Tim Wescott wrote:

snip

What this means is that in general your transmitter, like your wall socket,
can deliver more than it's rated power if you put the right load on it.
Normally you'd expect the final amplifier to be fairly "stiff" - the output
power should be inversely proportional to the resistor you put on it. Your
circuit doesn't act exactly like that, even if it doesn't follow the same
curve for a generator with a linear impedance. I suspect that with your
circuit the 7-pole filter is evening things out quite a bit. There's also a
good chance that the "strange results" you see with a load below 10 ohms are
the final amplifier going unstable and oscillating, or your output
transistor heating up and changing characteristics on you.

Yes there were bad things going on with very low value loads, the output was
collapsing and re-establishing. The srtange results may also be caused by the
loads with were high wattage resistors - no idea what inductance they might
have had.

So what it boils down to is that it is very important that your output stage
_sees_ the right impedance, but you shouldn't expect the output stage to
_deliver_ the same impedance that it needs to see. As long as you're
getting power out and your output transistor isn't letting all the smoke out
then you're fine.

So perhaps the fact that the transmitter is connected to a 50 ohm load and the
output transformer is 2:1 turns ratio means that the transistor is "seeing"
200 ohms?

Nevertheless I am still interested in how to verify that any circuit which I
design (copy!) does have a 50 ohm output impedance. There must be some way to
verify this figure.

--Gary