unexpected behavior of the matching components on the Smith chart / VNA

[med_a]

Hello,

I'm using the Smith chart on a VNA to determine the matching circuit for my PCB (an RF switch (LoRa, WiFi and GNSS) and an antenna (NN02-224)). I used theory to find the matching networks for GNSS and LoRa, and the VNA showed good results.

However, I couldn't get a good match for WiFi. I've used theory and several configurations (L-matching, T and Pi networks) and the first component behaves as expected (moving the point in the Smith chart to the intersection with the 1ohm circle). But as soon as I add the second component, the behavior is not as expected (instead of eliminating the reactance, the point moves up in the Smith chart).
I've changed the configuration: start with a series component (whether L or C) and then a shunt component, but the problem remains the same.

Can anyone tell me what the problem is?

Thanks

 

[noori.re]

Hello Med_a
I thnik the issue is that you has calibrated your VNA at the wrong point. You should calibrate it using Short/Open/through, exactly on the point on board where you want to have 50 ohm which is usually the pin of WiFi chip.

 

[virtualparticles]

I believe noori.re is correct. The reference plane of the measurement needs to be right at the component. If you can't do a real calibration right at the component, use port extension to move the reference plane from the calibration point to the component. You'll also find that the inductors you actually use are not ideal. They will have a parallel resonance. I usually do a first pass L-C match using ideal values and then substitute the S-Parameters for the inductor into the simulator to see how bad the change is. The capacitors can be assumed to be ideal for the most part. Here are some resources:

Non-Standard VNA Measurements - LCR Measurements

Learn how to use a vector network analyzers to make LCR measurements to determine reactance and resistance.

coppermountaintech.com

Eliminating Fixture Effects from Embedded Measurements

Introduction It is often necessary to measure a Device Under Test (DUT), which is mounted...

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In-Circuit Antenna Verification

Back VIDEOS In-Circuit Antenna Verification https://www.youtube.com/watch?v=yQoAlLPjiU8 In this video, Brian Walker, Senior RF Design Engineer...

coppermountaintech.com