Figure 4.20 The Field Pattern for a Half-Wavelength Dipole Antenna

Now, if you take a moment to contemplate the two equations presented above (4.19 and 4.20) youʼll see that the energy density (E, which is given in Watts per square meter, W/m2) is calculated based only on the trigonometric functions sine and cosine (with a Pi thrown in because there is spherical propagation taking place). Unless you have some background with electromagnetism (or some other physics that uses lots of trigonometric functions) you might find this odd. “How,” you might wonder, “can stuff that relates to triangles become units of W/m2?”

Earlier, when we discussed the basic field equation, we talked about a “unit vector”. An amazing thing happens in a physics equation when the length of a vectorʼs arrow is set to the value “one”.

Use of the Unit Vector

In the equations for electromagnetism we find the “unit vector” commonly used. This is because the “arrow” of a vector forms the hypotenuse of a right triangle relative to the axes of the coordinate system. Because the hypotenuse of the triangle in question is given as 1, the sine is equal to the length of the side opposite the angle and the cosine is equal to the length of the side adjacent to the angle. You can see this if you look at the basic trigonometric relationships. With the hypotenuse arbitrarily set to 1, the trigonometric relationships between the sine and cosine become representations of values on the x-, y-, or z-axis. When you see a reference to a sine (“SIN”), for example, youʼre seeing a representation of the y-axis value; cosine (“COS”) represents the x-axis value. Consequently, when the unit vector is used as the hypotenuse, the SIN and COS values can be treated as if they represented a particular unit of measurement (like W/m2 (watts per square meter), E, H, or anything else).

802.11 Site Considerations Related to Beamwidth

As an 802.11 network designer, itʼs important to be aware of each antennaʼs beamwidth. Itʼs also important to not get overly excited just because some part of the floor plan is outside the angle specified for the beamwidth. Remember that the beamwidth angle is always presented as the HalfPower Beamwidth (HPBW) and that means there will be signal coverage suitable for communication outside the specified beamwidth angle. It will simply be less than the signal at the limits of the HPBW.

An 802.11 radio lowers its data rate when itʼs not able to transmit at its maximum rate (11 Mb/sec for 802.11b and 54 Mbits/sec for 802.11a and g). The details of how bits are encoded in the 802.11 RF modulated signal is outside the scope of this discussion. Nonetheless, you should know that a single bit can be represented by an RF analog pattern that is of a longer or shorter duration. The shorter duration encoded bits result in a higher data transmission speed. The longer duration bits result in the data transmission speed being reduced. The transmitting radio determines whether or not it is capable

Copyright 2003 - Joseph Bardwell