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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

    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 Subscriber

    Umm, dipoles are 75 Ohms.
     
  3. AG3Y

    AG3Y Ham Member

    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

    Loose nut in front of the radio?
     
  5. KD0CAC

    KD0CAC Ham Member

    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

    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

    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

    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

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

    G8ADD Ham Member

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

    73

    Brian G8ADD
     
  11. RU9CA

    RU9CA Ham Member

    No problems - use the cable 75 ohm.
    All antenna, for cable 50 ohm, usually beautifully work and cable 75 ohm.
    Besides, you have an excellent possibility to connect parallel this cable and get the feeder 37.5 ohm - excellent for feeding the antennas GP.
    Or supply the frame ( the lazy delta ) through 2 cables in symmetrical cut-in. The feeder 150 ohm are Got.
    And certainly, always possible use the repeater. Take the cable to electric long equal half of the long wave. In this case - feeder repeats the resistance of antenna. And - his wave resistance already is not important much.
    For example - see figure
     
    Last edited: Sep 5, 2009
  12. K9ZMD

    K9ZMD Subscriber

    Money vs. SWR: This Perspective Bears Repeating

    "I would certainly use the free 75 ohm coax before I would use my social security check to buy 50 ohm coax" (With full credit to Cecil, W5DXP, 9/5/2009)
     
  13. K8ERV

    K8ERV Ham Member

    Right, I got it backwards, he got the 58. But it worked ok.

    TOM K8ERV Montrose Colo
     
  14. AG3Y

    AG3Y Ham Member

    I can remember when we re-wired the studios for Maryland Public Television. There were MILES of RG-59 cable in the cable ladders that ran through the tunnels beneath the various control rooms and studios. When the new equipment was put in place, we had to pull hundreds of feet of that stuff out, to make room for the new cables.

    I will never figure out why they didn't keep and use the old stuff! I suspect it had to do with "timing"which changes the phase of the chroma signal, but I will never know for sure ! All signals have to propagate the same distance, whether the studio is close by or far away , so every studio was wired with the same lengths of coax. Some of the cables were folded back on themselves a couple of times !

    That's your State Government funds at work !


    P.S. Tom, that's OK, I'll attribute it to a "Senior Moment" Coming closer and closer together, aren't they? ;) :eek:
     
  15. K8ERV

    K8ERV Ham Member

    Nah, just one big long continuous one. But it's better than the alternative.

    TOM K8ERV Montrose Colo
     
  16. AG3Y

    AG3Y Ham Member

    Ahhh, I see, NOW we are COUNTING NUMBER of OCCURANCES ! Well, I guess that puts you in a position ahead of me. I have several, you only have ONE !

    Now, I understand ! ! !
     
  17. KC9KXW

    KC9KXW Ham Member

    Frequency Limitations (?)

    I'm sorry to add a question to a question here, "BUT" is there any limitations as far as which frequencies or bands that this entire thread is talking about whether is be 160-6 meters, 2 meters or 440 MHz ? :confused:

    Thanks and 73 Jim
    KC9KXW
     
  18. K9STH

    K9STH Ham Member

    Theoretically 75 ohm coaxial cable has a lower attenuation versus frequency and therefore will be slightly better on the higher frequencies than 50 ohm cable. Basically, the diameter of the cable generally has the most effect on the attenuation. That is the larger the diameter of the cable the lower the attenuation.

    Using 75 ohm coaxial cable is pretty much universal in broadcast television reception. The reduced losses in the cable is definitely one of the reasons that this type of cable is used.

    Glen, K9STH
     
  19. KC9KXW

    KC9KXW Ham Member

    So I take it that 75 ohm coaxial cable is a little better for VHF/UHF Frequencies since TV stations run the higher bands, especially with the new (DIGITAL-TV) that's being used today.

    Please correct if I'm wrong, I'm trying real hard to understand what everyone is saying here........

    Thanks and 73 Jim
    KC9KXW
     
  20. KA9MOT

    KA9MOT Ham Member

    I run 75 Ohm out to my Fan dipole.....Works great.

    Hmmmmm.....are you guys tired of hearing about my Fan Dipole yet?
     
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