RG-6 RG-8 Questions

Discussion in 'Becoming an Amateur Radio Operator/Upgrading Privi' started by K0DNW, Sep 16, 2009.

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  1. K0DNW

    K0DNW Ham Member QRZ Page

    I have a ton of RG-6 but can't find any 8 or I think 58. Is there any issue with using RG-6 coax, and if so whats the issue? Can it be used in a pinch provided your not going to high on the watts used.
     
  2. K0CMH

    K0CMH Ham Member QRZ Page

    Your radios are made to output on 50 ohm impedance.

    RG6 is75 ohm impedance.

    RG8, RG58, RG8x, etc. are 50 ohm impedance.

    The closer the impedance of the coax cable is to your radio's output, the more power will get to your antenna, and the less power will be reflected back into your radio.

    The 75 ohms impedance of the RG6 will probably work well enough, but part of the power will be lost in the cable and in power reflected back into your radio. 50 ohm impedance coax cable is the stuff you want.

    For 2 meters and 70 cm, I recommend at least rg8x at less than 50 feet. If you must go more than 50 feet for your feed line, use low loss coax if you can offord it. Google for Beldon. They have a great list of 50 ohm coax on their site and it shows the loss at the 2 meter frequency.

    If you get a 2 meter "tuner", you will probably be able to use your 75 ohm RG6. A tuner will electronically convert the 75 ohm impedance close to 50 ohms (provided the tuner is of the quality to do that).

    In a simplified expliantion, the power flows the best across a connection of the same impedance. When your radio (50ohm impedance output) is connected to 50 ohm coax, the most power will cross that connection. Some coax cables are rated at 52 ohms. This is not enough difference to care about. 75 ohms may work well enough to not damage your radio but will eat up a lot of power.

    When the radio output impedance, the coax impedance and the antenna input impedance are all the same, then the most power goes into your antenna. It works the same way on receive. There is loss when the received signal must cross impedances that are different. The tuner does work both ways.

    Hope this helps.
     
  3. K0DNW

    K0DNW Ham Member QRZ Page

    Thats just what I needed to know thanks.
     
  4. K0CMH

    K0CMH Ham Member QRZ Page

    Keep watching this thread. Others may well jump in with even better information.
     
  5. K8JD

    K8JD Ham Member QRZ Page

    Pick up an issue of QST magazine, Most larger book stores have it in the magazine section.
    You will find many advertizers selling various kinds of coax cable used in our hobby.
    You can buy it in bulk, or specific lengths, even with connectors installed for you.
    They mail it right to your house ! :D
    I actually use RG6 or RG59, 75 ohm coax for making Z matching lines for some of my antennas
     
    Last edited: Sep 17, 2009
  6. AI3V

    AI3V Ham Member QRZ Page

    This is a good question, and the answer is,

    Maybe:D

    Some of the possibilities:

    No tune transmitter-usually solid state-nominally 50Ω output....If the transmitter is really 50Ω, and the coax and antenna are 75Ω about all you will see is a bit less power from the xmitter. hardly enough to measure, let alone tell on the air

    The same transmitter and coax, but a antenna that is 50Ω, well, that will depend on the length of the coax!!

    Tunable transmitter-usually tube type- 75Ω coax and a 75Ω antenna, and it will work as well as a 50Ω coax and antenna, the "load" control on the front of the transmitter will have to be moved a fraction

    Now you have the problem with determining if the antenna is a particular impedance, the ratio of the antenna and coax impedance will determine the VSWR, problem is, most VSWR meters are not accurate enough to reliably differentiate between 50 and 75Ω! you could be reading a "perfect" 1:1 VSWR on the 50Ω meter, and the impedance is really 75Ω

    Or, maybe the "50Ω" no tune transmitter is actually closer to 75Ω than 50Ω, then everything will be fine, except the reading on the VSWR meter!

    For most ham radio stuff, if you had 2 hunks of coax, one 50Ω, and one 75Ω, and neither had any markings,(and you didn't bother to otherwise test the coax), you will be VERY hard pressed to notice any difference in everyday, "on the air" operation.

    While I would not use the "wrong" coax if the "right" one was available, I also would not hesitate to use the "wrong" one in a pinch.

    A neat side note: Some of the biggest EME (moonbounce) "superstations" use nothing but 75Ω coax!!!!!, the reason being, they can get surplus cable tv coax for about scrap aluminum prices that has a lower loss than 50Ω costing maybe $1000(yes 1 kilo-dollar!), they simply re-tune the antenna and transmitter to 75Ω For moonbounce, the SLIGHT improvement is noticeable, for say, ragchewing on 75M phone you would NEVER tell over the air.

    Rege
     
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