# Forum

## Rain Notes [ For CWNA Folks ]

14 posts by 6 authors in: Forums > CWNA - Enterprise Wi-Fi Admin
Last Post: June 19, 2009:
• As a side note to the posting on rain, have put down a few notes about rain for interest for CWNA folks etc. It???¡é?¡é?????¡é???¡és just stuff that falls from the sky right ? Well actually it???¡é?¡é?????¡é???¡és a bit more complicated than that, and entire books have been written on it???¡é?¡é?????¡é???¡és affect on RF signals. Manchester rain is physically different from Singapore rain [ not just because it???¡é?¡é?????¡é???¡és colder ].

When you see skydivers on TV, you usually see them in one of two positions: belly down [ horizontal ] or nose down [ vertically diving ]. You might think that the higher up the skydiver goes, the faster he will become. This is true up to a point. That point is called the terminal velocity of the object. At that point, the force of gravity pulling down on the object matches the air resistance [ sort of pushing up ]. For the horizontal skydiver, this is about 125 mph. Provided nothing else changes, he will keep that velocity no matter how much higher he jumps from.

All raindrops start off as little spheres, nice and round. However they soon become changed in shape due to air resistance and quickly reach their terminal velocity. The raindrop becomes pear shaped. It???¡é?¡é?????¡é???¡és effect on RF signals varies depending on whether the wave goes through length wise, sideways or at another angle. It???¡é?¡é?????¡é???¡és properties out of the cloud are different than when it is in the cloud.

As the droplets fall, depending upon how high up they were formed [ low clouds or high clouds ], winds at different heights [ called velocity stratification ] ???¡é?¡é?????¡­?¡°tilt???¡é?¡é???????? the droplet to different angles. These angles are called ???¡é?¡é?????¡­?¡°canting angles???¡é?¡é????????.

The RF signal will be attenuated depending on a variety of factors:

1. Droplet size
2. Canting angle
3. Pollution [ little particles in the drop causing scattering ]
Etc.

Due to the complex mechanisms involved in tropical weather patterns, droplet sizes are much larger in Singapore than Manchester, with resulting different attenuation values. You can actually hear them pinging off tin roofs.
The canting angle is also important, as it determines ???¡é?¡é?????¡­?¡°depolarization???¡é?¡é????????. We know that Wi-Fi systems can send out vertically and horizontally polarized waves. When these waves hit raindrops, some of the energy in the vertical plane gets converted to the horizontal plane and vice versa. This is energy that is lost [ a form of attenuation ]. The physical temperature of the drops adds noise energy to the signal as well. Noise temperature is an important topic and I???¡é?¡é?????¡é???¡éll put some info on that in a future post if anyone is interested [ important for long distance links ].

Frozen rain, especially hailstones can cause serious problems with scattering and attenuation.

When planning for radio links, rainfall rates are important, and are usually gathered statistically from weather offices [ The Met Office in the UK ]. They have to be carefully analyzed however. All things being equal, which would give the highest attenuation for a radio link; The Sahara Desert or Manchester ? Bit of a trick question. The answer is the Sahara Desert. Over a year, the average rainfall rate is tiny, as it may only rain for a few days each year. However when it does rain, it comes down in buckets with rainfall rates [ mm/hour ] much , much higher than Manchester. The important factor is the amount of rain per cubic meter of air. This could mean complete blockage of all service for a whole day on long-distance microwave links at certain frequencies.[ as some have found to their expense in normally dry areas ???¡é?¡é?????¡é?€?? big oil companies etc]. If data is critical, some links have to be designed for worst case PER DAY or peak rainfall rates. Beware average and peak.

C-Band satellite communications works in the range of 3.625 or 3.7 to 4.2 Ghz on the downlink [ from the satellite ]. In the same ball park as 802.11a. I was called once to troubleshoot a satellite link in the tropics where the signal was dropping much, much more than calculations predicted. They had replaced every piece of equipment with new gear but to no avail. I was baffled until I took a look at the low elevation angle of the parabolic dish. It was white painted and when it rained, you couldn???¡é?¡é?????¡é???¡ét see very much. That night when it was raining, I took a flashlight and saw the problem. The rain literally couldn???¡é?¡é?????¡é???¡ét get off the dish fast enough. A thin sheet of water was absorbing the signal, scattering it and adding noise. Three quick holes with the Black and Decker and the network went back to behaving itself as calculations predicted.

On CWNA, we learn about the effects of possible interference from microwave ovens. A popular misconception is that these ovens heat up the food by vibrating the water molecules. It???¡é?¡é?????¡é???¡és actually another mechanism. Water vapor [ H2O ] actually resonates at 22.5 Ghz, well away from the 2.4 Ghz or so used by commercial ovens.

