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Why storms interfere with telecommunications?

  1. Aug 19, 2012 #1
    I am not referring to solar winds, just normal adverse atmospheric conditions.
    Thank you
    Regards
     
  2. jcsd
  3. Aug 19, 2012 #2

    Bobbywhy

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    Radio interference from natural causes can come from at least four sources:

    1. Atmospheric interference; electrical storms.
    2. Solar and cosmic interference; eruptions on the sun and other stars.
    3. Precipitation static from charged particles in the atmosphere. Particles may be rain, sleet, snow, sand, and smoke or dust. Dry particles produce greater charges than wet ones.
    4. Fading from disturbances in the medium through which radio waves are propagated.

    These interferences appear in electronic equipment as audible noise or errors in the output of some terminal equipments. Atmospheric interference is “wideband”, but the higher the frequency, the less noise interference.

    You may visit this NASA site to discover experiments you can do to observe these disturbances:

    “Lightning strikes produce radio frequency interference over a broad range of frequencies. Interference from lightning discharges can be heard long distances from the storms that generate them. Because of this, RFI from lightning is very common, especially during prime thunderstorm season in your region of the World. Many services, such as news organizations, governmental agencies, and commercial ventures have data communications links that use the high-frequency (shortwave) portion of the radio spectrum.”
    http://radiojove.gsfc.nasa.gov/observing/rfi_samples.htm
     
  4. Aug 19, 2012 #3
    Thank you for your answer! Can you develop the 4th point?
    Regards
     
  5. Aug 19, 2012 #4

    Bobbywhy

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    Here are a few sentences from the Wiki page on “Ionosphere”. Be sure to check out the “See Also” section for more.

    “The ionosphere is a part of the upper atmosphere, from about 85 km to 600 km altitude, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth.”
    “Lightning can cause ionospheric perturbations in the D-region in one of two ways. The first is through VLF (Very Low Frequency) radio waves launched into the magnetosphere. These so-called "whistler" mode waves can interact with radiation belt particles and cause them to precipitate onto the ionosphere, adding ionization to the D-region. These disturbances are called "lightning-induced electron precipitation" (LEP) events. Additional ionization can also occur from direct heating/ionization as a result of huge motions of charge in lightning strikes. These events are called Early/Fast.”
    “DX communication, popular among amateur radio enthusiasts, is a term given to communication over great distances. Thanks to the property of ionized atmospheric gases to refract high frequency (HF, or shortwave) radio waves, the ionosphere can be utilized to "bounce" a transmitted signal down to ground.”
    http://en.wikipedia.org/wiki/Ionosphere
     
  6. Aug 19, 2012 #5

    davenn

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    something that Bobbywhy didnt comment on in his expansion of point 4

    and much more closer to the ground than the ionosphere is, and more directly stormy weather related

    That is fading of the signal due directly to increased pathloss caused by absorption of radio signals by rain and increased presence of water vapour in general

    This pathloss effect is primarily on the microwave frequencies 1000 MHz (1GHz) and up, which dont use the ionosphere anyway.
    Designers of microwave links between say hilltop sites have to take pathloss due to moisture into their "link budgets". If they havent taken their pathloss calculations correctly or just havent taken into account very high pathlosses due to extreme weather

    I should also comment that, because satellite comms also use microwave frequencies, they are also very susceptable to dropout during extreme weather

    Dave
     
  7. Aug 20, 2012 #6
    davenn, thank you for your answer.
    So the "disturbances in the medium through which radio waves are propagated" are related to liquid water and water vapor. What about other particles, dust? I would therefore presume that simply very strong winds (by it self) have no reason to interfere with telecommunications. Is this right?´
    Taking this opportunity let me also ask about what determines the range of a walkie-talkie?
    Regards
     
  8. Aug 20, 2012 #7

    sophiecentaur

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    The answer to this involves many different variables. Walkie talkie systems are designed for close range and to avoid putting out too much interference for other users. The frequency you choose depends upon the required application. Also, the modulation system may be a factor. Basically, you have to use 'approved', off the shelf, units to comply with the regulations.
    The wavelength of many 'walkie talkies' is round about 2m (150MHz VHF) and the best propagation is more or less 'line of sight' because this wavelength will not diffract much around obstacles or over the horizon / hills. We are talking in terms of a couple of Watts of transmitter power (hand held), I assume. For two units at ground level, the range will be only a few km over water (no obstacles) but the horizon gets further way as you elevate your antenna, between tall ship's masts or high hills, the range can extend to several tens of km (for fixed installations with powers of up to 25W). The inverse square law applies so increasing the power by a factor of 10 would increase 'free-space' range by √(10) but there's not a lot of point in using such high powers at ground height. A yacht mast is good value for increasing comms range.
    The sensitivity of the receiver and the noise generated at its input can be relevant but there is always interference to contend with and that can't be eliminated except with a directive antenna or possibly filtering (to the possible detriment of the wanted signal too).
    Despite all this, you can end up with lousy range sometimes where there is some badly placed obstacle and unbelievably long range on other occasions (briefly) when atmospheric conditions can support very long range propagation in layers of cold air.

    Other walkie talkie systems use around 450MHz. They are very short range and tend to work well inside buildings where the waves tend to rattle around well between steel frames (same as with mobile telephones) - much better than VHF systems.
    I found this link, which is quite informative.
     
  9. Aug 20, 2012 #8
    Thank you for your answer.
    Beside line of sight and obstacles, what else contributes to weaken the signal? Some kind of lower atmosphere absorption? Water vapor is a factor?
     
  10. Aug 20, 2012 #9

    sophiecentaur

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    Water vapour has little effect until you get up to microwave. The attenuation of the space wave is largely 'beam spreading' at these lower frequencies.
     
  11. Aug 21, 2012 #10
    So low walkietalkie ranges are mainly due to low power and therefore high noise to signal ratio?
     
  12. Aug 21, 2012 #11

    sophiecentaur

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    They are 'type approved' which limits the max power. That is being good neighbourly. But, in most circs, their range is limited by terrain. If you used 100W power, the range would not usually increase by much but occasional interference OUT would spoil it for everyone else.
    There are many parameters involved and it's a compromise.
    The limited range does help with security, actually.
     
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