Sure, a couple of possibilities:
1) When your power supply is off it's a high impedance, and when it's on it's a low impedance, so you are at the very least switching in another inductor.
2) Semiconductor junction capacitance depends on applied voltage, and there are semiconductors in the amplifier block.
3) Your amplifier block is accepting input power (to amplify it) or not. You could think of this as an additonal load.
Without more details we can only speculate, but I'm not surprised at your results. I would certainly expect the input of the amplifier to change when it's powered up.
73, Willie N1JBJ
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On Jun 28, 2021, at 4:02 AM, Peter <yliroma@...> wrote:
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 ??
<Match_Meas.JPG>