Although at 4 ???¡é?¡é?????¡é?€?? 6 Ghz frequencies rainfall attenuation is ???¡é?¡é?????¡­?¡°not too bad???¡é?¡é???????? [ a few db usually at most ] at higher frequencies getting up towards the water vapor attenuation point, attenuation goes up at a very high rate. With many of the WMAN???¡é?¡é?????¡é???¡és coming into play, there is a move to higher frequencies such as 20 and 30 Ghz. Knowing the effects of rain on RF becomes extremely important.

As RF signals get higher in frequency, the signal running down the co-ax does a strange thing. Instead of being distributed equally, it tends to ???¡é?¡é?????¡­?¡°collect???¡é?¡é???????? on the outer surfaces. This is known as ???¡é?¡é?????¡­?¡°skin effect???¡é?¡é????????.

If adequate waterproofing is not provided, water moisture can get into the cable, affect these signals and attenuate them severely.

WATERPROOF, WATERPROOF, WATERPROOF.

This, grounding and proper lightning/surge protection are very important for outdoor links.

• Dave,

I'm not sure where you came from, but I vote you as poster of the month. :) Thanks a lot (again) for the great information. I knew a little bit of what you had in that post, but much I didn't know.

I've read some excerpts of a book that talked about how rain increases in temperature when it passes through an RF path.

Again, thanks for the great information!

GT

• Thank you

• By (Deleted User)

Yes, Dave, I concur with Gene. And, in the immortal words of Inigo Montoya, "who are you?"

• kevinsandlin Escribi?3:

Yes, Dave, I concur with Gene. And, in the immortal words of Inigo Montoya, "who are you?"

Classic. :)

GT

• By (Deleted User)

Dave,

I have to concur with Gene and Kevin - poster of the month. :)

Cary and I met Dave and his wife at the Ft. Lauderdale road show event, and hopefully he's going to be applying for instructor status soon. I think I can hear his thick Scottish accent while reading his forum posts! :D

Thanks again for the great information Dave. I would like to hear more about how microwave ovens work. You never finished that thought.

Devinator

• Hi Devin

In the region of 22.5 Ghz the water molecules resonate. The vibrate and absorb discrete amounts of energy "sucking" energy out of the radio wave. That is bad enough, but they then spew some of that energy back out in a sort of random manner.
That random energy comes out as noise, so the signal level decreases and the noise level increases.

As Oliver Twist would say "'orrible, bloody 'orrible!!"

At the 2.4 or so Ghz frequency of most ovens, the water molecules get sort of flipped back and forth [ not "excited" like at 22.5 Ghz ].

This is a little tutorial that I really like. It looks terrible on the first page but is really quite good at explaining a complex issue. If you jump to the "microwaves and water" section, it gives some neat animations.

For anyone with insomnia or who needs punishing, this section explains a bit more about the resonance phenomena:

Cheers

Dave

• Very nice post, Dave.

I have experience, working with stabilized antennas on yachts, where we lose a signal due to rain fade, for the very reasons outlined above.

When the crew tell us "but it's sunny here, not a cloud in the sky..." sometimes is difficult to get them to understand it's raining at the earth station, the other side of the link.

Anyhow, great post, Dave - are you based in Florida ?

Good luck with converting to CWNT, I love teaching these courses, I am sure you will too.

Neil Mac
CWNT

• 8-O ;-) :( :'(

Bugger me, when you put that context to 'rain' it makes you think a little more about not just getting wet, but dont stand under a radio link either!!!

Solution : Big tarpolin :D

Seriously though, where this link is, it rains more often than not and with the extreme weather we are experiencing it concerns me about the link long term viability...

I can confirm that when the links dropped, the rais was 'BOUNCING' off the ground.

Has anyone else experienced these issues in the UK?

• bigcontractboy Escribi?3:

8-O ;-) :( :'(

Bugger me, when you put that context to 'rain' it makes you think a little more about not just getting wet, but dont stand under a radio link either!!!

Solution : Big tarpolin :D

Seriously though, where this link is, it rains more often than not and with the extreme weather we are experiencing it concerns me about the link long term viability...

I can confirm that when the links dropped, the rais was 'BOUNCING' off the ground.

Has anyone else experienced these issues in the UK?

Didn't you say that one of your failing links were 100 meters? I will say this; If you have links under a mile, I cannot imagine rain failing that link at 5 GHz. Dave is of course the true expert, but I have put up a lot of wireless shots of great distance and have never lost a link due to rain.

GT

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