Help with what coax to use?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KF1X, Nov 30, 2012.

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  1. KF1X

    KF1X Ham Member QRZ Page

    Hello Everyone,

    Newbie here. I'm going to be taking my exam tomorrow, thus the lack of a call sign. ;)

    I've purchased a Yaesu FT-2900R and I will be installing a homemade 1/4 wave 2M antenna on the roof peak. The cable run will be about 50ft. I'm a little confused with all of the coax types, which one I should be getting. I was going to get LMR-400, but from reading these forums I'm thinking that might be overkill?

    I've seen a few people say RG-8 would be fine for similar setups, but I thought I'd see if ya'all could help clarify. Also, is RG-8 called something else? I don't see RG-8 at all on some of the bigger online retail sites.

    Thank you for your attention to this thread and any help ya'all can provide.

    Josh
     
  2. KO6WB

    KO6WB Premium Subscriber QRZ Page

    The difference between LMR-400 and a good quality RG-8 is so tiny it would be impossible to tell. There are many variations of RG-8. There's 9913, RG-213, RG-214 and a few others.
    There are loss calculators on the web that'll help you make the right choices. Keep in mind they are for new cable. Here's one that allows the addition of the SWR loss; http://www.arrg.us/pages/Loss-Calc.htm.
    Hope this helps
    73
    Gary
     
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Hope you passed the test okay.

    For 50' of cable on 2m, the "new and improved" version of RG-8/U is called RG-213/U and is very readily available. Nobody really makes "RG-8/U" anymore, although there are dozens of variants of the general "RG8" design. LMR400 is actually one of those variants.

    RG8X is "mini-8" and cheaper, lighter and more flexible than the larger variants and might be good enough for fifty feet on 2m FM. The "difference" between lightweight, cheap RG8X and much heavier and more expensive LMR400, in a 50 foot run on two meters, is only about a dB or so. As the run gets longer, the difference becomes larger.

    As someone who's operated 2m for 46+ years under a broad range of conditions, I'd say the antenna design, its height above ground, and its overall exposure (height above obstructions) is more important than the coax -- by far! The coax becomes more important when the antenna is far away from the station and everything else has already been optimized.

    A 2m vertical up 100' above the ground fed with lossy "mini-8" cable will dramatically outperform the same 2m vertical at 20' above the ground fed with the best, lowest-loss coax money can buy.
     
  4. KF1X

    KF1X Ham Member QRZ Page

    WB2WIK, the exams went great! I went ahead and took all three elements and passed. Now to learn how to use all this new found information.

    I appreciate the help with cable choice. I believe you guys just saved me some money as I would have over done it.

    Thank You!
     
  5. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Outstanding Josh, you went from zero to extra in one sitting.
    Let me give you some free downloads from the internet that'll be very helpful in the future.

    This is a simple quick presentation given at the Ohio State ARRL Convention; http://arrl-ohio.org/convention-2012/Wire%20Antenna%20Basics_ARRL_Ohio_8-11-12.pdf. Good information without a lot of details.

    This one is for our troops in the USMC; http://www.zerobeat.net/r3403c.pdf. and another USMC manual that's handy; http://community.marines.mil/news/publications/Documents/MCRP 3-40.3B Radio Operator's Handbook.pdf.

    This one has loads of ideas for antennas; http://www.eagle3.net/n4ywn/docs/PracticalAntennaHandbook-vol4.pdf.

    There's a bunch more information but this should be good enough for now.
    BTW there is no way you could over due it. Just ask the folks whose Christmas lights can be seen from the ISS:rolleyes:!!!
    Welcome and good luck
    73
    Gary
     
  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    Josh,

    Congrats on passing the ham exams.

    Generally, the difference in loss between cables of a given physical diameter is dependent upon the dielectric material -- the "insulation" between the center conductor and shield. Many believe RG-213 is always better than RG-8. Foamed dielectric always has less loss than its solid version. RG-213 ALWAYS has a solid polyethylene dielectric, and will have more loss per unit length (at a given frequency) than certain RG-8 types that have a foamed polyethylene dielectric. So, it is best to pay no attention to generic loss guesstimates, and use the manufacturer's datasheet when considering a particular feedline. My rule of thumb is when the loss is expected to exceed 2dB, I start looking for a better feedline. There are other concerns as well as loss:

    • Percentage of shielding. Better feedline will have ≥ 95% shielding. Some are braided wire, up to about 97%. Some use aluminum foil. Some have more than one shield.
    • UV resistance. Not all coaxial cable is suitable for use out of doors. Even up North, it matters (just not as much).
    • Flexibility. If the cable is expected to be repeatedly flexed, you should use a one with a stranded center conductor. Likewise, a cable that has only a foil shield won't take a lot of flexing before it fails.
    • Power handling. It decreases with increasing frequency. In your application, this is likely not an issue.

    You might be able to use RG-8X (aka mini RG-8) or LMR240UF, which is smaller than RG-8/RG-213/LMR400UF cables. It'll be more flexible and since it has a foamed polyethylene dielectric, should have acceptable loss. DX Engineering, Davis RF, The RF Connection, and The Wireman are popular sources.
     
  7. WA8UEG

    WA8UEG XML Subscriber QRZ Page

    That design is a oldie but goodie. I would recommend a small SS washer between the screw head and the radial as you will find the copper loop tends to open up when you tighten the screw down.
     
  8. KF1X

    KF1X Ham Member QRZ Page

    Gary/Bryan,

    Thanks for the information. I really appreciate all the feedback and assistance. I can't wait until I don't have this delayed posting anymore. :)
     
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