The Friis transmission equation can be given as:
Pr = Pt + Gt + Gr + 20 log10 (λ / 4πR)
Where the variables represent:
Pr: Power at the receiver Pt: Power at the transmitter (in e.g. dBm) Gt: gain of the transmission system (in decibels) Gr: gain of the receiver R: "radius" of signal, i.e. simply the distance between transmitter and receiver λ: the wavelength of the signal
The λ term is very surprising, isn't it? It very conveniently makes the units work out, but why should the frequency of a signal affect how "rapidly" it dissipates?
I know that ionspheric and other terrestrial effects tend to make HF signals propagate farther than VHF and higher, but what accounts for this "increased range" of low frequency signals even in theoretical free space?!