Ground Fill and track impedance

Hey Steve. Just to make sure, your stackup is signal on top, then a ground plane, then a middle plane that's either empty or has "stuff" -- yes?

And you're going to infill that middle plane with copper -- yes?

If so, then your "small amount" becomes "vanishingly small, of use only as an exercise in an advanced E&M class".

Thanks, Yes, I have Top L1 signal+Ground place

PCB manufacture wants me to put ground fill on layers 3 and 4 to balance co pper and was worried about impedance change. My saturns PCB microstrip calc ulator give 0.1mm track z0 as 60R on outer layer, if I use saturns PCB copl anar waveguide over ground calculator it gives z0 as 74R for 0.5mm clearanc e ground fill 0.1mm over ground plane, however as ground fill clearance inc reases impedance goes up which to me is illogical, if ground fill is a long way away it should have the same z0 as microstrip. Will take your advice a nd not worry about impedances changes due to ground fill.

Thanks again