Weather, or not?

30 posts by 4 authors in: Forums > CWNA - Enterprise Wi-Fi Admin
Last Post: July 5, 2009:
  • Hi CP

    Glad to see you're making progress. One other. problem that can occur is that very often in a city environment, the rain is slightly acidic [ from sulphurous emissions etc ]. When that rain gets into a cable which uses cheap connectors, the acidity can "eat" into the cheap outer covering and start affecting the interior wire. One of the many reasons to use the highest quality connectors possible. The technicians should make sure that the cable/connector assembly is perfectly dry before waterproofing as otherwise mositure can become trapped within the nice new waterproof "jacket".


  • Hi Dave

    Thanks for the advise.

    All the connectors are dry and didnt show signs of corrosion, however that may be because it has only been in a couple of months or the corrosion is very subtle. I did wipe all the connectors and the waterproofing was done when it was dry and about 28 degrees :-)

    Just had a call from a potential client who has an alvarion 11b point to point link (8+ years old) and has recently stoped worng in the afternoon when the sun is its warmest. The link is only around 400M and mounted on the school roof at both ends.
    They are looking at replacing it anyway as the 4Mb or so they get isnt really enough and leased lines are just too expensive. However I would like to know before hand that the issues arent likely toaffect any new link we may install!

    Other that the logs, any ideas of where to start looking?


  • Hi CP

    That's an unusual one. Usually it takes extreme heat to affect a link.I've seen it a few times on long-distance microwave links in the desert. But that's when you can actually "see" the stratification of the air [ you see objects "dancing" - appearing to move ]. You can see this on asphalt [ tarmacadam ] roads in the South in the US and elsewhere on very hot days. You can see the RSSI bouncing up and down on the microwave receiver. On a short link in the UK and with the degree [ or lack of ] heat there it is not very likely.

    Another thing that can happen is when outdoor electronics have a problem with [ say ] a badly soldered joint. Thermal expansion and contraction can cause problems.

    If the outdoor electronics are not "speced" for very high temperatures, problems can occur [ but I presume your kit is all indoor mounted except for cable and antenna ? ].Some electronic systems are designed to Shut down when the temperature gets too high. Occasionaly, an office building with shiny window coverings can reflect sunlight directly onto equipment, but usually that only increases the temperature a bit, and then drops as the sun moves on.

    Thermal expansion can also cause some connectors to "creep" away from each other [ especially if you have different metals in the connectors with different co-efficients of expansion. Again, all of this stuff I have seen before, but only in places where the temperatures were very high indeed - jungle, desert etc.

    Nowadays with money and travel/accommodation being a concern, you usually can't do this, but we used to always do a 24 hour stability test where we attached a recorder to a receiver and recorded the RSSI over a 24 hour period. Some software does the smae thing.

    Those stability tests were very useful indeed and gave us a "baseline" to compare potential future problems with.

    Let us know if the CCTV system remains stable after the waterproofing.


  • An interesting point for those using satellite TV. At C-Band frequencies [ about 3.7 - 4.2 Ghz coming down from the satellite ] , providing the dish has been set up properly and you are not at the edge of coverage, there are rarely outages from rainstorms.

    However, for those with Ku Band dishes [ 11/12 Ghz roughly ] you know that rain can "wipe out" reception almost completely.

    C-Band downlink frequencies are very close to the 5 Ghz frequencies we use in 802.11.

    Not a scientific comparison, but just something for you to "get a feel" of rain effects on 802.11.

    With an 802.11 5 Ghz link, correctly pointed with good clear-sky signal conditions and all connectors well water-proofed, if you get massive drops in signal level then it is NOT coming from rain ATTENUATION.

    If you have an antenna that is not quite pointed right, or on a sidelobe [ yes, it does happen, and some people wind up the power or put an illegal in-line amplifier to compensate - very, very bad idea for other neighbours ], when it rains, that signal level can drop down quite a bit. Similarly with an antenna with loose fittings on the support or a support post that is too thin, sway or twist can happen in heavy wind conditions, playing havoc with received signal strength.


  • Bye the way, not all waterproofing methods were created equally.

    This is a useful link:

    Without going into all the details, silicon gel can leave little gaps in some areas, which can either let water in or trap moisture. I have used a magnifying glass before to check this on some installations, and you could actually see small bubbles and areas where the gel was not in contact with the cable connector etc. Gel is a "no-no".

