Re: RF power

From: Mark (mzypt1_at_kolumbus.fi)
Date: 03/27/05 

Date: 27 Mar 2005 09:55:23 -0800

Andrew Holme wrote:
> mzypt1@kolumbus.fi wrote:
> > 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.
>
> 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.
>
> > 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.
>
> 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.
>
> > 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.
>
> 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.

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

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