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Choosing The Right Coax

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  • #8446

    Recently several members have asked what type of coax is best for their antenna installation. The answer is, of course, “It depends.” Coax loss increases with frequency and with length. For HF use into a matched load (low SWR) and for lengths of about 100 feet, nearly all common types of coax will attenuate very little. At UHF frequencies losses go up and so we are better off running the larger, lower loss coax like RG-8 or its variants: LMR-400 or 9913. Keeping the coax short will also reduce the attenuation of signals. Below I’ve compiled a table of common transmission lines and their approximate loss per 100 feet. For comparison I’ve included hard-line, LDF4-50 as well as common twinlead (like what was used to connect to old style TV antennas) and window line.

    Loss (attenuation) per 100 feet length into a matched load:

    There are other considerations when choosing your transmission line: Window line is low loss but it must be kept several inches away from large metal objects, like tower legs and masts, and away from the ground, so it cannot be buried. Hardline is also low loss but it is shielded coax so it can be placed nearly anywhere, but it is quite rigid and shouldn’t be used where it’s subject to flexing. Hardline would be a poor choice for portable operation, where it is coiled and uncoiled a lot.

    Some types of coax use solid insulation around the center conductor and will withstand some moderate abuse (they can pass through a door or window that might be casually closed on it) while coax with a foam insulator is much more easily damaged if someone were to step on it or pinch it in a door.

    All of the types of coax listed in the table will handle at least 100 Watts into a good match (low SWR.) If your antenna has high SWR (like 10:1) you should choose a lower loss transmission line to reduce your losses between the antenna match and the antenna. The larger, lower loss transmission line will also generally handle higher current and voltage which is important when SWR is high. For these reasons people will often use very low loss, window line to connect a non-resonant antenna to their antenna tuner/match. Between the antenna tuner and the radio should be low SWR and any coax rated for the power should be fine. Likewise, short jumpers used for interconnections can be higher loss types of coax since they are so short. The smaller size and flexibility are more important than the (very) small increased loss.

    #9675
    Joseph, K9JOE
    Participant

    Peter,
    Very nicely done. I would also like to see a companion article, Choosing the right connectors.

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