Here is Galen’s take on an air dielectric, in a discussion more specifically about thinly enamel-coated wires that’d essentially be equivalent to having air around them:
"There could be an advantage if the transient response of “air” at a given steady state capacitance is better (faster rise time). But the equation for Rs in a capacitor at typical frequencies isn’t differentiating what reaches a given rise time constant. Right now I’d say, no, air dielectric give a better INDUCTANCE at a given capacitance because the wires can be closer, and is benefiting inductance, not so much the capactance value, but the inductance value. This is why the IC cables are air core, to reduce BOTH L and C. Again, this is all held by the fundamentals I’ve described for you in the past.
Speaker cables are high current, and are tougher to make SAFE and consitent with air core and be affordable. Here, use magnetic field cancellations to drop inductance more than just the spacing of the wires, although that is optimized too."
Those of you who’ve mostly abandoned plastics and switched to air dielectrics for the whole cable assembly, what do you say, what does air bring to the table when compared to the best plastics and cotton/silk dielectrics? (Or even helium or vacuum, of which there are a few examples available)
Dielectrics can be a necessary evil and their lossy properties can also be utilized for advantage, such as what was done for the BAV power cords: using a dielectric with a high loss tangent (dissipation factor) for RF, thus turning unwanted frequency components into heat.