birt code 39

birt code 39

birt code 39

birt code 39

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Tropospheric Propagation At frequencies above about 30 MHz, the lower atmosphere bends radio waves toward the surface Tropospheric banding occurs because the index of refraction of air, with respect to EM waves, decreases with altitude The effect is similar to the way sound waves sometimes travel long distances over the surface of a calm lake in the early morning or early evening, letting you hear a conversation more than a mile away Tropospheric propagation makes it possible to communicate for hundreds of miles when the ionosphere will not return waves to the earth Another type of tropospheric propagation is called ducting It takes place when EM waves are trapped in a layer of cool, dense air sandwiched between two layers of warmer air Like bending, ducting occurs almost entirely at frequencies above 30 MHz Still another tropospheric-propagation mode is known as troposcatter This takes place because air molecules, dust grains, and water droplets scatter some of the EM field This effect is commonly seen at VHF and UHF Tropospheric propagation in general, without mention of the specific mode, is sometimes called tropo Auroral Propagation In the presence of unusual solar activity, the aurora (northern lights or southern lights) can return radio waves to the earth This is known as auroral propagation The aurora occur at altitudes of about 40 to 250 mi (65 to 400 km) Theoretically, auroral propagation is possible, when the aurora are active, between any two points on the surface from which the same part of the aurora lie on a line of sight Auroral propagation seldom occurs when one end of the circuit is at a latitude less than 35 north or south of the equator Auroral propagation is characterized by rapid and deep fading This almost always renders analog voice and video signals unintelligible Digital modes are most effective for communication via auroral propagation, but the carrier is often spread out over several hundred hertz as a result of phase modulation induced by auroral motion This severely limits the maximum data transfer rate Auroral propagation is often accompanied by deterioration in ionospheric propagation Meteor Scatter Meteors produce ionized trails that persist for approximately 05 s up to several seconds, depending on the size of a particular meteor, its speed, and the angle at which it enters the atmosphere This is not enough time for the transmission of much data, but during a meteor shower, multiple trails can result in almost continuous ionization for a period of hours Such ionized regions reflect radio waves at certain frequencies This is meteor scatter propagation It can take place at frequencies considerably.

if (windowXMLHttpRequest) { XMLHttpRequestObject = new XMLHttpRequest(); } else if (windowActiveXObject) { XMLHttpRequestObject = new ActiveXObject("MicrosoftXMLHTTP"); } }

above 30 MHz, and occurs over distances ranging from just beyond the horizon up to about 1500 mi (2400 km), depending on the altitude of the ionized trail and the relative positions of the trail, the transmitting station, and the receiving station

And then we check if we were successful in creating that object:

Moonbounce The moon, like the earth, reflects EM fields This makes it possible to communicate by means of earth-moon-earth (EME), also called moonbounce High-powered transmitters, sophisticated antenna systems, and sensitive receivers are needed for EME Some moonbounce communication is done by radio amateurs at frequencies from 50 MHz to over 2 GHz

function postDataDownloadText(url, dataToSend, callbackFunction) { var XMLHttpRequestObject = false; if (windowXMLHttpRequest) { XMLHttpRequestObject = new XMLHttpRequest(); } else if (windowActiveXObject) { XMLHttpRequestObject = new ActiveXObject("MicrosoftXMLHTTP"); } if(XMLHttpRequestObject) { } }

Careers in Programming Figure 31 Systems Programming Career Path Figure 32 Applications Programming Career Path

And if the XMLHttpRequest object exists, we open it, configuring it with the URL to access, and telling it to use the POST method:

Data can be transmitted over various media The most common are cable, radio (also called wireless), satellite links (a specialized form of wireless), and fiber optics Cable, radio/TV, and satellite communications use the RF spectrum Fiber optics uses IR or visible light energy

function postDataDownloadText(url, dataToSend, callbackFunction) { var XMLHttpRequestObject = false; if (windowXMLHttpRequest) { XMLHttpRequestObject = new XMLHttpRequest(); } else if (windowActiveXObject) { XMLHttpRequestObject = new ActiveXObject("MicrosoftXMLHTTP"); } if(XMLHttpRequestObject) { XMLHttpRequestObjectopen("POST", url); } }

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We also set the Content-Type HTTP header to "application/x-www-form-urlencoded", as discussed in 3, when you use POST: