Yes, though Z0=sqrt( (R+jwL)/(G+jwC) ) so a high R will boost the impedance; it’s not artificial. You may have to increase C to compensate. Though I would expect R to be very frequency dependent. Can you sweep it on a VNA, and see what happens at very low frequency such as 300 kHz? You may see a spike in Z0 at very low frequency, which will be exasperated by the R since you are getting sort of a divide by zero.

Please provide dimensional information. Is relative mu (permeability) of NbTi equal to one? A non unity value would increase impedance. Here mu_r=1.22 would give the result you are seeing.

At some point I would like to capture the ipedance formula that takes into account mu... let me know where there is a good explanation of that.

The microwave office TXLine calculator does show some effect on Z0 when resistance increased, but not enough to explain a 10% shift.

Steve

Z= Zo x sqrt(mu_r/epsilon_r). Skin depth also decreases with increasing mu_r which in turn increases resistivity (rho= L/A and A decreases with mu_r).

Not sure I understand where this comes from

Z= Zo x sqrt(mu_r/epsilon_r)

But I'll try to give it some more thought!

Thanks
Steve

Now seems like a good time to point out that Electroless Nickel (such as in ENIG finishes) is not pure Ni but instead contains "high phosphorus content" typically around 10% P, this effectively makes the Electroless Nickel alloy non-magnetic with a relative permeability mu_r near 1. This is a common electrical simulation mistake where designers might assume electroless nickel has the same properties as pure nickel with a mu_r in the many hundreds.

Another side note, the bulk conductivity is also much worse than pure nickel...

I personally use mu_r of 3, and a resistivity of 60 uOhm-cm.

Late to the topic but a good source for this is Pozar 4th Ed. Eq. 2.32.



In regards to Z= Zo x sqrt(mu_r/epsilon_r), I don't think so? Especially since the question regarded additional R (hence the conductors mu) but impedance formula uses medium mu. For what it's worth, in above, sqrt(mu/epsilon) = sqrt(mu_0 * mu_r/(espilon_0 * epsilon_r). Generally for coax impedance equations you see mu_r is assumed unity and the free space constants are evaluated out, so only sqrt(epsilon_r) is left in the denominator as a variable. So, if the mu_r of the medium happens to be non unity, the impedance should change by a factor of sqrt(mu_r).

Now for where conductor mu plays a role, of course Z0=sqrt( (R+jwL)/(G+jwC) )
For coax:

Where:

and finally we see conductor mu in skin depth:

Nickle naturally falls to ur=1 by 10 GHz, and is under ur=10 between 1 and 10 GHz; i.e. ur is very frequency dependent.

I've directly compared bare copper vs ENIG on the same coupon to 60 GHz and seen a good amount of dispersion between the two (of course the thin gold would cause that too), so I'm not entirely sold on ENIG as being harmless. I always use immersion silver, but that just means you need to store the bare boards properly to prevent oxidation prior to soldering.

I didn't know about the heavy frequency dependence, thanks for that! I agree that ImAg is definitely the best passivation for loss, but HASL is a close second (if you can put up with the non-planarity of the pads).

I totally agree that ENIG is not harmless. If the ur is ~1 at higher frequencies, I wonder if it comes down to the bulk conductivity of Ni not being great.

I once found this (single point experimental) data on the permeability of eNi which, looking back on it, supports your assertion of negligible permeability >10GHz

I've also attached an experiment I did with multiple platings on the same board geometry as a passivation loss comparison out to 40GHz. It's obvious that anything with nickel in it is worse. Also note how the loss slope of nickel platings is really steep until ~2GHz and then straightens out. Another piece of evidence of the 2GHz inflection of nickel's permeability!

Awesome plots! On the microstrip, notice the ENIG slopes are not linear until 5 GHz. Pretty sure that's the frequency dependent permeability. The permeability drops quite sharply in UHF, but the effect would be hard to see in thin plating.

I had read a paper from the 40's where they measured Nickle coaxial lines, generating a nice curve of permeability vs frequency through 10 GHz. I'd like to repeat the experiment, comparing two Maury airlines, but I need to fine a used airline I can effectively trash by Nickle plating it.

FWIW there is a company making an RF friendly paraleyne; have not tried it.

https://www.imperial.ac.uk/media/imperial-college/faculty-of-engineering/electrical-and-electronic-engineering/public/optical-and-semiconductor-devices/pubs/2008_06_PIERSO.pdf

https://iopscience.iop.org/article/10.1088/0370-1301/62/6/305/pdf