Here's an experament: Build a power output stage _only_. Don't put a
filter on it. You should have a transistor with the emitter grounded, a
choke to +12V, and some minimal network on the base to keep it biased. You
should be able to pick this out of your Tuna Tin II design. Capacitor
couple an appropriate resistor to the collector (200 ohms if you use your
Tuna Tin II).

Now feed it with enough RF so it's really into class C, and measure the RF
voltage on the resistor. It should be around 8V (supply * 0.35) give or
take a bit.

Now change the output resistor by a factor of two and measure the RF voltage
again. It should be about the same as before assuming that you've sized
your choke correctly (4x the design resistance). This shows that your
output stage is stiff (i.e. has a low equivalent Rs).

What this all means is that your average well-designed output stage does
_not_ have a matched Rs! Why? Because when Rs = Rl the power in Rl is
maximized _for that Rs_, but the power burnt into heat inside of Rs is also
maximized! Since you want the power in Rl to be much higher than the power
used to heat up your shack (unless it's really cold inside) this is a good
and desireable thing.

"Gary Morton" wrote in message
...
Over the Xmas break I constructed a simple 2 transistor QRP transmitter
using
the Tuna Tin II design.

Using a power meter and a 50 ohm dummy load it appears to show around
200mW, a
little less than the article suggested. Then again I had substituted the
final
stage 2N2222 transistor for a beefier (size wise) BFY51. I'm not too
sure
what exact difference this will make.

In the article it says that the 200 ohm output impedance of the final
stage is
transformed down to around 50 ohm (using a 2:1 turns ratio untuned
transformer wound on a toroid).

I have added a 7th order low pass filter to reduce the harmonics.

I am interested in verifying the output impedance.

If I understand the theory, when the output is terminated with a resistor
which matches the output impedance then the power transferred to the load
will
be maximised.

I set about loading the output will a series of resistors and measuring
the
peak to peak voltage across them in order to calculate the r.m.s. voltage
and
hence the power dissapation.

Resistors of value less than 10 ohm gave strange results.

load r.m.s. power (V*V/R)
voltage

10 1.272 0.162
15 1.767 0.208
27 3.005 0.334
33 3.253 0.321
39 3.676 0.346
47 4.066 0.352
56 4.384 0.343
68 4.666 0.320
100 5.303 0.281

The numbers work reasonably well and point towards 50 ohms-ish.

The power value looks too high - so I might have made a mis-calculation
somewhere!

Can anyone pass comments on the above?

I'm interested in where the 200 ohm figure comes from.

Any suggestions as to other methods of measuring the output impedance?

regards...

--Gary (M1GRY)