Port CH0 ideally presents a 50 ohm resistive impedance to anything
connected to it. If an attenuator is needed, it should have a Pi or Tee
form, so that a 50 ohm resistive impedance is still presented to
something connected to it. If a resistor is connected in series with
CH0, the impedance seen looking into it and CH0 would be 50 + R, clearly
a mismatch to the cal standards. If a Pi or Tee attenuator is used, do
the open-short-load-save calibration at the end of the attenuator not
connected to CH0. This way the calibbration will "take into account"
the presence of the attenuator.
David
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On 2023-08-24 18:35, Greg Strickland wrote:
If I may ask a follow up questionsabout Nano VNA-H.
1. Is port CH0 input a 50 ohm resistor load?
2. If #1 is correct, would proper attenuator implementation for the port be a series resistor, to form a L pad with the internal resistor?
3. If #2 is correct, would the open-short-load-done-save calibration be done with the measurement plane on the source end of the L pad?
4. If #3 is correct, would scale or another adjustment need to be done to account for a pad between Nano VNA and antenna under test.
btw- the parallel LC "trap" worked at the antenna tower and it reduced the nearby RF source down to less than 100 millivolts. Moving the measurement plane to "cancel out" the trap did not work for me. I was able to obtain presumably credible results by removing the measured impedance of the trap by itself from the measured impedance of the trap and antenna. What concerns me is DC resistance of the trap inductor in series with an antenna that might be 27 ohms at 55 ohms of capacitive or inductive reactance at the operating frequency.
Good news is I have not "blown up" the Nano VNA yet.
Thoughts or suggestions are appreciated. Thank you
