EIRP and Radiated Field Strength
Field strength has NOTHING to do with frequency - you can have a battery that generates a static electric field - no frequency involved, just DC. You can produce a constant magnetic field with a DC current and a coil. No frequency involved.

For the formula first consider this: -



Note that \$4\pi r^2\$ is the surface area of a sphere and \$120\pi\$ (377 ohms) is the impedance of free space.

So, if you plug in some numbers like 1 mW and 1 metre, E (volts per metre) is 0.173 volts per metre = 173,205 uV/m or 104.77 dBuV/m.

That is where the 104.8 comes from