At 2-3 GHz can even small additions such as an N-connector affect measured result, if not included in total VNA calibration.
With incorrect reading even before actual dut is connected, is it a bit hard to say anything about measured results.
For calibration of a HP VNA at these frequencies, check how I do **broken link removed**.
If you want to transform impedance from 140 to 50 Ohm with a quarter wave stub on a Helix, calculate it as a ~84 Ohm transmission line over ground.
I do normally not calculate these kind of transformers, I start with 10 mm wide cu tape, 30 mm long, and add more tape width until it looks good.
Sqrt (Z1*Z2) is the correct answer. My question is how did you build it? THere must be something wrong in the realization.,
If not very used with how your N-connectors behave at these frequencies, do I not recommend to include it at all if you really want to measure antenna impedance and nothing else. A N-connector is a rather big thing with lengths that can be complicated to compensate for.
I do this kind of stuff every day and have no problems. I propose that you calibrate VNA, including a thin coax cable. Check that calibration still is correct with the coax soldered at antenna ground plane and first then connect the antenna and nothing else.
Adjusting shape of the transmission line to something triangular can increase bandwidth, yes, but a cost of total gain at center frequency if this line also is a part of the antenna. As opposite, you can make the transmission line entirely at ground plane, allowing you to keep each turn on antenna more uniform, for best directivity and gain.
No, you do not even need a SMA connector, and should avoid it if you is able to follow the instructions at www.antune.net how to do a correct calibration. I do get correct result for this type of measurement, using same tools as you. If you get different result, you do something wrong or have faulty equipment!I don't have anything other than an N cal kit, so if I was to put an SMA connector rather than an N connector, I would have no way to calibrate the analyser.
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