably a couple hundred feet of 75 ohm coax, mostly RG6 under the house.

    I know there are some antenna designs that use a length of 75ohm coax as a matching network, but I was curious as to what other uses there might be for 75 ohm coax in Amateur Radio. If anyone can point to a design for any band that utilizes 75ohm cable or any other project I hate to let this much cable go to waste.

    Thanks
    Eric
    KJ4GXU
     
  2. W4HAY

    W4HAY Ham Member QRZ Page

    Umm, dipoles are 75 Ohms.
     
  3. AG3Y

    AG3Y Ham Member QRZ Page

    Not necessarily true. RESONANT dipoles may be somewhat close to that value, but the actual impedance will depend on many factors; height about ground, included angle of the elements ( inverted VEE vs. flat top ) surrounding objects that might reflect and distort the transmitted signal, etc. etc.

    It is quite possible to run 75 ohm coax to a dipole antenna and never notice the difference in performance between that, and a system fed with 50 ohm coax.

    There is entirely too much emphasis placed on SWR as a performance factor. Sure, you will have losses if you have an incredibly high SWR, but if that is the case, you should be checking to see what might be CAUSING that high SWR first of all !

    Feedline losses are pretty inconsequential, provided the SWR is somewhere below 2:1, and the radio or external tuner can match that load. Anything higher than that probably indicates some kind of problem. ( wrong length radiating elements, open or short, or just plain bad connections somewhere!

    Good luck! 73, Jim
     
  4. W6GQ

    W6GQ Swap Meet Moderator QRZ Page

    Loose nut in front of the radio?
     
  5. KD0CAC

    KD0CAC Ham Member QRZ Page

    I am somewhat new , and have seen lots of ham's put down 75 ohm , but are always modding all kinds of stuff , adding balun's , so no matter what you use , wouldn't making / winding you own balun , solve any mismatch ?
    Maybe I just have the wrong understanding of the issue ?
     
  6. W5DXP

    W5DXP Ham Member QRZ Page

    When I was first licensed in high school in the early 50's, a lot of hams used 75 ohm RG-11 coax as did I. Very few hams had an SWR meter. With a pi-net on the output of my Globe Scout tube transmitter, 50 ohms was nowhere to be found. When I finally bought a Heathkit SWR meter, I wired it for 75 ohms, not 50 ohms.

    75 ohm coax actually has an advantage over 50 ohm coax. With 50 ohm coax, one cannot affect the 50 ohm SWR by changing the length of the coax. But with 75 ohm coax, one can adjust the length and affect the 50 ohm SWR sometimes to a perfect match. For instance, 1/4WL of RG-11 will change the 50 ohm SWR from 2:1 at a 100 ohm antenna to 1:1 at the other end of the coax.

    Personally, I would certainly use the free 75 ohm coax before I would use my social security check to buy 50 ohm coax.
     
  7. K9STH

    K9STH Ham Member QRZ Page

    Prior to at least the mid 1960s using 75 ohm coaxial cable was just as popular as using 50 ohm cable. In fact, SWR bridges came in two flavors, 50 ohm and 75 ohm. The same thing with low pass filters, they were available in both 50 ohm and 75 ohm versions.

    Generally you can use 75 ohm cable in any amateur radio application which uses 50 ohm cable. Now there are a very few applications, like matching sections, that this doesn't work. However, in at least 95 percent of the cases 75 ohm cable works fine.

    If you have a "match" like a gamma match you can retune it to give a better match (lower SWR) to the 75 ohm cable.

    My first 40 meter dipole back in mid 1959 when I was a newly licensed Novice Class operator was fed with RG11/U cable that was given to me by K9BPV. It worked fine!

    Glen, K9STH
     
  8. K8ERV

    K8ERV Ham Member QRZ Page

    When I was wiring this house for tv, I asked my boy to get a roll of 58 at Dayton. Instead he got 59. I used it expecting some terminal reflections (ghosts). Worked perfectly.

    TOM K8ERV Montrose Colo
     
  9. AG3Y

    AG3Y Ham Member QRZ Page

    Tom, TV coax IS 75 ohm ! He got the right stuff !
     
  10. G8ADD

    G8ADD Ham Member QRZ Page

    If you read Louis Varney's paper, the coax he used in the G5RV was 80 ohms.

    73

    Brian G8ADD
     
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