Reading on power meter

Discussion in 'Amateur Radio Amplifiers' started by K7DVS, Mar 25, 2020.

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  1. K7DVS

    K7DVS Ham Member QRZ Page

    I have a yaesu ys-60 power meter. When I tune up my amp I tune it for maximum power which I know is around 700 watts. I move down the band where my Antenna is not as resonant and do the same thing but the meter tells me I’m putting out over a KW. Which I know is not true for this fl-2100b. Why does it do this? I am not using a tuner. I do have a separate tuner but was told no point in running the tuner just run that 2100b direct. I am wondering if he is wrong and I should be using the tuner?
     
  2. WG7X

    WG7X Platinum Subscriber Platinum Subscriber QRZ Page

    Your meter is summing up the forward and reverse power when presented with something other than a 1:1 or 1:1.5 SWR. To get the true power out you need meter that can show you the forward power and the reverse power. Then you simply subtract the reverse power reading from the forward power reading.
    That is the "true" power.

    This is one of the reasons that you see so many meters with either two separate meters or on meter with two needles.
     
    WQ4G, W4NNF and KW6LA like this.
  3. KW6LA

    KW6LA Ham Member QRZ Page

    YOU smile.. 1K and then know something is not RIGHT !!! lots of Hams go thru this. WG7X explained it very well. I posted this same answer on Eham and got flamed by the trolls Hi Hi.. They do make forward reading watt meters ? I am just too cheap to buy one, so you do the math or live with the bum readings. If you cant deal with that, buy a Bird #43 watt
    meter. It will set you back about $ 350 dollars.
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The Bird does the same thing.

    If there is any REFlected power, for the Bird FORward power indication to be correctly interpreted, you subtract the reflected indication from the forward one.

    If the meter shows 1000W forward and 300W reflected (SWR = 3.4), the actual forward power is (1000-300) = 700W.

    That's true as much for the Bird 43 as it is for any kind of in-line wattmeter.
     
  5. K7DVS

    K7DVS Ham Member QRZ Page

    Ok thanks for the replies that makes sense. so is it true that I do not need to be running a tuner? The pi network in the amp should take care of the slightly higher SWR when I am further down the band for example?
     
  6. AI3V

    AI3V Ham Member QRZ Page

    When you tune your amp, you are tuning for a certain plate current at the resonance. "Dip" ?

    And you figured out what that current should be by first tuning into a dummy load?

    If so, then you will be fine, you are using the variable capacitors in the amp to compensate for the difference in impedance of you actual antenna system from a pure 50 ohm load.

    And why does your "wattmeter" read differently?

    Because its really a voltmeter, with a scale calibrated in watts into a 50 ohm, non reactive load.

    Depending on the actual antenna system impedance, you can read either higher or lower power depending on whether the actual impedance is higher or lower than 50 ohms.

    Also, understand, that a 2:1 vswr can be (in the simple resistive case) either a 25 ohm or a 100 ohm load , and would make a "wattmeter" read either 1/2 the actual wattage (25 ohm load) or 2 times the actual wattage (100 ohm load)

    Rege
     
    WQ4G likes this.
  7. KM3F

    KM3F Ham Member QRZ Page

    And, and if the SWR causes the radio to begin to fold back (reduce power), that adds to the error in power readings.
    You simply need to know the Dummy load power at near 1:1 match, then sweep the antenna for match, then expect what you get from the match you see.
    Most of the time you cannot see return power of very low levels because the meter cannot resolve it when the meter needle is on or very near Zero.
    It becomes for all intent and purpose, a LOOP function.
    Good luck.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    With an FL-2100B, I'm not so sure about this. It's likely okay up to a ~3:1 VSWR but above that, maybe not.

    This is a matter of both "tuning range" and the voltage rating of the load capacitor and possibly even the bandswitch. Amplifier designs and materials vary a lot by model and I don't know how much mismatch the FL-2100B can really handle.

    I think G3YRO knows a lot about that amplifier and might want to chime in, if he's around.
     
  9. G3YRO

    G3YRO Ham Member QRZ Page

    It shouldn't be a problem . . just how wide a range of impedances the FL2100 is capable of matching varies from band to band.

    But if your Antenna impedance is too high, you simply won't be able to Load it to very high power . . . so you're unlikely to get any Arcing on the Load capacitor.

    As long as you can Load to get full power input, then you're within the range of the Pi-Tank.

    And as Rege said, nearly all Amateur Power Meters are really just reading the voltage on the feeder . . . so the power calibration is only correct when your antenna impedance is 50 ohms.

    Roger G3YRO
     
    WQ4G likes this.
  10. SM0AOM

    SM0AOM Ham Member QRZ Page

    Another repetition of the "urban myth".

    A directional wattmeter does read both forward power and reflected power by combining the voltage and current at the sampling point in a bridge circuit which may either lumped or distributed.
    The circuit uses the property that the current and voltage components of the standing wave along the line are 180 degrees out of phase.

    Actual delivered power to the load is formed by subtracting the reflected power from the forward power. When the mismatch is large, the directivity of the coupler may influence the accuracy of the delivered power value considerably, which makes it inadvisable to use this method when high accuracy is needed.

    The whole math behind this may be found in communications engineering textbooks, a summary is here:

    https://www.robkalmeijer.nl/technie...hniek/hambladen/qst/1959/04/page24/index.html,

    also in the 1980s Bird catalogs, and when I taught "analog radio" I required my students to derive the relations.

    73/
    Karl-Arne
    SM0AOM
     

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