Your microwave oven & my satellite system are cousins

A microwave ovenThe same principle that makes your microwave oven work is the one that give my satellite system nothing but headaches.  It all started in 1945.  Percy Spencer (http://en.wikipedia.org/wiki/Percy_Spencer) was working on magnetrons which create the radio signals for radar.  He noticed that the chocolate bar in his pocket started to melt when he stood in front of it.  Then he intentionally put popcorn in front of the transmitted and popcorn popped all around the room.

Water has a resonance frequency peaking at 22.24 GHz. This means that water will absorb transmissions at this frequency, and to a lesser extent spreading outward to in both directions to cover 18 to 26 GHz.  The absorbed energy of the radio waves is converted into heat energy, and heats the surrounding materials.  In Percy’s first case, the energy heated the chocolate bar causing it to melt.  (I do wonder about the rest of him.)

The frequency range of 18 to 27 GHz is referred to as the K-band.  Above it is the Ka band (27 to 40 GHz) and under it is the Ku band (12 to 18 GHz).  Yes, the a stands for above and the u stands for under.  K-band transmissions do not get through the atmosphere of the earth for any reasonable distance due to the moisture in the air.  The signal is absorbed by the water in the air.  The Ka and Ku bands are impacted by moisture in the air, but it takes more moisture since it is not at the peak resonance frequency.  Usually water will need to take the form of a heavy rain or snow event in the transmission path to degrade it to unusable.  Hence the generally used term of rain fade.

You’re probably wondering why we don’t shift to a different band.  Well, the other bands (like C band) have power limitations so it doesn’t interfere with terrestrial microwave signals.  The Ku band allows us a higher power which, in turn, allows us a smaller dish.

Next time your waiting for your popcorn to come out of the microwave, you should thank Percy for standing in front of his powerful transmitting antenna.  (An action that today is forbidden by FCC and OSHA guidelines.)  You will also remember that the microwave oven is a cousin to my satellite system’s transmitter.