Electromagnetic waves and Communication Theory: Facts and Fallacies
by Sanjay Mishra
I often run into interesting facts and fallacies about Electromagnetic waves and communication theory. Here are a few of them collected for your enjoyment.
Current is a flow of electrons - Wrong! This is a gross simplification taught in elementary Electrical Engineering courses. This simplification works great where the circuit elements are small in comparison to electric waves but fails for RF and high speed electronics. Electromagnetic energy is carried as electromangetic waves between wires or between wires and the ground plane. Once we get this picture in our heads a lot of things become easier- electromagnetic coupling, high speed PCB design, and RF device designn.
The maximum data rate is half the bandwidth - Wrong! I have actually had many experienced engineers tell me this. This is actually a misunderstanding of Nyquist Shannon theorem. The maximum symbol rate is half the bandwidth. Each symbol can encode multiple bits of data. The symbol rate is known as the baud rate while the bit rate is known as the bit rate.
For example a V.34 telephone line modem may operate at a baud rate 3420 bauds and each symbol can encode 10 bits resulting in a gross data rate of 34,200 bits per second. The net data rate after accounting for physical layer overhead is 33,000 bits per second.
Microwave ovens work because water resonates at electromagnetic waves at 2.4 GHz - Wrong! Microwave ovens are so efficient because they heat the entire volume of food. A conventional oven heats only the surface of the food and heat has to gradually transfer inside the food for the food to cook. Microwave overs are able to heat this volume of food because 2.4 GHz electromagnetic radiation penetrates inside water at 2.4 GHz to a depth (for a more accurate description look up skin depth) of 0.5 to 4.7 cm depending on the salt content.
We see in the visible spectrum because our eyes are water based and water is transparent only in the visible spectrum. Water becomes opaque very rapidly on either side of the visible spectrum.
Our eyes detect light in a non coherent fashion They use only the amplitude of the light received. Our eyes are in effect photon counters. Fiber optics use coherent detection of lasers to make high bandwidth communication possible
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