Characteristic impedance of vacuum

Empty space has an impedance. In a plane electromagnetic wave the electric field E and the magnetic field H are locked in step, and their ratio E/H is fixed at Z₀ = √(μ₀/ε₀) = μ₀c ≈ 376.73 Ω. It is a real resistance in every sense that matters: point a transmitting antenna at the sky and the power it radiates is genuinely dissipated into free space at this impedance, which is why an antenna's "radiation resistance" heats nothing yet consumes watts.

Antenna and microwave engineers round it to 377 Ω and design around it. A half-wave dipole in free space presents about 73 Ω, a folded dipole about 300 Ω, and matching networks exist to bridge those to the 50 Ω of coaxial cable. Resistive sheets manufactured at 377 Ω per square — Salisbury screens — absorb a normally incident wave almost perfectly, the oldest trick in radar-absorbing materials.