    Taping is the best method. Take your time, make sure that good even pressure is applied whilst laying the tape down. Most "botched" jobs are due to people rushing. I've taped hundreds of joints and have never had a problem of water ingress reported. This was entirely due to doing each connection slowly and carefully. Even if there were deadlines to be reached, I never rushed that part of the job.

    As far as heat-shrink is concerned, it CAN be used successfully, but it requires a lot of care and expertise. People usually end up taping over the heat-shrink at the end anyway.

    One big problem is that many heat-guns can really give out intense blasts of heat, which can actually start to soften and sometimes melt the plastic of the cable.


  • Speaking of cheap connectors etc, beware of cheap waterproofing tape.

    The sun emits all types of radiation from x-rays to C-Band frequencies [ This causes a problem twice a year in satellite links called "sun interference", where due to the rotation of the earth around the sun at a tilt angle, the sun, satellite station and the satellite are all "in a line". Communications can be affected or completely blocked by this .

    Pop quiz for CWNA Folks ???¡é?¡é?????¡é?€?? If the sun emits C-band frequencies which cause problems with satellite stations, why doesn???¡é?¡é?????¡é???¡ét it cause problems with 802.11 links ? [ The sun emits noise at 5 Ghz and 2.4 Ghz as well many many other Gigaherzesssses ???¡é?¡é???????|.. ]

    The thing that concerns us in the 802.11 world is the fact that the sun emits ultra-violet rays [ the same rays that cause health problems when too much exposure occurs ]. These rays are very powerful and react with the chemical compounds in the plastics [ such as polymers ] that make up the tape. What happens over time is that the tape becomes brittle and liable to crack, hence allowing water to come in. Cheap tapes will do this much more rapidly than good quality tapes.

    Even if good tape is used, a visual inspection of all outdoor weatherproofing connectors should be done at least once a year [ more often if the sun is very intense as in tropical climates ]. You should not only look, but also feel the tape. Is it still slightly pliable, or is it hard ? Are there any cracks in the tape ? Have the ends of the tape started to come loose ?

    Rodents like to chew on cables, and many a link has been hit by this, especially where the cables feed through plenums into a downstairs area. If this is a problem, there are chemical compounds that can be applied which helps repel them. They mainly chew on the cables [ at the copper ] to wear down their teeth [ which grow constantly ].

    Sharks can cause big problems with undersea cables. The first company I worked for owned half the world???¡é?¡é?????¡é???¡és undersea cables and some of their cables had ???¡é?¡é?????¡­?¡°shark tape???¡é?¡é???????? ???¡é?¡é?????¡é?€?? a thick copper shield around the main cable body.


  • Although rats chewing through cable may seem amusing, they can cause huge problems. I have seen several radio links which went off air completely [ carrying very large amounts of customer data ] due to chewing. If they have only partly chewed through the cable, they can cause intermittent loss [ retries, high BER etc ].


  • dave1234 Escribi?3:

    Pop quiz for CWNA Folks ???¡é?¡é?????¡é?€?? If the sun emits C-band frequencies which cause problems with satellite stations, why doesn???¡é?¡é?????¡é???¡ét it cause problems with 802.11 links ? [ The sun emits noise at 5 Ghz and 2.4 Ghz as well many many other Gigaherzesssses ???¡é?¡é???????|.. ]


    Because 802.11 links are usually inside buildings or are point to point, not pointed towards the sun, thus reducing the amount of interference actually received?


  • Hi Grant

    Yes, that is correct. The satellite antenna is normally a parabolic antenna. For Sun Interference to occur, all three must be lined up. Sun, Satellite and ground antenna. Most parabolic dishes outside point pretty much horizontally. However you do get some antennas pointing upwards to "Hit" a brother antenna perhaps on a hill top or tall building.

    As you go further north or south of the equator, the satellite dish elevation angle becomes quite low. There have been a few cases of "tilted" parabolic antennas for radio links in Northern Latitudes getting hit with sun interference for a short period and nobody could figure out where the burst of noise came from !!

    The noise levels are not strong enough to cause indoor problems for 802.11.

    I'll put a post on parabolic antennas later on. They're quite important to understand for many 802.11 and 802.16 etc links.

    Here???¡é?¡é?????¡é???¡és a picture of the sort of parabolic antenna I used to work with. They were 105 feet in diameter and weighed about 300 tons. This one is in Italy:


  • Here???¡é?¡é?????¡é???¡és someone setting fire to a piece of wood just using the sun???¡é?¡é?????¡é???¡és rays and a parabolic reflector. I???¡é?¡é?????¡é???¡éll cover the background theory later.

    The French can melt steel with their parabolic mirror assemblies in the pyranees.


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