dB loss is always negative… right???

A recent online posting set out to inform the masses:

The concept of free space path loss (FSPL) is widely misunderstood. Some university lectures and even texts get it wrong. …

The result is some dB of loss which is the ratio of the power received to the power transmitted expressed in dB form.

Errr no!

FSPL is a loss, and as a derivative of the basic meaning of loss (being \(Loss=\frac{Power_{in}}{Power_{out}}\)), \(FSPL=\frac{Power_{tx}}{Power_{rx}}\) (a ratio greater than unity).

We can express FSPL in dB as \(FSPL=10 log \frac{Power_{tx}}{Power_{rx}} \text{ dB}\) (a +ve dB value).

Let’s try an example.

Above is a FSC calculation of the uplink path for a LEO satellite (ignoring atmospheric loss). Because the “Extra Path Loss” is zero, the calculated link loss in the bottom of the window is simply FSPL, +136.3dB.