RF power

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.

 

[Mark]

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

 

[PeteS]

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

 

[Mark]

PeteS wrote:

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

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 .