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5 GHz and environmental moisture

joshuabardwell

Senior Member
Mentor
#1
In another thread, somebody wrote:

5.8ghz has some trouble penetrating clouds
I see this claim a lot, and I wanted to address it. FWIW, I'm a professional RF engineer, with experience designing, installing, and troubleshooting indoor and outdoor wireless systems, focusing primarily in the 2.4 and 5 GHz bands, but also other bands such as 24 GHz, 60 GHz, and 80 GHz.

First of all, let's take a look at this chart:

AR_4772_Fig02_L.jpg

This chart shows average atmospheric absorption of millimeter wave signals, under some nominal assumptions for environmental moisture, altitude, etc... The chart only goes down to 10 GHz, but it can be assumed that lower frequency signals have less-than-or-equal-to absorption showed in this chart. We can see that absorption generally goes up as frequency increases. There are a few places where there is an absorption peak, such as the oxygen peak at 60 GHz, but significant peaks such as these don't occur below 10 GHz.

The chart shows moisture for between 7.5 g/m^3 of water and 1 g/m^3. What kind of real-world conditions does this represent? A quick search on Wikipedia (reference) shows that a cloud may have between about 0.03 and 3 g/m^3 of moisture. A light cirrus cloud represents a moisture value even less than the "B" line on the graph. The "A" line on the graph represents more moisture than Wikipedia says a cloud can typically hold, so we assume that 7 g/m^3 represents unusual conditions.

Based on this, we can say that with light clouds, you could expect less than 0.004 dB/km of environmental attenuation. With heavy clouds, you could expect less than about 0.01 dB/km. Suffice it to say, these are completely negligible values over the distances most RC flyers use.

I know that some people will (validly) criticize the fact that the chart above doesn't actually show 2.4 and 5 GHz. So here is another chart that shows rain fade attenuation, and that includes down to 1 GHz.

get.jpg

The difference between this chart and the above one is that this chart is JUST the attenuation due to the rain, not the cumulative effect of atmospheric attenuation and moisture. In other words, if it were raining, you would add this chart's result to the above chart's result. That's why this chart doesn't have absorption peaks.

Notice that the Y axis starts at 0.1 dB/km, and even under "worse than monsoon" conditions (200 mm/hr), we don't hit that level until just before 4 GHz. Under "drizzle" conditions (0.25 mm/hr), we don't hit that level until between 12 and 13 GHz. Again, 0.1 dB/km is completely negligible under the conditions RC pilots usually fly. And since RC pilots seldom fly when it is actually raining, the environmental moisture absorption will be substantially less than even this minuscule number. Realistically, fluctuations in the environmental noise floor will probably cause more than 0.1 dB/km of difference in your SNR, making such small changes in your signal strength invisible.

In short, there is no truth to the claim that 5 GHz signals (as used by RC pilots) are noticeably affected by environmental moisture.
 
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Tritium

Amateur Extra Class K5TWM
#2
2.45GHz is just right for exciting a water molecule into rapid rotation and torsional stress. (Microwave oven). Resonance with a water molecule is in the Terahertz range.

Thurmond
 
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joshuabardwell

Senior Member
Mentor
#3
2.45GHz is just right for exciting a water molecule. (Microwave oven).
It's true, but the dB/km is still very small. Bear in mind that microwaves operate at something like 600-1200 watts. (EDIT: Input power. Typical efficiency around 65%, for output power of around 400 to 800 watts.) They need this massive power output in order to put enough energy into the water to heat it up specifically because absorption is relatively low even at 2.4 GHz. And it is exciting liquid water, which has many, many more molecules per unit volume compared to water vapor. Also, the oven is designed to create standing waves, which amplify the effect of the absorption; RF transmitters don't do this.
 
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#4
In another thread, somebody wrote:



I see this claim a lot, and I wanted to address it. FWIW, I'm a professional RF engineer, with experience designing, installing, and troubleshooting indoor and outdoor wireless systems, focusing primarily in the 2.4 and 5 GHz bands, but also other bands such as 24 GHz, 60 GHz, and 80 GHz.

First of all, let's take a look at this chart:

View attachment 36164

This chart shows average atmospheric absorption of millimeter wave signals, under some nominal assumptions for environmental moisture, altitude, etc... The chart only goes down to 10 GHz, but it can be assumed that lower frequency signals have less-than-or-equal-to absorption showed in this chart. We can see that absorption generally goes up as frequency increases. There are a few places where there is an absorption peak, such as the oxygen peak at 60 GHz, but significant peaks such as these don't occur below 10 GHz.

The chart shows moisture for between 7.5 g/m^3 of water and 1 g/m^3. What kind of real-world conditions does this represent? A quick search on Wikipedia (reference) shows that a cloud may have between about 0.03 and 3 g/m^3 of moisture. A light cirrus cloud represents a moisture value even less than the "B" line on the graph. The "A" line on the graph represents more moisture than Wikipedia says a cloud can typically hold, so we assume that 7 g/m^3 represents unusual conditions.

Based on this, we can say that with light clouds, you could expect less than 0.004 dB/km of environmental attenuation. With heavy clouds, you could expect less than about 0.01 dB/km. Suffice it to say, these are completely negligible values over the distances most RC flyers use.

I know that some people will (validly) criticize the fact that the chart above doesn't actually show 2.4 and 5 GHz. So here is another chart that shows rain fade attenuation, and that includes down to 1 GHz.

View attachment 36165

The difference between this chart and the above one is that this chart is JUST the attenuation due to the rain, not the cumulative effect of atmospheric attenuation and moisture. In other words, if it were raining, you would add this chart's result to the above chart's result. That's why this chart doesn't have absorption peaks.

Notice that the Y axis starts at 0.1 dB/km, and even under "worse than monsoon" conditions (200 mm/hr), we don't hit that level until just before 4 GHz. Under "drizzle" conditions (0.25 mm/hr), we don't hit that level until between 12 and 13 GHz. Again, 0.1 dB/km is completely negligible under the conditions RC pilots usually fly. And since RC pilots seldom fly when it is actually raining, the environmental moisture absorption will be substantially less than even this minuscule number. Realistically, fluctuations in the environmental noise floor will probably cause more than 0.1 dB/km of difference in your SNR, making such small changes in your signal strength invisible.

In short, there is no truth to the claim that 5 GHz signals (as used by RC pilots) are noticeably affected by environmental moisture.
Your analysis lacks the impacts of the exceedingly low signal strengths imposed by low ERP and horribly off axis antenna alignment. I fly FPV almost exclusively and have experienced the described fades that are coincident with wind gusts of high humidity air rolling off of the adjacent lake at our airfield. While moisture alone may not be attributable to the observed effects the thermal interfaces in colliding air masses can and do create reflective boundaries at high frequencies.