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Max power for RG58 coax cables?

Discussion in 'General Technical Questions and Answers' started by KE3FG, Mar 17, 2009.

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  1. KE3FG

    KE3FG Ham Member QRZ Page

    I ordered a new antenna switch box for my dipoles. I got 4 - 12ft. MFJ-5812 coax cables to run from the dipoles to the switch box. It appears I should have gotten RG8U cables since the RG58 is rated "low power". Occasionally I run my AL811H amp., so would it burn up these cables if I install them between the antennas and the switch box? What is the maximum power for the RG58 cables?
    Jim, KE3FG
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Depends on the quality of the cables and their connectors. But mil-spec RG58A/U or RG58C/U can handle a kilowatt on HF, into a matched load.

    If you run SSB-CW-PSK31, you should be in good shape unless you're transmitting into a real mismatch. For RTTY, I'd go for bigger cable.

  3. KZ4Y

    KZ4Y Ham Member QRZ Page

  4. KL7AJ

    KL7AJ Ham Member QRZ Page

    Don't tell anyone, but I've run 10 KW at 660KHz through a short length of RG-58. Not recommended, but it did keep an A.M. station on the air during a crucial polical season. :)

    (Glad to get rid of the coax AND the politicians after this one!)

  5. AI3V

    AI3V Ham Member QRZ Page

  6. KE3FG

    KE3FG Ham Member QRZ Page

    Looks like it'll be OK...

    Thanks for the link. I had searched for the answer using Google before I posted this question, but the only answer I saw regarding power was "low power". Here is the data for 10 Meters (lower freq have less attenuation and higher maximum power permitted):
    Coaxial Cable Attenuation & Power Handling Calculator

    Product RG-58
    Frequency 28 (MHz)
    Attenuation 2.384(db/100 feet)
    Average Power 0.57 (kW)
    Run Length 12 (feet)
    Total Run 0.3 Attenuation(dB)
    Efficiency 93.6(%)​

    So it looks like I will be OK for my CW and SSB operation. (I don't run RTTY) Thanks for all the answers.
  7. N7SGM

    N7SGM Ham Member QRZ Page

    Max Power For RG58


    It's funny you post this question because I always wondered about the power handling ability of not only RG58 but other ham related circuits as well.
    For instance, it is usually suggested to go with 12 awg or at least 14 awg, at a minimum, when building dipole antennas. With that in mind, I went with 12 awg stranded wire when I built mine. I used RG-58, as the plan suggested to build the choke balun. You and I both know that the center conductor of RG-58 is no where near the size of 12 awg wire. Here's the kicker, just by chance, I took the lid off the HF rig that puts out 100W and noticed this tiny wire soldered to the center conductor SO239 antenna jack. It may be 26 awg or at the most no bigger than 20 awg.

    I think you see where I am heading with this real scenario. I am an electrician, jet aircraft electrician, and an electronics tech and none of this BS makes sense to me. I'm sure glad I didn't wire my home like this. Can you imagine the look on the inspector's face when he sees this mixture of wire guages in a single circuit. I don't think he'd pass off on it for sure.

    Bottom line, almost any size conductor will pass a voltage but the key is just how much current (amperage) will it pass. Normally, when building circuits that require a lot of current, you must increase the size (diameter) of the wire (conductor).

    Then there's ham radio electronics - a whole different animal.:confused:

    Good luck Jim !!!

  8. KL7AJ

    KL7AJ Ham Member QRZ Page


    Though there is a good point to your being appalled at the inconsistency in R.F. wire gauges, you have to remember that the currents are NOT consistent in R.F. circuits....which is why they radiate!

    I do this neat trick (do NOT show this to your local electrical inspector!), where I can light a 100 watt incandescent bulb by holding the threaded part of the base VERY tightly between my thumb and finger, and touching the button to the output of a 10 meter transmitter. (If you try this, be SURE you grab the base tight, or you're in for a nasty R.F. burn!)

    Now, figure out the equivalent "gauge" of human skin for this!


  9. K7KBN

    K7KBN Ham Member QRZ Page

    Bob - that wire connecting to the SO-239 of your "100 watt" transmitter doesn't have to carry any more than 1.414 amperes, and this is what the manufacturer will size it for. That's determined by the power rating (100 watts) and the impedance (usually specified as 50 ohms).

    It's probably #20, which will handle it and more.
  10. G3TXQ

    G3TXQ Ham Member QRZ Page

    There's bad news and good news:

    Bad news: You need to de-rate the power rating by whatever VSWR is on the line. i.e. VSWR=2:1 you need to halve the power.

    Good news: It's an average power rating, so you can increase the rating by the duty cycle of the mode. Keep the overs real short and you could factor in the Tx/Rx duty cycle as well ;)

  11. K2WH

    K2WH Premium Subscriber QRZ Page

    And the frequency of operation. Using this cable at 900 mhz and 100 watts will eventually melt the coax.

  12. KC8VWM

    KC8VWM Premium Subscriber QRZ Page

    I wouldn't recommend using an amp with such cable.

    It might "work" but the question is for how long?

    A bad SWR on a rainy day is all it takes.

    Try to use "rated" feedline. I usually "overrate" feedlines as a safety margin.

  13. N7SGM

    N7SGM Ham Member QRZ Page

    KL7AJ - ERIC
    K7KBN - PAT

    Thanks guys for your replies to my uneducated response! I mean that in a sincere way. I simply don't understand some concepts with the appropriate wiring gauge used in ham radio electronics. While it remains somewhat abstract to me, I want to learn more about it. I base my opinions solely on my electrical wiring background with little or no knowledge of radio circuits.
    Thanks for keeping me straight! :)

    73 de Bob
  14. AB8ZL

    AB8ZL Ham Member QRZ Page

    The reason most people recommend 12 or 14 gage wire for an antenna is to make them mechanically sound, not for any electrical reasons. Higher gage (smaller) wire will brake to easily especially when ice, wind or broken tree branches are considered.
  15. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    ::Yep, that's why I said, "at HF."

    I've melted RG58/U on 2 meters with about 450 W. Oops.:p
  16. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    ::It wouldn't matter much. It's a very short length and as such cannot dissipate much power even if it were very thin wire (as long as it's a good conductor like silver, copper, aluminum or gold). Also, you can increase the conductivity of any crappy old wire (even if it were steel or iron) if you can silver plate it, since only the outer .001" (or much less) of wire actually conducts all the current at radio frequencies -- the "core" performs no function other than to support the skin.

    Power semiconductors (especially transistors, power ICs and hybrid circuits) routinely use about #24AWG to conduct several amperes internally -- you just can't see the wire, because it's inside a sealed device. That works fine because wire lengths are very short, resulting in almost no voltage drop even at high currents, and the connections are welded, so they will not "melt" until the wire itself does.

  17. KL7AJ

    KL7AJ Ham Member QRZ Page

    Hi Bob:

    No problem! By the way, you'll notice that nobody expressed any particular horror at my light bulb experiment. :) Guess we have a lot more adventuresome crowd on here than I thought. HI

  18. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    ::I can make a regular 40W 48" fluorescent lamp light up with no power source evident at all. I just have to walk up the hill to my local SWBC station KVOH (1.2 MW e.r.p. on HF) holding the tube end in one hand. As soon as I get within about 200 yards of the antennas, the bulb lights up. It's pretty cool at night.

  19. KL7AJ

    KL7AJ Ham Member QRZ Page

    We used to do that at HIPAS all the time. But a fluorescent lamp doesn't actually require any real power to light....just a lot of field voltage. The incandescent lamp, however, requires a real, genuine 100watts!

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