Hi Peter,
every amplifier (Transistor or Tube) will change its input (and output) impedance due to bias. So input (and output) impedance of impeadance matching network as shown in your picture will also change when bias to the amplifier is supplied.
73, Guenter, DK5DN
Am 28.06.2021 um 10:02 schrieb Peter:
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Hi there ...
Appreciating several features of this incredible instrument, I'm attempting to check the real Impedance of the Matching network INPUT (and/or out-put) value associated to a VHF RF Amplifier (medium-power abt 200W) ... I connected the VNA as shown on the picture and then, after related calibration, I perfomed the measure (sweep)....
Of course I didn't apply any RF source at input ... Nano-Vna only !
I found a strange Resonant Frequency ... quite far from the proper one (I expected to be around 140MHz...150MHz) .... But then , if I apply the related Power-Supply (including BIAS) , the situation changed ... the resonant frequency was really close to the expected one !
Is it normal ?? Really the Power-Supply/Bias could affect impedance values of Input matching net-work ?? This situation can be applied at Out-Put line as well ??
schweppe