Electronic – EIRP and Radiated Field Strength

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I have a question regarding Field Strength: I was told it is independent from the frequency, is that true? If so, why?

Moreover, I came accross this equation for converting the Field Strength to EIRP, but I do not get where the 104.8 comes from:

$$
E\ [dB \mu V/m] = EIRP\ [dBm] – 20 \log_{10}(d\ [m]) + 104.8
$$

Best Answer

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: -

enter image description here

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

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