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Acceptable SWR

Wayne VK4ACN
Wayne VK4ACN Member ✭✭

Not a PGXL owner yet but does anyone know what SWR the PGXL can take before getting upset. I will probably get the TGXL when they are readily available, but will start with the amp first. My Hex has a 2.02 swr on 12m.

Thanks

Wayne

VK4ACN

Best Answer

  • Alan
    Alan Member ✭✭✭
    Answer ✓

    Wayne

    Below is a well written explanation of high SWR and high power. I am not the author. It was was from a discussion on another forum on the cautions of using antenna tuners. The examples are either an open or a short, at the antenna, but everything else is somewhere between these two points.

    Alan. WA9WUD

    ==================

    Respectfully, I think this is best understood by simply applying Ohm's law and Bill's simplified SWR equation below. 

    If something goes wrong at the antenna end of a transmission line that causes a 10:1 VSWR on the line, it is because there is a bad (and probably unknown) terminating impedance right at the antenna terminals that is causing it. Simply by measuring the SWR, we don't know what this impedance is because there are an infinite number of termination impedances that could cause a 10:1 VSWR reading. Plotting these impedances on a Smith chart, they form circles that we know as Constant SWR circles. 

    I think you would agree that no matter what you do at the other end of the coax run (tuners, etc.) nothing will change the actual IMPEDANCE of the antenna as seen at the antenna terminals. For any given fault condition, that impedance is a constant. 

    It is easy to understand two points on the SWR circle where the faulty load impedance might be purely resistive. This of course would be highly coincidental but let's look at these points because they simplify things. 

    Purely resistive loads of 500 ohms and 5 ohms would both present a 10:1 VSWR on the 50 ohm coaxial line. Note that in my hypothetical fault condition these are real impedances. Now, if we were to somehow pump 1500 watts into these loads (for example by using a tuner at the other end to force power into the system) in one condition we would see much higher VOLTAGE (in the case of a 500 ohm fault) across the antenna terminals and in the other case we would see much higher CURRENT (in the case of 5 ohms) flowing into the antenna terminals. 

    Either way, too much voltage or too much current is not good for the balun which represents your load. The balun likely wasn't designed to operate under those conditions and bad things may result. 

    Bill is absolutely correct: using an antenna tuner with an antenna system that has a balun is bad practice. Tweaking in a 1.5:1 VSWR probably won't hurt anything, but what happens if something prevents the antenna from tuning properly? We all know that happens with these antennas. With a high power auto-tune amplifier in the circuit, you're running a great risk. Granted, most of these amplifier tuners can't handle anything higher than a 3:1 VSWR and you might get away with it ... but are you sure?


    Fred Glenn

    K9SO



Answers

  • Ian
    Ian Member ✭✭

    What I see with my PGXL is 2:1 SWR is no problem at all. After 2:1 power fold back starts to happen but I can go as up 3:1 and still run the amp. At 3:1 the amp faults off. I never use more than 1000 watts, usually less. With higher SWR the amp is working harder and the fans run more but I have had no issues.

    I got the TGXL now and all issues are now gone.

    I run SO2R 40% of my operating time and everything runs 99% perfectly. The 1% is when my network does something strange (I cannot identify what) and I have to reset either the amp or tuner and then all is ok.

    The 6600 / PGXL / TGXL / AGXL is a great combination of products


    73, Ian VE3JI

  • Wayne VK4ACN
    Wayne VK4ACN Member ✭✭
    edited April 3

    Thanks Ian for your reply

    So I guess the hex at 2.2 swr would still work. The SPE I have at the moment doesnt like above 1.5, but the tuner in it fixes that problem. Aiming to upgrade to all flex in the near future. Thanks

  • Alan
    Alan Member ✭✭✭

    Wayne

    A word of caution on higher SWR and higher power levels.

    As the SWR increases, the impedance remains the same at the antenna with or without a tuner.

    Depending on the values making up the antenna impedance, as the power increases, either the voltage or the current at the antenna will increase accordingly. At high power and high SWR, these values may cause damage to antenna components.

    So...even though your amplifier may be just fine with an SWR, or your tuner makes it look OK to your radio, you may still damage your antenna components.

    Alan. WA9WUD

  • Wayne VK4ACN
    Wayne VK4ACN Member ✭✭
    edited April 3

    Alan

    Interesting, not sure what on a Hex would get damaged, but I do have a polyphaser and 1x8 antenna genius in line from the SPE to antenna. These could be affected. I do notice on 12m at higher power, the swr does slowly creep up, (not a lot in the 15 sec tx) but guessing something is getting warm. Ill have to bring down the hex, one of these days, and try to sort out the high swr.

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