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It is the first time I am doing a 4-layer PCB with Bluetooth PCB antenna, that should be 50ohm coupled with the corresponding IC. As my layer stack is TOP-GND (layer)-POWER (plane) - BOTTOM, Altium actually used POWER plane as a reference in the impedance matching. I just wonder is it valid calculation in my case, because, as I know from RF electronics, microstrip impedance is always calculated in reference to GND ? Am I supposed to put GND plane just below the signal layer (TOP in my case) where the matching trace is placed ? I initially make the GND as layer because I have separated ground regions (Audio-analog GND, Digital GND, RF GNF) connected to the single point, and as Altium requires at least one plane for impedance matching, I put POWER as a plane. I am not sure what is the regular option..
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To keep the correct impedance, I always use the track geometry suggested by PCB manufacturer. I never rely on the numbers from Altium.
GND doesn't have to be the only reference plane (power planes sometimes can also be reference planes e.g. if you have a lot of decoupling capacitors between GND and Power plane). As I do not use impedance calculator in Altium, I can only guess, that Altium only recognize reference planes if you specify them as a plane, not as a layer. That is what is probably happening in your case.
And as you noted correctly, if you route your important track on the top layer (Layer 1), I would use GND as the layer below it (Layer 2) and that would be the reference plane.
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