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Polarization The orientation of the E field lines of flux is defined as the polarization of an EM wave If the E field flux lines are parallel to the earth s surface, you have horizontal polarization If the E field flux lines are perpendicular to the surface, you have vertical polarization Polarization can also be slanted, of course In some situations, the E flux lines rotate as the wave travels through space This is circular polarization if the E field intensity remains constant If the E field intensity is more intense in some planes than in others, the polarization is elliptical Rotating polarization can be clockwise or counterclockwise, viewed as the wavefronts approach The rotational direction is called the sense of polarization The Line-of-Sight Wave Electromagnetic waves follow straight lines unless something makes them bend Line-of-sight propagation can take place even when the receiving antenna cannot be seen from the transmitting antenna To some extent, radio waves penetrate nonconducting objects such as trees and frame houses The line-of-sight wave consists of two components: the direct wave and the reflected wave The direct wave: The longest wavelengths are least affected by obstructions At very low, low, and medium frequencies, direct waves can diffract around things As the frequency rises, especially above about 3 MHz, obstructions have a greater and greater blocking effect The reflected wave: Electromagnetic waves reflect from the earth s surface and from conducting objects like wires and steel beams The reflected wave always travels farther than the direct wave The two waves are usually not in phase at the receiving antenna If they re equally strong but 180 out of phase, a dead spot occurs This phenomenon is most noticeable at the highest frequencies At VHF and UHF, an improvement in reception can sometimes result from moving the transmitting or receiving antenna just a few inches In mobile operation, when the transmitter and/or receiver are moving, dead spots produce rapid, repeated interruptions in the received signal This is called picket fencing The Surface Wave At frequencies below about 10 MHz, the earth s surface conducts ac quite well Because of this, vertically polarized EM waves follow the surface for hundreds or even thousands of miles, with the earth actually helping to transmit the signals The lower the frequency, the lower the ground loss, and the farther the waves can travel by surface-wave propagation Horizontally polarized waves do not travel well in this mode, because horizontal E field flux is shorted out by the earth Above about 10 MHz, the earth becomes lossy, and surface-wave propagation is not useful for more than a few miles Sky-Wave EM Propagation Ionization in the upper atmosphere, caused by solar radiation, can return EM waves to the earth at certain frequencies The ionization takes place at three or four distinct layers The lowest ionized region is called the D layer It exists at an altitude of about 30 mi (50 km), and is ordinarily present only on the daylight side of the planet This layer absorbs radio waves at some frequencies, impeding long-distance ionospheric propagation.

sys.dm_io_*

Figure 8-7

sys.dm_qn_*

sys.dm_repl_*

The E layer, about 50 mi (80 km) above the surface, also exists mainly during the day, although nighttime ionization is sometimes observed The E layer can provide medium-range radio communication at certain frequencies The uppermost layers are called the F1 layer and the F2 layer The F1 layer, normally present only on the daylight side of the earth, forms at about 125 mi (200 km) altitude; the F2 layer exists at about 180 mi (300 km) over most, or all, of the earth Sometimes the distinction between the F1 and F2 layers is ignored, and they are spoken of together as the F layer Communication by means of F-layer propagation can usually be accomplished between any two points on the earth at some frequencies between 5 MHz and 30 MHz

8:

sys.dm_broker_*

To set the default button, add to the display dialog command the default button parameter followed by the button s name The following statement makes the One Window button the default, as you can see in Figure 8-8:

sys.dm_os_*

.

sys.dm_tran_*

display dialog "How many Word windows do you want to open " with title "Desktop Setup: Open Word Windows " buttons {"One Window", "Two Windows", "No Windows"} default button "One Window"

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