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A while back I read that heavy smog could interfere with microwave communication links.   I tried to search for more information on this, but the only thing I found were references to electro-smog, the RF "smog" supposedly caused by RF devices, including Wi-Fi networks. 

Right now, I'm reading a Ham Radio book entitled "Propagation and Radio Science".   The information here is mostly in reference to HF radio signal behavior in the ionosphere and atmosphere.   Very interesting from a Ham radio perspective, but alas, no mention of atmospheric smog effects.  

I was wondering if anyone familiar with long distance Wi-Fi links has seen any problem caused by good old "normal" smog.   I mean other than the expected optical alignment difficulties ?   Perhaps decreased SNR ?  Signal dropouts ?  Anything ?

Thank you.

I've heard of measuring atmospheric pollutants by their absorption of RF, but I'm pretty sure that was millimeter wave.

I doubt that there is going to be much of an effect below 10 GHz...

-MikeG

I looked into how rain effects RF many years ago and found some research that it was 0.5dB per km for heavy rain. It further defined heavy rain but I cant recall exactly what it was, something like 4 inches per hour which would be a monsoon.

Yes it can effect it but I would ask what distance you are looking at. Probably negligible.

I looked into how rain effects RF many years ago and found some research that it was 0.5dB per km for heavy rain. It further defined heavy rain but I cant recall exactly what it was, something like 4 inches per hour which would be a monsoon.

Yes it can effect it but I would ask what distance you are looking at. Probably negligible.

My interest is purely an academic one, and not specific to a particular range.

In the book on propagation I mentioned, it talks about the refractive index of air and how that affects the path of radio waves.   This is due to its refractive index, which is different for different frequencies. 

We also know from other studies, including the CWNP program, that a materials refractive index is related to its dielectric constant.

Upon further reading, I saw that by changing the gaseous makeup of air, either with water vapor, carbon dioxide, or other gas mixtures, we change its dielectric constant, and therefore its refractive index.

I was not able to determine the exact amount of variation due to smog, but clearly (no pun intended) a sufficient change in smog level, at some point in a radio beams path, could deflect it from its original target, and perhaps the signal level at the receiving antenna.

I had never considered anything other than diffusion from fog or smog.  Obviously it's more complicated than that and I'm still unsure as to the extent of smogs overall effect.

Where is Dave1234 when we need him ?

I'll take a shot at this:

The factor that would have the greatest affect on a radio wave would be the temperature inversion, i. e., a layer of warm air on top of a cold layer and a key component in the formation of smog. If, for example, we have a beam propagating through the inversion layer we can expect some kind of refraction.  How much the angle of refraction is going to be dependent on the temperature gradient thickness and the temperature difference.  If the angle is shallow enough, relative to the invasion layer boundary, we could conceivably end up with the beam being reflected off of inversion layer. 

A less important factor would be the particulates in the smog.  I'm presuming the origins of the word are as follows: smog = smoke + fog.  It's likely that particulates in the form of fog droplets and smoke particles could have a scattering effect, but I have doubts as a result of the microwaves are much, much bigger than the particulates (roughly 2.5 to 5 cm vs. 1 to 100 microns).   H2O is an exception here because the size of a droplet can vary enormously (0.1 µm up to 4 mm). 

The only other physical effect would be the gas components (i. e., O3, HC, NOx, SOx, CO, etc.) of smog interacting with the microwaves.  Since microwaves are non-ionizing, the actions that occur at the atomic level would be essentially monkeying around with the electron bonds in the form of vibration, stretching, and rotation.  At certain frequencies, the bonds would be excited.  I'm making a SWAG here, but I assume the energy would be re-radiated at the same frequency, but in a random directions (or converted into heat). That being said, even on the worst smog day, the amount of gases in the air would likely have a negligible affect on the beam (but not on your health).  IIRC, we're talking parts per million for most of theses gases.  I can't recall even on the worst day in LA during the 80s, that RF communications were ever impaired by smog.  Perhaps some Ham radio operator who operated at these frequencies could fill in the blanks. (I've got a Ham radio license [Extra Class] but can't recall any Ham intoning, "Hmm, smoggy day...let's shut down ol' Sparky and go to the mall)."

H2O, on the other hand, can be deleterious to a link and I'll leave that for other to explore the hows and whys.