On 6/28/21 5:43 AM, G8DQX list wrote:
Peter,
you haven't indicated whether the amplifier is linear (class A or AB or B) or class C (for FM or CW). The circuit operating conditions (class, signal level, bias, Vcc) make a big difference.
Input match
The input match is generally designed to present 50¦¸ to the input socket.
Output match
The output match is generally designed to deliver maximum power to a 50¦¸ load. The PA is *not* a conjugate match, which would reduce the output power by 6dB!
Take care
A nanoVNA (most VNAs for that matter) is not safe anywhere near a 200W amplifier. All it would take is a spurious oscillation in the PA, and the VNA is likely to see rather more than +10dBm (10mW) at a measuring port, with the danger of damaging the VNA electronics. (See also /g/nanovna-users/topic/77980973#18555 </g/nanovna-users/topic/77980973#18555>, for instance.)
If it breaks into oscillation (or picks up a spurious signal) it could easily push more than 10dBm *out* of the input port.
Even operating as designed, it could have surprisingly high power coming out the input.
In order to be stable, the S12 (reverse isolation) just has to be bigger than the forward gain (S21) (slight oversimplification). Let's say the amplifier gain is 10 dB,? S12 is -15 dB, and a mismatched load reflects -20dB, and the amplifier is putting out 100W (+50dBm) - the reflected power from the load is +30dBm, the amplifier (going backwards) reduces that to +15dBm.
