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RF power

Discussion in 'Electronic Basics' started by [email protected], Mar 26, 2005.

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  1. Guest

    Hi,

    Could someone explane me some basic issues of power tranfer in RF
    amplifier stages.
    1. How power is transferred from power transistor to antenna from
    physical point of view, is antenna acting as a load.. Can it be
    referred to AF amp and speaker.
    2. how can I be sure the power is transferred to antenna and not just
    to warm up the transistor when decreasing the value of series resistor
    (increasing power) in amlifier transistor circuit.
    3. Can successful impedance matching be measeured as emitter current
    increase. Is measured current higher ja vice versa when antenna length
    is corret and all transmitter power it transferred to air via antenna.
     
  2. Andrew Holme

    Andrew Holme Guest

    Yes, the antenna is a load. There is an impedance transforming / matching
    network between the transistor and the antenna. Ideally, the transistor
    should "see" a resistive load. The amount of power depends on the load
    "seen" by the transistor and the magnitude of current the transistor draws
    through it.
    This can be tricky. Possibilities include:

    1. Use a forward / reverse power meter
    2. Tune for a dip in PA current
    3. Connect a bulb in series with the antenna
    4. Check with a field strength meter
    5. Adjust the transmitter with a dummy load

    By "series resistor" do you mean a resistor in the emtter lead of the PA
    transistor? In the case of a Class C amplifier, you may be able to increase
    the power level by reducing this resistor, assuming the peak-to-peak AC
    voltage at the collector is not already at twice rail.
    Sometimes correct tuning is indicated by a dip in PA current, sometimes you
    must tune for maximum output. I don't think you want maximum emitter
    current. That should be governed by drive level.
     
  3. Mark

    Mark Guest



    Thanks for this info. Rf is more clear now. I'm trying to get familiar
    with rf technology but I found it quite difficult to understand
    comparing to "normal" analog circutary. This load thing is one issue I
    dont understand. Like how PA draws current to load, antenna. Ok I know
    that high frequency generates electromagnetic field and this needs
    power but still..
    By series resistor I ment class c emitter resistor as an example.
    Maybe I schould find some books to read first the basics. Or is there
    any good websites where this stuff it explaned in a beginner way.

    Mark
     
  4. PeteS

    PeteS Guest

    I would suggest looking up basic transmission line theory. A 'perfect'
    antenna is in fact a limiting point for transmission lines (either open
    circuit or short circuit at DC) and the length of the antenna counts -
    but the reason for that is in basic transmission line theory.
    A properly matched antenna 'looks' like a resistive (close, anyway)
    load to the RF PA, and does itself perform an impedance match (from the
    antenna Zo to free space Zo - about 377 ohms).
    Tuning, as noted, depends on a number of factors (importantly, the
    amplifier class of the PA stage, but there are other issues), so it's
    hard to know how to respond to that.
    Hope that gets the thought noodles going

    Cheers
    PeteS
     
  5. Mark

    Mark Guest


    This is getting clearer and clearer but somethin new issues comes up
    and confuses me as I learn more.
    I have studied tansmissionline theory, and yes theory, which is far
    beyond my understanding.
    But this is clear so far, in order to transfer power losslessly (can it
    be said this way) from stage to the next one we need inputs and output
    impedances match to 50ohm. But when calculating matchin LC network what
    is matched to 50 ohms. Like if we have an oskillator following with amp
    and these two element has to be matched together. How do we know
    oskillators output imedance which is to be matched to 50 ohm and this
    further to be matched with amps input impedance (which is?) If this
    oskillators 50 ohm matched output (lets assume we managed to calculate
    the network)is connected to transistors base, then I could somehow
    understand if the imput impedance is transistors base-emitter
    impedance. But so far I could't find this information from any
    datasheets.
    I have tried also to find ready calculated network examples with real
    citcuitry so I can figure out without Q and Z parameters and pages of
    calculations. There has to be more practical way of doing this .
     
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