On Sun, Jan 9, 2022 at 01:30 PM, Jeff Green wrote:
Goal:
Given my hardware, software, and less then stellar knowledge and skill set, is
it practical to measure the actual antenna input port impedance of several
general coverage receivers?
Jeff,
Yes you can measure the input impedance of an HF receiver with your NanoVNA but you need to be careful you do not overload the front end. The output of the NanoVNA is around 0 dBm which is too high.
The trick is to lower the NanoVNA output power as much as possible. If you have recent firmware by DiSlord you can set the power level to 2 ma. (which is the lowest possible) but still too high. The next step is to add a 20 dB attenuator to the S11 port and then calibrate with the attenuator in place. The attenuator will lower the signal into the receiver and reduce the dynamic range but you can still get reasonable SWR and RL measurements. The impedance measurement will be reasonable as well if close to 50 ohms. The further away from 50 ohms the worse the accuracy.
You will notice that the impedance will change rapidly when you are outside the passband of the receiver front end filters.
Roger
Roger Need