On Sun, 5 Jul 2015 20:58:58 +0100, (Roger Hayter)
wrote:

The impedance of a transmitter output will be nothing like 50 ohms
resistive, as this would result in an efficiency well below 50%, with
all the normal amplfier losses plus the actual RF power produced being
50% dissipated in the PA. This is why matching in the forward direction
coexists with a mjaor mismatch in the reverse direction. This is good
because if there is any reflected wave we don't want it to add yet more
to the PA dissipation. But it does explain what is happening, and why
there are increased losses in the feeder as well as the matching
networks.

I previously posted how to measure the output impedance of an RF power
amplifier. Methinks this would be a good time to repeat it...

It might be interesting to measure the output impedance of your HF
xmitter. All you need is a dummy load, and an RF voltmeter, RF probe
and voltmeter, or oscilloscope.
1. Turn down the xmitter RF output to some level where you won't blow
up your test equipment and so that it doesn't go into high VSWR
protect mode. My guess is about 10 watts is about right.
2. Measure the RF voltage across the output connector both with a
load (Vload) and without a load (Vno_load).
3. If measuring peak voltage, convert RMS by multiplying by 0.707. If
measuring peak-to-peak, divide by 2 and then multiply by 0.707.

Output_Impedance = 50 ohms (Vno_load - Vload) / Vload

Easy enough. Just don't let the RF output level or VSWR get high
enough to trigger the ALC or VSWR protection. I haven't done this in
a long time, and don't recall exactly what this simple measurement
produced on radios that I've designed. I do know that the output of
my (marine radio HF) PA's always went to a low pass filter, which was
designed to be terminated by 50 ohm in/out. If the PA didn't present
50 ohms to the filter input, the filter response would not be flat,
and there would be additional losses. Since that was not the case, I
can deduce that the amplifier output was at least close to 50 ohms.

Anyone wanna make the measurement? My bench is a currently a disaster
area so I can't do it without a major cleanup.




--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558