SWR Causing Failure

Discussion in 'CB Radio Forum' started by BTShepp, Jan 10, 2018.

  1. BTShepp

    BTShepp Blue Demon

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    So many of us are obsessed with obtaining the perfect SWR match. I see a lot of people insist that a moderately high SWR will make your radio explode on the spot. I recall seeing a video of a stock Cobra 25 with the microphone rubberbanded to dead key. There was no antenna or dummy load, and although the radio got very hot, nothing failed while it was transmitting continuously for six hours.
    A lot of ham radio articles actually argue that as long as the SWR reading is reasonable, there is no reason to struggle for a better match. They worry about other specs, such as coax loss (which can trick an SWR meter and radio into thinking the SWR is better than it really is).

    So I am curious. Has anyone ever blown a final on their radio? If so, was the radio stock? Were you using an amplifier?
     
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  2. mike5511

    mike5511 Road Train Member

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    Will follow this one......and my answer is NO, never blew a final, but I have a few extra of those 1969 thingies just in case. ;)
     
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  3. delta5

    delta5 Road Train Member

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    The ARRL handbooks have always said anything up to 2:1 is acceptable.
     
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  4. BTShepp

    BTShepp Blue Demon

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    In contrast, the bigger the hack of a technician, the more they seem to advocate that 1:1, or even “0”, SWR IMO.
    Yes I know 0 is not a real measurement.
     
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  5. Night Stalker10

    Night Stalker10 Light Load Member

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    Years ago, I had a buddy who is one of these guys that had to have a one to one match. It about drove me nuts to help him with his antennas. I have always been happy with a 1.5 for SWR. They say a quarter wave antenna on a mobile installation is between 32 and 36 ohms. If Your antenna impedance happens to be 36 ohms fed into a 50 ohms radio, it would be 50/36=1.388 or 1.4 for SWR. Personally I’ve always been more concerned about the reflective power rather than the SWR.
     
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  6. Meteorgray

    Meteorgray Light Load Member

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    All of us try to get the best longevity and performance possible out of our equipment. Perhaps the best "homemade" way to try achieving this is to concentrate on the SWR with our $20 meters, or the radio's built-in meter.

    I'm sure the SWR becomes a fetish to a lot of us because, well, it's one of the few things we feel we can somewhat control.

    However, several thoughts come to mind:

    1. The actual maximum resonance and thus RF efficacy from an antenna is not necessarily the very lowest SWR. Fortunately, the lowest attainable SWR gets you close, but pursuing that very last 0.1 factor might just be not only counterproductive, but can make you spill your beer, kick your dog and shake your fist at the radio gods in the sky. And, it takes a pretty expensive meter to find that perfect resonance, with relatively little payback especially for mobile radios. Because:

    2. The difference between a "perfect" SWR and an "imperfect" 1.5 SWR is only about 4% or so loss of RF transmission; not enough to worry about nor cause any damage. Even a "horrible" SWR of 2.0 is a loss of about 10% in RF efficacy, not good but probably not harmful either. Because:

    3. About the question of high SWR causing harm to the radio, that's a good question. I suppose "it depends." I've seen manufacturers who warn against having SWR too high. I've read that high SWR can burn out the "finals" in a radio. But then I've read reports from the mavericks out there who take no one's word for it and, and (gasp!) tie-down the trigger on a microphone that is connected to a radio that has no antenna at all and proceed to transmit for hours, and then come back and find the radio is undamaged and still working fine. "It depends," indeed.
     
    Last edited: Jan 13, 2018 at 8:42 AM
  7. Crude Truckin'

    Crude Truckin' Light Load Member

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    I've always been a "reflected power" kinda guy, because in my opinion, at 1500 watts and running a 1.5 swr results in about 1430 forward and 70 coming back at me in my shack. So I just treat every radio, weather it's a 4 watt Cobra or a Ten Tec Omni D running through an amp, the same. My 75 meter dipole is centered on 3860, so if I stray away from that, I throw the tuner to it to keep my equipment happy.
     
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  8. Powder Joints

    Powder Joints Subjective Prognosticator

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    The answer to the question is that I have repaired lots of blown finals, some with bad antennas , some with no antennas. But imho 2 or less is fine for cb radio. In the immature and commercial world we have things to take care of reflected power, most of the antennas on base stations or repeaters I use are ordered to a specific freq for the application.
     
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  9. Night Stalker10

    Night Stalker10 Light Load Member

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    You guys have made some good points in the last 3 posts. Of course I’m not saying to ignore the SWR of your radio and antenna system. I’ve just had good luck with running my radios around 1.5 for SWR. I’ve always been happy with how my radios perform, wether its a CB, HF, or VHF, they all seem to work as they were designed too. Of course now that I have a zero 5 vertical antenna for my HF rig, I use an external tuner at the base of my antenna. Even then sometimes the tuner is happy to run 1.4 or 1.5for the SWR reading.
     
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  10. Powder Joints

    Powder Joints Subjective Prognosticator

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    I use a screwdriver on my hf rig mounted off the rear of the cab
     
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