Underwater electromagnetic communications

Underwater electromagnetic communications Underwater communication using an em-wave:- Underwater electromagnetic communications have been investigated since the very early days of radio communication and again received considerable attention during the 1970s.in those days only the extremely low frequency (ELF) submarine communications system is believed to be the only successfully devolved subsea electromagnetic application. Its implementation was only one way communication to call individual at surface of higher bandwidth using terrestrial radio. Under water networking is a rather unexplored area although underwater communications have been experimented since 2nd world war. In 1945 an underwater telephone was developed in the United States to communicate with submarines. And also underwater sensors and nodes must possess self configuration capabilities like should b able to coordinate their operation by exchanging configuration, location and movement information, and relay monitored data to an onshore station. Water conductivity:- Water in its pure form is an insulator, but as found in its natural state, it contains dissolved salts and other matter which makes it a partial conductor, as so in case of the sea water its becomes more conductive medium than normal water. The higher its conductivity, the greater the attenuation of radio signals which pass through it. As the attenuation of the signal increases the signal strength decreases and its leads to difficulties in propagation of the signal. Conductivity of the water varies both with the salinity and temperature. Average conductivity of the sea is normally considered to be about 4 mhos per meter. What this means that one meter cube of sea water has a conductivity of 4 mhos or a resistance of 0.25 ohm, (its reciprocal). Attenuation:- Attenuation of radio waves in water (any conducting medium) increases both with increase in conductivity and increase in frequency. It can be calculated from the following formula: Attenuation (ÃŽ ±) in db/meter = 0.0173√ (fÏÆ') Where f = frequency in hertz And ÏÆ' = conductivity in mhos/metre From the figure below states Attenuation as a function of frequency for conductive saline water verses normal water .attenuation in saline or conductive water is very high. It is necessary to use very low frequencies (10 kHz -200 MHz) where attenuation would be very low as in order of 3.5 to 10db per metre. Underwater attenuation (VS) frequency Losses at the surface due to refraction or interfaces:- As the em waves had pass through the two different medium like water and air they undergo refraction when the wave travels from the air medium to the water medium and this are also important so they can be calculated as by the formula Refraction loss (db) = 20 log {(7.4586/106) x √ (f/ÏÆ')} As this losses plays a vital role in the performance of the system to avoid such looses we can connect an antenna to the terrestrial system hanging it into the water and could reduce the loose and in sea water this looses would be high and in the vicinity 60db for lower frequency. As compared in the sea and normal water the loss decrease with increase in frequency in the figure below we would illustrate the graph between the sea and normal related with the frequency. Air to water looses are very less in the normal water it would as lees as 27db and when in sea water it would be normally between 60 to 70 db. Wavelength in water:- The wavelength of the wave signal transmitted wouldnt be same in the water as would be in the air it would a fraction of the original expected signal as shown below Wavelength (ÃŽ ») in meters = 1000 √ {10 / (fÏÆ')} And hence taking all this effects on the wave due to the underwater communication. The communication through the underwater had been developing many process and methods to develop to communication underwater. either using the sound signal or the light signal as they the sound and light are used as means of communication in various fields and hence the method that is used with the sound signal is the acoustic communication and that used after the light is the optical communication they have got their own advantages and disadvantages due to several factors influencing them directly or indirectly. As to over the defects of each the other came into existence and the acoustic and the optical methods of communication would be referred later in our discussion and the latest and upcoming means of underwater communication is the electromagnetic wave propagation underwater. http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=arnumber=1353478isnumber=29739 http://www.docstoc.com/docs/2867858/RF-Communications-under-water http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5158896 http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=arnumber=5158896isnumber=5158801tag=1 http://delivery.acm.org/10.1145/1350000/1347372/p23-partan.pdf?key1=1347372key2=7686568521coll=GUIDEdl=GUIDECFID=56483254CFTOKEN=36576706 http://its-nl.org/doc/OIL__GAS_WELLS_RF_PROPAGATION_1st_Edition.pdf http://www.docstoc.com/docs/12533584/Introduction-to-RF-propagation As electromagnetic propagation through water is very different from propagation through air because of waves high permittivity and electrical conductivity like plane wave attenuation is high compared to air and increases rapidly with frequency Through water, full bandwidth, low range, analogue voice communication was found to be impractical and so rapidly it developed an opinion as electromagnetic signals had no application in the underwater environment Re-evolution of em capabilities:- As both the acoustic and optical wireless subsea systems have the disadvantages and so the electromagnetic wave subsea system comes into existence. As the electromagnetic signaling transmission using different mechanism. Many of the investigation had proved that electromagnetic signaling coupled with digital technology and signal compression techniques would provide many advantages which can be suitable for underwater application. As the electromagnetic waves would be more attenuated in water than that of the air. Conductivity of the sea water would be 4s/m were in fresh water its mille-s/m range comparatively less than the sea water. Losses in the water are mainly due to the effect of conduction on the conductive field component. The below parameters shows the variation frequency parameters in fresh and sea water in conductivity. Water relative permeability = Â µ = 1 Water relative permittivity = ÃŽ µ = 80 Sea water conductivity (typ) = ÏÆ' = 4 s/m Fresh water conductivity (typ) = ÏÆ' = 0.01 s/m

