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Over and above the NANOVNA issue: I presume you are dealing with a BJT
amplifier? Even with a FET PA, the following should be considered. The
input and output impedances of a power amplifier (PA) change noticeably
with drive power and corresponding out power, especially in the low-power
region before it reaches the linear region. The P(in) vs P(out) curve lof
a typical PA looks a bit like a forward biased diode VI characteristic.
Even if you connect the NANO directly to the input of the PA, the level of
RF from the NANO is too low to make a proper measurement of the input Z
which is representative of the PA at full power.

However, you're not out of luck. Insert an appropriate adjustable matching
network between your input to the amp (from a sig gen or transceiver - we
don't know the target power level of your PA or its rated input) and your
RF power source. Insert a GOOD SWR measurement device between the input to
the adjustable matching network and your RF source. Your RF source must be
from a good 50 ± j0 source - use good 50 Ω attenuators if needed to
stabilize the impedance. Adjust the variable matching network for 1:1
SWR. Then disconnect your PA and your RF source. In place of the RF
source, place a good 50 ± j0 resistor. Then connect the VNA to what was
the input from the matching network to your PA and measure the complex
impedeance. You're looking 'backward' into the required match for your
PA. The measured impedance made in this manner will be the complex
conjugate of the input to your PA. The resistance portion of the complex
measurement is real. The complex portion of the measurement must be
complemented with a reactive component of opposite sign to represent the
actual input impedance of your PA. This is the method the "big boys" use
to measure impedances of solid state power devices at RF frequencies.

Dave - W?LEV

On Sat, Aug 14, 2021 at 4:14 PM David Feldman via groups.io <wb0gaz=
[email protected]> wrote:

I need to measure S11 going into an RF power amplifier (1-15 MHz range of
interest.)

While the amplifier has proven stable (so far) when driven through a 10 dB
attenuator (from RF source), I am concerned that the impedance of the
Nanovna's S11 port (I am using a first-generation unit purchased mid 2019)
which is likely not 50 ohms could create a risk of oscillation (which could
put the nanovna's circuitry at risk.) The nanovna does not have separable
testset (like "option 011" on HP 8753 series), so I don't think I can apply
methods in HP app notes pertaining to measurements of RF power amplifier
circuits.

If I insert (example) 6 dB pad between nanovna S11 port and amplifier
input port, and calibrate the nanovna (open-short-load) at the "far" end of
the pad, what impact would this have on S11 measurements?

Are there other methods that would reduce risk when measuring S11 going
into a live amplifier?

Thanks for any suggestions/pointers.

--
*Dave - W?LEV*
*Just Let Darwin Work*