RF power

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.

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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

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

'perfect'

open

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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 .