Black Men and Public Spaces Essay

The name of the story is Black Men and Public Spaces. It was written by Brent Staples in 1986. Brent Staples is a black man who whenever in public is met with fear from others because of his race’s stereotype. Feeling like a potential perpetrator can be just as unnerving as feeling like a potential victim. Staples has the ability to alter public space by his physical behavior, his dress and his verbal reaction. Black men have a reputation of being rapist, murders and gang members, therefore many people get worried when a young black man is around. So Staples has learned how to change his perception or level of threat to others. He accomplishes this by his physical behavior. The way he walks or acts around others can change there mood. Now Staples notices if people are frightened by him. Brent Staples attempts to introduce people to something most all are guilty of, but pay little attention to. Using accounts from his own and others’ experiences, Staples essay portrays the racist tendency of people to assume black men are potentially violent and dangerous. Stereotypes are so common in our world today and people don’t even notice how bad they portray it. I hate seeing that a man had to force himself to change the way he acts and portrays himself in public. Why should someone change their ways? It made me think of a time when I was getting gas on Green River Road near Washington Avenue. I was almost done when a car with two black men pulled up and stepped out of their car. I was nervous so I stopped and got into my car real quick. Now that I think about it those men weren’t doing anything, just grabbing some late night food. How could I be that bad of a person? I know now that was wrong of me and I’m working on it. I hope someday society will realize what they are doing and change it. Not just for their sake but for mankind. Staples was viewed as a rapist, murder, and etcetera, all because he was a black man. But in reality he was just as uncomfortable as everyone who looked at him differently. Brent Staples tries to make people aware of what they are guilty of and someday all will realize.

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Why British Men Enlisted in the British Army in 1914 Essay

Why British Men Enlisted in the British Army in 1914 When Arch Duke Franz Ferdinand was assassinated, Europe erupted. The Austro-Hungarian Empire invaded Serbia; Russia sends troops to its borders with Austria and Hungary. Germany declares war on Russia and France and marches into Belgium and Britain declares war with Germany and her allies. Although Britain already had a well-trained professional army, it was far too small to be able to stand a chance against the huge German army that was more than three times its size. As a result of this Lord Kitchener immediately went on a recruiting campaign calling on all men aged between 19 and 30 to enlist in the British Army. There was a huge response,†¦show more content†¦Posters were very effective and created a picture of Germans who would run their bayonets through women and rape them. Lord Derby had the idea of pals battalions, these were battalions of people who volunteered to join the army in the same area. This scheme inspired local and civic pride, this meant that soldiers would not just be fighting for the British Empire, but for their town or city and friend and family. This was particularly effective as it encouraged friend to enlist at the same time, if one was left behind there would be no one they new. It had a similar effect as peer pressure - if you did not join up you considered a coward by your friends. This worked well in North England and Scotland, where places like Hull raised four regiments and three regiments from Liverpool went over the top of the trenches on the first day. Women too played a vital role in the recruitment process. The Order of the White Feather was a group that encouraged women to hand out white feathers to men not in uniform in public as a sign of cowardice - the white feather being a symbol a coward. This really pushed men to go as they would not want to be seen as a coward in public. 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