Wide band RF thermocouple ampmeter.

Discussion in 'Homebrew and Kit Projects' started by KM3F, Jul 25, 2018.

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  1. KM3F

    KM3F Ham Member QRZ Page

    I mounted a 3 amp rf thermocouple meter in a 4" electrical conduit box on a cover used for 240 volts round outlet by enlarging the round hole.
    The UHF chassis connectors are mounted to go through the back of the box, bolted fast and connected directly to the meter terminals with the shortest leads.
    Tested into a flat 50.8 ohm non inductive dummy load compensated for a flat match to 500 Mhz according to the analyzer.
    Antenna analyzer shows a flat match through the meter assembly to beyond 6 meters before the match goes up and is still useful at 150 MHz.
    I might even be able to do some compensation in this meter assembly to push it higher.
    Hand capacity changes at the meter face is very low when testing with the analyzer and no effect with power testing.. We don't normally have a hand on the meter during use. No reaction to hand on metal case housing during power testing..
    Power tested with rf at 100 watts 160m to 6m shows 1.3 to 1.4 amps depending on the radio's output on each band.. 1.4 sqed x 50.8 = 99.6 watts.
    The SWR in the radio remains flat on all bands.
    I have tested with a Bird 43 in line and they are very close.
    I believe this is as good as it gets without issues and becomes a useful tool.
    Did I just get lucky?
    Next tests will be to see what happens on the output of an amplifier driven to about 400 watts used to peak tune RF current, as a tuning aid, then the effects of a tuner in line to the antenna at about 2.8 amps.
    I realize there may be effects from the jumper cables and the tuner internals and take that into consideration for the results..
    W0GSQ and AI3V like this.
  2. KM3F

    KM3F Ham Member QRZ Page

    Tests with this meter connected between the amplifier output and the antenna tuner
    show the meter is subject to the same parameters as any other power meter a Ham station might use.
    Namely it is set up based on 50 ohm nominal impedance.
    The meter needs to be in a matched line condition.
    Otherwise it will react to SW conditions both directions.
    In an unmatched condition it will react to reverse power and add to the forward power by heating the element and raising the total current detected.
    Since the R is unknown, calculation of power is correct for the total if the impedance was known but not just forward power that would be in error.
    Also can see tuner losses assuming the tests are done to see the losses in a matches circuit
    The testing was done through an AL 80B, 75m, 20 watts drive and near 300 watts output
    into a tuner to antenna.
    Also tuner and amplifier tuning bandwidth can be seen before a retune should be done.
    That's all folks.
  3. VU2NAN

    VU2NAN Ham Member QRZ Page

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