Also, for frequencies above 5 to 6 GHz the effects of atmospheric
absorption become much higher and non linear with frequency. It has to
do with molecular resonance and atomic cross sections of the molecules.

Bob Bob wrote:

Hi all.

No doubt I could get this reading lots and lots of text books. It might
however be interesting to air a discussion on it..

Have been reading the free space pathloss formula from the ARRL antenna
handbook;

Loss in dB = K + 20logf + 20logD

Where f is the freq in GHz and D is the distance in miles

K is a constant of around 96.6dB

I have known about a "fixed K" loss in an antenna system for ages. It
even made mention I think in this NG recently when talking about a
passive repeater system. From my own exposure to path modelling
(EDX/Pathloss etc) I noted a very high dB loss per distance rate in the
first (say) 100 wavelengths when looking at graphs of same. (I wasnt
doing the actual job, just providing data to the engineer to compare
measured with predicted. Fascinating stuff!)

What I wonder is where roughly does the 96dB odd "come from". More
interestingly can it be reduced by any appreciable amount. Is it maybe a
antenna to "air" coupling loss, maybe even that a RX antenna cant
possibly extract all of the energy from the wave as it goes past. I
would appreciate any input on this.

My initial forays (as a young ham) into LOS paths went through the
isotropic/point source radiators and looking at the surface area of the
covered "sphere" containing all of the radiated power idea. Then the RX
antenna "aperture" area was used to calculate the actual received power.
Needless to say it never met the actual measured values!

Cheers Bob VK2YQA (in W5)